FUNCTIONALLY GRADED MATERIALS

INTRODUCTION• A new class of composite materials known as functionally

graded materials (FGMs) has drawn considerable attention of the scientific community.

• FGMs exist in nature like wood, bamboo, etc. Even human bones are also FGM.

• The aircraft and aerospace industry and the computer circuit industry make wide use of FGM that can withstand very high thermal gradients. This is normally achieved by using a ceramic layer connected with a metallic layer.

FUNCTIONALLY GRADED MATERIALS: AN OVERVIEW

FGMs are composite materials in which the composition or microstructure or both are locally varied so that a certain variation of the local material properties is achieved.

In FGMs, the different micro-structural phases have different functions. The overall FGMs attain the multi-structural status from their property gradation.

A typical FGM, with a high bending/stretching coupling effect is an inhomogeneous composite made from different phases of material constituents (usually ceramic and metal).

FUNCTIONALLY GRADED MATERIALS

Functionally Graded Materials can be defined as multi phase composites (generally two-phase particulate composites such as ceramic and metal alloy phases), synthesized such that they possess continuous spatial variations in volume fractions of their constituents to yield a predetermined composition profile.

These variations lead to the formation of a non-homogeneous macrostructure with continuously varying mechanical and/or thermal properties in one or more than one directions.

The microstructure of FGM is generally heterogeneous and the dominant type of failure in FGM occurs from crack initiation and growth from inclusions.

FGM STRUCTURE• The composition and microstructure of FGM vary in

space following a predetermined law. The gradual change in composition and microstructure gives place to a gradient of properties and performances.

COMPARISON TO TRADITIONAL COMPOSITE

• 1984-Sendai Group proposed a concept of FGM (Nino, Koizumi and Hirai).

• 1985-Establishing the concept of FGM.• 1986-Investigation and Research conducted for FGM (with

Special Coordination Funds for Promoting science and Technology).

• 1987-Launching a National Project called FGM Part I (with Special Coordination Funds for Promoting science and Technology) (to be ended in March 1991); Title of Research was “Research on Generic Technology of FGM Development for Thermal Stress Relaxation”; The 1st FGM Symposium was held.

• 1990-The First International Symposium on Functionally Graded Materials was held in Sendai, Japan.

BRIEF HISTORY OF FGM

BRIEF HISTORY OF FGM• A combination of materials used would serve the

purpose of a thermal barrier capable of withstanding a surface temperature of 2000 K and a temperature gradient of 1000 k across a 10 mm section. 

• In recent years this concept has become more popular in Europe, particularly in Germany.

• A transregional collaborative research center (SFB Transregio) is funded since 2006 in order to exploit the potential of grading monomaterials, such as steel, aluminium and polypropylen, by using thermomechanically coupled manufacturing processes.

CLASSIFICATION OF FGMs

FGMs may be compositionally or micro-

structurally graded.

The gradient is established through a transition

function (usually volume fraction as a function of

one or more spatial coordinates).

FGMs come in several types, depending on their

constituents (e.g. ceramic-metal, metal-metal…)

TYPES OF GRADATIONContinuous Gradation

TYPES OF GRADATIONStepped Gradation

VARIOUS TYPES OF FGMs

Ceramic-Metal

Metal-Metal/Intermetallic

Metal-Polymer

Single material (variation in porosity)

W-Cu, W-Mo, Al-Al3Fe

TiC-Ni, Mullite-Mo, Al-AlB2

Al-Polycarbonate

Others:

Glass - Ceramic

Ceramic - Ceramic

ADVANTAGES OF FGM OVER CONVENTIONAL MATERIALS

o FGM satisfies the working conditions for which it is specifically built.

o FGM is economical as it reduces material costs for particular engineering applications.

o FGM can reduce the magnitude of residual and thermal stresses generated under working conditions.

o FGM exhibits enhanced fracture toughness and bonding

strength as compared to a bi-material solution. This is normally achieved by using a ceramic layer connected with a metallic layer.

Metal- Ceramic FGMs

• Ceramic has high hardness, high resistance at high temperatures and good corrosion resistance.

• Metallic phase has good toughness, good weldability.

• With the gradual variation of the composition, no debonding phenomena happen between metallic and ceramic phases.

A TYPICAL METAL-CERAMIC FGM

SOME MORE ATTRIBUTES OF FGM

• High thermal resistance.• High abrasion resistance (ceramic face).• High corrosion resistance.• High impact resistance.• Weldable/boltable to metal supports.• Biological compatibility.

POTENTIAL APPLICATIONS OF FGM• Aerospace and aeronautics

Advanced aircraft and aerospace engines.Rocket heat shields.TBC.

• Power plantTBC.Heat Exchanger tubes.

• ManufacturingMachine tools.Forming and cutting tools.Metal casting and forging processes.

• Smart structuresFunctionally graded piezoelectric materials.Shape memory alloys.

• MEMS and sensors.• Electronics and optoelectronics

Optical fibers for wave high speed transmission.Computer circuit boards (PCB)Cellular phone.

POTENTIAL APPLICATIONS OF FGM

• BiomaterialsArtificial bones, joints.Teeth.Cancer prevention.

• OthersBaseball cleats.Razor blades.Titanium watches.

POTENTIAL APPLICATIONS OF FGM

• MedicineLiving tissues like bones and teeth are

characterized as functionally graded material from nature, to replace these tissues, a compatible material is needed that will serve the purpose of the original bio-tissue. The ideal candidate for this application is functionally graded material. FGM has found wide range of application in dental and orthopedic applications for teeth and bone replacement

POTENTIAL APPLICATIONS OF FGM

• DefenseOne of the most important characteristics of

functionally graded material is the ability to inhibit crack propagation. This property makes it useful in defense application, as a penetration resistant materials used for armor plates and bullet-proof vests

• EnergyFGM are used in energy conversion devices. They also

provide thermal barrier and are used as protective coating on turbine blades in gas turbine engine

POTENTIAL APPLICATIONS OF FGM

APPLICATIONS IN AIRCRAFT INDUSTRY

• Exhaust wash structure that separates exhaust gas from aircraft structure for vehicles which have internally exhausted engines, i.e., stealth aircraft and UAVs with engines that don’t exhaust directly to the atmosphere.

• Hot, high speed engine exhaust flows over the top surface of exhaust wash structures which, in turn, causes large deflections.

• An FGM patch applied to the underside of the exhaust wash structure can be designed such that thermally induced deflection of the FGM patch is in a direction opposite to the exhaust wash structure deflection.

FGMs have the added advantage that the metal side

can be bolted onto the airframe rather than bonded

as are the ceramic tiles used in the Orbiter.

Ceramic-metal FGMs are particularly suited for

thermal barriers in space vehicles.

Other possible uses include combustion chamber

insulation in ramjet or scramjet engines

APPLICATIONS IN AIRCRAFT INDUSTRY

APPLICATIONS OF FGM AS TBC

• One of the salient applications of FGM is in Thermal Barrier Coatings (TBC).

• TBCs find their widespread applications in automotive and aircraft industries. They are specifically designed to reduce heat loss from engine exhaust system components including exhaust manifolds, turbocharger casings, exhaust headers, downpipes and tailpipes.

TBC STRUCTURE

SUITABILITY OF FGMs AS TBC

• An FGM composed of ceramic on the outside surface and metal on the inside surface eliminates the abrupt transition between coefficients of thermal expansion, offers thermal/corrosion protection, and provides load carrying capability. This is possible because the material composition of an FGM changes gradually through-the-thickness; therefore, stress concentrations from abrupt changes in material properties (i.e., coefficients of thermal expansion) are eliminated.

OTHER APPLICATIONSFUEL CELL

TECHNOLOGY

By creating a porosity gradient in the electrodes of a fuel

cell, the efficiency of the reaction can be maximized .

OTHER APPLICATIONSNUCLEAR FUSION

REACTORSModification to heat exchangers in Tokamak fusion reactors

Reduction of interfacial stresses

→ prevention of delamination

effects → increase in lifetime

PROCESSING METHODS FOR FGM• Constructive processes

Powder densification processes (powder consolidation, infiltration, liquid phase sintering, etc.).

Coating processes (plasma spray forming, thermal spray deposition, vapor deposition processes, etc.).

• Transport based processesMass transport processes (Diffusion from surface,

interdiffusion).Settling and centrifugal separation.

MODELING OF FGM

Assuming a preset variation

• The properties in FGM may be imparted any gradation like linear, exponential, hyperbolic or reciprocal as per the requisition.

• FGMs lend themselves well to being optimized for various performance measures.

• Generally rule of mixtures is employed to predict the properties of a typical FGM.

• In complex cases Halpin-Tsai equations are used to model the properties. These take into account the volume fraction of the inclusion phase in the matrix phase.

• Other properties such as the Poisson ratio, fracture toughness and thermal expansion coefficient follow similar trends.

• Hardness of the resulting material is rather difficult to predict in some cases.

MODELING OF FGM

MODELING OF FGMSome researchers decided upon a basic unit to describe FGMs.

The maxel represents the smallest entity in which the composition of a

continuously graded FGM can be defined.

Pure Component B

Pure Component A

% Component A

% Component B

It is the equivalent of the build resolution in rapid prototyping processes

(quantized by voxels – hence maxel is material voxel)

MULTISCALE ANALYSIS AND DESIGN

• There are several homogenization methods that are commonly used to estimate the effective properties of two-phase functionally graded materials including the Mori-Tanaka and self-consistent schemes. These homogenization schemes, while simple to use, are based solely on the volume fractions of the constituent materials. In comparison, multiscale homogenization methods are more accurate since they take the microstructure of the composite material into account.

• Furthermore, the failure of the material can be investigated at the microscale. The multiscale method enables the accurate prediction of both the overall and local material properties and stresses in FGMs.

FATIGUE/ FRACTURE OF FGM

• The main applications of FGM as TBC are high temperature applications. Thus TBCs have a high tendency to thermal fatigue.

• Design of FGM does not necessarily stem from the point of view of strength. Hence, thermal fatigue/fracture should also be the implied criterion so that it is structurally safe.

• Thus to increase the spectrum of applications of TBC in thermal and mechanical fields the fatigue/fracture behavior of FGMs should be investigated.

RECENT RESEARCH TRENDS IN FGM• Lots of studies have been conducted on behavior of

functionally graded materials and the literature is very rich on this because of the wide areas of application of this noble material. Some of the current research traits are:

• Performance of FGM under localized transverse loading.• Fracture and fatigue behavior of FGM under different types of

cyclic loadings.• Thermoelastic behavior of FGM.• Investigation of geometric non linearity of structures made of

FGM.• There are still more to be done in terms of research to improve

the performance of manufacturing processes of FGM.

CONCLUSIONS

Functionally graded materials are still a very recent area of

research (and thus very active). More developments in this

field are in progress and many avenues are still left to be

explored.

Recent trends in research in the area of FGM is mainly

directed towards uncovering the complex nature of fracture

mechanics due to material inhomogeneity as well as in

developing/improving forming processes so that the target

gradient is achieved with precision.

• Functionally graded materials are very important in engineering and other applications but the cost of producing these materials makes it prohibitive in some applications.

• Research is still on the run to bring down the fabrication cost of these materials to broaden the spectrum of their applications.

CONCLUSIONS