New Structural Materials Technologies:Opportunities for the Use of Advanced

Ceramics and Composites

September 1986

NTIS order #PB87-118523

Recommended Citation:U.S. Congress, Office of Technology Assessment, New Structural Materials Technologies:Opportunities for the Use of Advanced Ceramics and Composites–A Technical IMemoran-dum, OTA-TM-E-32 (Washington, DC: U.S. Government Printing Office, September 1986).

Library of Congress Catalog Card Number 86-600551

For sale by the Superintendent of DocumentsU.S. Government Printing Office, Washington, DC 20402

Foreword

This technical memorandum responds to a joint request from the House Commit-tee on Science and Technology and the Senate Committee on Commerce, Science, andTransportation to analyze the military and commercial opportunities presented by newstructural materials technologies and outline the Federal research and development pri-orities which are consistent with those opportunities. This memorandum is part of alarger assessment which will address the impact of advanced structural materials onthe competitiveness of the U.S. manufacturing sector, and offer policy options for ac-celerating the commercial utilization of these materials.

New structural materials—ceramics, polymers, metals, or hybrid materials derivedfrom these, called composites—open a promising avenue to renewed international com-petitiveness of U.S. manufacturing industries. There will be many opportunities for useof the materials in aerospace, automotive, industrial, medical, and construction appli-cations in the next 25 years.

In recent years, several excellent studies have been carried out on both ceramicsand polymer matrix composites. This memorandum draws on this body of work andpresents a broad picture of where these technologies stand today and where they arelikely to go in the future.

OTA appreciates the assistance provided by the contractors, advisory panel, andworkshop participants, as well as the many reviewers whose comments helped to en-sure the accuracy of the report.

# A k 4 +c d.JOHN H. GIBBONSDirector

.,,///

New Structural Materials Technologies Advisory Panel

Rodney W. Nichols, ChairmanExecutive Vice President, The Rockefeller University

J. Michael Bowman Arthur F. McLeanDirector, Composites Venture Manager, Ceramics ResearchE.I. du Pent de Nemours & Co. Ford Motor Co.

Robert Buffenbarger Joseph PanzarinoChairman, Bargaining Committee Director, R&D of High Performance CeramicsInternational Association of Machinists Norton Co.

Joel Clark Dennis W. ReadeyAssociate Professor of Materials Systems Chairman, Ceramics Engineering DepartmentMassachusetts Institute of Technology Ohio State University

Laimonis Embrekts B. Walter RosenConsultant PresidentDix Hills, NY Materials Sciences Corp.

Samuel Goldberg Amy L. WaltonPresident, INCO-US Inc. Member, Technical StaffNew York, NY Jet Propulsion Laboratory

Sheldon Lambert Alvin S. WeinsteinConsultant ConsultantPiano, TX Concord, NH

James W. Mar Dick WilkinsProfessor, Department of Aeronautics and Director, Center for Composite Materials

Astronautics University of DelawareMassachusetts Institute of Technology

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OTA Project Staff

Lionel S. Johns, Assistant DirectorOTA Energy, Materials, and International Security Division

Peter D. Blair, Energy and Materials Program Manager

Richard E. Rowberg, Energy and Materials Program Manager, June 1985—December 1985

Gregory Eyring, Project Director, November 1985—present

Thomas E. Bull, Project Director, June 1985—November 1985

Joan Adams, Analyst, June 1985–January 1986

Laurie Evans, Analyst, June 1986—present

Administrative Staff

Lillian Chapman Linda Long

Contractors

Elaine P. RothmanMassachusetts Institute of TechnologyCambridge, MA

D.B. MarshallRockwell International Science CenterThousand Oaks, CA

J.E. RitterUniversity of MassachusettsAmherst, MA

J.J. MecholskyThe Pennsylvania State UniversityState College, PA

Julie M. ShoenungJoel P. ClarkMassachusetts Institute of TechnologyCambridge, MA

David W. RichersonCeramatec, Inc.Salt Lake City, UT

R. Nathan KatzAlfred L. BrozArmy Materials Technology LaboratoryWatertown, MA

Reginald B. StoopsR.B. Stoops & AssociatesNewport, RI

Carl ZwebenGeneral Electric Co.Philadelphia, PA

Kenneth L. ReifsniderVirginia Polytechnic Institute and

State UniversityBlacksburg, VA

John BuschFrank FieldMaterials Modeling AssociatesMassachusetts Institute of TechnologyCambridge, MA

Dale E. ChimentiThomas J. MoranJoseph A. MoyzisAir Force Materials LabWright Patterson AFBDayton, OH

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Ceramics Workshop Participants: Nov. 14-15, 1985

Alfred BrozArmy Materials Technology LaboratoryWatertown, MA

Joel ClarkMassachusetts Institute of TechnologyCambridge, MA

Tom HensonGTE ProductsTowanda, PA

R. Nathan KatzArmy Materials Technology LaboratoryWatertown, MA

Jack MecholskyPennsylvania State UniversityState College, PA

David RichersonCeramatec, Inc.Salt Lake City, UT

John RitterUniversity of MassachusettsAmherst, MA

Elaine RothmanMassachusetts Institute of TechnologyCambridge, MA

Composites Workshop Participants: Nov. 21-22, 1985

Joel ClarkMassachusetts Institute of TechnologyCambridge, MA

John DeVaultHercules Aerospace Co.Salt Lake City, UT

Joseph MoyzisWright-Patterson AFBDayton, OH

Thaddeus HelminiakWright-Patterson AFBDayton, OH

Kenneth ReifsniderVirginia Polytechnic Institute & State UniversityBlacksburg, VA

Reginald B. StoopsR.B. Stoops & AssociatesNewport, RI

Charles L. TuckerUniversity of IllinoisUrbana, IL

Carl ZwebenGeneral Electric Co.Philadelphia, PA

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Future Applications of Ceramics Workshop Participants:Dec. 9, 1985

Charles BergNortheastern UniversityBoston, MA

Bill KuhnZesto-ThermCincinnati, OH

Rustum RoyPennsylvania State UniversityState College, PA

Jim WimmerGarrett Corp.Phoenix, AZ

Albert PaladinoAdvanced Technology VenturesBoston, MA

Future Applications of Composites Workshop Participants:Dec. 10, 1985

Charles BergNortheastern UniversityBoston, MA

Bob HammerBoeing Commercial Aircraft Co.Kirkland, WA

D. William LeeArthur D. Little, Inc.Cambridge, MA

Joe LeesE.I. du Pent de Nemours & Co.Wilmington, DE

Paul McMahanCelanese Research Corp.Summit, NJ

Darrell H. RenekerNational Bureau of StandardsGaithersburg, MD

John RiggsCelanese Research Corp.Summit, NJ

Charles SegalOmniaRaleigh, NC

. . .Vlll

.

Acknowledgments

In addition to those cited above, the following are thanked for useful review comments:

Seymour NewmanFord Motor Co.

Jerome PershU.S. Department of Defense

Richard HelmuthPortland Cement Association

Charles WestResin Research Laboratories

Henry BrownSociety for the Advancement of Material and

Process Engineering

Joseph KirklandE.I. du Pent de Nemours & Co.

Craig BallingerFederal Highway Administration

Michael GreenfieldNational Aeronautics and Space Administration

Manfred KaminskySurface Treatment Science International

Harris BurteAir Force Materials Lab

Jim StephanCoors Biomedical

Maxine SavitzGarrett Corp.

Robert GottschallU.S. Department of Energy

Geoffrey FrohnsdorffNational Bureau of Standards

Bernard MagginNational Research Council

Robert MurphyBASF Structural Materials, Inc.

Glenn KuebelerHercules Aerospace Co.

Dick DauksysHexcel

E.R. BellShell Chemical

Clyde YatesU.S. Polymeric

Gail DiSalvoCiba-Geigy

David ForestFerro Corp.

Nick SpencerCiba-Geigy

ix

Contents

PageOverview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Market Opportunities for Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Research and Development Priorities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Market Opportunities for Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Research and Development Priorities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7General Prerequisites for the Use of Advanced Structural Materials . . . . . . . . . . . . . . . . . 7

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . . . 11

The Advanced Structural Ceramics Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Properties of Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Ceramic Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Ceramic Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Design, Processing, and Testing of Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Health Hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Applications of Structural Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Future Trends in Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Research and Development Priorities for Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43The Polymer Composites Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44Composite Constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Properties of Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Design, Processing, and Testing of Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Health and Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Applications of Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Future Trendsin Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58Research and Development Priorities for Polymer Matrix Composites . . . . . . . . . . . . . . . 62

General Prerequisites of the Use of Advanced Structural Materials . . . . . . . . . . . . . . . . . . . 66Education and Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66Multidisciplinary Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66Integrated Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Systems Approach to Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

List of TablesTable No. Page

1. Some Future Applications of Structural Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112. Comparison of Physical and Mechanical Properties of Common

Structural Ceramics With Steel and Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . 123. Fracture Toughness and Critical Flaw Sizes of Monolithic and

Composite Ceramic Materials Compared With Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . 144. Common Processing Operations for Advanced Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . 165. Selected Processes for the Production of Ceramic Coatings and for the

Modification of Ceramic Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196. Characteristics and Properties of Ceramic Coatings Often Considered Desirable . . . . 207. Comparison of the Mechanical Properties of Various Cements and Aluminum. . . . . . 218. Comparison of Some Possible Production-Level NDE Techniques

for Structural Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Contents—continued

Table No. Page

9. Future Diesel Engine Technology Development Scenario . . . . . . . . . . . . . . . . . . . . . . . . . 3210. Structural Ceramic Technology: Federal Government Funded R&D. . . . . . . . . . . . . . . . 3811. Breakout of the Fiscal Year 1985 Structural Ceramic Budget According to

the R&D Priorities Cited in the Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4112. Production Techniques for Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4913. NDE Techniques Appropriate for Production, Finished Product, Depot,

and Field-Level Inspections of Polymer Matrix Composite Structures . . . . . . . . . . . . . 4914. Budgets for Polymer Matrix Composite R&D in Fiscal Years 1985 and 1986 . . . . . . . . 6215. Hypothetical Multidisciplinary Design Team for a Ceramic Component. . . . . . . . . . . . 66

List of FiguresFigure No. Page

l. The Family of Structural Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92. Probability of Failure of a Ceramic Component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133. Estimated Scenario for Implementation of Ceramic Components

in Structural Application Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244, Projected U.S. Markets for Structural Ceramics in the Year 2000 . . . . . . . . . . . . . . . . . . 255. Load-Deflection Curve for a SiC/Glass-Ceramic Composite . . . . . . . . . . . . . . . . . . . . . . . 366. Composite Reinforcement Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437. Comparison of the Specific Strength and Stiffness of Various Composites and Metals 448. Comparison of General Characteristics of Thermoses and Thermoplastic Matrices. . . . 459. Composite Aircraft Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

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OVERVIEW

The past 25 years have witnessed remarkabledevelopments in structural materials technologies;new nonmetallic materials, such as ceramics andpolymer matrix composites, are replacing metalsin a wide variety of applications, ranging fromcutting tools to tennis rackets. The new materi-als represent both an opportunity and a challengefor U.S. manufacturing industries. In an era ofconcern about the long-term economic viabilityof the manufacturing sector, they open a prom-ising avenue to renewed international competi-tiveness, new industries, and employment oppor-tunities,

By the year 2000, large markets for ceramic andcomposite products will be found in military,aerospace, automotive, medical, and constructionapplications, However, these opportunities can-not be taken for granted. The United States doesnot enjoy a comfortable lead (indeed it often lags)in new structural materials technologies; key ad-vances continue to come from abroad. Moreover,through well-coordinated government-industry ef-forts, several countries have succeeded in bring-ing ceramic and composite products to the mar-ket years in advance of comparable U.S. products.

Japan in particular has initiated aggressive pro-grams to commercialize its evolving materialstechnologies, In order to realize the promise rep-resented by the new materials, a consensus is de-veloping in the materials community that U.S.policymakers in government, universities, and in-dustries will have to come together to define fea-sible goals and materials applications, coordinateresearch and development efforts toward thesegoals, and, especially, focus on reducing the bar-riers to commercial introduction of advanced ce-ramic and composite products.

This technical memorandum assesses the oppor-tunities for the use of structural ceramics and poly-mer matrix composites in the next 25 years, out-lines the research and development prioritiesimplied by those opportunities, and concludeswith a discussion of some key prerequisites fortheir realization. A parallel treatment of metal ma-trix composites, as well as a discussion of publicpolicy options for accelerating the developmentand commercialization of advanced structural ma-terials, is deferred until the final report. ]

‘Projected publication date September 1987.

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SUMMARY

In the past 25 years, new structural materials,such as ceramics, polymers, and composites, havebrought a revolutionary change to the field ofengineering. Never before have engineers had acomparable opportunity to design the materialsthey use as well as the structures they build. Infact, with advanced ceramics and composites, thevery concepts of materials and structures mergetogether, joined by the new concept of integrateddesign. Such designs consolidate discrete parts andfunctions into a single, multifunctional structure,leading to highly efficient use of materials andlower overall costs.

In the future, new structural materials will pro-vide a powerful leverage point for the manufac-turing sector of the economy: not only can ce-ramic and composite components deliver superiorperformance, they also enhance the performanceand value of the larger system in which they areincorporated. When this multiplier effect is takeninto account, the impact of advanced structuralmaterials on the gross national product (GNP),balance of trade, and employment could be dra-matic. All of the industrialized countries have rec-ognized the opportunities and are competing ac-tively for shares of the large commercial andmilitary markets at stake.

The Federal Government plays two importantroles in the research and development of advancedceramics and composites: one military, and theother nonmilitary. The Department of Defensefunds some 35 percent of Federal R&D on struc-tural ceramics and about 70 percent of that onpolymer matrix composites. The military andnonmilitary agencies have very different fundinggoals and materials requirements.

In the case of the military, the government isthe customer for materials technology and hard-ware. It has an interest in securing stable, domesticsources of material supply. Although materialscost is an important consideration, typically theperformance advantages obtained are more im-portant. Military applications often provide a first“window” for the use of advanced materials; how-ever, the technologies may or may not be “spunoff” into commercial applications. Because of the

emphasis on high performance and the fact thatproduction volumes are typically low, often nei-ther the materials nor the production methods areappropriate for commercial applications. For na-tional security reasons, restrictions are placed onthe dissemination of DOD-funded research resultsin scientific publications and at meetings; exportcontrols may also be placed on shipments of thematerials abroad. These restrictions tend to fur-ther discourage the commercial utilization of ma-terials technologies developed by the military. Inthe future, demand for high performance mate-rials in such applications as aircraft, missiles, andspace-based weapons systems will increase, mak-ing these among the fastest growing applicationsof structural ceramics and composites.

About half of total Federal spending for ad-vanced ceramics and polymer matrix compositesR&D is nonmilitary in nature, including most ofthat funded by the Department of Energy, theNational Aeronautics and Space Administration(NASA), the National Science Foundation, theNational Bureau of Standards, and the Bureau ofMines. These agencies generally do not act as theprocurers of hardware, but instead fund materi-als research ranging from basic science to tech-nology demonstration programs, according totheir mission objectives. Where appropriate, theyopenly seek to transfer materials technology tothe private sector. For a variety of reasons, thesuccess rate of these efforts has been low: the tech-nology may require significant further develop-ment prior to utilization; the market opportuni-ties for the technology may be too long-term orpoorly defined; or key decisionmakers within thereceiving company may not be committed to com-mercializing the technology. Federal nonmilitarymaterials R&D clearly represents a resource thatcan be exploited more effectively by U.S. com-panies in the future.

An important policy objective for the futurewill be to encourage industrial investment in re-search, development, and commercialization ofceramic and composite materials. Factors dis-couraging these investments include: fragmenta-tion of industries and markets, which precludes

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3

any one segment from undertaking the necessaryresearch; concerns that R&D investments cannotbe protected because of the high transferabilityof the technologies; and the relatively high costof capital in the United States compared with com-petitor nations such as Japan. Federal efforts tofacilitate collaborative R&D arrangements be-tween industries, universities, and governmentlaboratories, to tighten patent protection for in-dustrial products and processes, and to articulatepolicy clearly on export controls and on productliability issues, are likely to be very important.Ultimately, however, a strong U.S. competitiveposition depends on a judgment on the part ofmanufacturers that the new materials offer costand performance advantages in the marketplace.

Introduction

Structural materials may be classified as ce-ramics, polymers, metals, or hybrid materials de-rived from these, called composites, Wood andoyster shells are examples of composites that havebeen exquisitely engineered by nature to performtheir unique functions. Similarly, manmade com-posites are engineered materials whose propertiescan be tailored to match the design requirementsof a particular structure. The most common com-posites consist of a host (matrix) material whichis reinforced with particles, whiskers (short sin-gle crystals), or fibers of a second material; thus,there are ceramic matrix composites, polymer ma-trix composites, and metal matrix composites.

Although ceramics, polymers, and metals gen-erally have complementary combinations of prop-erties, there are many applications in whichdesigners can choose from several alternativematerials. A new structural material must there-fore demonstrate cost or performance advantages(often both) if it is to be chosen over a more fa-miliar material. Very often, ceramics and com-posites are competing with well-characterizedalloys of aluminum, titanium, steel, and high-temperature superalloy, Furthermore, new alloysare being developed and metal processing meth-ods are continually improving. Thus, the mini-mum cost and performance requirements whichmust be satisfied by a ceramic or composite arebecoming more stringent. This competitionamong different materials has two important

corollaries. First, the characteristics of structuralceramics and composites make them more suit-able for some applications than others; it is im-portant for government and industry policy-makers to identify the opportunities correctly.Second, the many technical and economic factorswhich could tip the balance in favor of one ma-terial or another make the market penetration ofceramics and composites difficult to predict.

Ceramics

Ceramics encompass such a broad class of ma-terials that they are more conveniently defined interms of what they are not, rather than what theyare. Accordingly, they may be defined as all solidswhich are neither metallic nor organic. Comparedwith metals, ceramics have superior wear resis-tance, high-temperature strength, and chemicalstability; they also generally have lower electri-cal and thermal conductivity, and lower tough-ness. The low toughness of ceramics (brittleness)causes them to fail suddenly when applied stressis sufficient to propagate cracks which originateat microscopic flaws (e.g., cracks, voids, or inclu-sions as small as 20 micrometers) in the material.The actual stress level at which this occurs canbe very high (e.g., 300,000 psi, or 2,069 MPa) andtheoretically could be much higher, if flaw sizescould be reduced. Unpredictable failure caused bypoor control over flaw populations is the mostserious handicap to the use of structural ceramicsin load-bearing structures.

Several approaches have been taken to improvethe toughness of ceramics. The most satisfactoryis to design the microstructure of the material toresist the propagation of cracks. Ceramic matrixcomposites, which contain dispersed ceramic par-ticulate, whiskers or continuous fibers, are anespecially promising technology for tougheningceramics. A different approach is the applicationof a thin ceramic coating to a metal substrate. Thisyields a component with the surface properties ofa ceramic combined with the high toughness ofmetal in the bulk.

Most advanced structural ceramics are fabri-cated by consolidation of a compact of pure,finely divided oxide or nonoxide powders in a fur-nace at high temperatures. However, another class

4 —

of ceramics, called chemically bonded ceramics,develops its strength at room temperature, typi-cally through chemical reactions involving the up-take of water. Traditional cement pastes and con-cretes fall in this category. Because these materialsnormally contain large flaws which limit theirstrength, they are not ordinarily considered “ad-vanced. ” However, in recent years the propertiesof chemically bonded ceramics have improveddramatically as new processing methods have re-duced the strength-limiting flaws. The low costand flexible fabrication methods of chemicallybonded ceramics could permit them to displaceplastics and metals in a wide variety of appli-cations.

Market Opportunities for Ceramics

Market demand for structural ceramics is notdriving their development in most applications atthe present time. In 1983, the world market foradvanced structural ceramics was estimated as$250 million, primarily in heat- and wear-resistantapplications. Projections to the year 2000 placethe U.S. market variously between $1 and $5 bil-lion, spread among many new applications dis-cussed below. The larger estimates assume sub-stantial utilization of ceramics in automotive heatengines, long predicted to be the most importantapplication of structural ceramics. The analysisbelow indicates that significant use of ceramicsin automotive heat engines is not likely by theyear 2000. However, large near-term opportuni-ties are available in other products, such as med-ical devices.

Japan is the principal competitor to the UnitedStates in advanced ceramics technology. Japanesecompanies have made a long-term commitmentto the development of advanced ceramics whichlooks beyond the current weak market demand.They have mounted a well-coordinated effortwhich includes: increasing the quality and avail-ability of raw materials; optimizing fabricationprocesses; installing the most advanced process-ing and quality control equipment; initiating man-ufacturing development; and aggressive market-ing of evolving materials. This commitment to thefuture of ceramics is likely to give Japanese com-panies the early lead in capturing the large near-term markets.

Current Production

Ceramics such as alumina, silicon nitride, andsilicon carbide are in production for wear parts,cutting tools, bearings, and coatings. The mar-ket share for ceramics in these applications is gen-erally less than 5 percent, but substantial growthis expected, and the U.S. markets for the ceramiccomponents alone could be over $2 billion by theyear 2000. Research and development funding iscurrently being provided by industry and is drivenby competition in a known market.

Ceramics are also in limited production (inJapan) in discrete engine components such asturbochargers, glow plugs, and precombustionchambers. Current military applications includeradomes, armor, infrared windows, and heatsources.

Near-Term Production

Near-term production (next 10 to 15 years) isexpected in advanced bearings, bioceramics, con-struction applications, heat exchangers, electro-chemical devices, discrete components in auto-mobile engines, and military applications. Thetechnology feasibility has generally been demon-strated, but scale-up, cost reduction, and designoptimization are required. Large markets are atstake: especially promising are bioceramics fordental and orthopedic implants, and chemicallybonded ceramics for construction applications.Wisely directed government funding will berequired to solve the remaining problems andachieve a production capability competitive withforeign sources.

Long-Term Applications

Long-term applications (beyond 15 years) arethose which require solution of major technicaland economic problems. These include an ad-vanced automotive turbine engine, the advancedceramic diesel (although ceramics could be usedin military versions of these engines at an earlierdate), some electrochemical devices, military com-ponents, and heat exchangers. A variety of otherturbine engines, especially turbines for aircraftpropulsion and for utility-scale power generation,should also be categorized as long term. In gen-eral, the risks are perceived by U.S. industry to

5

be too high to justify funding the needed research.Advances in these applications are likely to bedriven by government funding.

Research and Development Priorities

The following hierarchy of R&D priorities isbased on the technical barriers that must be over-come before ceramics can be used in the applica-tions discussed above.

Very Important

Processing Science .—This is the key to under-standing how processing variables such as tem-perature, composition, and particle size distribu-tion are connected to the desired final propertiesof the ceramic.

Environmental Behavior.—In many applica-tions, ceramics are required to withstand hightemperature and corrosive or erosive environ-ments. Information on the long-term behavior ofceramics in these environments is essential to pre-dict the service life of ceramics in those appli-cations.

Reliability .—The reliability of advanced ce-ramics and ceramic composites is the single mostimportant determinant of success in any applica-tion. Progress requires advances in brittle mate-rials design, process control, nondestructive eval-uation, understanding crack growth processes,and life prediction.

Ceramic Composites.—These novel materialsoffer an exciting opportunity to increase thestrength and toughness of ceramics.

Important

Joining. -Joining of ceramics to metals, glasses,and other ceramics is necessary when ceramiccomponents are incorporated into larger systems.The principles developed in joining research canalso be applied to coating technology.

Tribology.—This is the study of friction, wear,and lubrication of surfaces in relative motion. Itis especially relevant to understanding the degra-dation of ceramic wear parts, bearings, and thelubrication requirements of high-temperature ce-ramic engines.

Standardization and Testing.—Although stand-ards exist for ceramic refractories and brick, atpresent there is a lack of standards for advancedceramics, whether for testing procedures, com-positions, or processing history. Such standardsare a prerequisite for confident design and indus-try acceptance of ceramics.

Desirable

Chemically Bonded Ceramics.—These materi-als, which include advanced cement pastes andconcretes, represent an outstanding potential forlow-cost, net shape fabrication of ceramic struc-tures. The cost savings associated with the use ofimproved concretes in highways and bridges arelikely to be very large compared with the researchinvestment required.

Polymer Matrix Composites

Polymer matrix composites are organic poly-mers which have been reinforced with a varietyof short or continuous fibers to provide addedstrength and stiffness. The composite is designedso that the mechanical loads to which the struc-ture is subjected in service are supported by thereinforcement. The amount of reinforcement andits geometry can be varied across the structure,resulting in highly efficient use of material, con-solidation of parts, and weight savings.

Polymer matrix composites are often dividedinto two categories: reinforced plastics and “ad-vanced composites. ” The distinction is basedon the level of mechanical properties (usuallystrength and stiffness); however, there is no un-ambiguous line separating the two. Reinforcedplastics, which are relatively inexpensive, typi-cally consist of polyester resins reinforced withlow-strength glass fibers; they have been in usefor 30 or 40 years in applications such as boathulls, corrugated sheet, pipe, automotive panels,and sporting goods. Advanced composites, whichare used primarily in the aerospace industry, con-sist of fiber and matrix combinations which yieldsuperior strength and stiffness, They are relativelyexpensive and typically contain a large percent-age of high-performance continuous fibers (suchas high-strength glass, graphite, aramid, or otherorganic fibers). In this technical memorandum,

6

market opportunities for both reinforced plasticsand advanced composites are considered.

Chief among the advantages of polymer com-posites is their light weight coupled with high stiff-ness and strength along the direction of the rein-forcement. This combination is the basis of theirusefulness in aircraft, automobiles, and othermoving structures. Other desirable propertiesinclude corrosion and fatigue resistance. Onegeneric limitation of polymer matrix compositesis temperature; an upper limit for service temper-atures with present composites is about 600° F(316° C). With additional development, temper-atures above 8000 F (4270 C) might be achieved.

In addition to their superior physical and me-chanical properties, polymer composites offermany design and manufacturing advantages. Per-haps the most important of these is the opportu-nity to consolidate a large number of parts intoone, thus reducing assembly costs. This capabil-ity has already been demonstrated in aircraft andautomotive applications. For example, it has beenestimated that the 1,500 structural parts in an au-tomobile could be reduced to a few hundred usingcomposites, resulting in major savings in toolingand manufacturing costs.

Most advanced composites today are used inthe aerospace industry. They are fabricated by alaborious process called “lay -up.” This typicallyinvolves placement of sequential layers of poly-mer-impregnated fiber tapes on a mold surface,followed by heating under pressure to cure thelay-up into an integrated structure. Althoughautomation is beginning to speed up this process,production rates are still too slow to be suitableto high-volume, low-cost applications such asautomotive production lines. New compositefabrication methods which are much faster andcheaper will be required before composites cansuccessfully compete with metals in these appli-cations.

Market Opportunities forPolymer Matrix Composites

Fiberglass composites have been in use since the1940s, and about 85 percent of the materials usedin the reinforced plastics/composites industrytoday are glass fiber reinforced polyester resins.

Less than 2 percent of the materials are “ad-vanced” composites such as those used in aircraftand aerospace. Worldwide sales of advanced com-posites have been projected to grow by 15 per-cent annually for the remainder of the century,increasing from a 1985 value of $1.4 billion tonearly $12 billion by the year 2000. The industrycontinues to be driven by aerospace markets, withdefense applications likely to grow by as muchas 22 percent per year in the next few years.

Current Production

Aerospace applications of advanced polymercomposites account for about 60 percent of cur-rent sales. Sporting goods, such as golf clubs andtennis rackets, account for another 20 percent.The sporting goods market is considered mature,with projected annual growth rates of 3 percent.Automobiles and industrial equipment round outthe current list of major users of advanced com-posites, with a 15 percent share.

Near-Term Production

Composites were introduced into the horizon-tal stabilizer of the F-14 fighter in 1970, and havenow become the baseline materials in high per-formance fighter and attack aircraft. The next ma-jor challenge for composites will be use in largemilitary and commercial transport aircraft. Com-posites currently comprise about 3 percent of thestructural weight of commercial aircraft such asthe Boeing 757, but could eventually account formore than 65 percent. No technical barriers areforeseen which might prevent this scenario, al-though design and certification databases for civilapplications need to be established. Since fuel sav-ings are a major reason for the use of compositesin commercial aircraft, continued low fuel pricescould delay their exploitation.

The largest volume opportunity for polymercomposites is in the automobile. Composites cur-rently are in limited production in body panels,drive shafts, and leaf springs. By 1995, compos-ite unibody frames could be introduced in limitedproduction. The principle advantage of a com-posite unibody would be the potential for partsconsolidation resulting in lower assembly costs.If composites were to be used extensively in uni-bodies, the effects on the way automobiles aredesigned, built, and serviced would be dramatic.

7

However, production methods some 10 timesfaster than current methods will have to be de-veloped in order to take advantage of this oppor-tunist y.

Additional near-term markets for polymer com-posites include medical implants, reciprocating in-dustrial machinery, storage and transportation ofcorrosive chemicals, and military vehicles andweapons.

Long-Term Applications

Beyond the turn of the century, compositescould be used extensively in construction appli-cations such as bridges, buildings, and manufac-tured housing. Realization of this opportunity willdepend on development of cheaper materials andon designs which take advantage of compound-ing benefits of composites, such as reduced weightand increased durability. In space, a variety ofcomposites will be used in the proposed aerospaceplane, and composites are being considered forthe tubular frame of the space station. Compos-ites of all kinds, including metal matrix, ceramicmatrix, and polymer matrix, will be a central fea-ture of space-based weapons systems such as thoseunder consideration for ballistic missile defense.

Research and Development Priorities

Polymer matrix composites have achieved anexcellent service record and experience indicatesthat reliable composite structures can be fabri-cated. However, in many cases the technology hasoutrun the basic understanding of these materi-als. In order to generate improved materials andto design and manufacture composites more cost-effectively, the following needs should be ad-dressed.

Very Important

Processing Science.—Development of new,low-cost fabrication methods will be critical forcomposites. An essential prerequisite to this is asound scientific basis for understanding how proc-ess variables affect final properties.

Impact Resistance.—This property is crucial tothe reliability and durability of composite structures.

Delamination.—A growing body of evidencesuggests that this is the single most importantmode of damage propagation.

Interphase.—The poorly understood interracialregion between the fiber and matrix has a criticalinfluence on composite behavior.

Important

Mechanical Properties.—Better scientific under-standing and modeling in the following areaswould permit development of better materials andimproved designs: strength, fatigue, and fracture.

Environmental Effects.—The environments towhich composites are subjected can have a dele-terious effect on their long-term reliability. Inves-tigation of the effects of moisture and tempera-ture are particularly important.

Reinforcement Forms and Hybrid Compos-ites.—Innovations in both of these areas wouldprovide opportunities to improve properties andreduce costs. The large number of variables makesa purely empirical approach very inefficient.

Test Methods.—Lack of standardized test meth-ods contributes to the considerable variability inreported property values.

Desirable

Viscoelastic and Creep Properties.—Deforma-tion resulting from sustained loading can have anadverse effect on structural performance. Agreater database is required, especially for com-pressive loading.

General Prerequisites for the Use ofAdvanced Structural Materials

Education and Training

At present, the demand for scientists and engi-neers trained in ceramics and composites technol-ogies outstrips the supply. Education may be themost effective tool available to government foraccelerating the development of advanced mate-rials in the long run. Several aspects should beconsidered. Undergraduate introductory coursesin ceramics and composites should be offered to

8

engineering majors. Undergraduate courses spe-cializing in design with these materials should alsobe available to all students in applied mechanics,including mechanical and civil engineers. Provi-sion should be made through seminars and shortcourses for continuing education of engineers inother areas who want to learn more about ce-ramics and composites. Also, education of design-ers, planners, managers, and the general publicthrough newspapers, magazines, and televisionwill be especially important in creating demandfor products made from these high-technologymaterials.

Multidisciplinary Approach

In recent years there has been an increasing ap-preciation of the fact that many problems in sci-ence and technology are most effectively solvedthrough the combined efforts of specialists froma variety of disciplines. In no case is this moreapparent than with advanced materials. The na-ture of ceramics and composites is such that theprocess which transforms the raw materials intothe finished product integrates the discrete stepsof design, manufacturing, and testing into a co-herent whole. The familiar distinctions betweenthe product and the material from which it is madebecome blurred, in a way which is qualitativelydifferent from our experience with traditional ma-terials. This integrative property implies that amultidisciplinary team of experts is more likelyto solve the problems associated with the devel-opment and application of advanced materialsthan an alternative approach involving individ-ual researchers.

Integrated Design

The outstanding opportunities presented by ce-ramics and composites do not lie in part-for-partreplacement of metal in current designs; rather,new designs which incorporate many parts andfunctions into one are made possible. These newstructural shapes and systems for carrying loadsmay not look at all like metal designs. Such a de-sign capability will require the development of so-phisticated software for computer modeling and

analysis, as well as an extensive database on ma-terials properties. Perhaps the most important re-quirement is for a better scientific understandingof how the properties of the microscopic con-stituents determine the overall behavior of thestructure. Many of the research and developmentpriorities above are aimed at providing this in-formation.

Systems Approach to Costs

High cost is often cited as the largest barrierto the use of advanced ceramics and composites.Indeed, on a dollar-per-pound basis, today’s ce-ramics and composites often cost several times asmuch as the materials they would replace. Al-though costs can be expected to decline as pro-duction volumes increase and manufacturing tech-nologies mature, the baseline costs per pound ofmany ceramics and composites may always behigher than those of steel and aluminum. How-ever, rather than a barrier, cost may in fact bea potential advantage of the new materials, if the“system costs, “ including materials, manufactur-ing, and life cycle of a structure, are all taken intoaccount. Like multidisciplinary research and in-tegrated design, a systems approach to costs is im-plied by the holistic nature of advanced materials.

In conclusion, advanced structural materialsrepresent evolving technologies which will dra-matically affect the U.S. economy. The near-termmarkets for these materials are large, and the long-term opportunities are just beginning to be rec-ognized. Although the United States faces stiff in-ternational competition for the developing worldmarkets, the United States does possess severalnatural strengths which should enable it to excelin this competition. These include a strong univer-sity system, a well-developed computer and soft-ware industry, and the largest domestic marketfor materials in the world. The challenge to na-tional policy makers is to define a Federal rolewhich facilitates the maximum exploitation ofthese natural strengths. The most important pol-icy issues and options for addressing them willbe explored in the final report.

INTRODUCTION

The past 25 years have witnessed an unprece-dented explosion of new structural materials avail-able to the designer and engineer. Polymers,ceramics, and composites promise to bring a rev-olution in the performance of a wide variety ofengineering components and structures, and in theway those structures are designed and fabricated.Today the pace of technological change in thisfield is rapid, and the scope of the opportunitypresented by the new materials is only beginningto be recognized.

In addition to changing the engineering land-scape, advanced structural materials have alsoaltered the boundaries of traditional policy con-cerns relating to materials. Historically, the Fed-eral interest in materials has centered around theproblem of shortages of supply of certain “criti-cal” minerals. For example, the United States isdependent on foreign sources for virtually all ofits chromium, manganese, cobalt, and platinumgroup metals. These metals are important ingre-dients in a variety of steel alloys and superalloy.To a limited extent, ceramics and composites,which are made from plentiful raw materials, canalleviate this problem through substitutional How-ever, the potential of the new materials goes farbeyond simple substitution; they provide per-formance advantages in both military and com-mercial applications which cannot be achievedwith metals. Ceramics and composites are nowconsidered to be “critical” in their own right, bothfor national defense and future economic competi-tiveness. Therefore, the appropriate Federal rolein accelerating the development and commerciali-zation of these materials is likely to be a subjectof active debate in the years ahead.

Advanced materials are often ranked withmicroelectronics and biotechnology as the threemost promising “high-tech” industries of the fu-ture. Many countries around the world have rec-ognized the exceptional promise of these materialsand have targeted them for priority development,with the result that the United States, having pi-

‘U. S. Congress, Office of Technology Assessment, Strategic Afa-terials: Technologies To Reduce LI. S. Import L’ulnerability, OTA-ITE-248 (Washington, DC: U.S. Government Printing Office, May1985],

oneered many of the basic technologies, now findsitself in a heated race with Japan and several coun-tries in Europe to supply the growing world mar-kets. Fundamental advances in these technologiesare now being made overseas, so that the UnitedStates cannot afford to be complacent if it intendsto be competitive. It is important to note that inmost cases ceramic and composite structures donot stand alone, but are instead components thatenhance the performance of much larger systems,such as automobiles or aircraft. Thus, the mate-rials have a significance to the economy extend-ing far beyond the value of the materials them-selves.

Structural materials may be classified as ce-ramics, polymers, or metals, as shown in figure1. Two or more of these materials can be com-bined together to forma composite having prop-erties superior to those of the constituents alone.

Figure 1.— The Family of Structural Materials

The family of structural materials includes ceramics, poly-mers, and metals. Reinforcements added to these materialsproduce ceramic matrix composites (CMCs), polymer matrixcomposites (PMCs), and metal matrix composites (MMCs).Materials in the shaded regions are discussed in this report.

SOURCE: Off Ice of Technology Assessment

9

10

The most common form of composite is a hostmaterial (matrix) which is reinforced with particlesor fibers of a second material. Composites areclassified according to their matrix phase; thus,there are ceramic matrix composites (CMCs),polymer matrix composites (PMCs), and metalmatrix composites (MMCs). Wood, jade, and oys-ter shells are natural composites which displayremarkable strength and durability. A commonmanmade composite is glass fiber reinforced plas-tic (FRP), which combines the strength of the glasswith the toughness and corrosion resistance ofplastic,

Each type of structural material has its own ad-vantages and disadvantages. For example, metalsare strong, tough, inexpensive, and readily form-able, but are also heavy, chemically reactive, andlimited to service temperatures below about1,900° F (1,038° C). Ceramics are hard, chemi-cally stable, and useful at high temperatures, butare also brittle and difficult to fabricate. Polymersare light and easy to fabricate, but are relativelyweak and restricted to service temperatures be-low about 600° F (316° C).

Frequently, composites offer an excellent op-portunity to eliminate some of the undesirableproperties while retaining the desirable ones. Me-tals can be reinforced with ceramic fibers to ex-tend their useful strength to higher service tem-peratures; ceramics can be reinforced with ceramicfibers to reduce their brittleness; and polymers canbe reinforced with ceramic or organic fibers tomake them, pound for pound, the strongest ma-terials known. The great value of composites is

that the reinforcement type and geometry can bevaried to optimize the performance and minimizethe cost of the overall structure.

In the face of the challenge from ceramics andcomposites, metals technologies have not re-mained static. New steel and aluminum alloys andnew processes such as rapid solidification andpowder metallurgy techniques have been devel-oped. Metal matrix composites promise to in-crease the hardness and service temperature of me-tals without requiring a large investment in newmanufacturing plant and equipment. Also, vari-ous coatings and surface modification techniquescan be used to protect the metal so that it will notdeteriorate in hostile environments.

The diversity of structural materials availabletoday offers many options to the designer. Typi-cally, a given structure can be fabricated from anyof a variety of candidate materials. Only rarelywill the performance requirements dictate a sin-gle choice. Nor does the challenge to the primacyof metals come only from the most “advanced,”most expensive composites or ceramics: unrein-forced engineering polymers such as nylons orpolyamides, and chemically bonded ceramics (highperformance cements) offer an inexpensive andeffective alternative to metals in many applica-tions. This capacity for materials substitution alsocomplicates the task of projecting the future sharesof materials in various markets. However, one canpredict with confidence that the properties andmanufacturing advantages of ceramics and com-posites will make them as familiar in the 21st cen-tury as metals are today.

CERAMICS

Ceramics may be defined as nonmetallic, in-organic solids. By far the most common of ter-restrial materials, ceramics in the form of sandand clay have been used for many thousands ofyears to make brick, pottery, and artware. Mod-ern structural ceramics bear little resemblance tothese “traditional” materials; they are made fromextremely pure, microscopic powders which areconsolidated at high temperatures to yield a dense,durable structure.

The Advanced StructuralCeramics Industry

The world market for advanced structural ce-ramics in 1983 was only $250 million. z Of this,the Japanese market made up nearly half. Theseceramics were primarily used in heat- and wear-resistant applications. The limited current mar-kets for ceramics do not exert a strong “marketpull” on the technology; at present, the situationis characterized by “technology push. ” In the next10 to 15 years, however, the market opportuni-ties for structural ceramics are expected to expandrapidly (table l), such that by the year 2000 U.S.markets are projected variously between $1 bil-

2G .B. Kenney and H .K, Bowen, “High Tech Ceramics in Japan:Current and Future Markets, ” American Ceramic Society Bulletin62(5):590, 1983.

Table 1 .—Some Future Applications ofStructural Ceramics

Application Performance advantages Examples

Wear parts High hardness. low frictionsealsbearingsvalvesnozzles

Cutting tools High strength, hot hardnessHeat engines Thermal insulation, high

diesel components temperature strength, fuelgas turbines economy

Medical Implants Biocompatibility, surfacehips bond to tissue, corrosionteeth resistancejoints

ConstructIon Improved durability, lowerhighways overall costbridgesbuildings

SOURCE Off Ice of Technology Assessment

Sillcon carbide,alumina

SIllcon nitrideZlrconia, silicon

carbide, sili-con nitride

Hydroxylapatite,bioglass,alumina,zirconia

Advanced ce-ments andconcretes

lion and $5 billion.3 This estimate reflects onlythe value of the ceramic materials and compo-nents, and does not include the value of the over-all systems in which they are incorporated. Whenthis multiplier effect is taken into account, a re-cent study has indicated that the country whichleads in the development of advanced ceramicswill reap large benefits in the form of jobs andeconomic expansion.4

Properties of Ceramics

The properties of some common structural ce-ramics are compared with those of metals in ta-ble 2. In general, ceramics have superior high-temperature strength, higher hardness, lower den-sity, and lower thermal conductivity than metals.The principal disadvantage of ceramics as struc-tural materials is the sensitivity of their strengthto extremely small flaws, such as cracks, voids,and inclusions. Flaws as small as 10 to 50 microm-eters can reduce the strength of a ceramic struc-ture to a few percent of its theoretical strength.Because of their small sizes, the strength-control-ling flaws are usually very difficult to detect andeliminate.

The flaw sensitivity of ceramics illustrates theimportance of carefully controlled processing andfinishing operations for ceramic components.However, even with the most painstaking efforts,a statistical distribution of flaws of various sizesand locations will always exist in any ceramicstructure. Even “identically prepared” ceramicspecimens will display a distribution of strengths,rather than a single value. Design with ceramicsis therefore a statistical process, rather than a de-terministic process, as in the case of metals. Thesituation is illustrated in figure 2.

The curve on the right of figure 2a representsthe distribution of strengths in a batch of identi-cally prepared ceramic components. The curve onthe left is the distribution of stresses to which these

3Gr~~ Fischer, “Strategies Emerge for Advanced Ceramic Busi-ness, ” American Ceramic Society Buffetin 65(1):39, 1986.

4Larry R. Johnson, Arvind P.S. Teotia, and Lawrence G. Hill,“A Structural Ceramic Research Program: A Preliminary EconomicAnalysis, ” Argonne National Laboratory, ANL/CNSV-38, 1983.

11

12

Table 2.—Comparison of Physical and Mechanical Properties of Common Structural Ceramics WithSteel and Aluminum Alloys. SiC: silicon carbide; Si3N4: silicon nitride; ZrO2: zirconia

Strength a ThermalDensity a Room temperature at 1,095” C Hard nessb conductivity

Material (g/cm3) strength (M Pa) (M Pa) (kg/mm z) 250/1 ,100° (W/mC)

Various sintered SiC materials . . . . 3.2 340-550 (flexure) 340-550 (flexure) 2,500-2,790 85/1 75Various sintered Si3N 4 materials . . 2.7-3.2 205-690 (flexure) 205-690 (flexure) 1,366 17/60Transformation toughened ZrO, . . . 5.8 345-620 (flexure) — 625-1,125 1.713.5Steels (4100, 4300, 8600, and

5600 series) . . . . . . . . . . . . . . . . . . 7-8 1,035-1,380 (tensile yield) useless 450-650 43Aluminum alloy . . . . . . . . . . . . . . . . . 2.5 415-895 (tensile yield) useless 100-500 140-225NOTE. 1 MPa = 145 psi = O 102 Kg/mm Y,SOURCES: aR. Nathan Katz, “Applications of High Performance Ceramics in Heat Engine Design, ” Materials Science and Engineering 71 ‘227-249, 1985.

bElaine P Rothman, “Ultlmate Properties of Ceramics and Ceramic Matrix Composites, ” contractor report for OTA, December 1985

components are subjected in service, The over-lap between the two curves, in which the stressin service exceeds the strength of the ceramic, de-termines the probability that the part will fail.

There are several ways to reduce the probabil-ity of failure of the ceramic. One is to shift thestrength distribution curve to the right by elimi-nation of the larger flaws (figure 2b). A secondapproach is to use nondestructive testing or prooftesting to weed out those components that havemajor flaws. This leads to a truncation of thestrength distribution, as shown in figure 2c. Al-though proof testing of each individual compo-nent is expensive and can introduce flaws in thematerial which were not there originally, it iswidely used in the industry today.

A third approach is to design the microstruc-ture of the ceramic to have some resistance to frac-ture (increased “toughness”), and hence, some tol-erance to defects. Toughness is a measure of theenergy required to fracture a material in the pres-ence of flaws. For a ceramic component understress, the toughness determines the critical flawsize which will lead to catastrophic failure at thatstress. In fact, the critical flaw size increases withthe square of the toughness parameter; thus, anincrease in the material toughness of a factor of3 leads to a ninefold increase in the flaw sizetolerance. Reduction in the flaw sensitivity of cer-amics is especially important for applications in-volving a hostile environment which can intro-duce strength-degrading defects, and thus negateall efforts to ensure reliability by identifying oreliminating the largest preexisting flaws. Three re-cent developments have been shown to improvethe toughness of ceramics: microstructure design,

transformation toughening, and composite re-inforcement.

Microstructure Design

The toughness of monolithic ceramics can beimproved considerably by refinement of the poly-crystalline grain size and shape. The presence ofelongated fibrous grains, especially in ceramicsbased on silicon nitride, has been shown to in-crease toughness by as much as a factor of 2 overother monolithic ceramics, such as silicon carbideand aluminum oxide.5 Numerous mechanismshave been proposed to account for the observedtoughening: crack deflection, microcracking, re-sidual stresses, crack pinning, and crack bridg-ing. It is likely that several of these mechanismsoperate simultaneously in these materials. Thehigh toughness is accompanied by high strength,both of which result from the modified micro-structure. Commercial activity in the siliconnitride-based materials is projected to expand rap-idly by 1990 and many new applications shouldbe established by 1995.

Transformation Toughening

Transformation toughening is a relatively newapproach to achieving high toughness andstrength in ceramics and has great potential forincreasing the use of ceramics in applications re-quiring wear resistance. The key ceramic mate-rial is zirconium oxide (zirconia). Zirconia goesthrough a phase transformation from the

‘Nitrogen Ceramics, F.L. Riley (ed, ) (Boston and The Hague: Mar-tinus Nijhoff Publishers, 1983).

13

a)

b)

c)

Figure 2.— Probability of Failure ofa Ceramic Component

d Strength

The probability of failure of a ceramic component is the over-lap between the applied stress distribution and the materialstrength distribution, as shown in (a). This probability can bereduced by reducing the flaw size (b), or truncation of thestrength distribution through proof testing (c).

SOURCE R Nathan Katz, “Applications of High Performance Ceramics in HeatEngine Design,” Materials Science and Engineerfrig 71:227.249, 1985

tetragonal to the monoclinic crystal form whilecooling through a temperature of about 1,1500C (2,102o F). This phase transformation is accom-panied by an increase in volume of 3 percent, sim-ilar to the volume increase that occurs when waterfreezes. By control of composition, particle size,

and heat treatment cycle, zirconia can be densi-fied at high temperature and cooled such that thetetragonal phase is maintained down to room tem-perature. When a load is applied to the zirconiaand a crack starts to propagate, the high stressesin the vicinity of the crack tip catalyze the trans-formation of adjacent tetragonal zirconia grainsto the monoclinic form, causing them to expandby 3 percent. This expansion of the grains aroundthe crack tip compresses the crack opening, pre-venting the crack from propagating. The resultis a ceramic that is both tough and strong.

Ceramic Matrix Composites

A variety of ceramic particulate, whiskers(high-strength single crystals with length-to-diam-eter ratios of 10 or more), and fibers maybe addedto the host matrix material to generate a compos-ite with improved fracture toughness. The pres-ence of these additives appears to frustrate thepropagation of cracks by at least three mecha-nisms. First, when the crack tip encounters a par-ticle or fiber which it cannot easily break or getaround, it is deflected off in another direction.Thus, the crack is prevented from propagatingcleanly through the structure. Second, if the bondbetween the reinforcement and the matrix is nottoo strong, crack propagation energy can be ab-sorbed by “pullout” of the fiber from its originallocation. Finally, fibers can bridge a crack, hold-ing the two faces together, and thus preventingfurther propagation.

Table 3 presents the fracture toughness and crit-ical flaw sizes (assuming a typical stress of 700MPa, or about 100,000 psi) of a variety of cer-amics and compares them with some commonmetals. The toughness of monolithic ceramics gen-erally falls in the range 3 to 6 MPa-m’ 1, cor-responding to a critical flaw size of 18 to 74micrometers. With transformation toughening orwhisker dispersion, the toughness can be increasedto 8 to 12 MPa-m’ 2 (critical flaw size 131 to 294micrometers); the toughest ceramic matrix com-posites are continuous fiber-reinforced glasses, at15 to 25 MPa-m’ 1. It is important to note that inthe latter composites the strength appears to beindependent of preexisting flaw size, and is thusan intrinsic material property. Metals such as steelhave toughnesses above 40 MPa-m1 (some al-

14

Table 3.—Fracture Toughness and Critical FlawSizes of Monolithic and Composite Ceramic

Materials Compared With Metals.a

A12O3: alumina; LAS: lithium aluminosilicate;CVD: chemical vapor deposition

Fracture Criticaltoughness flaw size

Material (MPa. m’/2) (micrometers)

Conventional microstructure:Al 203. . . . . . . . . . . . . . . . . .

Sintered SiC. . . . . . . . . . . .Fibrous or interlocked

microstructure:Hot pressed Si3N4 . . . . . . .Sintered Si3N4 . . . . . . . . . .SiA10N . . . . . . . . . . . . . . . .

Particulate dispersions:Al2 05-TiC . . . . . . . . . . . . . .SiC-Ti B2 . . . . . . . . . . . . . . .Si3N4-TiC. . . . . . . . . . . . . . . .

Transformation toughening:ZrO2-MgO . . . . . . . . . . . . . .ZrO 2-Y2O3 . . . . . . . . . . . . . .A1203-zro2 . . . . . . . . . . . . .

Whisker dispersions:Al2O3,-SiC . . . . . . . . . . . . . .

Fiber reinforcement:b

SiC in borosilicate glass .SiC in LAS . . . . . . . . . . . . .SiC in CVD SiC . . . . . . . . .

Aluminum c . . . . . . . . . . . . . . . . .Steel c . . . . . . . . . . . . . . . . . . . . .

3.5-4.0 25-333.0-3.5 18-25

4.0-6.04.0-6.04.0-6.0

4.2-4.5

4.5

9-126-9

6.5-15

8-10

15-2515-258-15

33-4444-66

33-7433-7433-74

36-41

41

165-29474-16586-459

131-204

aAssumes a stress of 700 MPa ( - l@,000 Psi).bThe strength of these composites is independent of preexisting flaw size.cThe toughness of some alloys can be much higher.

SOURCES David W. Richerson, “Design, Processing Dewdopment, and Manufac-turing Requirements of Ceramics and Ceramic Matrix Composites, ”contractor report for OTA, December 1985, and Elaine P. Rothman,“Ultimate Properties of Ceramics and Ceramic Matrix Composites,”contractor report for OTA, December 1985

loys may be much higher), more than 10 timesthe values of monolithic ceramics.

The critical flaw size gives an indication of theminimum flaw size which must be reliably de-tected by any nondestructive test in order to en-sure reliability of the component. Most NDEmethods cannot reliably detect flaws smaller thanabout 100 micrometers, corresponding to a tough-ness of about 7’ MPa-m’ I. Toughnesses of 10 to12 MPa-m” would be desirable for most com-ponents.

Ceramic Coatings

The operation of machinery in hostile environ-ments (e. g., high temperatures, high mechanicalloads, or corrosive chemicals) often results in per-formance degradation due to excessive wear and

friction, and productivity losses due to shutdownscaused by component failure. Frequently, thecomponent deterioration can be traced to dele-terious processes occurring in the surface regionof the material. To reduce or eliminate such ef-fects, ceramic coatings have been developed toprotect or lubricate a variety of substrate mate-rials, including metals, ceramics, and cermets (ce-ramic-metal composites).

The coating approach offers several advan-tages. One is the ability to optimize independentlythe properties of the surface region and those ofthe base material for a given application. A sec-ond advantage is the ability to maintain closedimensional tolerances of the coated workpiece,since very thin coatings (of the order of a fewmicrometers) are often sufficient for a given ap-plication. Further, cost savings are obtained byusing expensive, exotic materials only for thincoatings and not for bulk components. This cancontribute to the conservation of strategically crit-ical materials. Finally, it is often cheaper to recoata worn part than to replace it.

In view of these advantages, it is not surpris-ing that ceramic coatings have found wide indus-try acceptance. For example, coatings of titaniumnitride, titanium carbide, and alumina can en-hance the useful life of tungsten carbide or highspeed steel cutting tools by a factor of 2 to 5.6 In1983, annual sales of coated cutting tools reachedabout $1 billion.7 Ceramic coatings are also find-ing wide applications in heat engines. Low ther-mal conductivity zirconia coatings are now be-ing tested as a thermal barrier to protect the metalpistons and cylinders of advanced diesel engines.In turbine engines, insulative zirconia coatings im-prove performance by permitting combustion gastemperatures to be increased by several hundreddegrees (F) without increasing air-cooled compo-nent metal temperatures or engine complexity.8

Ceramic coatings have also been used to providean oxidation barrier on turbine blades and rings.

‘David W. Richerson, “Design, Processing Development, andManufacturing Requirements of Ceramics and Ceramic Matrix Com-posites, ” contractor report prepared for the Office of TechnologyAssessment, December 1985.

7U. S. Department of Commerce, Bureau of the Census, Censusof Manufacturing, Fuels, and Electric Energy Consumed, 1984.

‘Tom Strangman, Garrett Corp., personal communication, Au-gust 1986.

15

Progress in the use of ceramic coatings in theseand other applications suggest that further re-search on new coatings and deposition processesis likely to yield a high payoff in the future.

Design, Processing, and Testingof Ceramics

It is in the nature of advanced structural mate-rials that their manufacturing processes are ad-ditive rather than subtractive. They are notproduced in billets or sheets which are later rolled,cut, or machined to their final shape. Rather, thegoal is always to form the material to its finalshape in the same step in which the microstruc-ture of the material itself is formed. Because ofthe severity of joining problems, the designer isalways conscious of the need to consolidate asmany components as possible together in a sin-gle structure. To be sure, these goals are not al-ways realized, and expensive grinding or drillingis often required. However, to a great extent, thepromise of the advanced materials lies in the pos-sibility of net shape processing, thereby eliminat-ing expensive finishing and fastening operations.

Ceramics Design

Designing with ceramics and other brittle ma-terials is much different from designing with me-tals, which are much more tolerant of flaws. Inpractice, ceramic structures always contain a dis-tribution of flaws, both on the surface and in thebulk. Ceramic designs must avoid local stress con-centrations under loading which may propagatecracks originating at the flaws.

Two serious barriers to the use of ceramics arethe lack of knowledge among designers of theprinciples of brittle material design, and the poorcharacterization of ceramic materials of interest.Greater emphasis needs to be placed on brittle ma-terials in college curricula. Courses at the collegelevel and minicourses for continuing education ondesign for brittle materials should be offered andpublicized. ’

The characterization of commercially availableceramics for design purposes is generally poor.The poor quality of the data is due to the fact thatmechanical, thermal, and chemical properties ofceramic materials vary with the method of man-ufacture as well as test method. Both carefullycontrolled and documented processing proceduresand standard test methods will be required to givedesigners the confidence that consistent proper-ties at a useful level can be obtained at a predict-able cost.

Computer-aided design (CAD) techniques havebecome essential to the design process, particu-larly in the analysis of mechanical stresses, ther-mal stresses, and life prediction of the part in serv-ice. Sophisticated software to calculate mechanicaland thermal stresses is available; however, theseprograms require detailed data on properties suchas thermal expansion coefficient, heat capacity,and elastic modulus, which often are not avail-able. Current techniques for predicting the serv-ice life of ceramics are limited by lack of data onthe behavior of these materials in various envi-ronments of interest. These techniques are espe-cially unreliable in high temperature, high stressregimes where several failure mechanisms areoperating simultaneously.l”

It is not particularly unusual that there shouldbe a lack of reliable design data in a field like struc-tural ceramics, which is very new and which isconstantly producing new materials. It can evenbe argued that the imposition of codes and speci-fications would be premature, given that there isno consensus on the best materials or processes.Although there is a danger in narrowing the fieldtoo soon, there is also a danger in keeping it gen-eral for too long. It may be appropriate to chooseone or two materials which seem most promis-ing and concentrate on producing uniform, high-quality components from these. Silicon carbideand silicon nitride would be two possible candi-dates, because they have already received a largeamount of research funding over the years for heatengine applications. These materials have a broadrange of potential uses, but designers cannot com-pare them or use them without a reliable data-

9Specific proposals to improve ceramics education are given inthe Report of the Research Briefing Panel on Ceramics and CeramicComposites (Washington, DC: National Academy Press, 1985).

IOR, Nathan Katz, “Applications of High Performance Ceramicsin Heat Engine Design, ” Materials Science & Engineering 71 ;227-249, 1985.

16

base on standard compositions having specifiedproperties.

Processing of Ceramics

The production of most ceramics, includingtraditional and advanced ceramics, consists of thefollowing four basic process steps: powder prep-aration, forming, densification, and finishing. Themost important variations on these steps are givenin table 4.

Powder Preparation.—Although the basic ce-ramic raw materials occur abundantly in nature,they must be extensively refined or processed be-fore they can be used to fabricate structures. Theentire group of silicon-based ceramics (other thansilica) does not occur naturally. Silicon carbide,silicon nitride, and sialon (an “alloy” of siliconnitride with aluminum oxide in which aluminumand oxygen atoms substitute into silicon and ni-trogen lattice positions, respectively) compositionsmust all be fabricated from gases or other ingre-dients. Even minerals which occur naturally, suchas bauxite, from which alumina is made, and zir-con sands, from which zirconia is derived, mustbe processed before use to control purity, parti-cle size and distribution, and homogeneity.

In recent years, the crucial importance of pow-der preparation has been recognized. Particle sizes

Table 4.—Common Processing Operations forAdvanced Ceramics

Operation Method

Powder prepa-ration Synthesis

SizingGranulatingBlendingSolution chemistry

Forming Slip castingDry pressingExtrusionInjection moldingTape castingMelting/casting

Densification SinteringReaction bondingHot pressingHot isostatic pressing

Finishing MechanicalChemicalRadiationElectric

Examples

SiCSi3,N4

Zr02

GlassesCombustors, statorsCutting toolsTubing, honeycombTurbocharger rotorsCapacitorsGlass ceramicsAl ,03

S13N4Si3N4, SiC, BNSi3N,, SiCDiamond grindingEtchingLaser, electron beamElectric discharge

SOURCE Off Ice of Technology Assessment

and size distributions are critical in advanced ce-ramics to produce uniform green (unfired) densi-ties, so that rearrangement can occur to producea fully dense, sintered, ceramic part.

Various dopants or sintering aids are added toceramic powders during processing. Sinterabilitycan be enhanced with dopants, which can con-trol particle rearrangement and diffusivities. Thesedopants allow sintering at lower temperaturesand/or faster rates. Dopants are also used to con-trol grain growth or achieve higher final densi-ties. The use of dopants, while providing manybeneficial results, can also influence the material’sproperties. Segregation of dopants at the crystal-line grain boundaries can weaken the final part,and final properties such as conductivity, strength,etc., may differ significantly from those of the“pure” material.

Forming.—Ceramic raw materials must beformed and shaped before firing. The formingprocess often determines the final ceramic prop-erties. The processing variables in the forming stepaffect particle shape, particle packing and distri-bution, phase distribution, location of pores,shrinkage, and dry and fired strength.

Forming processes for ceramics are generallyclassified as either cold or hot forming. The ma-jor cold forming methods include slip casting, ex-trusion, dry pressing, injection molding, tape cast-ing, and variations. The product of cold formingtechniques is called a “green body, ” which maybe machined before firing. The homogeneity ofthe cold formed part determines the uniformityof shrinkage during firing.

Hot forming methods combine into one step theforming and sintering operation to produce sim-ple geometric shapes. These techniques include hotpressing and hot isostatic pressing.

Densification.— Sistering is the primar y

method for converting loosely bonded powderinto a dense ceramic body. Sintering involves con-solidation of the powder compact by diffusion onan atomic scale. Moisture and organics are firstburned out from the green body and then, at thetemperature range where the diffusion process oc-curs, matter is moved from the particles into thevoid spaces between the particles, causing den-sification and resulting in shrinkage of the part.

.-

Combined with forming techniques such as slipcasting, sintering is an economic method for pro-ducing intricate ceramic components. Its draw-back lies in the need to use additives and long sin-tering times to achieve high densities. Thecomplications introduced by the additives havebeen noted above in the discussion of powders.

Finishing.—These techniques include grindingand machining with diamond and boron nitridetools, chemical etching, and laser and electric dis-charge machining. The high hardness and chem-ical inertness of densified ceramics make the fin-

ishing operations some of the most difficult andexpensive in the entire process. Grinding alone canaccount for a large fraction of the cost of the com-ponent, In addition, surface cracks are often in-troduced during machining, which reduce thestrength of the part and the yields of the fabrica-tion process.

Near Net Shape Processing (NNSP).—This de-scribes any forming process which gives a finalproduct that requires little or no machining. Typi-cally, ceramics shrink to about two-thirds of theirgreen volume upon sintering. This shrinkagemakes it extremely difficult to fabricate ceramicsto final net shape. However, if the green ceramicis machined prior to densification, a near netshape part can be obtained. Hot isostatic press-ing and ceramic coatings also yield parts whichdo not require subsequent machining.

Near net shape processes which are currentlyused for metals include powder metallurgy andadvanced casting techniques. Since metals are indirect competition with ceramics in many appli-cations, near net shape processing of ceramics willcontinue to be a high priority research area whichwill be critical to their cost competitiveness,

Ceramic Matrix Composites

Ceramic matrix composites may consist of con-tinuous fibers oriented within a ceramic matrix,ceramic whiskers randomly oriented within aceramic matrix, or dissimilar particles dispersedin a matrix with a controlled microstructure. Thepotential benefits of ceramic composites includeincreased fracture toughness, hardness, and im-proved thermal shock resistance. Processing meth-ods for particulate-reinforced composites are sim-ilar to those for monolithic ceramics.

50µM

Photo credit: Los Alarnos Nafional Laboratory

Fracture surface of a composite formed by hot-pressing silicon nitride powder with 30 percent by

volume silicon carbide whiskers.

Whisker Reinforcement.—Ceramic whiskersare typically high-strength single crystals with alength at least 10 times the diameter. Silicon car-bide is the most common whisker material. Cur-rent dispersed whisker composites are fabricatedby uniaxial hot pressing, which substantiallylimits size and shape capabilities and requires ex-pensive diamond grinding to produce the final

18

part. Hot isostatic pressing (HIP) has the poten-tial to permit fabrication of complex shapes atmoderate cost, but procurement of expensive cap-ital equipment and extensive process developmentare required.

Continuous Fiber Reinforcement.—The pri-mary fibers which are available for incorporationinto a ceramic or glass matrix are carbon, siliconcarbide, aluminum borosilicate, and mullite. Cur-rently, glass matrix composites are more devel-oped than their ceramic analogs. These compos-ites are far tougher than unreinforced glasses( t a b l e00 to 1,3000 F ( 5 9 3o to 7 0 4o C). S e r v i c etemperatures up to 2,0000 to 2,2000 F (1,093 o to1,2040 C) may be obtained with “glass-ceramic”matrices which crystallize upon cooling from theprocess temperature.11 Carbon matrix compositeshave the highest potential use temperature of anyceramic, exceeding 3,5000 F (1,927° C). However,these composites oxidize readily in air at temper-atures above about 1,100° F (s930 C), and requireprotective ceramic coatings if they are to be usedcontinuously at high temperature.12

ilK~r] M, Prewo, J .J. Brennan, a n d G.K. Layden, “ F i b e r -Reinforced Glasses and Glass-Ceramics for High Performance Ap-plications,” American Ceramic Society Bulletin 65(2):305, 1986.

‘zJoel Clark, et al., “Potential of Composite Materials To ReplaceChromium, Cobalt, and Manganese in Critical Applications, ” con-tractor report prepared for the Office of Technology Assessment,1984.

Photo credit’ United Technologies Research Center

Tensile fracture surface forNicalon SiC fiber-reinforced glass-ceramic

Fabrication of continuous fiber ceramic com-posites is currently of a prototype nature and veryexpensive. Several approaches are under de-velopment:

● The fibers are coated with ceramic or glasspowder, laid up in the desired orientation,and hot pressed,

Fibers or woven cloth are laid up, then areinfiltrated by chemical vapor deposition(CVD) to bond the fibers together and fill ina portion of the pores,

● Fibers are woven into a three-dimensionalpreform, then infiltrated by CVD.

● A fiber preform is infiltrated with a ceramic-yielding organic precursor, then heat treatedto yield a ceramic layer on the fibers. Thisprocess is repeated until the pores areminimized.

Considerable research and development will benecessary to optimize fabrication and to decreasethe cost to levels acceptable for most commercialapplications.

Ceramic Coatings

Many different processes are in use for the fabri-cation of ceramic coatings and for the modifica-tion of surfaces of ceramic coatings and ofmonolithic ceramics. Table 5 lists some of themore important techniques. The choice of a par-ticular deposition process or surface modificationprocess depends on the desired surface properties.Table 6 lists some of the coating characteristicsand properties which are often considered desira-ble. Additional considerations which can influ-ence the choice of coating process include: the pu-rity, physical state, and toxicity of the materialto be deposited; the deposition rate; the maximumtemperature which the substrate can reach; thesubstrate treatment needed to obtain good coat-ing adhesion; and the overall cost.

For most coating processes, the relationshipsbetween process parameters and coating proper-ties and performance in various environments arepoorly understood. Coating providers tend to relyon experience gained empirically. Work is inprogress to establish these relationships for cer-tain processes, e.g. ion beam- or plasma assisted-physical vapor deposition. However, in view of

19

Table S.—Selected Processes for the Production of Ceramic Coatings and for the Modification of Ceramic Surfaces

Process category Process class Process

Ceramic coating processes:Low gas pressure (’(vacuum”) Chemical vapor deposition (CVD) Pyrolysis

processes Reduction (plasma assisted)Decomposition (plasma assisted)Polymerization (plasma induced)

Physical vapor deposition (PVD) Evaporation (reactive, plasma assisted)Sputtering (reactive, plasma assisted)Plasma-arc (random, steered)Ion beam assisted co-deposition

Plasma discharge spraying

Processes at elevated gas pressures

Liquid phase epitaxy processes

Electrochemical processes

Low pressure plasma spraying

Plasma sprayingFlame spraying

Wetting process

Spin-on coatings

Electrolytic deposition

Electrophoretic depositionAnodization

Electrostatic deposition

Processes for the modification of ceramic surfaces:Particle implantation processes

Densification and glazing processes

Chemical reaction processes

Conversion processes

Etching processes

Mechanical processes

Direct particle implantation

Recoil particle implantation

Laser beam densification andglazing

Electron beam densification andglazing

Gaseous anodization processes

Plasma arc sprayingCombustion flame spraying

Dip coating (e.g., Sol-Gel)Brush coating

Reverse-roller coating

Cation deposition

Charged colloidal particle depositionAnion oxidation in electrolytes

Charged liquid droplet deposition

Energetic ion or atom implantation insolids

Recoil atom (ion) implantation in solids

CW-laser power depositionPulsed-laser power deposition

Energetic electron beam powerdeposition

Ion nitridingIon carburizingPlasma oxidation

Deposition of molecular speciesFormed in gas phase

Diffusion of material from surface intobulk of substrate

Acidic solutions; lye etching

Disproportionation processes

Thermal diffusion

Chemical etching

Ion etching

GrindingPeeningPolishing

Sputter process

SOURCE: Manfred Kaminsky Surface Treatment Science International, Hinsdale. IL

the current widespread use of coated machinery Chemically Bonded Ceramicscomponents, and projected future requirementsfor components with advanced coatings, research Hardened cement pastes and concretes fall inin processing science for ceramic coatings remains the category of chemically bonded ceramicsan important priority. In addition, improved (CBCs), because they are consolidated throughdeposition processes are required, particularly for chemical reactions at ambient temperatures ratherthe coating of large components or those having than through densification at high temperature.a complex shape. Due to their low strength compared with dense

20

Table 6.—Characteristics and Properties ofCeramic Coatings Often Considered Desirable

Good adhesionPrecise stoichiometry (negligible contamination)Very dense (or very porous) structural morphologyThickness uniformityHigh dimensional stabilityHigh strengthHigh fracture toughnessInternal stresses at acceptable levelsControlled density of structural defectsLow specific densityHigh thermal shock resistanceHigh thermal insulating propertiesHigh thermal stabilityLow (or high) coefficient of frictionHigh resistance to wear and creepHigh resistance to oxidation and corrosionAdeauate surface topography

SOURCE Manfred Kaminsky, Surface Treatment Science International, Hinsdale,IL.

ceramics, concrete and other cementitious mate-rials are not normally considered “advanced. ”However, in recent years, new processing meth-ods have led to significant improvements in thestrength of chemically bonded ceramics, and fur-ther development will provide additional im-provements. In fact, the ultimate limits on ten-sile and compressive strength have not yet beenapproached.

Cements.—Cements are chemically activebinders which may be mixed with inert fillers suchas sand or gravel to form concrete. Cement pastescontaining minor additives such as organic poly-mers can also be used as structural materials. Byfar the most common cement used in makingCBCs is portland cement. Portland and relatedcements are hydraulic; i.e., they react with waterto form an relatively insoluble aggregate. In hy-draulic cements, excess water is usually added toimprove the working characteristics, but thiscauses the hardened structures to be porous (theminimum porosity of fully hydrated cements isabout 28 percent) 13 and of low strength. In a re-cent advance, a high-shear processing techniquetogether with pressing or rolling are used to re-move pores from a low-water calcium aluminatecement paste containing 5 to 7 percent by weightorganic polymers to improve workability. Thedense paste, which is sometimes called macro-defect-free or MDF cement, has the consistency

Richard A. He]muth, portland Cement Association, perSOIM]

communication, August 1986.

—— -—..-.-——— ——.——. -— —-—

I

I II

—.. .- . . -——.— -JPhoto credits” /mperia/ Chemica/ /ndustries PLC

Top photo: Conventional Portland cement pastemicrostructure, showing large flaws (pores).

Bottom photo: Advanced cement paste microstructure,illustrating the absence of large pores

(magnification x1OO).

Photo credit” CEMCOM Research Associates, Inc

This tool, made from a cement-based composite,is used for autoclave forming of a fiber-epoxy

jet engine component.

of cold modeling clay, and can be molded or ex-truded by techniques similar to those used forplastics. The hardened cement paste has a strengthapproaching that of aluminum (table 7) and muchlower permeability than ordinary portland cementpaste. 14 Although MDF cement pastes cost 20 to

I iAccording t. product literature supplied by ImPerial ChemicalIndustries, “New Inorganic Materials. ”

21

Table 7.—Comparison of the Mechanical Properties of Various Cements and Aluminum

Material Density (g/cm3) Flexural strength (psi)a Compressive strength (psi) Fracture energy (J/m2)Portland cement paste . . . . 1.6-2.0b 725-1,450 4,000-5,000 b

Cement/asbestos . . . . . . . . . . 2.3 5,075Advanced cementsc . . . . . . . . 2.3-2.5 14,500-21,750 22,000-36,000Aluminum . . . . . . . . . . . . . . . . 2.7 21,750-58,000 42,000

20300

300-1,00010,000

a1 MPa = 145 psi.bAccording to information supplied by the Portland Cement AssociationcThe advanced cement has the following composition. 100 parts high alumina cement; 7 parts hydrolyzed Poly (vinylacetate),

SOURCE Imperial Chemical Industries.

30 cents per pound compared with portland ce-ment paste at 3 cents per pound, they are still sig-nificantly cheaper than metals and plastics.

Processing of hydraulic CBCs is very cheap,since it involves only adding water, mixing, cast-ing or molding, and allowing the material to setat room temperature or slightly elevated temper-ature. Very little dimensional change occurs dur-ing the set and cure, so that parts can be madeto net shape. Due to the low processing tempera-ture, a wide variety of reinforcing fibers canbe considered, including metal fibers. Furtherwork is needed to improve the long term stabil-ity of these materials, and the presence of organicsmakes them unsuitable for use above about 2000 F(93° c).

Concrete.—As chemical additives, such as or-ganic polymers, have improved the properties ofcement pastes, chemical and mineral additiveshave had a similar effect on concrete. Mineralssuch as fly ash and microscopic silica particles helpto fill in the pores in the concrete and actually im-prove the bonding in the cementitious portion.This results in greater strength and reduced per-meability. In a recently developed concrete, mol-ten sulfur is used as a binder in place of cement.Sulfur concrete has superior corrosion resistancein acidic environments and can be recycled byremelting and recasting without loss of the me-chanical properties.

The compressive strength of typical concretestoday is around 5,OOO psi (34 MPa), although con-cretes with strengths of 10,000 to 15,000 psi (69to 103 MPa) are becoming common. Under lab-oratory conditions, compressive strengths of atleast 45,000 psi (310 MPa) have been achieved,

“W. C. McBee, T.A. Sullivan, and H.L. Fike, “Sulfur Construc-tion Materials, ” Bulletin 678, U.S. Department of the Interior, Bu-reau of Mines, 1985.

10-12 parts water

and there is no indication that the ultimatestrength is being approached. 16 In concrete high-rise buildings, the higher compressive strengthspermit use of smaller columns, with consequentsavings in space and materials.

Two deficiencies in concrete as a structural ma-terial are its low tensile strength and low tough-ness. A typical concrete has a tensile strength be-low 1,000 psi (7 MPa). Steel reinforcement barsare added to the concrete to provide tensilestrength. In prestressed concrete, high strengthsteel wires under tension are used to keep the con-crete in a state of compression. To improvestrength and toughness, a variety of reinforcingfibers, including steel, glass, and polymers, havebeen tried, with varying degrees of success. Fi-ber reinforcement can increase the flexuralstrength by a factor of 2.5 and the toughness bya factor of 5 to 10 above unreinforced materials. 18

This technology, which dates back to the straw-reinforced brick of the ancient Egyptians, requiresfiber concentrations which are sufficiently low(usually 2 to 5 percent by volume) to preserve theflow characteristics of the concrete, and a chem-ically stable interface between the fiber and theconcrete over time. Asbestos fibers served thisfunction for many years; however, because of thehealth hazards, new fibers are being sought. Afully satisfactory reinforcing fiber for concrete hasyet to be discovered.

Nondestructive Evaluation (NDE) of Ceramics

Nondestructive evaluation refers to techniquesfor determining properties of interest of a struc-

‘6 Sidney Mindess, “Relationships Between Strength and Nlicrmstructure for Cement-Based Materials: An Overview’, ” Alateriak Zi’e-search Society S~’mposium Proceedings, 42:53, 1985.

“Ibid.18American Concrete Institute, “State-o-the-Art Report on Fiber-

Reinforced Concrete, ” Report No. ACI 544.11 <-82, 1982.

22

ture without altering it in any way. NDE has his-torically been utilized for flaw detection in ceramicmaterials to improve the reliability of the finalproduct. In the future, NDE will be utilized fordefect screening, material characterization, in-process control, and lifecycle monitoring. Thesetechniques will be applied to the starting materi-als, during the process, and to the final product.

A key goal for the future will be the evolutionof NDE techniques amenable to automation andcomputerization for feedback control. Powderand green body characterization will be criticalfor materials processed from powders. For in-process characterization, the relation betweenmeasurable quantities, obtained through the useof contact or noncontact sensors, and desiredproperties is crucial. This will require develop-ments in sensor technology as well as theorieswhich can quantitatively relate the measured NDEsignal to the properties of interest.

In the past, a great deal of emphasis has beenplaced on the sensitivity of an NDE technique,i.e., the size of the smallest detectable flaw. Amore relevant criterion for reliability purposes isperhaps the size of the largest flaw that can goundetected. There has been very little emphasison the reliability of NDE techniques, i.e. the prob-ability of detecting flaws of various sizes. Experi-ence has shown that most quality problems re-sult not from minute flaws, but from relativelygross undetected flaws introduced during thefabrication process.19

Cost of production estimates for high perform-ance ceramic components typically cite the inspec-tion costs as approximately 50 percent of the man-ufacturing cost .20 Successful NDE techniques forceramic components should meet two major cri-teria: they should reliably detect gross fabricationflaws to ensure that the material quality of thecomponent is equal to that of test specimens, andthey should be able to evaluate the quality of acomplex-shaped component in a practical man-ner. No single NDE technique for ceramics com-pletely satisfies these criteria. However, thosewhich could be cost-effective for production levelinspections are described in table 8.

Health HazardsThe most serious health hazard associated with

ceramics appears to be associated with ceramicfibers. Studies carried out at the National Can-cer Institute have indicated that virtually all dura-ble, mineral fibers having a diameter of less than1 micrometer are carcinogenic when introducedinto the lining of the lung of laboratory rats.2l Thecarcinogenicity drops with increasing diameter,such that fibers having diameters greater than 3micrometers do not produce tumors. Recent studieson commercially available aluminosilicate fiberssuggest that animals exposed to the fibers developan increased number of lung cancers over timecompared with a control group .22 No data on theeffects of ceramic fibers on humans are available,and no industry standards for allowable fiberconcentrations in the workplace have been estab-

IQR Nathan Katz and Alfred L. Broz, “Nondestructive Evalua-tion Considerations for Ceramics and Ceramic Matrix Composites, ”contractor report prepared for the Office of Technology Assessment,November 1985.

‘“Ibid.z IMear] F. Stanton, et al., Journal of the National Cancer hsti-

tute 67:965-975, 1981.Zzphi]ip J. Lan&-igan, M. I)., The Mount Sinai Medical Center,

personal communication, August 1986.

Table 8.—Comparison of Some Possible Production-Level NDE Techniques for Structural Ceramics

Adaptability to Extent of developmentNDE technique Detected flaw type Sensitivity complex shapes required for commercialization

Visual (remote) . . . . . . . . . . surface fair good noneDye penetrant . . . . . . . . . . . surface good good noneRadiographic . . . . . . . . . . . . bulk 1-20/0 of specimen excellent none

thicknessUltrasonic . . . . . . . . . . . . . . . bulk and surface good poor someHolographic . . . . . . . . . . . . . surface good fair largeThermographic. . . . . . . . . . . surface poor excellent someProof test . . . . . . . . . . . . . . . any good, but may excellent none

introduce flawsSOURCE: Office of Technology Assessment

23

lished. Until such data become available, the ani-mal studies suggest that these fibers should be con-sidered carcinogenic, and treated in a mannersimilar to asbestos fibers.

Applications of Structural Ceramics

Figure 3 shows an estimated timetable for theintroduction of ceramic products in various cat-egories. It shows that some advanced structuralceramics are in production, others have near-termpotential for production, and some are far awayfrom production.

Current Production

Ceramics such as alumina, silicon nitride, andsilicon carbide are already well established in pro-duction for many structural applications in thecategories of wear parts, cutting tools, bearings,and coatings. The ceramics portion of the mar-ket is currently small (generally less than 5 per-cent) .23 Substantial growth in ceramics produc-tion is expected to occur over the next 25 yearsby growth of the overall markets, by achievingan increase in the market share, and by spin-offapplications.

Ceramics are also in limited production (inJapan) in discrete engine components such asturbochargers, glow plugs, and precombustionchambers. Current military applications includeradomes, armor, infrared windows, and heatsources.

Funding for known near-term markets is cur-rently being provided by industry. Much of thefunding is directed toward development of newor improved ceramic or ceramic matrix compos-ite materials. Key objectives are to achieve im-proved toughness, higher reliability, and de-creased cost. Development of silicon nitride,transformation toughened ceramics, and compo-sites has yielded materials with enhanced tough-ness and reliability, but costs are still high andreliability remains a problem. Progress in resolv-ing these limitations is occurring, and large in-creases in the market share for ceramics are pro-jected.— — . .

‘‘U.S, Department of Commerce, A Corrtpetiti;re Assessment ofthe U.S. Advanced Ceramics Industry’ (Washington, DC: U. S, C,o\r-ernment Printing Oftice, March 1984), pp. 38-39.

The United States is competing with other coun-tries, especially Japan, for the growing markets.Japan has a well-coordinated effort which in-cludes: increasing the quality and availability ofthe raw materials; conducting material fabrica-tion process optimization; installing the most ad-vanced processing and quality control equipmentfor prototype and production facilities; and ag-gressively marketing their evolving materials.

Near-Term Production

Near-term production is projected in advancedconstruction products, bearings, bioceramics, heatexchangers, electrochemical devices, isolated com-ponents for internal combustion engines, and mil-itary applications. The technology feasibility hasgenerally been demonstrated, but scale-up, costreduction, or design optimization are required.Although much of the feasibility demonstrationhas occurred in the United States, foreign indus-try and government-industry teams, particularlyin Japan, have more aggressive programs to com-mercialize the near term applications. Large mar-kets are at stake; for example, the world marketfor all biocompatible materials has been projectedto be as much as $6 billion by the year 199524;ceramics could capture 25 to 30 percent of this. 25Foreign dominance of these markets would ad-versely affect the U.S. balance of trade. Govern-ment, industry, and universities need to team to-gether in focused programs to solve the remainingproblems and achieve production capability com-petitive with that of foreign countries.

Long-Term Applications

Some potential applications of ceramics requiresolution of major technical and economic prob-lems. These high risk applications are categorizedas long term (greater than 15 years away). Theultimate payoff may be large, but it is not possi-ble to predict confidently that the problems willbe overcome to achieve the benefits.

Long-term applications include the automotivegas turbine engine, the advanced diesel, some elec-

j~Larry L. Hench and June Wilson, “Biocompatibility of Silicatesfor Medical Use, ” Silicon Biochemistry, CIBA Foundation Sympo-sium No. 121 (Chichester: John Wiley & Sons, 1986), pp. 231-246,

JsLarw L, Hench, university of Florida, persona] communication,August 1986.

Figure 3i.—Estimated Scenario for Implementation of Ceramic Components in Structural Application Categories

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Ai,o, SiC PSZ Si3N4. CompositesTTA TZP Si3N4-BNWear par ts

A d v a n c e d construction products I Chemically Compositesbonded

Military applications

Bearings

Bioceramics

Heat exchangers

Electrochemical devices

Heat engines:

Gasoline automotive

Diesel automotive

Automotive turbine

Other turbines

Coatings

BC Armor Coatings Bearings Diesels TurbinesIsolated components

Ai203 Military Commercial

AI203, FDA hip Orthopedic A d v a n c e d

cl in ical approval and dental mater ia ls

Recuperated Rotary Tubular Cogenerat ion F ixedfurnaces mi l i tary industr ia l bounda ry

O 2 sensors Elect rochlorination Na-S bat tery Fuel ce l l02 pump

Exhaust port liner Piston pinTurbochargerCam follower

Pre-chamber , coat ings Iso la ted par tsglow plug Uncooled engine

Isolatedc o m p o n e n t s

Wear and Cutting Turbine Minimum-cooledcorrosion tools components dieselres is tance

SOURCE: David W. Richerson, ""Design, Processing Development, and Manufacturing Requirements of Ceramics and Ceramic Matrix Composites, ” contractor report prepared for the Officeof Technology Assessment, December 1985.

—.

25

trochemical devices such as fuel cells, some heatexchangers, and some bearings. A variety of otherturbines, especially those for aircraft propulsionand utility scale power generation, should also becategorized as long term.

Substantial design, material property, and man-ufacturing advances are necessary to achieve pro-duction of applications in the long-term category.In general, risk is perceived by industry to be toohigh and too long range to justify funding theneeded developments. Advancement will likelybe driven by government funding. In many ofthese categories, military use will predate com-mercial use.

Markets for Advanced Structural Ceramics

Estimated U.S. markets in the year 2000 for sev-eral of the categories listed above are shown infigure 4.

Wear Parts.—Wear parts include such appli-cations as seals, valves, nozzles, wear pads, grind-ing wheels, and liners. The Department of Com-merce has estimated that by the year 2000 ceramicscould capture roughly 6 percent of the wear partsmarket, currently dominated by tungsten carbidecermets, and specialty steels. With the total mar-ket estimated at $9 billion, the ceramic portionwould be $540 million. 26

Cutting Tools .—Ceramics have demonstrateda capability as a cutting tool, especially in com-petition with tungsten carbide-cobalt cermets (ce-ramic-metal composites) as inserts for metal turn-ing and milling operations. The advantage ofceramics compared with carbides is retention ofhigh hardness, strength, and chemical inertnessto temperatures in excess of 1,000° C (1,8320 F).This allows use of the ceramics at much highermachining speeds than can be tolerated by car-bides. However, the ceramics have lower tough-ness than the carbide materials, and have onlybeen used successfully in the limited operationsof turning and milling. A further impediment tothe use of ceramics, especially in the United States,

Figure 4.— Projected U.S. Markets for StructuralCeramics in-the Year 2000 (billions of dollars)

1,0

0.5

—m

(,-l

—

—n

— l-l

—

—mcoa—

u

SOURCES aU.S. Department of Commerce, “A Competitive Assessment of theU.S. Advanced Ceramics Industry” (Washington, DC, U S Government Printing Office, March 1984)

bHigh Technology, March 1986, P. 14.cLarry L. Hench and June wilson, “Blocompatablility of Silicates for

Medical Use, ” CIBA Foundation Symposium 121, pp. 231-246(Chichester, John Wiley & Sons, Publishers, 1986) Ceramics are as.sumed to capture 30°/0 of the estimated $3 billion U S blomaterialsmarket

‘David W. Richerson, “Design, Processing Development, andManufacturing Requirements of Ceramics and Ceramic Matrix Com.posites, ” contractor report prepared for the Off Ice of TechnologyAssessment, December 1985

has been equipment limitations. Much of the pro-duction metal machining equipment does not havethe rigidity or speed capability to utilize ceramics.

In spite of the current limitations of ceramicsand equipment, an assessment by the Departmentof Commerce estimates steady growth of ceramicsfor cutting tool applications. It projects a growthof the total U.S. shipments from $2.2 billion in1980 to $8 billion in the year 2000, with thegrowth of the ceramics portion from $45 million(2 percent) to $960 million (12 percent) .2’

16U s, Department of Commerce, Op. cit. , March 1984, P. 35. “Ibid.

26

Photo credit: Norton Co

Silicon nitride anti-friction bearing components offerimproved wear resistance and fatigue life compared

to most bearing steel.

Bearings

High-performance ceramic bearings have beendeveloped for military applications such as mis-siles. The primary candidate material is hotpressed silicon nitride. Ceramics offer resistanceto low temperature corrosion, high temperaturestability, low density, and the ability to operatefor a moderate length of time with little or no lu-brication. Hot isostatic pressing (HIP) is being de-veloped to improve properties and to decreasecost by allowing near-net-shape fabrication.

The military developments will yield a technol-ogy base that can be applied to commercial prod-ucts such as instrumentation bearings, hydraulicand pneumatic activator systems, and ceramiccoatings on the foils in gas bearings. Potential ce-ramic bearing markets have been estimated to be$300 million per year.28

Coatings

Ceramic coatings provide a variety of benefits,including abrasion resistance, thermal protection,corrosion resistance, and high-temperature lubri-cation. Applications include ultrahard coatings forcutting tools, thermal insulation and lubricatingcoatings for adiabatic diesel engines and cooled

‘“High Technology, March 1986, p. 14.

gas turbines, and bioactive glass coatings for metalorthopedic implants. The list could be expandedto include other sectors such as mining (e.g.,drills), utilities (e.g., turbine-generator sets, heatexchangers), agriculture (e. g., plows and tillers),and aerospace (e. g., bearings, power transferassemblies, and actuator drive systems).

The availability of advanced ceramic coatingsis expected to be a significant benefit to the U.S.economy. The value of the market for ceramiccoatings is not easily assessed, because the rangeof applications is so wide. One estimate is for a$1 billion market worldwide for all coating ma-terials, about 60 to 70 percent of which is domes-tic.29 This estimated market includes jet engine,printing, chemical, textile, and tool and die ap-plications. As we have seen, this list could begreatly expanded to include wear parts, bearings,biomaterials, heat exchangers, and automotivecomponents in the future. Ceramic coatingsshould be considered an extremely important tech-nology for extending the performance of metalcomponents, and, in some cases, coated metalstructures may be an excellent alternative tomonolithic ceramics.

Advanced Construction Products

Potential applications of advanced cement-based materials include floors, wall panels, androof tiles, in addition to pipes, electrical fittings,and cabinets. The cements can be laminated withwood or foam to form hard, decorative, and pro-tective surfaces. so Advanced cement pastes cost20 to 30 cents per pound compared with 75 centsto 2 dollars for metals and plastics, and could dis-place them in the future in many common uses.

The development of a cost-effective, durable,high tensile and compressive strength concretewould have dramatic implications for the infra-structure of the United States. It has been esti-mated that between 1981 and the end of the cen-tury the nation will spend about $400 billion forreplacement and repair of pavements, and about$103 billion to correct bridge deficiencies .3’ Cost

29Richerson, op.cit., December 1985.“Imperial Chemical Industries, op. cit.31National Research Council, Transportation Research Board,

“America’s Highways: Accelerating the Search for Innovation, ” Spe-cial Report 202, 1984.

27

A spring made from a high-strength cement, formed by extrui

Photo credits” CEMCOM Research Associates, Inc.

sion processing. Left: natural length. Right: compressed.The spring is not intended for any practical use, but demonstrates the versatility and resilience of the material.

28

savings of about $600 million per year could re-sult from implementing new technologies .32 Inaddition to reducing repair and maintenance costs,new materials would provide other benefits. Forexample, high compressive strength concrete canbe used to reduce the number and thickness ofconcrete bridge girders, significantly reducing thestructural weight;33 in concrete high-rise buildings,its use permits a reduction in the diameter of thecolumns, thus freeing up additional floor space.

The barriers to the development and implemen-tation of new technologies in the construction in-dustry are high. Some of those most often citedare industry fragmentation (e.g. some 23,000 Fed-eral, State, and local agencies operate the Nation’shighway industry), a system which awards con-tracts to the lowest bidder, and low industry in-vestment in research as a percentage of sales (thesteel industry invests 8 times more) .34 The low in-vestment is due in part to the fact that the prin-cipal benefits of the use of better materials accrueto the owner of the highway or bridge (the tax-payer) rather than to the cement producer.35

Legislation is currently before Congress (theFederal-Aid Highway Act of 1986, S.2405) whichwould set aside 0.25 percent of Federal-aid high-way funds for a 5-year, $150 million StrategicHighway Research Program. The program, whichwould be administered by the National ResearchCouncil, has targeted six priority research areasfor support: asphalt characteristics, $50 million;long-term pavement performance, $50 million;maintenance costs, $20 million; concrete bridgecomponents, $10 million; cement and concrete,$12 million; and snow and ice control, $8 million.

Bioceramics

Bioceramics, or ceramics for medical applica-tions such as dental or orthopedic implants, rep-resent a major market opportunity for ceramicsin the future. The overall worldwide market forbiocompatible materials is currently about $3 bil-lion, and this is expected to double or triple in

321bid.33J.E. Carpenter, “Applications of High Strength Concrete for

Highway Bridges, ” Public Roads 44:76, 1980.“National Research Council, op. cit., 1984.‘51bid.

the next decade .3’ Ceramics could account for 25to 30 percent of this market .37

Bioceramics may be grouped into three cate-gories: nearly inert, surface active, and resorb-able. 38 Nearly inert ceramics can be implanted inthe body without toxic reactions. These materi-als include silicon nitride-based ceramics,transformation-toughened zirconia and transfor-mation-toughened alumina.

Surface-active ceramics form a chemical bondwith surrounding tissue and encourage ingrowth.They allow the implant to be held firmly in placeand help prevent rejection due to dislocation orto influx of bacteria. Surface-active ceramicswhich will bond to bone include dense hydroxy -apatite, surface-active glass, glass-ceramic, andsurface-active composites.

The function of resorbable bioceramics is toprovide a temporary space filler or scaffold whichserves until the body can gradually replace it.Resorbable ceramics have been used to treat max-illofacial defects, for filling periodontal pockets,as artificial tendons, as composite bone plates, andfor filling spaces between vertebrae, in bone,above alveolar ridges, or between missing teeth.An early resorbable ceramic was plaster of paris(calcium sulfate), but it has been replaced by triso-dium phosphate, calcium phosphate salts, andpolylactic acid/carbon composites.39

Any new material intended for use in the bodymust undergo extensive testing before it is ap-proved. Preclinical testing, clinical studies, andfollowup may take as long as 5 years to com-plete. 40 However, ceramics have been in clinical usefor some 15 years, and are gaining acceptance.Industry interest in bioceramics has increasedsince 1980; however, no data were available onaggregate R&D spending either in industry or theFederal Government. In contrast, France, Germany,and especially Japan have aggressive programs ofgovernment support for the commercialization of

36 Hench and Wilson, op. cit., 1986.JT~rV L. Hench, University of Florida, persona] communication,

August 1986.3“J.W. Boretos, “Ceramics in Clinical Care, ’’American Ceramics

Society 13ulfetin 64(8):630-636, 1985.391bid.dOEduardo March, Food and Drug Administration, persona] com-

munication, August 1986.

29

Photo credit’ R/chards Medical Co

Total hip system including ceramic femoral head andacetabular cup, with metal femoral stem, The systemis manufactured in West Germany and marketed by

Richards Medical Co.

bioceramics.” All researchers interviewed agreedthat the Japanese effort in bioceramics is consider-ably larger than the U.S. effort.

Heat Exchangers

Ceramic heat exchangers are of great interestbecause they can utilize waste heat to reduce fuelconsumption. Heat recovered from the exhaustof a furnace is used to preheat the inlet combus-tion air, so that additional fuel is not required forthis purpose. The higher the operating tempera-

~ I I{lchers(ln, Op, c1 t., December 1985.

ture, the greater the benefit. Ceramic systems havepotential for greater than 60 percent fuel savings.’2

Ceramic heat exchangers may be used in a va-riety of settings, including industrial furnaces, in-dustrial cogeneration, gas turbine engines, andfluidized bed combustion. The size of the unit,manufacturing technique, and material all varydepending upon the specific application. Sinteredsilicon carbide and various aluminosilicates havebeen used in low pressure heat exchangers becauseof their thermal shock resistance; however, theservice temperature of these materials is currentlylimited to under 2,200° F (1,2040 C), Silicon car-bide is being evaluated for higher temperatures,but considerable design modifications will be nec-essary. 43

Government support has been necessary toaccelerate development of the ceramic materialsand system technology for heat exchangers, inspite of the design projections of significant fuelsavings and short payback time. The materialmanufacturers, system designers, and end usershave all considered the risks too high to investtheir own funds in the development and imple-mentation of a system. Specific concerns include:the high installed cost (up to $500,000 for a 20MBTU per hour unit), which represents a signifi-cant financial risk to the user for a technology thatis not well proven; the fact that many potentialend users are in segments of industry that pres-ently are depressed; and the fact that designs varyaccording to each installation, leading the user towant a demonstration relevant to his particularsituation.

Many of the ceramic heat exchanger programswere initiated in the 1970s when there was a keensense of urgency concerning the “energy crisis. ”In recent years, declining fuel prices have gener-ally reduced this sense of urgency. If the currentlow fuel prices persist, this could delay the wide-spread implementation of ceramic heat exchangersfor waste heat recovery.

42s ,M. Johnson and D.J. Rowcliffe, SRI International Report toEPRI, “Ceramics for Electric Power-Generating Systems, ” January1986.

tlRicherson, Op. cit,, December 1985.

30

Electrochemical Devices

Though not strictly structural applications ofceramics, devices in this category utilize ceramicsfor both their electrical and structural properties.Typically, the ceramic, such as zirconia or betaalumina, serves as a solid phase conductor for ionssuch as oxygen or sodium. Examples include oxy-gen sensors, oxygen concentration cells, solid ox-ide fuel cells, the sodium sulfur battery, sodiumheat engine, and electrodes for metal winning andelectrochlorination cells. As a group, these appli-cations could comprise a market of over $250 mil-lion for ceramics by the year 2000.44

Heat Engines

The advantages of using ceramics in advancedheat engines have been widely publicized. Theseinclude increased fuel efficiency due to higher en-gine operating temperatures, more compact de-signs, and reduction or elimination of the cool-ing system .45 Ceramics are being considered inthree general categories: discrete components suchas turbochargers in metal reciprocating engines;coatings and monolithic hot-section componentsin advanced diesel designs; and all-ceramic gasturbine engines.

A number of sources have predicted that com-ponents for heat engines will be the largest areaof growth for structural ceramics over the next25 years. Market estimates have varied widely.Kenney and Bowen stated:

The potential demand for ceramic enginescould reach $9 billion in Japan and $30 billionworld wide. If ceramics are used only to partiallyreplace metals as hot parts in the engine, the de-mand would be from $1 to $5 billion.46

The Department of Commerce has estimated moreconservatively a U.S. market of $56 million by1990 and $840 million by 2000.47 A study byCharles River Associates estimates U.S. consump-tion of ceramic heat engine parts at $25 to 45 mil-lion in 1990 and $920 million to $1.3 billion by

2000.48 Some structural ceramic components arealready in limited production for heat engines. Ce-ramic precombustion chambers and glow plugsfor diesels, and ceramic turbochargers, are nowin production in current model Japanese cars.These markets will grow, but not to a level thatwill account for the projected $1 billion sales forheat engine components in the year 2000. Growthto this level would require material and designtechnology breakthroughs, as well as manufac-turing scale-up and cost reduction. In view ofthese technical and economic barriers, the moreconservative estimates are likely to be be the moreaccurate.

Gasoline Engines.—The automotive internalcombustion engine offers a vast market for ma-terials. Total sales for 1985 of cars and trucks inthe free world have been estimated at 38.7 mil-lion units.49 Any part replacement or new partwould represent a volume market with substan-tial sales, even if the unit price were small. How-ever, the current engine designs are considered byautomotive companies to be mature, reliable, andcost-effective. Very few incentives for change ex-ist. Cost reduction remains a significant incentive,but this is extremely difficult to satisfy for a newmaterial, whose introduction may require redesignof adjacent parts, retooling, and modification ofthe production line.

Another incentive is to develop new technol-ogy which may be applicable to advanced designs.This consists of both generic and directed R&Dwith the primary objective of maintaining a com-petitive position. Ceramics within this categorywhich have potential for production include ex-haust port liners, cam followers, and turbochargercomponents. To date, U.S. firms have not intro-duced these products, although R&D programscontinue. Ceramic turbocharger rotors, glowplugs, precombustion chambers, and rocker armwear pads are currently in limited production inJapan.

The United States is well behind Japan inprocuring the advanced production equipment to

“Ibid.45 Katz, op. cit., 1985.46 Kenney and Bowen, Op. Cit., 1 9 8 3 .

“U.S. Department of Commerce, op. cit., March 1984.

“Charles River Associates report prepared for the National Bu-reau of Standards, ‘Technological and Economic Assessment of Ad-vanced Ceramic Materials, Vol. 2: A Case Study of Ceramics inHeat Engine Applications, ” NBS-GCR 84-4760-2, August 1984,

4’Richerson, op. cit., December 1985.

-. ——..— —

31

Phofo credit The Garrett Corp

Ceramic turbocharger turbine rotors

produce ceramic turbocharger rotors. U.S. au-tomotive companies do not appear to have thesame level of confidence as the Japanese that aceramic turbocharger market will develop. Thisis an institutional barrier based on perceived risk.In spite of this, U.S. industry is still funding con-siderable R&D on ceramic turbocharger rotors.One study has estimated that if a ceramic rotorprice of $15 can be reached, and if performanceand reliability are acceptable, a worldwide mar-ket of $60 million could be generated for ceramicturbocharger rotors by the year 2000.50

50 Elaine P. Rothman, “Advanced Structural Ceramics: Techni-cal Economic Process Modeling of Production and a Demand Anal-ysis for Cutting Tools and Turbochargers, ” Materials Systems Lab-oratory, Massachusetts Institute of Technology, August 1985.

The ceramic turbocharger has a significance farbeyond its contribution to the performance of theengine. It is regarded as a forerunner technologyto far more ambitious ceramic engines, such asthe advanced gas turbine. Design, fabrication, andtesting methods developed for the turbochargerare expected to serve as a pattern for subsequentceramic engine technology efforts.

Diesel Engines.—There are several levels atwhich ceramics could be incorporated in diesel en-gines, as shown in table 9. The first level involvesa baseline diesel containing a ceramic turbo-charger and discrete ceramic components. Thesecond level adds a ceramic cylinder and piston,and eliminates the cooling system. The ceramicused at this level would provide high-temperature

32

Table 9.—Future Diesel Engine Technology Development Scenario

Technology level Engine configuration Potential ceramic components Potential payoffs

1 State-of-the-art engine, Turbocharger Improved performanceturbocharged Valve train components Reduced cost?

Prechamber, glow plugs Early manufacturing experience

3

2 Uncooled, non-adiabatic Turbocharger Reduced weight - efficiency gain(no water or air cooling) Valve train components Gives option to improve aero-(no turbo-compounding) Piston, cap dynamics - efficiency gain

Cylinders, liners Reduced maintenanceReduced engine systems cost?Flexibility of engine placement

Adiabatic turbo-compound Turbocharger Very significant reduction inTurbo-compound wheel specific fuel consumptionValve train components Improved aerodynamicsPiston, cap Reduced maintenanceCylinders, linersExhaust train insulation

Minimum friction technology Air bearings Lower specific fuel consumption(could be combined with 1, High-temperature rings2 or 3) High-temperature bearings

Nongalling wear surfacesLow friction liquid, lubricant-freebearings

SOURCE R Nathan Katz," Applications of High Performance Ceramics in Heat Engine Design, ” Material Science and Engineering 71227.249, 1985

strength, rather than thermal insulation. The thirdlevel would utilize ceramics for thermal insula-tion in the hot section as well as in the exhausttrain. Turbocompounding would be used to recy-cle energy from the hot exhaust gases to the drivetrain. The fourth level would use advanced min-imum friction technology to improve the perform-ance of the engine. Appropriate aspects of thiscould be utilized at levels one, two, or three,

The four levels listed above place different de-mands on the ceramic materials. Levels one andtwo require a low-cost, high-strength material, butwithout insulating properties; sintered silicon ni-tride or silicon carbide would be possibilities here.It has been suggested that level two represents thebest compromise for light duty ceramic dieselssuch as those in automobiles. 51 Level three wouldrequire an insulating ceramic, probably zirconiaor a zirconia-based composite. This is the levelwhere the most significant improvements in fuelefficiency would be realized. The current zirco-nia and alumina-zirconia transformation tough-ened ceramics are not reliable at the high stressof the piston cap and do not have a low enoughcoefficient of friction to withstand the sliding con-tact stress of the rings against the cylinder liner.

51 Katz, Op. cit., 1985.

These materials do seem to have adequate prop-erties, however, for other components such as thehead plate, valve seats, and valve guides.52

Emission requirements will likely affect the sizeof the diesel market for passenger cars and trucks.Diesel engines generally produce a high level ofparticulate emissions. The higher operating tem-peratures of the adiabatic diesel could reduce emis-sions or allow emission control devices to oper-ate more efficiently. The market for ceramicscould also be affected in another way: a majorcandidate for diesel emission control is a ceramicparticle trap. However, such a trap is likely tobe expensive, and could raise the price of the au-tomobile to an unacceptable level.

Ceramic coatings may be an alternative tomonolithic ceramics in diesel applications. Zir-conia coatings can be plasma-sprayed onto metalcylinders to provide thermal insulation. In a jointprogram between the United States Army TankAutomotive Command and Cummins EngineCo., the combustion zone of a commercial Cum-mins NH diesel engine was coated with a zirconia-based ceramic and installed in a 5 ton Army truck,minus the cooling system. The engine accumu-

52 Richerson, op. cit., December 1985.

.

33

lated over 15,000 km of successful road testing.53

The current state-of-the-art thickness of zirconiacoatings is 30 to 50 thousandths of an inch, It isestimated that thicknesses of 125 thousandths willbe required to provide thermal insulation com-parable to monolithic zirconia,54 The coating isnot as impermeable nor as resistant to thermalshock as the monolithic zirconia. However, thecoated metal part has higher strength and tough-ness than the all-ceramic part.

In the past, confusion has arisen because iden-tical configurations of ceramic and metal engineshave not been compared. It is important to sepa-rate out the configuration options, such as turbo-charging, turbocompounding, heat recovery,cooling, etc., from the material options in orderto isolate the benefits of the use of ceramics. Fail-ure to do this has led to overestimation of the ben-efits of ceramics. For example, a recent study ofa ceramic diesel design funded by DOE indicatesthat “a practical zirconia-coated configurationwith a cooled metal liner, intercooled, with com-bined turbocompounding and Rankine cycle ex-haust heat recovery, provides a 26 percent in-crease in thermal efficiency over a metallic,cooled, turbocharged, intercooled, baseline en-gine. ”55 The bulk of the performance improve-ment was attributed to the turbocompoundingand the Rankine cycle exhaust heat recovery,Only 5.1 percent was attributed to the improvedthermal insulation.

A recent study by Charles River Associates pre-dicts that the uncooled ceramic diesel engine sys-tem will be the first to be commercialized.56 I tprojects that the initial introduction will be in thelate 1980s to early 1990s, and could account for5 percent of new engines manufactured in 1995,This projection is more optimistic than the abovediscussion would imply. Zirconia materials do notyet exist which can be used for level three tech-nology, where the greatest fuel efficiencies are ex-pected. It remains to be seen whether the elimi-

nation of the cooling system will provide sufficientincentives to U.S. automakers to commercializelevel two ceramic technology. However, Japan inparticular has very active programs both in ma-terial and diesel engine development. The Japa-nese company Isuzu has reported greater than 300miles of road testing and is projecting 1990 pro-duction. ”

Automotive Gas Turbines.—The major incen-tive for the use of ceramics in turbines is the pos-sibility of operating the engine at turbine inlet tem-peratures up to about 2,500° F (1,3710 C),compared with superalloy designs, which arelimited to about 1,900° F (1,038° C) without cool-ing. This temperature difference translates into anincreased thermal efficiency from around 40 per-cent to nearly 50 percent. ’g Power increases of 40percent and fuel savings of around 10 percent havebeen demonstrated in research engines contain-ing ceramic components .59 Other potential advan-tages include reduced engine size and weight, re-duced exhaust emissions, and the capability toburn alternative fuels, such as powdered coal.

Structural ceramics are an “enabling technol-ogy” for the automotive gas turbine; i.e., with-out the use of ceramics, an automotive turbinecannot be designed and manufactured that cancompete in cost or performance with current gas-oline and diesel engines. Extensive design, mate-rials, and engine efforts have been made over thepast 15 years in the United States, Europe, andJapan. These efforts have resulted in significantprogress in design methods for brittle materials,the properties of silicon nitride and silicon car-bide materials, fabrication technology for largerand more complex ceramic components, NDE andproof testing, and engine assembly and testing.

Ceramic components have been operated inprototype turbine engines in Germany, Sweden,Japan, and the United States. The tests in theUnited States have been highly instrumented de-velopment engines in test cells. 60 Current pro-

‘]Katz, op. cit., 1985.s4Bi]] Mand]er Cummins Engine Co. , personal communication,

August 1986. ‘SST More] et a],, ‘‘Ana]ysis of LOW Heat Rejection Engine COn-

cepts, ” Proceedings of the 23rd Automotive Technology Develop-ment Contractors Coordination Meeting, to be published by the So-ciety of Automotive Engineers in 1986.

“Charles Rivers Associates, op. cit., August 1984.

sTRicherson, op. cit., December 1985.‘“John Mason, Garrett Corp., presentation at the Society of Au-

tomotive Engineers International Congress and Exposition, Febru-ary 1986.

“David W. Richerson and KM. Johansen, “Ceramic Gas Tur-bine Engine Demonstration Program, ” Final Report, DARPA Nav}rcontract NOO024-76-C-5352, May 1Q82.

fi”D.~’. Richerson, “Evolution in the U.S. of Ceramic Technol-ogy for Turbine Engines, ” American Ceramics Societ}’ Bulletin64(2 1:282-286, 1985.

34

1. Flow Separator Housing 2. Turbine Shroud 3. Turbine Rotor

4. Inner Diffuser HSG 5. Outer Diffuser HSG 6. Turbine Backshroud

8. Turbine Transition Liner 9. Combustor Baffle

10. Bolts 11. Regenerator Shield 12. Alumina-Silica Insulation

Photo credit The Garrett Corp

Prototype ceramic gas turbine engine components

35

grams have achieved over 100 hours of operationat temperatures above 2,0000 F (1,0930 C). 61

These achievements, while impressive, are still farfrom the performance required of a practical gasturbine engine, which will involve continuousoperation above 2,5000 F (1,3710 C). The limit-ing component in these engines appears to be therotor, which must spin at about 100,000 rpm atthese temperatures. The best rotors availabletoday are Japanese. ’z The target automotive gasturbine engine is designed to provide about 100horsepower with fuel efficiencies of about 43 mpgfor a 3,000 lb. automobile. 63

The automotive gas turbine would be a morerevolutionary application of ceramics than thediesel. The diesel engine is a familiar technologyand incorporation of ceramics can occur in stages,consistent with an evolutionary design. The gasturbine, on the other hand, represents a com-pletely new design requiring completely new tool-ing and equipment for manufacture. Because ofthe remaining technical barriers to ceramic gas tur-bines and the fact that they represent a completedeparture from current designs, it is unlikely thata ceramic gas turbine passenger car could beproduced commercially before 2010. 64 In view ofthis long development time, it appears possiblethat this propulsion system could be overtakenby other technologies, including the ceramicdiesel. One factor which would favor the turbineengine would be dramatically increased fuel costs.In the case that traditional fuels became scarce orexpensive, the turbine’s capability to burn alter-native fuels could make it the power plant ofchoice in the future.

In summary, the outlook for ceramic heat en-gines for automobiles appears to be highly uncer-tain. The performance advantages of ceramic en-gines are more apparent in the larger, moreheavily loaded engines in trucks or tanks thanthey are in automobiles. Ceramic gas turbines andadiabatic diesel designs do not scale down in size

as efficiently as reciprocating gasoline engines. 65

Thus, if the trend toward smaller automobilescontinues, reciprocating gasoline engines are likelyto be favored over advanced ceramic designs.

The prevailing approach of U.S. automakersis to wait and see if an clear market niche for ce-ramics develops before investing heavily in thetechnology. This is likely to mean that previousforecasts of the U.S. ceramics heat engine mar-ket, which cluster in the $1 to $5 billion range bythe year 2000, are too optimistic. On the otherhand, the Japanese approach, in which ceramicsare steadily being incorporated in engines on amore experimental basis, reflects greater faith inthe future of the technology. If a substantial au-tomotive market for ceramics does develop, heatengine applications for ceramics would be one ofthe most highly leveraged in terms of economicbenefits and jobs.66 67

Military

Production of ceramics for military applicationsis projected to expand substantially during thenext 25 years. 68 Near-term growth is expected forarmor, radomes and infrared windows, bearingsfor missiles, and rocket nozzles (carbon-carboncomposites and ceramic-coated carbon-carboncomposites). New applications are likely to be la-ser mirrors, gun barrel liners, rail gun compo-nents, and turbine and diesel engine components.Ceramics and ceramic composites in many casesoffer an “enabling” capacity which will allow ap-plications or performance that could not other-wise be achieved. Some of the resulting technol-ogy will be suitable for commercial spinoff ifacceptable levels of fabrication cost and qualitycontrol cost can be attained.

Diesels.—In military diesels, ceramics providemuch the same benefits as in commercial diesels.Of particular interest to the military is the elimi-nation of the cooling system to achieve smallerpackaging volume and greater reliability, Con-

“Mason, op. cit., January 1986.“Richard Helms, General Motors Corp., presentation at the So-

ciety of Automotive Engineers International Congress and Exposi-tion, Detroitr MI, February 1986.

“Katz, op. cit., 1985.“Richerson, op. cit., December 1985,

65U s Conge=, Office of TechnoloW Assessment, Increased A u-. .tomobile Fuel Efficiency and Synthetic Fuels: Alternatives for Re-ducing Oil Zmports, OTA-E-185 (Washington, DC: U.S. Govern-ment Printing Office, September 1982), p, 145.

“Johnson, Teotia, and Hill, op. cit., 1983.bTChar]es River Associates, Op. cit., August 1984.

~gRicherson, op. cit., December 1985.

36

siderable progress has been made through the useof ceramic coatings, Monolithic ceramics havealso been tried, but have only been successful ina few components, and require further develop-ment. A military diesel with minimum coolingachieved primarily with ceramic coatings couldbe produced within 5 years. Engines containingmore extensive ceramic components are not likelyto appear before 1995 to 2000. 69

Turbines.—Turbine engines are in widespreaduse in the military for aircraft propulsion, aux-iliary power units, and other applications. Theyare being considered for propulsion of tanks,transports, and other military vehicles. Ceramicshave the potential to enable advanced turbines toachieve a large increase in performance: as muchas 40 percent more power, and 30 to 60 percentfuel savings.70 In addition, they offer lowerweight, longer range, decreased critical cross sec-tion, and decreased detectability.

Design and material technologies are availablein the United States to produce high-performance,ceramic-based turbine engines for short life ap-plications such as missiles and drones. Further-more, it appears that these engines have poten-tial for lower cost than current superalloy-basedshort life engines.71 Longer life engines will requireconsiderable development to demonstrate ade-quate reliability. This development must addressboth design and materials in an iterative fashion.While the use of ceramic thermal barrier coatingsin metal turbines is well underway, new turbinesdesigned specifically for ceramics are not likelyto be available before the year 2000.

Future Trends in Ceramics

Ceramic Matrix Composites

Dramatic improvements in the fracture prop-erties of ceramics have been obtained by reinforc-ing with continuous, high-strength fibers. Opti-mum microstructure result in composites that donot fail catastrophically, and therefore have me-chanical properties that are very different from

b91bid.701bid.“Ibid.

those of monolithic ceramics, as shown in figure5. The most developed composites to date are sili-con carbide fiber-reinforced glass/ceramic ma-trices.

Recent analysis suggests that fibers resist theopening of a matrix crack by frictional forces atthe matrix-fiber interface .72 One of the importantresults is that the strength of the composite be-comes independent of preexisting flaw size. Thismeans that strength becomes a well-defined prop-erty of the material, rather than a statistical dis-tribution of values based on the flaw populations,

Ceramic matrix composites present an oppor-tunity to design composites for specific engineer-ing applications. This will require a detailed un-derstanding of the micromechanics of failure andexplicit quantitative relations between mechani-cal properties and microstructural characteristics.The most important breakthrough in ceramiccomposites will come with the development ofnew high-temperature fibers that can be processedwith a wider range of matrix materials. Newchemical methods to form fibers and matrices to-gether could yield even more revolutionary ad-vances.

72D. B. Marshall and A.G. Evans, “The Mechanics ofCracking in Brittle-Matrix Fiber Composites, ” Acta Metal].2013-2014, 1985.

Figure 5.— Load-Deflection Curve fora SiC/Glass-Ceramic Composite

400 r300

-u(u

3100

00 0.25 0,5

Deflection (millimeters)

Matrix33(11):

7 600

500

400,-~

300 gT

200 x

100

0

Dotted line indicates deflection at which a single crack haspassed completely through the matrix, but remains bridgedby intact fibers,

SOURCE: D.B. Marshall and A,G, Evans, “The Mechanics of Matrix Cracking inBrittle-Matrix Fiber Composites,” Acta Mefa//. 33(1 1):2013-2021, 1985

37

Chemical Processing of Ceramics

In the future, chemical approaches to the fabri-cation of ceramics will probably be preferred tothe traditional methods of grinding and pressingof powders. Chemical methods, such as sol-gelprocessing, 73 are already used extensively in theproduction of electronic ceramics, and will be-come increasingly important in structural ceramicsas well. These techniques afford greater controlover the purity of the ceramic and over its micro-structure. It has been estimated that by the year2010, 50 percent of structural ceramics will beprocessed chemically .74

Ceramics Factory of the Future

Because of the extreme reliability and reprodu-cibility problems of ceramics, production proc-esses in the future must be highly automated,Processing equipment will be microprocessor con-trolled, including automatic load/unload devicesand numerically controlled grinding and finish-ing machines.

Computer-process control of critical steps com-bined with nondestructive, in-line test methodssuch as acoustic flaw detection and radiographywill form the heart of the manufacturing opera-tion. In the near term, off-line proof testing of crit-ical components will be required.

Quality assurance will be integrated into theprocess and will range from incoming raw mate-rials inspection to automated dimensional inspec-tion techniques such as computer controlled co-ordinate measuring machines of either a contactor noncontact (optical) configuration. Great em-phasis will be placed on the characteristics of thestarting powders, and use of clean rooms will bethe rule in the initial forming steps.

Perhaps the most important processing goal inthe future will be forming to net shape. This willreduce the costs of ceramics by eliminating costlygrinding steps, and by making more efficient useof the material. Moreover, it will increase the

“See for example: David W. Johnsonr Jr., “Sol-Gel Processingof Ceramics and Glass, ” American Ceramic Society Bulletin64(12):1597-1602, 1985.

‘*R. Nathan Katz, in presentation at the Society of AutomotiveEngineers International Congress and Exposition, Detroit, MIr Feb-ruary 1986.

reliability and yields, by eliminating flaws whichare introduced in the finishing operations.

Biotechnology

Structural ceramics could have a significant in-teraction with the developing field of biotechnol-ogy in the future. Ceramics could be used exten-sively in fermenters, and are likely to be importantin a broad range of product separation tech-nologies.

Fermenters .—Most current fermenters are madeof 316 grade stainless steel. 75 Steel fermenters suf-fer from several disadvantages, including contami-nation of the cultures with metal ions, corrosioncaused by cell metabolizes or reagents, and leaksaround gaskets and seals during sterilization andtemperature cycling. Glass-lined steel bowls aresometimes used, especially for cell cultures, whichare more sensitive to contamination than bacterialcultures. However, the thermal expansion mis-match between the glass lining and the metalcauses problems during steam sterilization. Ce-ramics offer a solution to these problems, becauseof their chemical stability and low thermal expan-sion. Ceramics could be used in the bowl and agi-tator as well as in the peripheral plumbing jointsand agitator shaft seal to prevent leaking.

Separation Technology .—Separation andpurification of the products of cell and bacterialcultures is a key aspect of biotechnology. In gen-eral, the separation techniques are based less onfiltration than on active interactions between asolid phase and the liquid mixture, as in chro-matography. For example, biologically producedinsulin is now being purified with a chiromato-

graphic process based on a modified silica mate-rial, 76 Silica or alumina particles can also be usedas a solid support for attaching monoclinal anti-bodies, which bind to specific proteins and effecta separation by affinity chromatography .77 Thestrength and hardness of the ceramics are key tothe avoidance of deformation of the ceramic par-ticles under conditions of high throughput .78 In

“Richard F. Geoghegan, E.I. du Pent de Nemours & Co., Inc.,personal communication, August 1986.

“Joseph J. Kirkland, E,I. du Pent de Nemours & Co., Inc., per-sonal communication, August 1986.

“George Whitesides, Department of Chemistry, Harvard Univer-sity, personal communication, August 1986.

“Ibid.

38

the future, the most efficient processes could behybrids based on both filtration and chromatog-raphy. 79 As these processes are scaled up to pro-duction units, a large increase in the demand forspecially modified silica and alumina columnpacking materials can be anticipated.

Energy Production

Power Turbines.—The performance require-ments for power turbines are much greater thanthose for automotive gas turbines. 80 Power tur-bines must have a lifetime of 100,000 hours, com-pared with about 3,000 hours for the auto tur-bine. In addition, the consequences of powerturbine failure are much greater. In light of thesefacts, a recent report has concluded that powerturbines will be commercialized after automotivegas turbines.

81 While some of the processing tech-nology developed for the auto turbine may beapplicable to power turbines, the scale-up froma rotor having a 6 inch diameter to one havinga much larger diameter may require completelynew fabrication techniques. The larger ceramicstructures may also require different NDE tech-niques to ensure reliability. Thus, use of mono-

“Michelle Betrido, Celanese Research Corp., personal commu-nication, August 1986.

80 John50n and Rowcliffe, op. cit., January 1986.‘*Ibid.

lithic ceramics in the critical hot section compo-nents of power turbines is not anticipated in thenext 25 years. However, ceramics may find ap-plications in less critical structures, such as com-bustor linings, and ceramic coatings could be usedto augment the high-temperature resistance ofcooled superalloy rotor blades .82

Research and Development Prioritiesfor Ceramics

In 1986, the U.S. Government spent about $60million on structural ceramics research and de-velopment (table 10). R&D expenditures in pri-vate industry may be roughly comparable .83 TheDepartments of Energy and Defense spent thelargest fractions, at 47 percent and 36 percent, re-spectively. Funding for fiscal year 1987 is expectedto decrease by over 20 percent, with 50 percentreductions in the DARPA program and in theDOE vehicle propulsion program. The followinghierarchy of R&D priorities is based on the op-portunities identified above. These are then cor-related with the actual spending on structural ce-ramics R&D in fiscal year 1985.

azlbid.“Charles River Associates, op. cit., August 1984, p. 38.

Table 10.—Structural Ceramic Technology: Federal Government Funded R&D (in millions of dollars)

FY 1983 FY 1984 FY 1985 FY 1986 Estimated FY 1987

Department of Energy:Conservation and renewable energy:

Vehicle propulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Advanced materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Industrial programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Energy utilization research . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fossil energy:Advanced research and technology development . . . . . . .

Energy research:Basic energy science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

NASA:Lewis Research Center . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

National Science Foundation. . . . . . . . . . . . . . . . . . . . . . . . .National Bureau of Standards . . . . . . . . . . . . . . . . . . . . . . . . . .Department of Defense:

Defense ARPA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .U.S. Air Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .U.S. Army . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .U.S. Navy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Total . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .alnCIUdeS $1.6 for Manpower Salaries.blncludes $4.0 for TA C O M Diesel.

SOURCE: Robert B. Schulz, Department of Energy

11.43.21.00.5

1,0

3.0

3.02.9

7.73.04.71.2

42.6

12.7 11.9 10.05.1 6.0 8.72.3 1.7 1.51.5 1.8 1.8

1,1 1.5 1.2

4.4 4.6 4.5

2.5 5.4 4.5a

3.3 3.6 3.62.2

8,2 9.4 10.03.4 4.7 4.76.0 2.5 4.4b1.3 1.4 2.3

51.8 54.5 59.4

5.26.50.81.0

0.9

4.5

4.6a

3.73.0

5.05.04.7b

2.3

47.2

39

Very Important

Processing Science.—There is a great need forgeneric research to support the development ofpractical manufacturing technologies withinindustry. The agenda for such research is long,but

●

●

●

●

●

includes such topics as:

development of near-net-shape processes;development of pure, reproducible powders,whiskers, and fibers which can be formedand densified with a minimum of interme-diate steps; role of solution chemistry inpowder preparation and control of interfaceproperties in composites;development of practical in-process inspec-tion devices to identify problems at theearliest possible stage in the process;iterative development of new equipmentsuch as hot isostatic presses (HIP) and multi-stage processes such as sinter-HIP, with em-phasis on scaling up to commercially viablesize; andunderstanding the relationships betweencoating process variables and final proper-ties and performance of ceramic coatings.

Environmental Behavior.—Many of the appli-cations for ceramics mentioned above requirelong-term performance in severe environments.In order to develop higher temperature, corrosionresistant materials, it is necessary to understandthe long-term behavior of candidate ceramic ma-terials in the anticipated environments.

For heat engine applications, the generalrequirement is to understand the mechani-cal and chemical behavior of advancedceramics such as silicon nitride, siliconcarbide, zirconia, and composites based onthese materials in environments of 1,0000 to1,4000 C (1,8320 to 2,5520 F) in air, carbonmonoxide, or carbon dioxide.In ceramic wear parts, it will be necessaryto understand the relationship between wear,erosion, and toughness in the presence of lu-bricating fluids and gases. Generally, wearparts include hard materials like tungstencarbide, titanium boride, and materialswhich have good lubrication characteristics,like silicon nitride.

●

●

●

Since heat exchangers generally fail as a re-sult of long-term corrosion at high tempera-tures, it is important to understand thechemical processes of corrosion in environ-ments like salts of sodium, potassium, mag-nesium, calcium, vanadium, and mixedmetals. In addition to existing materials (e.g.silicon carbide, cordierite, and zirconiumsilicate), newer materials, including siliconnitride and composites, should be investi-gated. Corrosion-resistant ceramic coatingsmay become important here.Considering the large size of the potentialmarkets in bioceramics, it will be critical tounderstand the long-term effects of bodyfluids on chemical structure and mechanicalproperties. In view of the many years whichceramic implants must serve without failure,the interaction between slow crack growthand the body environment should be inves-tigated.The long-term environmental stability of ad-vanced chemically bonded ceramics will becrucial to their effectiveness in constructionand other applications. The deterioration ofthe properties of some advanced cements inthe presence of moisture remains a problem,and the chemical degradation of the fiber in-terface in reinforced cements and concreteshave limited the structural uses of these ma-terials.

Reliability. -No factor is more important to thesuccess of ceramics in all of the applications dis-cussed than reliability. Since the performancespecifications and environment of each applica-tion are different, it will be necessary to estab-lish the most appropriate and cost-effective non-destructive testing methods for each one.

In order to facilitate design, models need to bedeveloped for predicting the service lifetime of ce-ramic parts containing various kinds of flaws.Such models must depend heavily on informationderived from the categories of environmental be-havior of ceramics and composite failure mecha-nisms discussed above. Beyond a dependence onintrinsic flaws in the material, however, lifetimealso depends on the location and nature of theflaw relative to the structure itself. It is not suffi-

40

cient to characterize the behavior of a coupon ofthe material from which a structure is made; ei-

ther the structure itself must be tested, or addi-

tional models must be available to predict the ef-fects of a particular flaw on a particular structure.

Interphase in Composites.—The interphase be-tween ceramic fiber and matrix is crucial to thestatic strength, toughness, and long-term stabil-ity of the composite. Very little is known aboutthe relationship between the properties of the in-terphase and these overall composite properties.The capability to modify the surface chemistryof ceramic fibers and whiskers to provide opti-mum compatibility between reinforcement andmatrix could yield remarkable improvements inceramic performance and reliability,

Important

Joining of Ceramics. -In most applications, cer-amics are not used alone; rather, ceramic com-ponents are part of larger assemblies. Therefore,the ceramic must be joined to more conventionalmaterials in the assembly to function properly.Broad research in joining of ceramics to metals,glasses, and other ceramics could have a decisiveimpact on a future use of monolithic ceramics,coatings, and composites. The key to joining isan understanding of the surface properties of thetwo materials and of the interface between them.In general, the interface is a critical point of weak-ness in discrete ceramic components such as thosein heat engines, in ceramic coatings on metal sub-strates, and in ceramic fibers in composites. Prin-cipal needs in this area are in the strengtheningand toughening of joints, an understanding oftheir high-temperature chemistry, and improvedresistance to corrosion in the environments of in-terest. As with solution methods in powder prep-aration, chemistry will make a crucial contribu-tion to this field.

Tribology of Ceramics.—Tribology, the studyof friction, wear, and lubrication of contiguoussurfaces in relative motion, is a field of key im-portance to ceramic wear parts and heat enginecomponents. Lubrication is a particularly seriousproblem in ceramic engines because of their highoperating temperatures. Ordinary engine oils can-not be used above about 3500 F (177° C). For

operating temperatures of 500° to 7000 F (2600to 371 o C), synthetic liquids such as polyol es-ters are available, but are extremely expensive andrequire further development. In a low-heat rejec-tion (“adiabatic”) ceramic engine, cylinder linertemperatures may reach 1,000° to 1,700° F (5380to 927° C), depending on the insulating effective-ness of major engine components. For this ele-vated temperature regime, synthetic lubricantscannot be used effectively. One approach is to usesolid lubricants which would become liquid at ele-vated temperatures; however, no such lubricantsexist for use in the environments envisioned (hightemperature, corrosive gases). Moreover, the dis-tribution of solid lubricant around the engine isa persistent problem .84

A second approach involves modifications ofthe surface of the component to produce “self-Lubrication. ” The lubricant can be introducedthrough ion implantation directly into the surface(to a depth of several micrometers) of the com-ponent; it then diffuses to the surface to reducefriction. Some metal or boron oxides show prom-ise as lubricants. Another alternative is to use sur-face coatings of extremely hard ceramics such asthe carbides and nitrides of zirconium, titanium,or hafnium, without any lubrication. At present,these techniques all lead to sliding friction coeffi-cients which are roughly four times higher thanthose achieved at low temperatures with engineoils.85 Further research is needed to improve thissituation.

Failure Mechanisms in Composites.—Ceramicmatrix composites offer the best solution to theproblem of the brittleness of ceramics. However,this field is still in its infancy, and research is char-acterized by a very empirical approach to mix-ing, forming, densification, and characterizationof fiber-powder combinations. Fundamental un-derstanding of the failure mechanisms in compos-ites would provide guidance for development ofnew, tougher ceramics. This would include an in-vestigation of: multiple toughening techniques,such as transformation toughening and whiskerreinforcement; the role of interphase properties

giManfred Kaminsky, Surface Treatment Science International,personal communication, August 1986.

‘sIbid.

41

in fracture; and failure mechanisms in continu-ous fiber composites.

Standardization and Testing.—Standardizedtests and methods of reporting test data are fun-damental to the use of any engineering material;however, a consensus on even these basic princi-ples has eluded the ceramics community. It is notsurprising that in a new field like structural ce-ramics, which is constantly inventing new mate-rials and processes, there should be a lag in thedevelopment of standards and test methods.Standardization of materials must inevitably fol-low the formation of a consensus that certain ma-terials are worth pursuing. While there is a dan-ger in prematurely narrowing the possibilities,there may also be a danger in not developing thematerials we already have. This is particularlytrue of the relatively well-studied materials: sili-con nitride, silicon carbide, and zirconia.

In the case of ceramic composites, the specifica-tion of standard tests is more difficult, becausethe relationship between the micromechanics ofthe fibers and matrix and the macroscopic com-posite behavior is not well understood. In con-tinuously reinforced composites, for example,which display progressive rather than catastrophicfailure behavior, the stress at which the compos-ite “fails” is ambiguous.

There is a growing interest, both nationally andinternationally, in the development of standardsfor ceramics .8’ Areas which could benefit fromstandards include: statistical process control, non-destructive testing, analytical procedures, me-chanical properties, performance characteristicsin various environments, design procedures andterminology .87 Standards are essential for the gen-eration of design data, and for reliability speci-fications for ceramic materials sold domesticallyor abroad,

““Concerned organizations include the American Ceramic Soci-ety, the U. S. Advanced Ceramic Association (USACA), the Amer.ican Society for Testing and Materials (ASTM), the Versailles Projecton Advanced Materials and Standards (VAMAS ), and the NationalBureau of Standards,

‘-Proposal developed by S. J. Schneider, as described in the Ce-ramic Technology Newsletter, Oak Ridge National Laboratory}’, No,10, Februrary-April 1986.

Desirable

Chemically Bonded Ceramics.–Chemicallybonded ceramics offer great promise for low-cost,net shape fabrication of structures in such appli-cations as wear parts and construction. Recent im-provements in the tensile strength of CBCs sug-gest that the limits of this key engineering propertyare far from being realized, Further research is re-quired in flaw size reduction, long-term stabilityin various environments, and the properties of theinterphase in fiber-reinforced cements and con-cretes.

Table 11 shows that the actual structural ce-ramics R&D spending in fiscal 1985 for all gov-ernment agencies corresponds roughly with thepriority categories recommended above, althoughspecific projects differ. Processing research ac-counted for the lion’s share, with 76 percent. Noseparate estimate was available of research on theinterphase in composites; no doubt a portion ofthis work was included under composite fabrica-tion, listed in table 11 as a subcategory of proc-essing. Also, no separate figure was obtained for

Table 11 .—Breakout of the Fiscal Year 1985 StructuralCeramic Budget According to the R&D Priorities

Cited in the Text

FY 1985 –

Research area budget percentageProcessing:

Powder synthesis . . . . . . . . ... ... 4Monolithic fabrication . . . . . . . . . . 32Composite fabrication . . . . . . . . . . . 32Component design and testing . . . . 4Coatings . . . . . . . . . . ... ... ... 4Machining ... . . . . . . . . . . . . . . . . . <1

Subtotal . . . . . . . . . . . . . . . . . . . . . 76Environmental behavior ., . . . . ... . 4Reliability:

Modeling ., . . . . . . . . . . . . . . . . . . ... 2Time dependent behavior . . . . . . . . 1Nondestructive evaluation . . . . . . . . 3Micros t ruc ture eva luat ion . . . 4

Subtotal , . . . . . . . . . . . . . . ... 10Interphase in composites ... ... . no separate figure

Tribology.

Joining . .

Fracture .

Standards

TotalSOURCE S J

.. . . . . . . . . . . . . . . . . . . . . . . . . 2

. . . . . . . . . . . . . . . . . . . . . . . . . 3

. . . . . . . . . . . . . . . . . . . . . . . . . 5

. . . . . . . . . . . . . . . . . . . . . . . . . <1

. . . . . . . . . . . . . . . . . . . . . . . . . 100 ”/0Dapkunas, Department of Energy

Federa l expendi tures on new cements and con- suming that the current breakdown of Federa lcretes. In 1983 , however , to ta l U .S . Government ceramics research is similar to that in f iscal 1985,

and industry funding of cement research was esti- a comparison of table 11 with the priorities abovemated at only $1 million, compared with a port- suggest that greater emphasis should be placed onland cement sales volume over $1 billion.88 As- standards development, joining, tribology, and

cement-based materials.88National Research council, Transportation Research Board, op.cit., 1984.

POLYMER MATRIX COMPOSITES

Unlike a ceramic matrix composite, in whichthe reinforcement is used primarily to improve thefracture toughness, the reinforcement in a poly-mer matrix composite (PMC) lends strength andstiffness to the relatively weak matrix. The com-posite is designed so that the mechanical loads towhich the structure is subjected in service are sup-ported by the reinforcement. The function of thematrix is to bond the fibers together and to trans-fer loads between them. As with ceramic matrixcomposites, the reinforcement may consist of par-ticles, whiskers, fibers, or fabrics, as shown in fig-ure 6.

Polymer matrix composites are often dividedinto two categories: reinforced plastics, and “ad-vanced composites. ” The distinction is based onthe level of mechanical properties (usually strengthand stiffness); however, there is no unambiguousline separating the two. Reinforced plastics, whichare relatively inexpensive, typically consist ofpolyester resins reinforced with low-strength glassfibers (E-glass); they have been in use for 30 to40 years in applications such as boat hulls, cor-rugated sheet, pipe, automotive panels, and sport-ing goods. Advanced composites, which have beenin use for only about 15 years, primarily in theaerospace industry, consist of fiber and matrixcombinations which yield superior strength andstiffness. They are relatively expensive and typi-cally contain a large percentage of high-perform-ance continuous fibers, such as high-strength glass(S-glass), graphite, aramid, or other organic fibers.In this report, market opportunities for both rein-forced plastics and advanced composites are con-sidered.

The properties of the composite depend on thematrix, the reinforcement, and the boundary layerbetween the two, called the “interphase.” Conse-quently, there are many variables to consider whendesigning a composite. These include the type ofmatrix, the type of reinforcement, their relativeproportions, the geometry of the reinforcement,and the nature of the interphase. Each of thesevariables must be carefully controlled in order toproduce a structural material optimized for theconditions under which it is to be used.

Figure 6.—Composite Reinforcement Types

Particulate

SOURCE: Carl Zweben, General Electric Co

43

44

The use of continuous fiber reinforcement con-fers a directional character (“anisotropy”) to theproperties of composites. PMCs are strongestwhen stressed parallel (O O) to the direction of thefibers (longitudinal direction), and weakest whenstressed perpendicular (90o) to the fibers (trans-verse direction). In practice, most structures aresubjected to complex loads, necessitating the useof fibers oriented in several directions (e. g., 0°,+ / –45 o, 900). However, PMCs are most effi-ciently used in applications which can take advan-tage of the inherent anisotropy of the materials,as shown in figure 7, which compares the strengthand stiffness of composites with various metals.

When discontinuous fibers or particles are usedfor reinforcement, the properties tend to be moreisotropic, since the reinforcement direction is notas well defined. Such composites lack the outstand-ing strength of continuous fiber PMCs, but theycan be produced more cheaply, using the technol-ogies developed for unreinforced plastics, such as

extrusion, injection molding, and compressionmolding. Sheet molding compound (SMC, figure7) is such a material, which is widely used in theautomotive industry.

The Polymer Composites Industry

The reinforced plastics/composites industrytoday encompasses a very broad range of materi-als, products, and manufacturing processes. About85 percent of the material used is glass fiber-rein-forced polyester resin (often called fiber-reinforcedplastic, or FRP). 1 FRP is fabricated by a numberof processes to produce a great variety of prod-ucts, some of which are noted above.

Less than 2 percent of the material used in thereinforced plastics/composites industry goes into

‘Reginald B. Stoops, R.B. Stoops & Associates, Newport, RI,“Manufacturing Requirements of Polymer Matrix Composites, ” con-tractor report prepared for the Office of Technology Assessment,December 1985.

Figure 7.–Comparison of the Specific Strength and Stiffness of Various Composites and Metals (Steel: AISI 4340;Aluminum: 7075”T6; Titanium: Ti-6Al-4V) Specific Properties are Ordinary Properties Divided by Density.

Angles Refer to the Directions of Fiber Reinforcement

4

3

2

1

(0°) Graphite/epoxy

/+

/’

(0°) Kevlarlepoxy

i+ (O”,90c) Graphitelepoxy

/

+/’Steel

Aluminum ~ +Titanium

(0° t 45° 90°) (0”) S-glass/epoxy

IGraphite/epoxy

t Sheet molding compound (SMC)

1 2 3 4

Specific tensile strength (relative units)

SOURCE Carl Zweben, General Electr!c Co

“advanced composites” for use in “high-tech” ap-plications such as aircraft and aerospace.2 In 1984,the world produced some 22 million pounds ofadvanced composite materials, mostly in theUnited States. The total value of fabricated partswas about $1.3 billion split among four major con-suming industries: aerospace (60 percent), sportsequipment (20o percent), and industrial and au-tomotive (15 percent ).’

It has been estimated that advanced polymercomposites will grow at the relatively high rateof about 15 percent per year in the next few years,with the fastest growing sector being the aerospaceindustry at 22 percent. By 1995, consumption isforecast to be 110 million pounds with a valuein 1985 dollars of about $6.5 billion. By the year2000, consumption is estimated to be 200 millionpounds valued at about $12 billion.4

If we divide these numbers to get a rough ideaof the cost per pound of advanced composite ma-terial, we find a value of about $60/pound. Thiscompares with a value of about $1/pound for steelor $1 .50/pound for FRP. If these forecasts are cor-rect, it is clear that over this period advanced com-posites will be used primarily in high value-addedapplications which can support this level of ma-terial costs. However, we shall see that use of com-posites can lead to cost savings in manufacturingand service. Thus, the per-pound cost is rarelya useful standard for comparing advanced com-posites with traditional materials.

‘Ibid, These are primarily epoxy matrices reinforced with carbonfibers.

3According to the market resdarch firm, Strategic Analysis (Read-ing, PA) as reported in High Technology, May 1986, p. 72. Ad-vanced composites were defined as those reinforced with S-glass orsuperior fibers.

“’Industry News, ” SAMPE Journal, July August 1985, p. 89.

Composite

Matrix

45

Constituents

The matrix determines the resistance of the com-posite to most of the degradative processes whicheventually cause failure of the structure, includ-ing impact damage, delamination, water absorp-tion, chemical attack, and high-temperature creep.Thus, the matrix is typically the “weak link” inthe composite structure.

The matrix phase of commercial composites canbe classified as either “thermoset” or “thermoplas-tic. ” The general characteristics of each matrix typeare shown in figure 8; however, recently devel-oped matrix resins have begun to change this pic-ture, as noted below.

Thermosets. -Thermosetting resins include pol-yesters, vinylesters, epoxies, bismaleimides, andpolyamides. Thermosetting polyesters are com-monly used in FRPs, and epoxies make up mostof the current market for advanced compositeresins. Initially, the viscosity of these resins is low;however, these matrix materials undergo chemi-cal reactions which crosslink the polymer chains,and thus connect the entire matrix together in athree-dimensional network. This process is called“curing. ” Thermoses, because of their three-dimensional crosslinked structure, tend to havehigh dimensional stability, high temperature re-sistance, and good resistance to solvents. Recently,considerable progress has been made in improvingthe toughness of thermosets.5

‘Norman J. Johnston, “Synthesis and Toughness Properties (>f Re\-ins and Composites, ” CP 2321, National Aeronautics and SpaceAdministration, 1984.

Figure 8.—Comparison of General Characteristics of Thermoset and Thermoplastic Matrices

Process Process Use SolventResin type temperature time temperature resistance Toughness

Thermoset . . . . . . . . . . . . . . . . . . . . . . . . . Low 1 High I High II High I Low

Toughened thermoset . . . . . . . . . . . .

Lightly crosslinked thermoplastic . . . . . t 1 t t 1

Thermoplastic. . . . . . . . . . . . . . . . . . . . . . High I Low I Low Low I High I

SOURCE Darrel R Tenney NASA Langley Research Center

46

Thermoplastics.—Thermoplastic resins, some-times called engineering plastics, include somepolyesters, polyetherimide, polyamide imide, poly-phenylene sulfide, polyether ether ketone (PEEK),and liquid crystal polymers. They consist of long,discrete molecules which melt to a viscous liquidat the processing temperature, typically 5000 to700° F (260° to 3710 C), and, after forming, arecooled to an amorphous, semicrystalline, or crys-talline solid. The degree of crystallinity has astrong effect on the final matrix properties. Un-like the curing process of therrnosetting resins, theprocessing of thermoplastics is reversible, and, bysimply reheating to the process temperature, theresin can be formed into another shape if desired.Thermoplastics, while generally inferior to ther-mosets in high-temperature strength and chemi-cal stability, are more resistant to cracking andimpact damage. However, it should be noted thatrecently developed high-performance thermoplas-tics such as PEEK, which have a semicrystallinemicrostructure, exhibit excellent high-temperaturestrength and solvent resistance.

Thermoplastics offer great promise for the fu-ture from a manufacturing point of view, sinceit is easier and faster to heat and cool a materialthan it is to cure it. This makes thermoplastic ma-trices attractive to high-volume industries such asthe automotive industry. Currently, thermoplas-tics are used primarily with discontinuous fiberreinforcements such as chopped glass or carbon/graphite. However, there is great potential forhigh-performance thermoplastics reinforced withcontinuous fibers. For example, thermoplasticscould be used in place of epoxies in the compositestructure of the next generation of fighter aircraft.

Reinforcement

The continuous reinforcing fibers of advancedcomposites are responsible for their high strengthand stiffness. The most important fibers in cur-rent use are glass, graphite, and aramid (other or-ganic fibers, such as oriented polyethylene, arealso becoming important). Advanced compositescontain about 60 percent of reinforcing fiber byvolume. The strength and stiffness of some con-tinuous fiber composites are compared with those

of sheet molding compound and various metalsin figure 7. For example, unidirectional, highstrength graphite/epoxy has over three times thespecific strength and stiffness of common metalalloys.

Of the continuous fibers, glass has a relativelylow stiffness; however, its tensile strength is com-petitive with the other fibers and its cost is dra-matically lower. This combination of propertiesis likely to keep glass fibers the most widely usedreinforcement for high-volume commercial com-posite applications. Only when stiffness or weightare at a premium will aramid and graphite fibersbe used.

Interphase

The interphase of composites is the region whereloads are transmitted between the reinforcementand the matrix. The extent of interaction betweenthe reinforcement and the matrix is a design vari-able, and may vary from strong chemical bond-ing to weak frictional forces. This can often becontrolled by using an appropriate coating on thereinforcing fibers. Generally, a strong interracialbond makes the composite more rigid, but brit-tle; a weak bond decreases stiffness, but enhancestoughness. If the interracial bond is not at leastas strong as the matrix, fracture and delamina-tion will occur at the interphase under certain load-ing conditions. Frequently, the most desirablecoupling is intermediate between the strong andweak limits. The bond is also critical to the long-term stability of the composite, playing a key rolein fatigue properties, environmental behavior, andresistance to hot-wet conditions.

Properties of Polymer Composites

Composites are designed materials. This is reallythe fact that underlies their usefulness. Given thespectrum of matrix and reinforcement materialsavailable, the composite properties can be op-timized for a specific application. A composite canbe designed to have zero coefficient of thermalexpansion. It can be reinforced with combinationsof fiber materials (hybrid composites) and geom-etries in order to optimize performance and min-imize cost. The inherent anisotropy of the mate-

47

rial means that the composite can have differentproperties in different directions. The design op-portunities of PMC materials are just beginningto be realized.

Design, Processing, and Testing ofPolymer Matrix Composites

Composites Design

Because of their larger number of componentsand anisotropic properties, polymer compositesare inherently more complex than metals. In fact,composites are more accurately characterized ascustomized structures rather than materials. Whilethe engineering properties of the homogeneousresins and fibers can be determined, the proper-ties of the composite depend on the composition,fiber geometry, and the nature of the interphase.The categories of mechanical and physical prop-erties used to characterize composites are carriedover from the long engineering experience withmetals. In many cases, however, properties whichare meaningful in metals are not meaningful incomposites. “Toughness” is such a property. Inmetals, where the dynamics of crack propagationand failure are relatively well understood, tough-ness can be defined relatively easily. In a com-posite, however, toughness is a complicated func-tion of the matrix, fiber, and interphase, as wellas the reinforcement geometry. The shear andcompression properties of composites are alsopoorly defined.

In spite of many years of discussion about stand-ardized test methods for measuring engineeringproperties, the composites community has notbeen able to reach a consensus on what it is thatthe existing tests actually measure. The Army’sMaterials Technology Laboratory (Watertown,Massachusetts) is currently developing MIL-17,a handbook on advanced composites which willbegin to address the questions of how data on com-posites can be obtained and reported. At present,each company qualifies its material for each sep-arate aerospace or defense application accordingto its own individual tests and procedures. Dataon material properties are often developed undergovernment contract (costing $100,000 to $200,000and taking about 1 year), and companies are reluc-tant to share the results. Even when data are re-

ported in the literature, the type of test used andthe statistical reliability of the results are not re-ported with the data. While the lack of standardsprobably does not inhibit the expert designer ofcomposite aerospace structures, standards couldencourage the use of composites in industries suchas construction, where designers have no familiar-ity with the materials.

Computer-Aided Design.—Inasmuch as com-posites are tailored to meet the design requirementsof a particular structure, designers must rely oncomputer models to analyze and predict the be-havior of composite structures using a databasecontaining the properties of the resins, fibers, andunidirectional composites. Such models presup-pose a scientific understanding of the underlyingphysical, chemical, and mechanical processeswithin the composite at a microscopic level, andthe relations between these and the macroscopicproperties. In general, this understanding does notexist; the most critical needs in this area are dis-cussed below in the section on research and de-velopment priorities.

A number of models exist today to address theneeds of composites designers. Elastic behaviormodeling is fairly advanced, with programs avail-able to analyze stress distributions and elasticdeformation in complex three-dimensional shapes.Several models exist to predict failure in compos-ite structures where the characteristics of the crit-ical flaws are known. These are based on LinearElastic Fracture Mechanics (LEFM), a body of the-ory developed to predict fracture in metals. Ex-tension of LEFM to composites is controversialtoday, because the failure modes of compositesare more complex than metals. Very little workhas been done in modeling the long-term behaviorof composites; for example, in the areas of fatigue,creep, and environmental degradation. Part of thereason for this is the lack of data on which to testand validate models.

In spite of the lack of modeling capability, ex-perience to date has shown that designers and man-ufacturers can produce reliable composite struc-tures. This is no doubt due to two factors. First,in the face of uncertainty, designers tend to “over-design”; that is, to be too conservative in theiruse of material, to avoid any possibility of mate-

48

rial failure. Second, composite structures havebeen extensively tested before use, ensuring thatany potential problems show up during the tests.Thus, the composite materials themselves havebeen proven, in the sense that structures can befabricated which are reliable and meet all designcriteria. However, both overdesign and empiri-cal testing are costly and drive up the prices ofcomposites. The principal benefit of enhancedmodeling capability will be to help to make com-posites more cost competitive.

Manufacturing of PolymerComposite Structures

Given the many different fibers and matricesfrom which composites can be made, the subjectof PMC manufacturing is an extremely broad one.However, more than any other single area, lowcost manufacturing technologies are required be-fore composites can be utilized more widely. Thebasic steps include: impregnation of the fiber withthe resin; forming of the structure; curing (thermo-

set matrices) or thermal processing (thermoplas-tic matrices); and finishing. Depending on theprocess, these steps may occur separately or simul-taneously. For example, the starting material formany composites is a “prepreg”; i.e., a fiber tapeor cloth which has been preimpregnated with resinand partially cured. Therrnoset prepregs may bestored in the freezer for up to 12 months prior touse. Some of the more important fabrication proc-esses for composites are listed in table 12.

Nondestructive Evaluation (NDE)of Polymer Matrix Composites

In general, polymer composites do not have asgreat a tendency to brittle fracture as do ceramics.This means that the critical flaw size in large PMCstructures may be of the order of centimeters, whilein ceramics it is some tens of micrometers. PMCstructures are increasingly used in life-criticalstructures such as aircraft wings and fuselages.

Photo credit: Hercules, Inc.

Filament winding of a rocket motor case.

49

Table 12.— Production Techniques forPolymer Composites

Technique Characteristics Examples

Sheet molding Fast, flexible, 1 -2” SMC automotive bodyfiber panels

Injection molding Fast, high volume Gears, fan bladesvery short fibers,thermoplastics

Resin transfer Fast, complex parts, Automotive structuralmolding good control of fiber panels

orientationPrepreg tape Iayup Slow, Iaborious, Aerospace structures

reliable, expensive(speed improved byautomation)

Pultrusion Continuous, constant l-beams, columnscross-section parts

Filament windlng Moderate speed, Aircraft fuselage,complex geometries, pipes, drive shaftshollow parts

Thermal forming Reinforced thermoplastic All of above(future) matrices, fast, easy

repair, joiningSOURCE Off Ice of Technology Assessment

This places a special burden on NDE, both in thefactory and in the field.

Although NDE is now used primarily for thedetection of defects in finished structures, in thefuture it will be used increasingly for monitoringthe status of composites at intermediate steps inthe production process. Progress in this field willrequire development of sophisticated sensors andfeedback control systems.

Requirements for NDE of composites differsomewhat from those of ceramics. While the flawsizes to be detected are not as small, the area ofstructure to be investigated is frequently muchlarger, up to hundreds of square feet. Thus, tech-niques are required which can rapidly scan largeareas for flaws or damage. While there are nu-merous NDE techniques which may be useful inthe laboratory for testing of small specimens forresearch purposes, relatively few are appropriateto production or field-level inspection. Several ofthe more important NDE techniques which arerelevant to production, end product, and field levelinspection are listed in table 13. Excellent progresshas been made in production-level techniques suchas ultrasonics, and manufacturers are confidentthat large composite surface areas can be inspectedreliably and economically for such flaws as bulkdelaminations.

The inspection and repair of composite struc-tures (e.g., aircraft components) at the depot andfield levels will require a substantial training pro-gram for inspectors unfamiliar with composites.All procedures must be standardized and straight-forward, since in general composite experts willnot be available. In the future, as inspection proc-esses become fully computerized, this will be anexcellent application for expert systems which canguide the operator through the process and alerthim to any detected anomalies.

Table 13.—NDE Techniques Appropriate for Production, Finished Product, Depot,and Field-Level Inspections of Polymer Matrix Composite Structures

N DE technique Flaw type Sensitivity

Production:Visual (remote) ... ... , . . Fiber orientation good

Foreign materialUltrasonic ., . . . . . . . . . . . . . . Porosity, viscosity good

during cureDielectrometry . . . . . . . . . . Degree of cure good

End product:Visual . . . . . . . . . . . . . . . . . . surface goodUltrasonic . . . . . . . . . . . . . . . bulk goodRadiographic . . . . . . . . . . . . . bulk fairAcoustic emission ., ... . bulk fair

Depot level:Ultrasonic ., . . . . . . . . . ... bulk delamination good

Field level:Ultrasonic . . . . . . . . . . . . . . . . bulk delamination fair

Complex Development forshapes commercialization

good

poor

good

goodpoor

excel lentgood

poor

Door

none

extensive

some

nonesomenone

extensive

some

extensiveSOURCE Joseph A. MoyzIs, et al , “Nondestructive Testing of Polymer Matrix Composites,” a contractor report prepared for

OTA, December 1985

50

Health and Safety

There are a number of unique health and safetyissues associated with the manufacture of com-posite materials. The health hazards stem fromthe fact that chemically active materials are usedand workers handling them may breathe harmfulfumes or come in contact with irritating chemi-cals. The chemical of greatest concern is the sty-rene monomer used in polyester resins. The prob-lem is most severe when the resin is sprayed, andthe monomer evaporates into the air. Inhalationof styrene monomer can cause headaches, dizzi-ness, or sore throat; some people become sensi-tized to the vapors and can no longer work in areinforced plastics plant.

The Occupational Safety and Health Adminis-tration has specified that styrene monomer con-centrations in a plant should not exceed 100 partsper million .6 In a plant where spray systems areused, extensive air handling equipment, spraybooths, and air masks are required to maintainthese standards. Where polyester resins are usedfor compression molding, resin transfer molding,or other enclosed mold systems, the problem canbe dealt with by simple exhaust systems.

A new safety hazard was introduced with theadvent of carbon fibers, which tend to float arounda plant where they are used and, because they areelectrical conductors, can get into unprotected elec-trical devices to cause short circuits. The fiber con-centration in the air can be controlled by a nega-tive pressure exhaust system in the area where theyare used, but all electrical devices in the areashould be sealed (explosion proof).

Recycling and Disposal of Composites

Most polymer composite materials in use todayhave thermosetting matrices, and after they havebeen cured, have no apparent scrap value. Al-though attempts have been made to grind themup and use them as fillers, this has not proven tobe economically practical. The reuse of uncuredFRP composites offers little economic incentive.Most scrap is simply discarded. One of the po-tential advantages of composites with thermoplas-tic matrices is that the scrap can be recycled.

‘World of Composites, quarterly publication of the SPI ReinforcedP]astics/Composites Institute, winter 1986.

Cured composites present no particular prob-lem with disposal; they are chemically inert andcan be used for land fill. Burning them can gener-ate toxic smoke, and so is generally avoided.

The principal problem of disposal arises withuncured composites. Wet lay-ups, prepregs, SMC,etc., are still chemically active and pose both healthand safety problems. If used in landfill, the activechemicals can leach out and cause contaminationof the soil or water. A more serious problem isthat the catalyzed resins may go on to cure andgenerate an exotherm that causes spontaneouscombustion or self-ignition. The safe way to dis-pose of uncured composite material is to first bakeit until it is cured and then dispose of it.

Applications of PolymerMatrix Composites

Polymer matrix composites are a more maturetechnology than structural ceramics. With the ex-perience gained in military applications such asfighter aircraft and rocket motor casings begin-ning in the 1970s, composites now have a solidrecord of exceptional performance and reliabil-ity, They are rapidly becoming the baseline struc-tural material of the defense aerospace industry.

Becaues of their high cost, diffusion of ad-vanced composites into the civilian economy islikely to be a “top-down” process, progressingfrom relatively high value-added applications suchas aircraft to automobiles and then to the rela-tively “low-tech” applications such as construc-tion. On the other hand, there is also a “bottom-up” process at work in which savings in manu-facturing costs permit unreinforced engineeringplastics and short fiber-reinforced composites toreplace metals in applications where high strengthand stiffness are not required. Use of sheet mold-ing compound for automobile body panels is oneexample of this phenomenon.

Markets for Advanced PolymerMatrix Composites

Aerospace.—This sector has been estimated toconsume as much as 80 percent of all high-per-formance PMCS.7 Growth projections for aero-

7According to the market study conducted by Business Commu-nications Co., “Advanced Composites: An Evaluation of Commer-cial Prospects, ” Stamford, CT, as reported in World of Composites,a publication of the Society of Plastics Industries, winter 1986, p, 4.

-—— .--—

51

space usage of composites have ranged from 8.5percent per year8 to 22 percent per year.9 The pri-mary matrix materials in aerospace applicationsare epoxies, and the most common reinforcementsare carbon/graphite, aramid (e. g., Du Pent’s“Kevlar 49”), and high-strength glass fibers. How-ever, high-temperature thermoplastics such asPEEK are considered by many to be the matricesof choice for future aerospace applications. Com-posites are used extensively today in small mili-tary aircraft, military and commercial rotorcraft,and prototype business aircraft. The next majoraircraft market opportunity for composites is inlarge military and commercial transport aircraft.

The principal advantages of PMCs in aerospaceapplications are their superior specific strengthand stiffness compared with metals, resulting inweight savings of 10 to 60 percent over metal de-signs, with 20 to 30 percent being typical.l0 Thisweight reduction can be used to increase range,payload, maneuverability and speed, or reducefuel consumption. It has been estimated that apound of weight saved on a commercial transportaircraft is worth $100 to $300 over its service life,depending on the price of fuel, among other fac-tors.11 This high premium for weight saved isunique to this sector, and explains why it leadsall others in growth rate. Additional advantagesof PMCs are their superior fatigue and corrosionresistance, and vibration damping properties.

Military Aircraft.—Advanced composites havebecome essential to the superior performance ofa large number of fighter and attack aircraft (fig-ure 9). Indications are that composites may ac-count for up to 40 percent of the structural weightof the Advanced Technology Fighter (ATF),which is still in the design phase. Because the per-formance advantages of composites in militaryaircraft more than compensate for their high cost,

Figure 9.—Composite Aircraft Structure (by percent)

V-22 Rotor Craf t

TAVTEK 400 C o r p A / C

Beech ‘H x

Starship

Com DosltesE;o.-(JU~ g 40“Inc . - S-76(UC

2 ~ 20 -n F-15

F-14

n B-767~

:ntatlon

Adv

2

Fighter &

Z Fighter Attack A/C/.

dA320Large Commercial &

fra Mllltary A/C

1970 1980 1990

Year of first flight

Business aircraft and rotorcraftM)l,ta~ rotorcraftF!ghler and attack aircraftLarge commercial and mllltary a!rcraft

Bomber

SOURCE. Richard N Hadcock, Grumman Aircraft Systems Dlvlslon, “Status andViabillty of Composite Materials in Structure of High Performance Air-craft, ” a presentation to the National Research Councfl, Aeronauticsand Space Engineering Board Naval Postgraduate School, Monterey,CA, Feb 10, 1986

this is likely to be the fastest growing market foradvanced composites over the next decade. Oneestimate, which assumes only existing productionplus the ATF, projects a growth from about 0.3million pounds per year in 1985 to 2 millionpounds per year in 1995. ’2

Commercial Aircraft. —If glass fiber reinforcedcomposites are included, the volume of compo-sites used in commercial and business aircraft isabout twice that used in military aircraft. 13 In cur-rent commercial transport aircraft, such as theBoeing 767, composites make up about 3 percentof the structural weight, and are used exclusivelyin the secondary (not flight-critical) structure. 14However, several companies, including Beech andAvtek, are awaiting FAA approval of “all com-posite” business aircraft prototypes, which utilizecomposites in the wings, empennage, and fuse-lage. 15

‘According to the market study conducted by Frost & Sullivan,‘Worldwide High Performance Composites, ” New York, as reportedin World of Composites, a publication of the Society of Plastics In-dustries, winter 1986, p. 4.

‘According to a market study by Charles H. Kline & Co., “Ad-vanced Polymer Composites, ” Fairfield, NJ, as reported in PlasticsEngineering, June 1985, p. 62.

IOcar] Zweben, “polymer Matrix Composites, ” hontiers h ~a-terials Technologies, M.A, Meyers and O.T. Inal (eds. ) (The Nether-lands: Elsevier Science Publishers, 1985), p. 365.

llBob Hammer, Boeing Cornrnercid Aircraft CO., personal com-munication, August 1986.

IZRichard N, HadcWk, Gmmman Aircraft Systems Division, “Sta-tus and Viability of Composite Materials in Structures of High Per-formance Aircraft, ” a presentation to the National Research Coun-cil, Aeronautics and Space Engineering Board, N’aval PostgraduateSchool, Monterey, CA, Feb. 10, 1986.

“Ibid.I ~Darre] R, Tenney, NASA Langley Research Center, “Advanced

Composite Materials: Applications and Technology Needs, ” pres-entation to the Metal Properties Council, Inc., Miami, FL, Dec. 5,1985.

lsHadcock, op. cit. , Feb. 10, 1986

5 2

built by 1995, has placed the U.S. composite com-

\l\

‘*

Photo credit” Hercules, Inc.

A modern lightweight fighter incorporating 64 differentcomponents—more than 900 pounds of composite

structure per airplane.

Although growth of the business aircraft fleetover the next 5 years is expected to be only around10 percent, two categories (turboprops and turbo-jets) are expected to grow significantly. ” Theseaircraft are also the best candidates for compos-ite fuselages. The estimated value (derived fromcost and volume estimates) of composite fuselages,assuming all business aircraft manufacturers adoptthis technology, is about $100 million a year.17

These fuselages could account for 1.2 millionpounds of graphite/epoxy consumption annually.Large transport or commercial aircraft fuselageswill probably not be made from advanced com-posites until the technology is demonstrated inbusiness aircraft.

By the year 2000, composites could make up65 percent of the structural weight of commer-cial transport aircraft.18 Estimating a structuralweight of 75,000 pounds per aircraft and produc-tion of 500 aircraft per year, this application aloneshould account for 24 million pounds of advancedcomposite per year. Assuming a starting materialvalue of $60/lb, the market in the year 2000 isvalued at about $1.5 billion for the composite ma-terial alone. A more conservative estimate, whichassumes that no new commercial aircraft will be

“Materials Modeling Associates, “Properties, Costs, and Appli-cations of Polymeric Composites, ” Massachusetts Institute of Tech-nology, a contractor report prepared for the Oftice of TechnologyAssessment, December 1985.

‘“Ibid.IRTenney, op. cit. , Dec. 5, 1985.

mercial airframe production at between 1 and 2million pounds in that year.19

Helicopters.–With the exception of the “all-composite” business aircraft prototypes, which arestill awaiting certification, composites have beenused more extensively in helicopters than in air-craft. Military applications have led the way, andthe advantages of composites are much the sameas in aircraft: weight reduction, parts consolida-tion, and fatigue and corrosion resistance. Overthe past 15 years, composites have become thebaseline materials for rotors, blades, and tailassemblies. Sikorsky’s S-76 commercial model,which is about 25 percent composite by weight(figure 9), was certified in the late 1970s. Futuremilitary helicopters, such as the Army’s LHX(with major airframe design teams at Bell/Mc-Donnell Douglas and Boeing/Sikorsky), or theNavy’s tilt-rotor V-22 “Osprey” (designed byBell/Boeing), have specifications which forcedesigners to consider composites, which are likelyto comprise up to 80 percent of the structuralweight (figure 9). Materials such as graphite/ep-oxy are likely to be used in the airframe, bulk-heads, tail booms, and vertical fins, while the lessstiff glass/epoxy composites will be used in therotor systems. As with aircraft, there could be along-term trend away from epoxy resins andtoward thermoplastic resins.

Automotive Industry .—The automotive indus-try is widely viewed as being the industry in whichthe greatest volume of advanced PMC materialswill be used in the future. Because the industryis mature and highly competitive, the principalmotivation for introducing composites is cost sav-ings. In contrast to the aircraft industry, there isno clear-cut premium associated with a pound ofweight saved. Nevertheless, Detroit continues tobe interested in saving weight as it pursues theconflicting goals of larger automobiles and higherfuel efficiency. Automakers are looking to the ve-hicle skin/frame systems to provide the next bigleap in weight reduction. Other potential advan-tages of composites, such as corrosion resistance,appear to be secondary to the cost issue.

1~Hadcock, op. cit., Feb. 10, 1986.

— . . . — ———

By far the greatest volume of composite mate-rial is sheet molding compound (SMC) used in ex-terior panels .20 The most visible automotive useof SMC in recent years is the Pontiac Fiero, whichhas an all composite exterior. The Fiero is con-structed with a steel “space frame” superstructureto which the composite body panels are attached.By using different composite exterior panels, au-tomakers can achieve model differentiation forlimited production runs (100,000 to 200,000 units)while avoiding the prohibitive tooling costs whichwould be involved with use of steel. Automobilecompanies have adopted the space frame conceptand will be using PMC materials for exteriorpanels in the future. In the short run, these willbe mostly SMC.

The next major opportunity for composites inautomobiles is in structural components,21 Twostructural components currently in service are thecomposite drive shaft and leaf spring. Some 3,000

‘“Materials Nlodeling Associates, op. cit., December IQ85.21P. Beardmore, “Composite Structures for Automobiles, ” Com-

posite Structures 5:163-176, 1986.

53

drive shafts manufactured by filament windinggraphite and E-glass fibers in a polyester resin areused annually in the Ford Econoline van. 22 Also,glass fiber reinforced plastic leaf springs in theCorvette and several other models are in produc-tion at the rate of approximately 600,000 per year.Leaf springs are regarded as a very promising ap-plication of composites, and are expected to showstrong growth, especially in light trucks. Proto-type primary body structures have been con-structed with weight savings of 20 percent ormore.

Advance engineering groups within the BigThree automobile producers are targeting the mid-1990s to launch composite unibody vehicles .23 Ini-tially, composite unibodies will appear in limitedproduction, in models like the Corvette or Fiero.Annual production should be in the range of25,000 to 100,000 vehicles. It is likely that a com-posite unibody would not look like a conventionalmetal unibody, but would have a shape more con-sistent with the requirements of the manufactur-ing process used; resin transfer molding or fila-ment winding would be two candidate Drocesses.However, an estimate of the amount of compos-ite material required can be obtained from the fol-lowing. A typical mid-size, stamped steel unibodyweighs between 500 and 600 pounds. It is expectedthat the equivalent composite system will weigh25 to 45 percent less. Based on these assumptions,consumption of composites will amount to about12,000 tons in the first full year, which representsabout 10 percent of current reinforced plastic con-sumption in the domestic automotive sector. Esti-mating the value of composite unibodies at $6/lb,the estimated market would be about $150 mil-lion for these components alone.24

The automakers are exploring composite uni-bodies for a variety of reasons. Composite vehi-cles would enable designers to reduce the numberof parts required in assembly; some manufacturersare looking into a one-piece composite structure.By reducing the number of parts, better consistencyof parts can be achieved at considerably reduced

Photo credit Ford Motor Co

Compression molded composite rear floor panprototype for the Ford Escort. Ten steel components

were consolidated into a single molding, with15 percent weight savings.

zz~though composite drive shafts are technically successful, Fordwill take them out of production in 1987 in favor of a new alumi-num design

“Materials Modeling Associates, op. cit., December 1985.241bid.

54

assembly costs. Composites also offer substantialimprovements in specific mechanical properties,with the possibility of reducing weight while in-creasing strength and stiffness. Finally, PMCs of-fer greatly improved corrosion resistance oversteel or galvanized steel, since they do not rust.Observers have estimated that composite automo-biles could last 20 or more years, compared tothe current average vehicle lifetime of 10 years .25

Without question, the major technical barrierto use of PMCs in the automotive industry is thelack of manufacturing technologies capable ofmatching the high production rates of metalstamping technology. The fact that steel parts canbe stamped in seconds while plastics mold in min-utes has led to lower production costs for steel.A major reason behind the increased use of plas-tics in automobiles is that the time of manufac-ture has fallen, not the cost of the equipment,tools, or materials. The fastest current technol-ogies can process material at the rate of tens ofpounds per minute, while true economy will re-quire rates of 100 pounds per minute or more.26

Thus, there is a gap of roughly an order of mag-nitude between current and economical rates.

If composites are used extensively in automo-bile unibodies, there could be large, secondaryeconomic effects. These would involve the auto-mobile producers and suppliers, the service net-work, and those involved in disposal of auto-mobiles.

Auto producers will have to build new facil-ities or modify existing physical plant in order tofabricate composite unibodies in-house. Alterna-tively, they may elect to outsource the moldingoperations. This would lead to the creation of newspecialized composites molding businesses, simi-lar to the custom molders of plastics which nowserve Detroit. Production lines would have to bealtered to accommodate different processes for join-ing components together. Segments of the auto-mobile service network would be directly affectedby a change to composite unibodies. Body shopswould have to change their repair procedures, orswitch from repairing to replacing damaged parts.

‘s Ibid.z~char]es Sega], President, Omnia, Raleigh, NC, in OTA work-

shop on “Future Applications of Advanced Composites, ” Dec. 10,1985.

Rust-proofing services might be eliminated al-together.

Finally, an all-composite unibody will greatlyreduce the supply of steel scrap and may renderthe recycling of automobiles unprofitable. Shouldthis occur, disposing of millions of auto hulks peryear could become a national problem. In addition,since steel minimills use steel scrap as their start-ing material, they might be placed in jeopardy.

Reciprocating Equipment.—PMC materialshave considerable potential for use in many differ-ent kinds of high-speed industrial machinery. Cur-rent applications include such components as cen-trifuge rotors, weaving machinery, hand-heldtools, and robot arms. All of these applicationstake advantage of the low inertial mass, but theyalso benefit to various extents from the tailorableanisotropic stiffness, superior strength, low ther-mal expansion, fatigue life, and vibration damp-ing characteristics of composites.

The productivity of robotic work stations andflexible manufacturing cells could be improved ifrobots could operate at higher speeds with moreaccurate endpoint positioning, Stiffness is the keymechanical property, since the endpoint accuracyis limited by bending deflections in the beam-shaped robot members. With metal designs, stiff-ness is obtained at the cost of higher mass, whichlimits the robot response time. Consequently, theratio of the weight of the manipulator arm to thatof the payload is rarely lower than 10:1.27 Becauseof their superior stiffness per unit weight, com-posites are a promising solution to this problem.At present, only one U.S. company28 has mar-keted a robot incorporating composites, althoughthere are several Japanese models and a numberof other countries are funding research.

Although the benefits of using composites inreciprocating equipment are clear, initial attemptsto penetrate this market have been disappointing.The market is a highly fragmented one, and equip-ment manufacturers, who tend to be oriented

Z7B s Thompson and C .K. Sung, “The Design of Robots and In-. .telligent Manipulators Using Modern Composite Materials, ” A4ech-anism & Machine Theory 20: 471-482, 1985.

2’Graco Robotics of Lavonia, Michigan manufactures a spraypainting robot with a hollow graphite/epoxy arm. However, thisarm is being phased out in favor of a new aluminum design.

5 5

toward metals, have shown a reluctance to con-sider the use of a higher cost material (particularlywhen its use requires new processes and tooling)even when performance advantages are demon-strated. No attempt has been made to quantita-tively estimate future markets. Compositepenetration is likely to be slow but steady.

Shipping and Storage.—Plastics have long beenknown to be inert to chemical attack from a widevariety of substances. This has made them use-ful as containers for many corrosive chemicals.With the addition of fiber reinforcement, the in-creased strength makes them suitable for largestructures such as large diameter pipe, holdingtanks, and pressure vessels. One generic limita-tion of composites for these applications is tem-perature. As the structures are generally made outof glass fiber reinforced polyester or epoxy, themaximum service temperature is about 3000 F(1490 C). A further potential problem is that theglass fibers are subject to corrosive attack if thematrix cracks and exposes them to water or otherchemicals. Assuming fiber cost reductions can beachieved, use of corrosion-resistant graphite orceramic fiber reinforcement could be a promis-ing solution to this problem in the future.

Pipe. -Glass fiber reinforced plastic (FRP) pipeshave major applications in the oil and the chemi-cal process industry. The demand for compositepipe is directly affected by the level of economicactivity within these industries. Chemical manu-facturing expenditures have been decreasing overthe past few years, and low oil prices havebrought acquisition of new oil field equipment toa halt. Consequently, the demand for fiberglass-reinforced pipe in these sectors has been declin-ing. 29

Industry sources predict that any growth in thedemand for composite pipe will come from thepetroleum industry. One high-volume applicationwhich has shown strong growth is pipe used inpumping of gasoline. It is expected that fiberglass-reinforced pipe will penetrate 15 to 30 percent ofthe oil field market in such applications as suckerrods, downhole tubing, and casing, However,growth in these areas is predicated on rising oilprices. The FRP pipe industry is approximately

Material Modeling Associates, op. cit., December 1985.

30 years old. It saw tremendous growth until1980, but is now leveling off. A maximum of 5percent annual growth is expected .30

Storage Tanks.—FRP has captured about 40percent of the market for underground tanks forstoring fuel and corrosive chemicals and could in-crease its market share to about 80 percent overthe next 5 to 10 years.

31 FRP underground tanksare expected to last at least 30 years, comparedwith 13 to 15 years for bare steel.

Above-ground tanks for storage of oil, chem-icals, and food represent a large potential mar-ket for FRP. Above-ground tanks smaller thanabout 12 feet in diameter or less than 30 feet highare predominantly FRP, but for larger tanks, thedifficulty of manufacturing, handling, and ship-ping with FRP makes steel the more economicalmaterial.

Composites could be used widely in the futurefor portable storage tanks and pressure vessels.The two properties of greatest importance are cor-rosion resistance and light weight. In Europetoday there are currently some 20,000 tankertrucks which use an FRP shell reinforced by a steelframe. In the United States there is one prototypetanker which utilizes a reinforcement combina-tion of chopped glass fiber and glass filamentwound layers.32

T h ee s t a n d a a r d t a n k e r d e s i g n f o rhauling corrosive chemicals (a business which ac-counts for about 5 percent of the total shippingmarket), is stainless steel lined with rubber, ep-oxy, or glass. These liners must be replaced every1 to 5 years, at considerable expense. The compos-ite tanker can be produced for approximately thesame price as the steel design, and, based on theEuropean experience and the performance recordof underground composite tanks, it is expectedto last longer, reduce costs, and increase safety.As of September 1984, composite tanker truckswere approved by the Department of Transpor-tation for travel on U.S. roads; however, the dif-ficulty of obtaining insurance continues to be aserious barrier to use of composite tankers .33

301bid.s] ~orj~ of Composites, Op. cit,, winter 1986, P. 8.JzJowph M. Plecnik, et al., “Composite Tanker Trucks Have Made

the Grade, ” Plastics Engineering 41(3):63-65, March 1985.JJJowph M. pl=ik, Department of Civil Engineering, Long ~ach

State University, personal communication, August 1986.

5 6

Photo credit: The CUMAGNA Corp.

Computer trace of fiber paths in a three-dimensional braided l-beam. This process yields a composite l-beam which isstrong in all directions, but which weighs much less than steel.

Construction.—Construction applications arepotentially a very significant opportunity for com-posites. For example, most highway bridges in theUnited States are over 35 years old, and most rail-road bridges are over 70 years old.34 Replacingor refurbishing even a small fraction of these withcomposite materials would involve a substantialvolume of fiber and resin. However, significanttechnical, economic, and institutional barriers ex-ist to the implementation of this technology, suchthat construction opportunities should be viewedas long term.

j*John Sca]zi, National Science Foundation, persona] communi-cation, August 1986.

A potentially high-volume market for compos-ites is in construction of buildings, bridges, andhousing. Additional applications include lampposts, smokestacks, and highway culverts. Con-struction equipment, including cranes, booms,and outdoor drive systems, could also benefitfrom use of composites. Because of the many in-expensive alternative building materials currentlybeing used, cost of the PMC materials will be thekey to their use in this sector. The chief advan-tage of composites would be reduced overall sys-tems costs for erecting the structure, includingconsolidation of fabrication operations, reducedtransportation and construction costs due to

57—

l ighter we ight s t ruc tures , and reduced mainte -

nance and lifetime costs due to improved corro-sion resistance. 3 5

Bridges are likely to be the first large scale con-struction application for polymer composites. TheU.S. Department of Transportation is currentlyevaluating composites for use in bridge deckingand stay cables.36 Fiberglass tendons are also be-ing used in place of steel in prestressed concretebridge structures. ” Other countries which haveactive programs in this area include the Peoples’Republic of China, England, West Germany, Is-rael, and Switzerland. Because the largest loadwhich must be supported by the bridge is its owndead weight, use of light weight composites wouldallow the bridge to accommodate increased trafficor heavier trucks. Decking materials are likely tobe relatively inexpensive vinylester or epoxy res-ins reinforced with continuous glass fibers. Ca-bles would probably be reinforced with graphiteor aramid fibers, because of the high modulus andlow creep requirements.

The manufactured housing industry is an espe-cially intriguing potential opportunity for com-posite materials. In 1984, almost half of all newhousing units were partially manufactured; i.e.,large components were built in factories, ratherthan assembled onsite.38 In the future, factorymanufacture of housing promises to reduce hous-ing costs while still maintaining options for dis-tinctive designs. Composite manufacturing tech-niques such as pultrusion and transfer moldingcould be used to fabricate wall structures contain-ing structural members and panels in a single step,Components such as I-beams, angles, and chan-nels can also be economically produced by thesetechniques. In spite of the opportunities, however,Japan and several countries in Europe are farahead of the United States in composite housingconstruction technologies.

An important research need affecting the useof composites in construction applications has to

“Howard Smallowitz, “Reshaping the Future of Plastic Buildings, ”Civil Engineering, May 1985, pp. 38-41.

‘eAccording to information provided by Craig A. Ballinger, Fed-eral Highway Administration, August 1986.

“Engineering News Record, Aug. 29, 1985, p. 11.“Thomas E. Nutt-Powell, “The House That Machines Built, ”

Technology Review, Nov. 1985, p. 31.

Photo credit Cofnpos/te Technology, /nc

Interior of a nonmetallic, nonmagnetic structure builtfor Apple Computer, Inc., to house a microwave devicetesting facility. The building components, including

fasteners, are entirely made of fiber-reinforcedcomposites.

do with adhesion and joining. The joining of com-posite materials to other materials for the purposeof load transfer, or to themselves for the purposeof manufacturing components, requires advancesin technology beyond present levels. This is a par-ticular obstacle when joining may be done by un-skilled labor.

An additional technical barrier is need for de-velopment of design techniques for integrated,multifunctional structures. Window frames, whichare made by joining together several pieces ofwood, can be replaced by molded plastic andcomposite structures having many fewer pieces,lower assembly costs, and better service perform-ance. The flexibility of the design and manufactureof composite materials could also be used to inte-grate a window frame into a larger wall section,again reducing the number of parts and the costof manufacture, This has been done for certainexperimental bathroom structures where 1avato-

5 8

ries, shower rooms, and other structural compo-nents have been integrated in a single molding.39

The principal barriers to the adoption of newmaterials technologies in the construction indus-try in the United States are not so much techno-logical as institutional and economic. Like thehighway construction industry discussed above,the housing construction industry is highly frag-mented. This makes the rate of R&D investmentand adoption of new technology very low. Theperformance of housing materials is regulated bythousands of different State and local building andfire safety codes, all written with conventionalmaterials in mind. Further, engineers and contrac-tors lack familiarity with the composite materi-als and processes. Finally, composites must com-pete with a variety of low-cost housing materialsin current use. As a result, composites used inmanufactured housing are not likely to be “ad-vanced”; rather, they might consist of wood fiberspressed with inexpensive resins or laminated struc-tures involving FRP skin panels glued to a foamor honeycomb core.

Medical Devices.—PMC materials are currentlybeing developed for medical prostheses and im-plants. The impact of composites on orthopedicdevices is expected to be especially significant.While medical devices are not likely to providea large volume market for composites, their so-cial and economic value are likely to be high. Thetotal estimated world market for orthopedic de-vices such as hips, knees, bone plates, and in-tramedullary nails is currently about 6 millionunits with a total value of just over $500 million.40

Estimates of the U.S. market for all biocompati-ble materials by the year 2000 have been quotedabove as up to $3 billion per year.41 Polymer com-posites could capture a substantial portion of that,sharing the market with ceramics and metals.

Metallic implant devices of this type, such asthe total hip unit which has been used since theearly 1960s, suffer a variety of disadvantages: dif-ficulty in fixation, allergic reactions to variousmetal ions, poor matching of elastic stiffness, and

jqKenneth L. Reifsnider, Materials Response Group, Virginia poly-technic Institute, “Engineering Research Needs of Polymer Compos-ites, ” a contractor report prepared for the Office of TechnologyAssessment, December 1985.

‘“Ibid.tlHench and Wilson, oP. cit.

mechanical (fatigue) failure. Composite materi-als have the potential to overcome many of thesedifficulties. Not only can the problem of metalion release be eliminated, but composite materi-als can be fabricated with stiffness which is tai-lored to the stiffness of the bone to which theyare attached, so that the bone continues to bearload, and does not resorb (degenerate) due to ab-sence of mechanical loading. This is a persistentproblem with metal implants. It is also possibleto create implants from biodegradable compos-ite systems which would provide initial stabilityto a fracture but would gradually resorb over timeas the natural tissue repairs itself. Finally, compos-ites can be designed to serve as a scaffold for theinvasive growth of bone tissue as an alternativeto cement fixation. This leads to a stronger andmore durable joint.

Research in composite orthopedic devices is cur-rently being carried out on a relatively small scalein the labs of orthopedic device manufacturers.Further research is required to improve in situstrength and service life, stress analysis, and fabri-cation and quality control technologies.

In order to overcome the remaining technicalbarriers, a cooperative effort of an interdiscipli-nary team is required. At a minimum, the teammust include expertise in design, engineering,manufacturing, and orthopedic surgery. Signifi-cant strides in this field are being made in Japan,the United Kingdom, France, Germany, Italy,Canada, and Australia, as well as in the UnitedStates .42

Future Trends in PolymerMatrix Composites

Novel Reinforcement Types

Rigid Rod Molecules. —Composites can be rein-forced with individual rigid rod-like molecules orfibers generated from these molecules. One exam-ple is poly (phenylbenzobisthiazole), or PBT. Ex-perimental fibers made from this material havespecific strength and modulus on a par with themost advanced fibrous reinforcement, exceedingthe properties of commercially available metals,

tzReif5nider, Op. Cit” ~ December 1985.

5 9

including titanium, by over a factor of 10.43 Aparticularly exciting possibility is dissolving themolecular rods in a flexible polymer, and thus cre-ating a composite reinforced by individual mole-cules. Such a homogeneous composition wouldmitigate the problem of matching the thermal ex-pansion coefficient between the reinforcement andthe matrix, and would virtually eliminate thetroublesome interface between them. The futureof this technology will depend on solving theproblems of effectively dissolving the rods in the“matrix” and on orienting them once dissolved.

Novel Matrices

Because the matrix largely determines the envi-ronmental durability and toughness, the greatestimprovements in the performance of polymercomposites in the future will come from new ma-trices, rather than new fibers. Perhaps the mostsignificant opportunities lie in the area of molecu-lar design; chemists will be able to design poly-mer molecules to have the desired flexibility,strength, high-temperature resistance, and adhe-sive properties .44 Some of the more promisingdirections are discussed below.

Oriented Molecular Structures. -At present theanisotropic properties of most composites are de-termined by the directions of fiber orientation. Inthe future, it may be possible to orient the indi-vidual polymer molecules during or after poly-merization to produce a “self-reinforced” struc-ture. The oriented polymers will serve the samereinforcing function as fibers do in today’s com-posites. In effect, today’s organic fibers (e.g., DuPent’s Kevlar or Allied’s Spectra 900), which con-sist of oriented polymers and which have amongthe highest specific stiffness and strength of allfibers, provide a glimpse of the properties whichtomorrow’s matrices might have.

Recently developed examples of oriented poly-mer structures are the liquid crystal polymers(LCPs). They consist of rigid aromatic chainsmodified by thermoplastic polyesters (e.g., poly -ethyleneterephthalate, or PET), or polyaramids,

tJThaddeus Helminiak, “Hi-Tech Polymers From OrderedMolecules, ” Chemical Week, Apr. 11, 1984.

~tcharl= p, Wet, R~in Research Laboratories, Inc., Newark,NJ, personal communication, August 1986.

and have a self-reinforcing fibrous character whichimparts strength and modulus comparable tothose of reinforced thermoplastic molding com-pounds, such as 30 percent glass reinforced ny-lons.” The fiber orientations of current LCPS arehard to control (current applications includemicrowave cookware and ovenware, which re-quire high-temperature resistance but not highstrength) and this represents a challenge for thefuture.

High-Temperature Matrices.—The maximumcontinuous service temperature of organic poly-mers in an oxidizing atmosphere is probablyaround 7000 F ,46 although brief exposures tohigher temperatures can be tolerated. Currently,the most “refractory” matrices are polyamides,which can be used at temperatures of 600° F(316° C), although slow degradation occurs.” Ifstable, high-temperature matrices could be de-veloped, they would find application in a vari-ety of engine components and advanced aircraftstructures.

Thermotropic Thermosets. -These hybrid ma-trices are designed to exploit the processing ad-vantages of thermoplastics and the dimensionalstability and corrosion resistance of thermoses.The molecules are long, discrete chains whichhave the latent capacity to form crosslinks. Proc-essing is identical to thermoplastics in that the dis-crete polymers are formed at high temperaturesto the desired shape. Then, however, instead ofcooling to produce a solid, the structure is givenan extra “kick,” with additional heat or ultravio-let light, which initiates crosslinking between thepolymer chains. Thus, the finished structure hasthe dimensional stability characteristic of a ther-moset .48

isRegina]d B, Stoops, R.B. Stoops & Associates, Newport, RI,“Ultimate Properties of Polymer Matrix Materials, ” a contractor re-port prepared for the Office of Technology Assessment, December1985.

qbpaul McMahan, Ce]anese Research Corp., in an OTA work-shop, “Future Opportunities for the Use of Composite Materials, ”Dec. 10, 1985.

d7Aerospace America, May 1986, p. 22.4aJohn Riggs, Celanese Research Corp. in OTA workshop cited

in footnote 46.

60

The Composites Factory of the Future

It is in the nature of advanced composites thatthey do not lend themselves to standard processesor manufacturing procedures. The very conceptof tailoring a material to each specific applicationmilitates against standardization. It is thereforeunlikely that we will see completely automatedfactories in the near future in which manufactur-ing instructions are fed in at one end and a com-posite part pops out the other, untouched by hu-man hand.

Computer-Aided Design.—Computer-aideddesign (CAD) systems currently focus on three-dimensional graphics, and are often coupled withthe capability for stress analysis of a structure.However, a comprehensive CAD system whichwould facilitate the process of choosing suitablematerials, reinforcement geometry, and methodof fabrication is still far in the future. Such a sys-tem would require an extensive database on fi-ber and resin properties, as well as a processingdatabase which would permit modeling of themanufacturing steps necessary to fabricate thepart, including the costs of those operations. Theprincipal advantage of an integrated CAD systemwould be to clearly define the options and trade-offs associated with various production strategies.

Computer-Aided Manufacturing.—Computer-aided manufacturing (CAM) would alter the proc-essing of composites in several ways. Based onthe instructions generated by the CAD system,numerically controlled machine tools would auto-matically machine molds contoured to the pre-cise outline of the part. CAD instructions wouldguide automated lay-up of the part, and sensorsembedded in the molds or in the materials them-selves would make the entire manufacturing proc-ess an interactive one in which the computer con-troller would automatically adjust the processingvariables based on real-time information aboutthe state of the structure being fabricated. Withthe advent of robot-assisted manufacturing, NDEduring the production cycle, performed withprcbes manipulated by the robot and analyzedby artificial intelligence, could become the wayall high-value composite components are produced.Integration and miniaturization of sensors willlead to improvements in the quality and reliabil-

ity of the information obtained from a nonde-structive test.

Nondestructive Evaluation.—Experience todate gives no indication that composite structuresin service will require more frequent inspectionthan metal structures. In the future, it will bejudged inconvenient to take composite systemsout of service merely to inspect them for struc-tural integrity. For such cases it will be necessaryto have NDE performed continuously by devicesthat can sense changes in the mechanical proper-ties which would precede a structural failure.These sensors would be included in the criticalcomponents during manufacture, and their sig-nals would be examined regularly by a computerwhich would integrate the data, using AI, and de-cide whether to alert a human operator. A can-didate NDE test for such a system is acoustic emis-sion, in which the sound emitted by a compositestructure under stress can be used to characterizethe flaw population. Another possibility wouldbe to include optically or electrically conductingfibers with the structural reinforcement; the de-gree of transmission of light or electric current bythese fibers could be related to fiber breakage inthe overall structure.

Thermal Forming.—With the increased use ofadvanced thermoplastic composites in the future,new thermal forming technologies will become im-portant. The thermoplastic could be applied ei-ther as a hot melt or as a prepreg, which is rela-tively fast and easy to make with thermoplastics.Either method will require the development ofnew equipment capable of placing the fibers andcompacting the structure while maintaining theprocess temperature.

Space

Space Transportation Systems.—over 10,000pounds of advanced composites are used on theNASA space shuttle. ” Polymer composites arealso being considered in designs for the proposedaerospace plane, although such an aircraft prob-ably will not be available until after the year 2000.The primary limitation on the use of polymercomposites in this application is high tempera-

‘* Tenney, op. cit. , Dec. 5, 1985.

61— —

to the sun. This thermal cycling produces radialcracks in graphite/epoxy tubes which can reducethe torsional stiffness by as much as 30 percentafter only 500 cycles. 52 To reduce the effects ofthermal cycling, the composite tubes would becoated with a reflecting, thermally conductinglayer to equalize the temperature throughout thetube. The layer would also protect the compos-ite from atomic oxygen, which is a major causeof material degradation in low earth orbit, as wellas from solar ultraviolet radiation.

Military .—Composites of all types, includingceramic, polymer, and metal matrix composites,are ideal materials for use in space-based militarysystems, such as those associated with the Stra-tegic Defense Initiative (SDI). ” Properties suchas low density, high specific stiffness, low coeffi-cient of thermal expansion, and high temperatureresistance are all necessary for structures whichmust maneuver rapidly in space, maintain highdimensional stability, and withstand hostile at-tack. A program devoted to the development ofnew materials and structures has recently beenestablished within the SDIO. 54

Reference configuration for the NASA manned space Bioproductionstation. Composites could be used in the tubular struts

which make up the frame. Living cells can synthesize polymeric moleculeswith long chains and complex chemistries which

ture. ’” Flying at speeds exceeding Mach 7, the cannot be economically reproduced in the labora-lower surfaces and leading edges would experi- tory. For example, crops and forests are an im-ence temperatures of 2,0000 to 3,000 F (1,093 portant source of structural materials and chern-to 1,6490 C) .51 If PMC materials were available ical feedstocks. A number of crops such aswhich could retain high strength and stiffness up crambe, rapeseed, and various hardwood trees areto 800° F (427° C), they could be used extensively now being evaluated for commercial productionfor the cooler skin structure and most of the sub- of lubricants, engineering nylons, and compos-structure. ites. 55 For some materials, such as natural rub-

Space Station.—Graphite /epoxy compositesand aluminum are both being considered for thetubular struts in the space station reference de-sign. The goal of reducing launch weight favorsthe use of composites; however, their lower ther-mal conductivity compared with aluminum couldcreate problems in service. The most serious envi-ronmental problem faced by the composite is tem-perature swings between –250’ F ( –121” C) and+200” F ( +93 o C) caused by per iodic exposure

ber, the United States is totally dependent on for-eign sources. Congress has mandated (“CriticalAgricultural Materials Act, ” Public Law 98-284,1984) that the Department of Agriculture estab-

—.—‘ 2 Tenney, op cit . , Dec. 5, 1Q851‘Jerome Persh, ‘ \later]als and Structure,\ S[ ](,nc ( ,ln~j T(’c hn~~l-

ogy Requirement\ k(lr the DOD St ratcg]c I )ei en~c’ 1 n I t la t 1 i(’ .’l J?lt’l -ican Ceramic Societ~’ Bullet~r7 04 t 4 }, 555-55Q 1 Q85

‘dThe effort Inclucie\: IIghtwe]ght \tructure\, thermal and CIL,C tr)ca 1 ma terlals, opt ] ca 1 rnater]a 1> d nd proc e~se~, t rlhologlca” I ma t(, r 1-a]s, and materials durab] ] ] tv. C u rrent budxets are about S2 ml I I I on

62

lish an Office of Critical Materials to evaluate thepotential of industrial crops to replace key im-ported materials. Several demonstration projectsof 2,000 acres or more are planned for 1987.

With the possible exception of wood, it is un-likely that biologically produced materials willcompete seriously with polymer matrix compos-ites in the structural applications discussed in thisreport. Although natural polymers such as cellu-lose, collagen, or silk can have remarkably highstrength, their low stiffness is likely to limit theiruse in many structures. Nevertheless, their uniquechemical and physical properties make them ap-propriate for certain specialty applications. Forexample, collagen is a biologically compatible ma-terial which is being used to generate artificialskin. 56

Biotechnology may offer a novel approach tothe synthesis of biological polymers in the future.Genetically engineered bacteria and cells havebeen used to produce proteins related to silk.57

In the future, production rates could be acceler-ated by extracting the protein synthetic machin-ery from the cells and driving the process withan external energy source, such as a laser or elec-tric current.58 The flexibility inherent in such ascheme would be enormous; by simply alteringthe genetic instructions, new polymers could beproduced.

Research and Development Prioritiesfor Polymer Matrix Composites

Federal R&D spending for polymer matrix com-posites in fiscal years 1985 and 1986 is shown intable 14; roughly 65 to 70 percent in each yearwas spent by the Department of Defense. Thelarge drop in Federal expenditures from 1985 to1986 does not reflect a sharp cut in compositesR&D; rather, it can be attributed to the comple-tion of large research programs in 1985 and thetransitioning of the technology (particularly for

“J. Burke, et al., “The Successful Use of Physiologically Accept-able Artificial Skin in the Treatment of Extensive Bum In jury,” An-nals of Surgery 194:413-428, 1982.

57 Dennis ~ng, Syntro Corp., La Jolla, CA, pmorkd communi-cation, August 1986.

SsTerence Barrett, FJationa] Aeronautics and Space Administra-tion, persona] communication, August 1986.

Table 14.—Budgets for Polymer Matrix CompositeR&D in Fiscal Years 1985 and 1986 (millions)

Agency FY 1985 FY 1986

Department of Defensea (6.1, 6.2, 6.3A), 55.9 29.2National Aeronautics Space Agencyb 23 8.7N a t i o n a l S c i e n c e F o u n d a t i o nc ., ... ., 1-2 1-2National Bureau of Standardsd . . ., ., 0.4 0.4Department of Transportatione ., ., — 0.4

Total . . ., ., 80-81 40-41SOURCES aJerome Persh, DOO

bMlchael Greenfleld, NASAcRanga Komandurl, NSF‘Leslie SmKh, NBS‘Cratg Ballmger, FHWA

carbon fiber composites) to private industry. De-fense applications continue to drive the develop-ment of advanced composites, which are used inan estimated $80 billion of weapons systems .59

Reliable composite structures can now be de-signed which satisfy all of the engineering require-ments of a given application. However, scientificunderstanding of composites has lagged behindengineering practice; in order to design more effi-ciently and cost effectively, and to develop im-proved materials, it will be necessary to under-stand and model several important aspects ofcomposites. Based on the opportunities outlinedabove, some research and development prioritiesfor composites are suggested below.

Very Important

Processing Science. —The primary goal of proc-essing science is to be able to control the fabrica-tion process to assure complete and uniform cure,minimize thermal stresses, control resin content,and assure accurate fiber placement. This requiresa model which can predict the influences of keyprocess variables and techniques for monitoringthese variables so that pressure and temperaturecan be adjusted accordingly. Such models wouldalso provide useful guidelines for tool design. Atpresent, modeling is in a very early stage of de-velopment.

The existence of low-cost fabrication processeswill be critical to the use of new composite sys-tems, such as low-cost, high-performance ther-moplastics reinforced with continuous fibers. For

SeKenneth Foster, Assistant for Materials po]icy, Department ofDefense, personal communication, August 1986.

.— —

6 3

example, methods for fabricating shapes withdouble curvature are needed. Another importantproblem is the impregnation and wetting of fiberbundles by these relatively viscous plastics. Theeffect of processing on microstructure requires fur-ther study, as does the influence of residual ther-mal stresses. The latter is a particular concern forresins processed at high temperatures. Finally, asfor thermoses, process models are required.

Impact Damage.—The impact damage resis-tance of a composite structure has a critical ef-fect on its reliability in service. It has been shownthat impact damage that is barely visible to thenaked eye can cause a reduction in strength of as

6 0 I m p a c t r e s i s t a n c e i s e s p e-

much as 40 percent.cially important in primary aircraft structures andother safety-critical components. Tougher ther-moplastic matrix materials promise to improvethe impact resistance of aircraft structures nowmade with epoxy matrices.

The complexity of the impact damage processmakes modeling very difficult. However, it wouldbe very desirable to be able to relate the extentof damage to the properties of the matrix, fiber,and interphase, along with factors like reinforce-ment form. This would facilitate the developmentof more reliable materials and structures. In addi-tion, an understanding of impact damage mech-anisms would aid in developing protocols for re-pair of composite structures, a field which largelyrelies on empirical methods.

Delamination. -There is a strong body of opin-ion that delamination is the most critical form ofdamage in composite structures (particularly thoseproduced from prepregs). As noted above, impactdamage barely visible on the surface can causedramatic reductions in strength through localdelamination. Delamination may prove to be aproblem of increasing severity in the future, be-cause of the trend toward higher working strainsthat tend to accentuate this mode of failure.

Analyses to date have concentrated on growthand failure associated with compressive loading.These need to be verified and refined. In addition,the effects of combined loading, resin fracture

““Carl Zweben, General Electric Co., “Assessment of the ScienceBase for Composite Materials, ” a contractor report prepared for theOtfice of Technology Assessment, December 1985.

toughness, reinforcement form, and environmentneed to be investigated.

Interphase.—The interphase has a critical in-fluence on the composite, since it determines howthe reinforcement properties are translated intothe composite properties. The characteristics ofthis little-studied region merit thorough investi-gation. The objective would be to develop a bodyof knowledge that would guide the developmentof fiber surface treatments, matrices, and fibercoatings that will optimize composite mechani-cal properties and provide resistance to environ-mental degradation.

Important

Strength.—The excellent strength properties ofcomposites are one of the major reasons for theiruse. However, as with many properties of com-posites, their strong heterogeneity makes strengthcharacteristics very complex. This heterogeneitygives rise to failure modes that frequently haveno counterpart in homogeneous materials. Evenin the simplest composites, unidirectional lami-nates, the relationships between axial and trans-verse loading (parallel and perpendicular to thefiber direction) and failure are not well under-stood. In more complex composites, containingseveral fiber orientations and various flaw pop-ulations, efforts to model strength have beenlargely empirical. It will be important in the fu-ture to have analytical models for the various fail-ure modes of unidirectional composites andlaminates that relate strength properties to basicconstituent properties.

Fatigue.—Fatigue of composites is an impor-tant design consideration. Fatigue resistance is amajor advantage which composites enjoy overmetals; however, the traditional models for ana-lyzing fatigue in metals do not apply to compos-ites. The risks associated with fatigue failure arelikely to increase because of the trend toward useof fibers with higher failure strains, and the de-sire to use higher design allowable for existingmaterials. Ideally, it would be desirable to be ableto predict composite fatigue behavior based onconstituent properties. A more realistic near-termobjective is to understand fatigue mechanisms,and how they are related to the properties offibers, matrix, interphase, loading, and environ-

64

ment of the composite. Important topics whichhave not received adequate attention are the fa-tigue properties of the reinforcements and matrixresins, and compression fatigue of unidirectionalcomposites. In view of the increasing interest inthermoplastics, fatigue of these materials alsodeserves study.

Fracture.—One of the most important modesof failure in metals is crack propagation. Arisingat regions of high stress, such as holes, defects,or other discontinuities, cracks tend to grow un-der cyclic tensile load. When cracks reach a criti-cal size, they propagate in an unstable manner,causing failure of the part in which they are lo-cated. This, in turn, may result in failure of theentire structure. In contrast, failure of compositesoften results from gradual weakening caused bythe accumulation of dispersed damages, ratherthan by propagation of a single crack.

In view of the significant differences in failuremodes between metals and composites, use of lin-ear elastic fracture mechanics (LEFM) to describefracture in these complex materials is controver-sial. It is open to question whether there areunique values of fracture toughness or criticalstress intensity that describe the fracture charac-teristics of composites. In order to develop relia-ble design methods and improved materials in thefuture, it will be necessary to develop a body offracture analysis which is capable of accountingfor the more complex failure mechanisms.

Environmental Effects.—The environments towhich composites are subjected can have a sig-nificant effect on their properties. Environmentsthat are known to be especially damaging arethose of high temperature and moisture underload, ultraviolet radiation, and some corrosivechemicals. The key need in the environmental areais to develop a thorough understanding of degra-dation mechanisms for fibers, resins, and inter-phases in the environments of greatest concern.This knowledge will lead to more reliable use ofexisting materials and provide the information re-quired to develop new, more degradation-resistantones.

Reinforcement Forms and Hybrid Composites.—There are two main reasons for the interest innew reinforcement forms: improved through-the-

thickness properties, and lower cost. A pervasiveweakness of composite laminates of all kinds isthat the out-of-plane strength and stiffness, be-ing dependent primarily on the matrix, are muchinferior to the in-plane properties. This is becausein convertional laminates there is no fiber re-inforcement in the thickness direction.

New reinforcement forms under developmentinclude triaxial fabrics, multilayer fabrics, two-dimensional braids, three-dimensional braids, andvarious kinds of knits. In addition, laminates havebeen reinforced in the thickness direction bystitching. From an analytical standpoint, the ma-jor drawback to fabrics, braids, and knits is thatthey introduce fiber curvature, which can causesignificant loss of strength, compared to a uni-directional laminate. However, multidirectionalreinforcement appears to confer increased frac-ture toughness on the composite.

Use of several types of fibers to reinforce com-posites will be driven by the desire to obtain prop-erties that cannot be achieved with a single fiber,and to reduce cost. For example, glass fibers,which are cheap but have a relatively low tensilemodulus, can be mixed with more costly, highmodulus graphite fibers to achieve a compositewhich is both stiff and relatively cheap.

With both new reinforcement forms and hybridreinforcement, there is a great need for analyti-cal methods to identify the configurations requiredto produce desired properties. Without such tools,it will be necessary to rely on intuition and time-consuming, costly empirical approaches.

Test Methods.—Reliable and standardized testmethods are critical to the evaluation of all ma-terials. For homogeneous materials such as me-tals, such methods are fairly straightforward. Incomposites, however, the macroscopic mechani-cal behavior is a complex summation of the be-havior of the microconstituents. Consequently,there has been great difficulty in achieving a con-sensus on what properties are actually being meas-ured in a given test, let alone what test is mostappropriate for a given property. There are cur-rently numerous test methods in use throughoutthe industry, and numerous private databases.This has resulted in considerable property varia-bility appearing in papers and reports. The prob-

.

6 5

lem is particularly severe for toughness, bending,shear, and compression properties.

The movement to standardize composites test-ing began in 1969, but has stalled in recent years. 61

Standardized methods of testing and reporting re-sults are a prerequisite to the establishment ofcomposite properties databases for design andprocess modeling. The lack of standards is alsoa significant barrier to the transfer of test resultsfrom one structural application to another.

Desirable

Creep Fracture.—Materials subjected to sus-tained loading fail at stress levels lower than theirstatic strengths. This is called creep fracture.Topics which require study include tensile and

“Kenneth L. Reifsnider, Virginia Polytechnic Institute, personalcommunication, August 1986.

compressive loading of both unidirectional com-posites and laminates, and the influence of tem-perature and environment. Since the time-depen-dent degradation of the matrix and interphaseproperties are typically greater than those of thefibrous reinforcements, particular attention shouldbe paid to transverse matrix cracking and delami-nation.

Viscoelastic and Creep Properties.—The occur-rence of significant deformation resulting fromsustained loading can have an adverse effect onstructural performance in some applications, suchas reciprocating equipment, bridges, and build-ings. Consequently, creep behavior is an impor-tant material characteristic. This subject couldbenefit from development of a database for creepproperties of various fibers, matrices, and com-posites, especially for compressive loading, whichhas received relatively little attention.

GENERAL PREREQUISITES OF THE USEOF ADVANCED STRUCTURAL MATERIALS

Education and Training

The expanding opportunities for ceramics andcomposites will require more scientists and engi-neers with broad backgrounds in these fields. Atpresent, only a handful of universities offer com-prehensive curricula in ceramic or composite ma-terials. There is also a shortage of properly trainedfaculty members to teach the courses. The jobmarket for graduates with advanced degrees inceramic or composite engineering is good, and canbe expected to expand in the future. Stronger rela-tionships between industry and university labora-tories are providing greater educational and jobopportunities for students.

There is a great need for continuing educationand training opportunities for designers and engi-neers in industry who are unfamiliar with the newmaterials. In the field of polymer composites, forexample, most of the design expertise is concen-trated in the aerospace industry. Continuing edu-cation is especially important in “unsophisticated”industries such as construction, which must pur-chase, rather than produce, the products they use.Universities and professional societies are offer-ing seminars and short courses to fill this gap;these opportunities should be publicized and mademore widely available.

Beyond the training of professionals, there isa need for the creation of awareness of new mater-ials technologies among technical editors, manag-ers, planners, vice presidents, and the general pub-lic. In recent years, the number of newspaper andmagazine articles about the remarkable proper-ties of ceramics and composites has increased, ashas the number of technical journals associatedwith these materials. The success of compositesports equipment, including skis and tennisrackets, shows that new materials can have a“high-tech” appeal to the public, even if they arerelatively expensive.

Multidisciplinary Approach

Progress in materials development often requiresa team effort. For example, for a typical ceramiccomponent, the team might consist of one or morespecialists from each of the disciplines in table 15.Materials research lends itself naturally to col-laborative institutional arrangements in which therigid disciplinary boundaries between differentfields are relaxed. The successes of numerousmultidisciplinary materials centers based at uni-versities and national laboratories across the coun-try reflect this fact. A consensus is developingwithin the materials community that the explo-ration of new mechanisms for collaborative workbetween university, industry, and governmentlaboratory scientists and engineers would have asalutary effect on the pace of advanced materialsdevelopment and utilization. This topic will beexamined in greater detail in the final report.

Table 15.—Hypothetical Multidisciplinary DesignTeam for a Ceramic Component

Specialist Contribution

Systems engineer . . . . . . . Defines performanceDesigner . . . . . . . . . . . . . . . Develops structural conceptsStress analyst . . . . . . . . . . Determines stress for local

environments and difficultshapes

Metallurgist . . . . . . . . . . . . Correlates design with metallicproperties and environments

Ceramist . . . . . . . . . . . . . . . Identifies proper composition,reactions, and behavior fordesign

Characterization analyst. Utilizes electron microscopy,X-ray, fracture analysis, etc.to characterize material

Ceramic manufacturer . . . Defines production feasibilitySOURCE J.J Mecholsky, “Engineering Research Needs of Advanced Ceramics

and Ceramic Matrix Composites, ” contractor report for OTA, Decem-ber 1985.

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Integrated Design

Advanced ceramics and composites shouldreally be considered structures rather than mate-rials. Viewed in this light, the importance of a de-sign process which is capable of producing highlyintegrated and multifunctional structures becomesclear. Polymer matrix composites provide a goodexample. Perhaps the greatest single economicadvantage of composites, beyond their superiorperformance, is the potential for reduction in thecost of manufacture achieved by reducing thenumber of parts and operations required in fabri-cation. For example, a typical automobile hasabout 1500 structural parts. It has been estimatedthat, with an integrated composite design, thistotal could be reduced to a few hundred parts,with major savings in tooling and manufacturing

costs. 62 Because composites can be tailored in so

many ways to the various requirements of a par-t i c u l a r e n g i n e e r i n g c o m p o n e n t , t h e k e y t o o p -timizing cost and performance is a fully integrated

design process which is capable of balancing all

o f the re levant des ign and manufac tur ing var ia -

bles. Such a design process will require an exten-sive database on matrix and fiber properties, aswell as sophisticated software capable of model-ing fabrication processes and three-dimensionalanalysis of the properties and behavior of the re-sulting structure. Development of expert systemssoftware for the design of both materials andstructures will also be important, particularly forengineers who are new to the field. Perhaps themost important element in the development of in-tegrated design algorithms will be an understand-ing of the relationships between the constituentproperties, microstructure, and the macroscopicproperties of the structure. Many of the researchand development priorities listed above are in-tended to provide this information.

The view of ceramics and composites as struc-tures also sheds light on the often-stated need fora “database” for design purposes. The discussionabove suggests that the “customized” nature ofadvanced structural ceramics and composites mili-tates against the very idea of standardized struc-tures with specified properties which could be cat-aloged in a database. However, depending on the

blReifsnider, op. cit., December 1985.

application, a two-tracked approach to standard-ization and data-collection may be the best over-all solution. For design of integrated, multifunc-tional structures, the best approach may be adatabase containing the properties of constituentsand unidirectional composites, coupled withstandardized algorithms for modeling and anal-ysis. Destructive testing of the finished structurewould provide data to validate the models. Forapplications in which standard ceramic or com-posite components are desired, such as brackets,elbows, or beams, the properties and composi-tion of the structures would be specified andstandardized,

Systems Approach to Costs

It is often stated that the three biggest barriersto the increased use of advanced materials arecost, cost, and cost. In a narrow sense, this ob-servation is correct. If advanced materials are con-sidered on a dollar-per-pound basis as replace-ments for steel or aluminum in existing designs,they cannot compete. This is very often the per-spective of potential user industries, who are ori-ented toward metals processing, and has been thesource of great frustration among ceramics andcomposites suppliers. However, the per-poundcost and part-for-part replacement are rarely validbases for comparison. This is the reason that thecosts of ceramics and composites have not beenstressed in this report. A more fruitful approachis an analysis of the overall systems costs of a shiftto advanced materials, including integrated de-sign, fabrication, installation, and lifecycle costs.

The high per-pound cost of advanced materi-als is largely a result of the immaturity of the fab-rication technology and the low productionvolumes. Large decreases in materials costs canbe expected as the technologies mature. For ex-ample, a pound of standard high strength carbonfiber which used to cost $300 now costs less than$20, and new processes based on synthesis frompetroleum pitch promise to reduce the cost evenfurther. 63 If high-strength carbon fibers in the $3

bgThe Wor]d’s foremost manufacturer of pitch-based carbon fi-ber, Kureha Chemical Industry Co. of Japan, has developed a newhigh-strength fiber said to cost one-third as much as polyacryloni-trile (PAN) -based fibers. See IrorI Age, June 20, 1986, p. J6.

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to $5 per pound range became available, majornew opportunities would open up for compositesin automotive, construction, and corrosion-resistant applications.

Two other economic factors which can affectthe competitiveness of ceramics and compositesdeserve mention: they are energy costs and laborcosts. The cost of the energy required to manu-facture ceramic and composite components is anegligible fraction of the overall production costs;however, energy savings in service are a majorreason for the use of advanced materials in suchapplications such as heat exchangers, engines, andtransportation structures. Persistent low energycosts are likely to reduce the incentives to intro-duce advanced materials into these applications.

Throughout this report, the vision presented ofthe materials factory of the future has been oneof extensive automation and computer-controlledprocessing. The reliability and reproducibility re-quirements of ceramic and composite structuresdemand a minimum of human intervention. These

considerations indicate that the ceramics or com-posites factory of the future will not be a laborintensive operation. In fact, labor costs, which arecurrently a large fraction of overall productioncosts of advanced structures, are a key target forfuture cost reduction. Thus, rising labor costscould accelerate the utilization of ceramics andcomposites.

Technical, economic, and institutional factorswill all influence the incorporation of ceramics andcomposites technologies into commercial produc-tion. Many of these factors have been discussedin this interim report. However, several key is-sues are mentioned without discussion. These in-clude: the need for improved cooperation betweenindustry, universities, and government labs; thedramatic changes which new materials will bringto manufacturing; and government policy optionswhich could enhance the international competi-tiveness of United States advanced materials usersand suppliers. The final report will explore theseissues in detail.

—

GLOSSARY

adiabatic: Referring to any process in which there isno gain or loss of heat.

aggregate: Inert filler material such as sand or gravelused with a cementing medium to form concrete ormortar.

alloy: A material having metallic properties and con-sisting of two or more elements.

anisotropic: Showing different physical or mechani-cal properties in different directions.

aramid: Lightweight polyaromatic amide fibers hav-ing excellent high temperature, flame, and electri-cal properties. These fibers are used as high-strengthreinforcement in composites.

brittle fracture: A break in a brittle material due tothe propagation of cracks originating at flaws.

carbon/graphite: These fibers, which are the dominantreinforcement in “advanced” composites, are pro-duced by pyrolysis of an organic precursor, e.g.polyacrylonitrile (PAN), or petroleum pitch, in aninert atmosphere. Depending on the process temper-ature, fibers having high strength or high elasticmodulus may be produced.

cement: A dry powder made from silica, alumina,lime, iron oxide, and magnesia which forms a hard-ened paste when mixed with water; it may be usedin this form as a structural material, or used as abinder with aggregate to form concrete.

ceramic: An inorganic, nonmetallic solid.chemically bonded ceramics: Used here to distinguish

advanced cements and concretes, which are consoli-dated through chemical reactions at ambient tem-peratures (generally involving uptake of water) fromhigh-performance ceramics, such as silicon nitrideand silicon carbide, which are densified at high tem-peratures.

chromatographic adsorption: Preferential adsorptionof chemical compounds (gases or liquids) accord-ing to chemical affinity onto a solid adsorbent ma-terial.

composite: Any combination of particles, whiskers,or fibers in a common matrix.

compressive stress: A stress that causes an elastic bodyto shorten in the direction of the applied force.

concrete: A mixture of aggregate, water, and a binder(usually portland cement) which hardens to stone-like condition when dry.

consolidation of parts: Integration of a number of for-merly discrete parts into a single part which encom-passes several functions; a key advantage of engi-neered materials such as ceramics and composites.

continuous fiber: A reinforcing fiber in a compositewhich has a length comparable to the dimensionsof the structure.

creep: A time-dependent strain of a solid, caused bystress.

cross-linking: The formation of chemical bonds be-tween the formerly separate polymer chains.

crystal: A homogeneous solid in which the atoms ormolecules are arranged in a regularly repeatingpattern.

deflection: Deformation of a material produced with-out fracturing the material.

deformation, plastic deformation: Any alteration ofshape or dimensions of a body caused by stresses,thermal expansion or contraction, chemical or met-allurgical transformations, or shrinkage and expan-sions due to moisture change.

delamination: Separation of a layered structure intoits constituent layers.

dielectric: A material which is an electrical insulatoror in which an electric field can be sustained witha minimum dissipation in power.

diffusion: The movement of mass, in the form of dis-crete atoms or molecules, through a medium.

dispersion: Finely divided particles of one material heldin suspension in another material.

ductility: The ability of a material to be plasticallydeformed by elongation without fracture.

E-glass: A borosilicate glass most used for glass fibersin reinforced plastics.

elasticity: The property whereby a solid material de-forms under stress but recovers its original config-uration when the stress is removed.

engineered materials: Materials whose physical andmechanical properties and function are tailored fora particular application.

extrusion: A process in which a hot or cold semisoftsolid material, such as metal or plastic, is forcedthrough the orifice of a die to produce a continu-ousl y formed piece in the shape of the desiredproduct.

failure: Collapse, breakage, or bending of a structureor structural element such that it can no longer ful-fill its purpose.

fatigue: Failure of a material by cracking resulting fromrepeated or cyclic stress.

filtration: A process of separating particulate matterfrom a fluid, by passing the fluid carrier througha medium that will not pass the particulate.

finishing: Final processing operations on a part.

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70

flexure: Any bending deformation of an elastic bodyin which the points originally lying on any straightline are displaced to form a plane curve,

fracture stress: The minimum stress that will causefracture, also known as fracture strength,

glass: A state of matter that is amorphous or disor-dered like a liquid in structure, hence capable of con-tinuous composition variation and lacking a truemelting point, but softening gradually with increas-ing temperature.

glass-ceramic: Solid material, partly crystalline andpartly glassy, formed by the controlled crystalliza-tion of certain glasses.

grain: One of many crystallite comprising a poly -crystalline material.

green state, greenware: A term for formed ceramic arti-cles in the unfired condition.

hardness: Resistance of a material to indentation,scratching, abrasion, or cutting.

heat treatment: Heating and cooling of a material toobtain desired properties or conditions.

holography: A technique for recording and later recon-structing the amplitude and phase distributions ofa wave disturbance.

hot isostatic pressing: A form of ceramic or powdermetallurgical forming or compaction process inwhich the mold is flexible and pressure is appliedhydrostatically or pneumatically from all sides,

hot pressing: Forming a metal powder compact or aceramic shape by applying pressure and heat simul-taneously at temperatures high enough for sinter-ing to occur.

impact strength: Ability of a material to resist shockloading,

inclusion: A flaw in a material consisting of a trappedimpurity particle.

injection molding: Forming metal, plastic, or ceramicshapes by injecting a measured quantity of the ma-terial into shaped molds.

internal stress, residual stress: A stress system withina solid (e. g., thermal stresses resulting from rapidcooling from a high temperature) that is not depen-dent on external forces.

interphase, interface: The boundary layer between thematrix and a fiber, whisker, or particle in a com-posite.

lay-up: A process for fabricating composite structuresinvolving placement of sequential layers of matrix-impregnated fibers on a mold surface.

load: The weight that is supported by a structure, ormechanical force that is applied to a body.

matrix: The composite constituent that binds the rein-forcement together and transmits loads betweenreinforcing fibers.

metal: An opaque material with good electrical andthermal conductivities, ductility, and reflectivity;

properties are related to the structure in which thepositively charged nuclei are bonded through a fieldof mobile electrons which surrounds them, forminga close-packed structure.

microstructure: The internal structure of a solid viewedon a distance scale on the order of micrometers.The microstructure is controlled by processing, anddetermines the performance characteristics of thestructure.

modulus of elasticity: A parameter characterizing thestiffness of a material, or its resistance to deforma-tion under stress. For example, steel has a relativelyhigh modulus, while Jello has a low modulus.

monolithic: Constructed from a single type of material.near-net-shape The original formation of a part to a

shape which is as close to the desired final shape aspossible, requiring as few finishing operations aspossible.

nondestructive testing, evaluation: Any testing methodwhich does not involve damaging or destroying thetest sample; includes use of X-rays, ultrasonics, mag-netic flux, etc.

phase: A region of a material that is physically dis-tinct and is homogeneous in chemical composition.

plasticity: The property of a solid body whereby it un-dergoes a permanent change in shape or size whensubjected to a stress exceeding a particular value,called the yield value.

polymer: Substance made of giant molecules formedby the union of simple molecules (monomers); forexample, polymerization of ethylene forms a poly-ethylene chain.

pore, porosity: Flaw involving unfilled space inside amaterial which frequentl y limits the materialstrength.

prepreg: Fiber reinforcement form (usually tape, fab-ric, or broadgoods) which has been preimpregnatedwith a liquid thermosetting resin and cured to a vis-cous second stage. Thermoplastic prepregs are alsoavailable.

proof test: A predetermined test load, greater than theintended service load, to which a specimen is sub-jected before acceptance for use.

radiography: The technique of producing a photo-graphic image of an opaque specimen by transmit-ting a beam of X-rays or gamma rays through it ontoan adjacent photographic film; the transmitted in-tensity reflects variations in thickness, density, andchemical composition of the specimen.

radome: A strong, thin shell made from a dielectricmaterial, used to house a radar antenna,

reciprocating (engine or machinery): Having a motionthat repeats itself in a cyclic fashion.

refractory: Capable of enduring high-temperature con-ditions.

S-glass: A magnesia-alumina-silicate glass that pro-

7 1

vides very high tensile strength fiber reinforcement.Often regarded as the reinforcement fiber dividing“advanced” composites from reinforced plastics.

shearing stress: A stress in which the material on oneside of a surface pushes on the material on the otherside of the surface with a force which is parallel tothe surface.

sintering: Method for the consolidation and densifi-cation of metal or ceramic powders by heating with-out melting.

slip casting, slip, slurry: A forming process in the man-ufacture of shaped refractories, cermets, and othermaterials in which slip is poured into porous plas-ter molds. Slip or slurry is a suspension of ceramicparticles in water with a creamy consistency.

strain: Change in length of an object in response toan applied stress, divided by undistorted length.

stress: The force acting across a unit area in a solidmaterial in resisting the separation, compacting, orsliding that tends to be induced by external forces.

structural materials or assembly: Those parts of a sys-tem that support most of the loading on the wholesystem.

substrate: Base surface on which a material adheres,for example a surface to be coated.

systems approach (to cost or to design): Considera-tion of product design, manufacture, testing, andlifecycle as an indivisible whole; see comoliclationof parts.

tensile strength, ultimate tensile strength: The maxi-mum stress a material subjected to a stretching loadcan withstand without breaking.

thermal conductivity: The rate of heat flow understeady conditions through unit area per unit tem-perature in the direction perpendicular to the area—the ability of a material to conduct heat.

thermoplastic resin: A material containing discretepolymer molecules that will repeatedly soften whenheated and harden when cooled; for example, poly-ethylene, vinyls, nylons, and fluorocarbons.

thermosetting resin: A matrix material initially hav-ing low viscosity that hardens due to the formationof chemical bonds between polymer chains. Oncecured, the material cannot be melted or remoldedwithout destroying its original characteristics; ex-amples are epoxies, phenolics, and polyamides.

toughness: A parameter measuring the amount ofenergy required to fracture a material in the pres-ence of flaws.

tribology: The study of the phenomena and mecha-nisms of friction, lubrication, and wear of surfacesin relative motion.

turbocharger: A centrifugal air compressor driven bythe flow of exhaust gases and used to increase in-duction system pressure in an internal combustionreciprocating engine.

ultrasonic testing: A nondestructive test method thatemploys high-frequency mechanical vibration ener-gy to detect and locate structural discontinuities ordifferences and to measure thickness of a variety ofmaterials.

unibody: Integrated structure containing the chassisas well as elements of the body of an automobile.

viscoelasticity: Property of a material that is viscousbut which also exhibits certain elastic properties suchas the ability to store energy of deformation, andin which the application of a stress gives rise to astrain that approaches its equilibrium value slowly.

wear: Deterioration of a surface due to material re-moval caused by relative motion between it andanother material.

nettability: The ability of any solid surface to be wet-ted when in contact with a liquid.

whisker: A short, single crystal fiber with a length-to-diameter ratio of 10 or more, often used to im-prove the fracture toughness of ceramics.

yield strength: The lowest stress at which a materialundergoes plastic deformation. Below this stress, thematerial is elastic.