Materials and Design 32 (2011) 1717–1732

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Materials and Design

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Review

Glasses as engineering materials: A review

Eugen Axinte ⇑Gh. Asachi Technical University of Iasi, Faculty of Machine Manufacturing & Industrial Management, 59A, Prof. Dimitrie Mangeron Blvd., Romania

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 September 2010Accepted 24 November 2010Available online 28 November 2010

Keywords:GlassesConstitutionEnvironmental performance

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.11.057

⇑ Tel./fax: +40 232217290 (office), mobile: +40 722E-mail address: axintee@tcm.tuiasi.roURL: http://www.cm.tuiasi.ro/html/ro/tcm.htm

Glass products have applications in design engineering, and they can solve many special problems. Thesematerials can work in situations in which plastics and metals would fail and need to be part of designer’srepertoire. In some situations, by using these materials, some difficult problems would be solved. Thispaper contains a number of chapters as follows: a brief about ceramics family, a short history of glass,a brief about physics and the technology of glass fabrication, recently developed glasses with special des-tinations, testing methods and news about glass parts processing (grinding, waterjet processing, lasercutting, nanoimprint lithography, etc.). The last chapter of this review paper contain some strategic linesof glass usage in industry and estimations about the future of glass development.

� 2010 Elsevier Ltd. All rights reserved.

1. The ceramics family

Cermets and ceramics are becoming the tool materials for thepresent and future. By using the cemented carbides at wood work-ing tools (as saw blades, cutting wheels), the wear was reduced sig-nificant. Coated cemented carbides displaced the high-speed steelfor cutting tools and also high production press dies use the ce-mented carbide tooling. Ceramics are taking a lot of high-temper-ature machine tasks, are substrates for computer chips, and areused for prosthetic devices. Glasses and carbon products haveapplications in design engineering and they can solve many specialproblems.

These materials can work in situations in which plastics andmetals would fail and need to be part of designer’s repertoire;sometimes, using these materials, some difficult problems wouldbe solved. Ceramics are defined as solids composed of compoundsthat contain metallic and/or non-metallic elements, and the atomsof compounds are held together with strong atomic forces (ionic orcovalent bonds). The spectrum of ceramic uses is presented inFig. 1.

The ceramics with high strength and the best toughness (as alu-minum, zirconias, oxides, silicon carbides) were named in 1980 as‘‘structural ceramics’’. In Japan, these ceramics were called ‘‘fineceramics’’. In 1990, the (ASTM) Committee for ceramics (C28)named this ceramics ‘‘advanced ceramics’’. The definition givenby C28 for this class of materials is highly engineered, high-perfor-mance, predominantly non-metallic, inorganic, ceramic materialhaving specific functional attributes (by standard ASTM C1145) [1].

ll rights reserved.

892926.

Glasses are not ceramics (by previous definition of ceramics),but they are used for similar type of things as ceramics and havesome properties that are typical for ceramics. The most commonproperty of ceramics, glasses, and cements is brittleness.

The measure of crack propagation tendency, fracture toughness,is lower at ceramics family than at metals, as is shown in Fig. 2.

2. Brief history of glass: past and present of glass

It is not exactly known when, where, or how humans firstlearned to make glass. The legends tells us that a Phoenician sailor(by other historians, a Roman sailor), cooking the evening meal ona beach, sets the pots on top of stones of natron (a natural mixtureof sodium carbonate decahydrate, sodium bicarbonate along withsmall quantities of household salt). As the cooking fire heated boththese stones and the sand below, an unknown liquid began to flowand that was the origin of man-made glass.

In [2] is demonstrated and argued that in ancient times, sodaglasses with high alumina concentrations are quite rare aroundthe Mediterranean area or in the Middle East. The few availableexamples include European Iron-Age dark blue glass colored withcobalt-rich alum that contains up to 8% of alumina. Mineralsoda–alumina (m-Na–Al) glass has been found across a vast areastretching from Africa to East Asia. m-Na–Al glass appears aroundthe 5th c. B.C. and is relatively common for periods as late as the19th c. A.D. It is particularly abundant in South Asia, where rawmaterials to produce m-Na–Al glass are readily available and waslikely manufactured there; however, the number and the impor-tance of the manufacturing centers are unknown as archaeologicalinformation is extremely scarce. The interpretation of data ob-tained using compositional analysis on a large corpus of artifacts(486) shows that at least five subgroups of m-Na–Al glass can be

Fig. 1. The spectrum of ceramics uses.

Fig. 2. The fracture toughness of different materials.

1718 E. Axinte / Materials and Design 32 (2011) 1717–1732

identified using the concentrations of calcium, magnesium,uranium, barium, strontium, zirconium, and cesium measuredwith laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS). From this data, it is possible to infer the existence ofseveral m-Na–Al glass-making centers, not all of them located inSouth Asia as previously assumed. They were operating over differ-ent time periods and were connected to different exchangenetworks.

It is historically accepted that the first manufactured glass wasin the form of a glaze on ceramic vessels, about 3000 B.C. The firstglass vessels were produced about 1500 B.C. in Egypt and Mesopo-tamia. The glass industry was extremely successful for the next300 years and then declined. It was revived in Mesopotamia inthe 700 B.C. and in Egypt in the 500 B.C. For the next 500 years,Egypt, Syria, and the other countries along the eastern shore ofthe Mediterranean Sea were glassmaking centers. Glass manufac-turing developed in the Roman Empire and spread from Italy toall Roman provinces. The first four centuries of Christian Era iscalled the First Golden Age of Glass. In the middle ages, by the timeof the Crusades, glass manufacture had been revived in Murano is-land, near Venice, where soda lime glass, known as crystal, was

developed (this period is known as the Second Golden Age of Glass).In North America (United States), the first factory known was aglass plant built at Jamestown, Virginia, in 1608, but failed withina year. The Jamestown colonists tried glassmaking again in 1621,but an Indian attack in 1622 and the scarcity of workers ended thisattempt in 1624. The glass industry was reestablished in Americain 1739, when Caspar Wistar built a glassmaking plant in SalemCounty, New Jersey. This plant operated until 1780.

In 1820, Bakewell, Page, & Bakewell Co. from Pittsburgh, Penn-sylvania, introduced the first real development in production glass-blowing since the blowpipe, a development that would changehow glass was used forever. They patented a process of mechani-cally pressing hot glass. After 1890, glass uses and manufacturingdevelopments increased so rapidly as to be almost revolutionary.The late 1900s brought new important specialty glasses. Amongthe new specialty glasses were transparent glass–ceramics, whichare used to make cookware, and chalcogenide glass, an infrared-transmitting glass that can be used to make lenses for night-visiongoggles. The science and engineering of glass as a material wasmuch better understood, and in the late 1950s, Sir Alastair Pilking-ton introduced a new revolutionary production method (float glassproduction), by which 90% of flat glass is still manufactured today.In the 1970s, optical fibers were developed for use as ‘‘light pipes’’in laser communication systems. These pipes maintain the bright-ness and intensity of light being transmitted over long distances.Types of glass that can store radioactive wastes safely for thou-sands of years were also developed during the 1970s.

3. Brief about physics of glass: how it is made

Generally, solids (metals) have a three-dimensional periodicstructure (crystalline structure). But also exists solids with a ran-domized three-dimensional structure – these solids are calledamorphous or glassy (Fig. 3).

A lot of materials such as organic polymers and metal alloys areable to form under special conditions amorphous structures. Aninorganic amorphous or glassy solid is a high-speed cooled liquid(cooled fast enough to prevent crystallization).

Fig. 3. Crystalline structures (a) amorphous randomized structures (b) and the molecular structure of silica-based glass (c).

Fig. 4. Dynamic viscosity of glass vs heating temperature.

E. Axinte / Materials and Design 32 (2011) 1717–1732 1719

The relationship between the oxygen and the cation of the oxidecompound essentially influences the glass-forming ability of anoxide. There are four types of oxides used in glass fabrication(see Table 1).

At room temperature, glasses are very viscous structures(1018 Pa � s – glass viscosity vs 1 Pa � s – water viscosity). Viscousflow of glass at room temperature occurs in a geological timescale.With increasing temperature, the glass viscosity decreases as isshown in Fig. 4.

There are five very important temperatures, called ‘‘standardpoints’’, associated with the viscous flow (melting) of glass. Thestrain point is the maximum temperature supported by glass forstructural applications. A review and the description of standardpoints is given by Table 2.

After the glass was formed, it is cooled to a temperature nearlyabove the strain point, where it will retain its shape and resist flow.At this point, the glass parts are annealed to relieve the internalstresses. A glass part that is incorrectly annealed will fracture orcrack at ambient temperature.

The behavior of specific density of glass as a function of thetemperature is illustrated by graph from Fig. 5.

It is observed that at glass transition temperature exists aninflexion point. After this point, the glass viscosity decreases,specific volume abruptly increases, and the specific density has agreater rate of decreasing. This behavior has major consequencesfor design and manufacture of molds. In [3], Fluegel developedan accurate glass viscosity model relevant to commercialapplication through statistical analysis and based on all composi-tion–property data available in SciGlass. The viscosity model forpredicting the complete viscosity curve of glass was developedusing a global statistical approach and more than 2200 composi-tion–viscosity data for silicate glasses collected from over 50 yearsof scientific literature, including soda–lime–silica container and

Table 1Types of oxides used in glass fabrication.

No. Oxide type Characteristics

1 Main glass former oxides Suitable structures and low cr

Form glass under slow cooling

2 Conditional glass formers oxides Form glass under certain cond3 Intermediate oxides Cannot form glass themselves4 Network modifier oxides Cannot form glass themselves

They can modify the propertie

float glasses, TV panel glasses, borosilicate fiber wool and E-typeglasses, low expansion borosilicate glasses, glasses for nuclearwaste vitrification, lead crystal glasses, binary alkali silicates, andvarious other compositions.

All that is required to make glass is sand, soda, a little lime, anda lot of heat. The typical process of glasses fabrication is shown inFig. 6.

Examples

ystallization rates SiO2

B2O3

rates GeO2

P2O5

itions Al2O3; Bi2O3 WO3; MoO3

but form glass in mixture with former oxides TiO2; ZnO; PbO; Zr2O3

nether in mixture with former oxides MgOCaONa2O

s of glass by affecting Si–O bonds K2O

Table 2The standard points of glass.

No. Standard pointname

Viscosity(Pa s)

Temperature descriptions

1 Working point 103 At this temperature, the viscosity is sufficiently low for glass forming. Casting processes are possibly below 10 Pa s viscosity2 Softening point 106.6 At this temperature, the viscosity is sufficiently low for glass to slump under own weight. Near below this temperature glass

is stiff, but a little effort is necessary for yield and flow3 Glass transition

temperature1012 Range of temperatures at which glass transitions from super cooled liquid in a solid state

4 Annealing point 1013.4 Internal stresses are relieved in minutes5 Strain point 1013.6 Internal stresses are relieved in hours

1 Pa s (SI) = 0.1 P (P – poise, physics system of units).

Fig. 5. The evolution of specific density vs temperature (———— specific volume vstemperature).

1720 E. Axinte / Materials and Design 32 (2011) 1717–1732

The mixture of refined sand (SiO2) and additional basic oxide isheated in furnace (gas or electric) at a temperature higher than1200 �C. Essentially, the main role of this additional oxides (asCaO, Na2O, K2O) is to reduce the working point of the mixture. Apure silicone oxide (quartz) is refractory, with a softening pointnear 1500 �C or higher. Then, the melted composition is manufac-

Fig. 6. Basic technology for glass fabrication�

tured in glass pieces by different techniques as blowing, molding,casting, injection, extrusion, and wire drawing.

Commercial glasses are silica-based glasses with additional oxi-des (see Table 1). The presence, type, and the quantity of one ormany oxides give the glass type and also have major influenceson the glass properties and utilization. For example, a colored glassis obtaining by addition of a metallic oxide (iron oxide for greenglass, cobalt oxide for blue glass). Crystalline glass–ceramics areobtained by the introduction of titanium oxide in melted glass.Titanium oxide initiates the crystallization and the material ob-tained is up to 96% crystalline [1].

The mechanical and physical properties of glasses are essen-tially determined by their composition, but a general view can begiven:

a. Glasses are harder than metals.b. Glasses have tensile strength in the range 24–69 MPa.c. Glasses are brittle and have low ductility.d. Glasses have a low coefficient of thermal expansion.e. Glasses have a low coefficient of thermal conductivity.f. Glasses are good electrical insulators.g. Glasses are resistant to acids, solvents, chemicals, water and

saline water and alkaline solutions.h. Some glasses can be used at high temperatures (700 �C –

soda lime for windows; 1580 �C – fused quartz–silica).

Selection of the main mechanical and physical properties ofamorphous glass and crystalline glass–ceramic is presented in

(�integrated with the recycling process).

Table 3Mechanical properties of glasses vs other engineering materials.

Material Flexuralstrength(MPa)

Compressivestrength(MPa)

Hardness,HV

Youngmodulusof elasticity,E (GPa)

Silica (amorphousglass)

98 1860 600 69

Crystallineglass–ceramic

103 344 250 64

Plastic(polycarbonate)

89.6 2.28

Carbon steel 275 275 100 200Cemented carbide 1400 4000 1000 612

Table 4Physical properties of glasses vs other engineering materials.

Material Density(kg/m3)

Thermalconductivity(W/m �C)

Coefficientof thermalexpansion,20–100 �C(10�6 m/m �C)

Electricalresistivity(O m)

Silica (amorphousglass)

2304 34 1 107

Crystallineglass–ceramic

2592 33 9.4 1012

Plastic(polycarbonate)

1296 0.2 6.7 8 � 1014

Carbon steel 8064 47 11.9 20Cemented carbide 16,000 86 7.4 6 � 10�8

E. Axinte / Materials and Design 32 (2011) 1717–1732 1721

Tables 3 and 4. For comparison, in tables are presented some prop-erties of a plastic (polycarbonate), a carbon steel, and a cementedcarbide WC with 6% cobalt.

Optical properties of glasses make them preferable in the con-struction of lenses and windows. The very low coefficient of ther-mal expansion of high silica glasses makes them extremelyresistant at thermal shocks and also makes them favorite for labo-ratory glass instruments (tubes, retorts) and light bulbs. The crys-talline glass with elasticity modulus above 130 � 103 MPa hasgood shock resistance.

4. Advances in glass family development

The glass family is huge and is in continuous enrichment. Yearafter year, new types of glass with new properties extend their uti-lization domain. The utilization domain of glass in engineering de-sign have a lot of facets: from banal windows or bottles toantinuclear radiation containers; from architectural and structuralglasses to photosensitive glass devices used in machine controls;from food preparation tanks to newest optical fibers.

The enormous variety of existent glass types, rapid develop-ment of new and innovative glasses, developments in glass fabrica-tion and development of glass-manufacturing processes make theclassification of glasses extremely difficult. After the product types,a first classification can be done for the main glass industries, as inTable 5 (based on http://www.glassonweb.com/ description-insection directory).

Table 5Classification of main glass industries.

Main glass industries

Flatglass

Hollowglass

Automotiveglass

Artglass

Opticalglass

Glassfibers

GlassWool

A list of glass types in the flat glass industry is starting from ba-sic glass types as float glass, mirrors, and trough security types tospecial glass types like electrochromic and photovoltaic glass(see Table. 6). A large spectrum of designer’s interests for theseglass products are also observed.

A simplified classification is based on the chemical compositionand follows the crystallization model (Table 7).

4.1. Advances in glass and glass–ceramics design and manufacture

In [4], Hampshire revealed some advances in the domain of oxy-nitride glasses. This work is a complete review of the developmentof oxynitride glasses and outlines the effect of glass composition,especially nitrogen content and also cation ratios, on propertiesand relates this to structural features within the glass. Nucleationand crystallization studies are also outlined. Oxynitride glassesare prepared by mixing appropriate powders – silica, alumina,the modifying oxide(s) plus silicon nitride or aluminum nitride –in isopropyl alcohol in a ball mill with sialon milling media, fol-lowed by evaporation of the alcohol. Glasses (50–60 g) are meltedin boron nitride-lined graphite crucibles under 0.1 MPa nitrogenpressure at 1600–1750 �C for 1 h, after which it is quickly removedfrom the furnace and poured into a preheated graphite mold at�850–900 �C. The glass is annealed at this temperature for 1 h toremove stresses and then slowly cooled. The main conclusions ofthe study are that oxynitride glass formation occurs in a numberof M–Si–O–N, M–Si–Al–O–N and M–Si–Mg–O–N systems, andusing normal melting processes, up to �30 equiv.% nitrogen canbe dissolved in the glass. An innovative approach, using metal pre-cursors, allows much more N to be dissolved into some M–Si–O–Nglasses. As nitrogen content increases, properties such as glasstransition temperature, elastic modulus, viscosity, hardness, andslow crack growth resistance increase while thermal expansioncoefficient decreases as a result of increased cross-linking of nitro-gen within the glass network. Spectroscopic studies have identifiedthe structural features of glasses and the role of nitrogen as a net-work ‘‘former’’. Many studies on crystallization of oxynitrideglasses have been carried out, which have identified suitabletwo-stage heat treatments for nucleation and growth of crystalphases, to form glass–ceramics with significant increases instrength and elastic modulus over the parent glasses. Addition offluorine extends glass formation in oxynitride systems and allowsdissolution of higher levels of nitrogen into glasses. Fluorine lowersglass transition temperature but does not have any effect on elasticmodulus or microhardness. Nitrogen may be dissolved in phos-phate glasses with consequent improvements in chemical durabil-ity and increases in many physical and mechanical properties [4].

New advances in structural glasses domain are described byRoyer-Carfagni and Silvestri [5]. The authors developed an innova-tive point-fixing system (called Gecko system) for frameless glassglazing that exploits the enhanced mechanical properties of anew generation of ionoplast polymer interlayers. Laminated glassconnected with the new device exhibits a noteworthy resistanceand interesting post-glass-breakage performances. To achieve afail-safe performance, the key point is to attach the polymericinterlayer of laminated glass directly to metallic holders, so thatthe interlayer itself may act as a confining membrane even afterglass-breakage. This technology is possible only if the polymericmaterials present sufficiently high mechanical properties and ad-here well to both glass and metals. The Gecko system is presentedin Fig. 7 (from [5]).

After the testing of the system in different conditions (tests atroom temperature, tests on aged samples and tests at low and hightemperatures), the authors have demonstrated that the polymercan be easily curved by slightly heating the material, and evenwhen the radius of curvature is very small, no considerable decay

Table 7Simplified glasses classification.

No. Type Description/key words

1 Ordinary – amorphousglasses

a. Quartzb. Silica glassc. Na–Ca

silicate glass

Amorphous structure; silica base; high commercial spectrum

2 Glass–ceramics Glass–ceramics are manufactured through the controlled crystallization of a specially formulated glass – a high density of crystallinenuclei (Ti, Zr, P2O5) is generated in the molten glass. Glass–ceramics are useful in thermally hazardous conditions. Good resistance toerosion and pressure and the excellent hardness make glass–ceramics widely used in industrial purposes. Moreover, glass–ceramicsare very good electrical insulators

3 Bulk metallic glassesBMGs

Bulk metallic glasses (BMGs) are metallic materials with a disordered atomic-scale structure, produced directly from the liquid stateduring cooling (the rapid cooling, on the order of millions of degrees a second, is too fast for crystals to form and the material is ‘‘lockedin’’ a glassy state), are called ‘‘glasses’’, or amorphous metals and they are commonly referred as ‘‘metallic glasses’’ or ‘‘glassy metals’’.BMGs have been paid great attentions for its theoretical and practical reasons since the bulk amorphous Pd–Cu–Si and Pd–Ni–P alloyswas first synthesized by water quenching method in 1974. More recently, batches of amorphous steel have been produced thatdemonstrate strengths much greater than conventional steel alloys. The most useful property of bulk amorphous alloys is that they aretrue glasses, which means that they soften and flow upon heating. This allows for easy processing, such as by injection molding, inmuch the same way as polymers. Bulk metallic glasses have been commercialized for use in medical devices, and as cases for electronicequipment

Table 6Glass types in flat glass industry and the designers spectrum of interests.

Designer’s interests Automotive/aerosapce

Architectureconstruction

Physics/chemistry/electronics

Flat glassindustry

Common glasses (basic anddecorative)

Float glass xBody tinted glass xReflective glass x xLow glass x x xMirror x x xInsulating glassEnameled/screenprinted glass

x

Pattern glass x xAntique mirror x

Special glass types Photovoltaic glass x x xX-ray protection glass x x xElectrically heated glass x x xElectrochromic glass x x xLiquid crystal glazing x xSelf-cleaning glass x x xSand-blasted glass xAcid-etched glass x xBent glass x xTempered glass x x xLaminated glass x x xFire-resistant glass x x xWired glass x x xAlarm glass x x xAntireflective glass x x x

1722 E. Axinte / Materials and Design 32 (2011) 1717–1732

of the mechanical properties has been observed. In any case, theviscoelastic nature of the polymer renders the response stronglydependent upon the load duration and environmental tempera-ture. However, the confinement effect produced by the attachedglass or metal enhances the mechanical strength of the bentappendix, which results much higher than that of the plain poly-mer especially at relatively high temperatures. The post-glass-breakage testing shows that the interlayer remains attached tothe metallic holders even after complete breakage of both glassplies, thus acting as a confining membrane that prevents thedetachment of the glass fragments.

In [6], Fujimoto describes a new infrared luminescence frombismuth (Bi)-doped glass. In this work, the author will introducethe basic properties of Bi-doped silica glass (BiSG), such as a phasediagram and spectroscopic properties, and then mainly talk aboutthe origin of luminescent center. After the discovery of a new infra-red luminescent bismuth center, several research groups started to

study its applications, such as optical amplification or laser oscilla-tion using Bi luminescent materials. Optical amplification around1.3 lm with Bi-doped multicomponent glass fiber is useful formetropolitan area network optical amplifiers [7].

A large study about advances in multicomponent silicateglasses and their glass–ceramics derivatives for dental applicationsis presented by ElBatal et al. [8]. X-ray diffraction patterns revealthe formation of lithium disilicate as a major phase together withother subsidiary phases precipitated during the crystallization pro-cess according to the other constituent oxides. Infrared spectrashow mainly characteristic bands due to silicate network.

In [9] is statistically analyzed the relation between the chemicalcomposition and the density of silicate glass melts at temperaturesof 1000–1400 �C. The analysis was carried out on all 140–260available values in the SciGlass information system for composi-tions containing more than 40 mol% silica, less than 40 mol% boronoxide, varying amounts of Al2O3, Li2O, Na2O, K2O, MgO, CaO, PbO,

Fig. 7. The Gecko system with indication of the bent polymeric interlayer [5]. (a) Section; (b) front view of a prototype.

E. Axinte / Materials and Design 32 (2011) 1717–1732 1723

and other minor components. A model based on multiple regres-sion was developed. The 95% confidence interval of the mean mod-el prediction on the density was 0.5–3%, depending on thecomposition of interest. The prediction of density as a function oftemperature made possible the estimation of the coefficient ofthermal expansion in the molten state to within 20–40% error witha 95% level of confidence. The effects of the composition of silicateglass melt density and thermal expansion are investigated: boronoxide, B2O3, decreases the density of silicate glass melts based onits low molecular mass; lithium oxide, Li2O, slightly decreasesthe density of silicate glass melts; alumina, Al2O3, clearly increasesthe glass melt density at high temperatures (1200–1400 �C); so-dium oxide, Na2O, does not have a strong influence on the glassmelt density within the studied temperature range because ofthe interplay between its medium molecular weight, its influenceon the thermal expansion, and component interactions. At1400 �C, adding Na2O appears to decrease the density, whereas atlower temperatures, the influence of Na2O addition is not readilyrecognized; potassium oxide, K2O, decreases the density of silicateglass melts; magnesium oxide, MgO, might not decrease the glassmelt density despite its low molecular weight, because it does notappear to increase the thermal expansion coefficient significantly;

Fig. 8. Glass viscosity example curves, derived from expe

calcium oxide increases the glass melt density due to its relativelyhigh molecular weight and the moderate influence on the thermalexpansion coefficient; lead oxide, PbO, has a very high molecularweight; therefore, it increases the glass melt density significantly.An example for glass designer’s use is given in Fig. 8 (from [9]).

The authors recommend that for glass design through propertymodeling, evaporation losses during glass batch melting and possi-ble influences of the oxidation states of transition metal oxidesmust be taken into account.

Advances in environmental glass industrialization are extractedfrom [10–12] and are listed as follows: Ref. [10] presents glassesobtained from melting mixtures of industrial wastes (panel glassfrom cathode ray tubes, mining residues from feldspar excavation,and lime from fume abatement systems of the glass industry). Mi-cro- and macrocellular sintered glass–ceramics were manufac-tured. Microcellular glass–ceramics, with a closed porosity, wereprepared by the direct foaming of the glass mass, determined byviscous flow sintering of fine powders (<37 lm), due to the addi-tion of a SiC-based waste (from polished glass articles). The surfacecrystallization of glass, upon sintering, limited the porosity (beingabout 50%), but imparted a remarkable crushing strength to theproducts (up to about 80 MPa), useful for construction applica-

rimental data (except lead crystal glass) – from [9].

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tions. Micro- and macrocellular glass–ceramics, with an openporosity and very low relative density (from 40% to less than10%), were prepared by the sintering of fine glass powders mixedwith Polymethyl Methacrylate micro-beads or deposited on poly-urethane sponges. The crystallization, besides imparting a goodmechanical strength, allowed the maintenance of the open-celledmorphology, useful for filtering applications. The production of cel-lular sintered glass–ceramics, both by direct foaming (exploiting aSiC-based waste) and by the usage of sacrificial polymeric materi-als, is also illustrated.

Ref. [11] summarizes the progresses and points out the direc-tions for the proper uses of waste glasses in Portland cement andconcrete. The use of recycled waste glasses in Portland cementand concrete has attracted a lot of interest worldwide due to theincreased disposal costs and environmental concerns. Being amor-phous and containing relatively large quantities of silicon and cal-cium, glass is, in theory, pozzolanic or even cementitious in naturewhen it is finely ground. Thus, it can be used as a cement replace-ment in Portland cement concrete. The use of crushed glasses asaggregates for Portland cement concrete does have some negativeeffect on properties of the concrete; however, particle applicabilitycan still be produced even using 100% crushed glass as aggregates.The main concerns for the use of crushed glasses as aggregates forPortland cement concrete are the expansion and cracking causedby the glass aggregates.

National Academy of Engineering (NAE – Washington, DC) iden-tified glass and glass–ceramics as central to many of the great engi-neering achievements of the 20th century, as development of solidstate lasers and optical glass fibers, biomaterials, glasses for imag-ing technologies, glass films in microelectronic devices [12]. In2008, NAE also identified The Grand Challenges for Engineeringfor 21st Century (Fig. 9).

Some of these challenges offer great opportunities for glass sci-entists and manufacturers to develop their role of improving thehuman condition, to contribute to the development of new andsustainable sources for energy and to develop techniques that en-hance the environment.

A complete survey of environmental, energy-saving and pro-duction applications of glasses is given by Brow and Schmitt[12]. In this paper, authors review some of the opportunities forthe development and use of glass to address future energy andenvironmental challenges. Some of these applications will require

Fig. 9. Schematics of The Grand Challenges for Eng

large-scale manufacturing of glasses that are closely related to cur-rent products, and other applications will require materials re-search to develop new compositions and forms that meetstringent engineering requirements.

4.1.1. Glasses and solar energySolar energy applications include thermal, photovoltaic, and

photochemical energy conversion, and the scale of solar systemsrange from power plants to portable, individual power units.Intrinsic physical, chemical, and mechanical properties of glasses(optical transparency, chemical durability, strength, thermalexpansion coefficient, manufacturability, etc.) make them indis-pensable for all solar energy systems. The specific function of theglass component is common to all solar systems: glass transmitsdesirable solar radiation to an active component (photovoltaic cell,thermal storage unit, etc.) while providing chemical and structuralprotection of that active component from the ambient conditions.

The engineering conditions for glasses in solar energy systemsare synthesized in Table 8 (adapted from [12]).

The performances of solar power units can be improved byimproving the design of the glass unit. Glass covers for photovol-taic units can be patterned to help concentrate and guide the solarenergy to the photovoltaic layer. Reflective layers coated on thepatterned, back surface of the cover further guide solar radiationto the photovoltaic cells by means of total internal reflection. Mir-rors are commonly used in solar power units, and researchers tryto find the best solutions for improving their durability and effi-ciency (silvered glasses, polymer-coated sheet aluminum, silveredpolymer films). Other solar power applications drive to importantglass development and manufacturing as building-integratedphotovoltaic (BIPV). BIPV have been developed to incorporateenergy-efficient structures into building fronts and facades andare produced by encapsulating and alternating with transparentresin solar cells between glass sheets. In this way is producing elec-tricity and is reduced solar heat gain while still allowing daylightto enter the building. Because of their large thermal inertia, glassesare used in other forms to rapidly capture solar energy and torelease them slowly: hollow glass beads, glass wool, etc. The inte-gration of inorganic synthetic methods with a size reduction to thenanoscale has lead to the creation of a new class of optical report-ers, called quantum dots. These semiconductor quantum dot nano-crystals have emerged as an alternative to organic dyes and

ineering for 21st Century (adapted from [12]).

Table 8The engineering conditions for glasses in solar energy systems (adapted from [12]).

Desiredproperties

Descriptors Research, development and design challenges

Optical properties � Maximum transparency over desired wavelength ofsolar light (UV, IR)

Compositional control for trace transition metal contents

� Reduced refractive index (for minimum reflection loss) Nanocomposites, glasses and glass–ceramics with engineered transparencies� Filtering non-desirable UV wavelengths New materials for optically active substrates (infrared luminescence)� No solarization effects

Mechanicalproperties

� Minimum density at maximum strengths Stronger, ‘less brittle’ glasses and glass–ceramics� Improve fracture toughness Thermal/chemical/mechanical treatments to improve strength and resistance at

environmental events (sand storms, etc.)� Thermal expansion characteristics of system

Chemicalproperties

� Maximized weathering resistance Improved resistance at corrosion to acid rains or environmental salinity

Manufacturingproperties

� Viscosities, thermal stabilities that complement specificmanufacturing process

New manufacturing techniques for customized designs and compositions

E. Axinte / Materials and Design 32 (2011) 1717–1732 1725

fluorescent proteins and are brighter and more stable againstphotobleaching than standard fluorescent indicators. Quantumdots have tunable optical properties that have proved useful in awide range of applications from multiplexed analysis such asdetection of nucleic acids (DNA) and cell sorting and tracking tomost recently demonstrating promise for in vivo imaging and diag-nostics [14]. By introducing quantum dots (crystalline, semicon-ductor nanostructures, as CdSe/ZnS, which absorb light and emitphotons of lower frequencies) directly in transparent glass matrixor in films incorporated by glass sheets are obtained solar collec-tors that can be integrated in building facades.

4.1.2. Glasses and water purificationSolar-driven technologies make use of glass as substrates for

photocatalytic decontamination methods, and glass windows andtubes enable the operation of both photocatalytic and desalinationsystems for producing clean water. Photocatalysis using glass sub-strates – the electrophoretic coating method – was used to deposittitania onto glass substrates that had been coated with indium-doped tin oxide to improve conductivity. The glass substrates werefound to have a significant advantage over other options due totheir transparency to UV-A light and their chemical stability. Thistransparency allows for illumination from either side of the coatingand increases the surface area available for photocatalysis. Furtherimprovements in efficiency could be realized when glass sheets arereplaced by glass beads and fiberglass mesh to increase the activesurface areas of the photo-reactors.

Fig. 10. Double basin solar still for desalina

4.1.3. Glass for water desalination and hydrogen productionTransparency to solar radiation is essential for solar collection

systems and thus solar stills generally consist of glass covers. Theglass covers also act as condensation surfaces and are usuallyangled to aid in distillate collection. The most basic types of solarstills are single and double basin (Fig. 10 – adapted and modifiedfrom [12]).

In [13] authors describes the using of amorphous silicone car-bide for construction of an photoelectrode for hydrogen production(SiC:H) using the sun light. The simulation results which indicatethat a SiC:H as a photoelectrode in a photovoltaic (PV – SiC:H)structure could lead to a efficiency of solar heat conversion greaterthan 10% are also presented.

4.1.4. Glasses and wind energyGlass fibers reinforced composites are commonly used for wind

turbines. The fatigue resistance of these composites is crucial forthe maintenance and reliability of turbine. That gives the opportu-nity for designers to create new, ultra-high strength, high-modulusglass compositions for these applications, including environmen-tally friendly, boron-free glasses. In [15], authors made a compar-ison at chemical attack for basalt and E-glass fibers, under acomplex chemical attack. Basalt and glass fibers were treated withsodium hydroxide and hydrochloric acid solutions, respectively, fordifferent periods of time. Both the mass loss ratio and the strengthmaintenance ratio of the fibers were examined after the treatment.Some results, obtained under acid attack conditions are presentedin Figs. 11 and 12 (reproduced from [15] – �Elsevier).

tion (adapted and modified from [12]).

1726 E. Axinte / Materials and Design 32 (2011) 1717–1732

For the basalt fibers, the acid resistance was much better thanthe alkali resistance. Nevertheless, for the glass fibers, the acidresistance was nearly the same as the alkali resistance.

Technical report [16] proposes a hybrid glass–reinforced epoxycomposite as a good material for Passenger Car Bumper Beam. Envi-ronmental constrains make the hybridization of synthetic withagro-based structures become a reality in the near future. Thesehybrids are designed for non-structural and semi-structural com-ponents. Experimental setup from [16] is focused on a hybrid of ke-naf/glass fiber to enhance the desired mechanical properties for carbumper beams as automotive structural components with modi-fied sheet molding compound (SMC). In this research, authors de-velop a hybrid material using natural kenaf and synthetic glassfiber as reinforcement. Kenaf is extracted from the bast of the an-nual fast-growing plant named Hibiscus cannabinus. The main con-stituents of kenaf are cellulose (45–57 wt.%), hemicelluloses(21.5 wt.%), lignin (8–13 wt.%), and pectin (3–5 wt.%). E-glass fiberhas been extensively applied as a reinforcement in polymer com-posite, and its good mechanical properties and low price areimportant considerations. Epoxy is selected as the matrix for thehybrid material. A specimen without any modifier is tested andcompared with a typical bumper beam material called glass mate-rial thermoplastic (GMT). The results indicate that some mechani-cal properties such as tensile strength, Young’s modulus, flexuralstrength, and flexural modulus are similar to GMT, but impact

Fig. 11. Mass loss ratio-treating time behavior (acid treatment) (reproduced from[14]).

Fig. 12. Strength maintenance ratio-treating time behavior (acid treatment)(reproduced from [15]).

strength is still low and shows the potential for utilization of hy-brid natural fiber in some car structural components such as bum-per beams. The comparison charts (Fig. 13) show some mechanicalproperties advantages (higher elasticity and tensile/flexuralstrengths) and disadvantages (lower impact resistance and highermass density) of hybrid material vs the common bumper beammaterial (GMT).

4.1.5. Glasses as immobilizers for nuclear wastesA big challenge in nuclear-related industries is that the radioac-

tive wastes generated must be isolated and safely stored. Glass isthe material of choice for waste forms because it can be designedwith good chemical durability, good mechanical properties, andsuperior radiation and thermal stability than other waste forms.The vast compositional variability of glass makes it possible toincorporate the many different nuclear waste compositions, andglass forming and processing allow for easy, large-scale productionof waste forms. For these dangerous wastes, as immobilizers aregenerally used the borosilicate glasses. Borosilicate glass is aparticular type of glass, known under the brand names Pyrex orKimax. It was first developed by German glassmaker Otto Schottin the late 19th century and sold under the brand name ‘‘Duran’’in 1893. After Corning Glass Works developed Pyrex in 1924, it be-came a synonym for borosilicate glass in the English-speakingworld [17]. Borosilicate glass is the oldest type of glass to haveappreciable resistance to thermal impact and higher temperaturesand also has excellent resistance to chemical attack. In this glassstructure, a little of the SiO2 is replaced by boric oxide B2O3 (13%).

Typical algorithm process in vitrification facilities for radioac-tive wastes is shortly presented below: radioactive elements areseparated from the waste stream, mixed with a borosilicate glassfrit, and melted at �1200 �C; the molten waste glass is cast intolarge stainless steel canisters (approximately 1500 kg each); thecanisters are decontaminated, welded shut, and stored at the vitri-fication facility; the canisters are moved to geological repositories[18]. Recent, the iron phosphate glasses are investigated as alterna-tive for nuclear waste immobilizers and are considered a ‘‘high pri-ority for US Department of Energy – DOE’’ [19]. Studies revealedthat iron phosphate glasses have excellent chemical durability,and some compositions have corrosion rates 1000 times lowerthan borosilicate waste glass.

4.1.6. Glasses as energy storage systemsThe possibilities to engineer and fabricate a material with a par-

ticular set of thermal, electrical, and chemical properties, into astrictly defined geometry, make from glass a crucial material to en-able these technologies for energy storage.

Glasses play important roles in smaller power supplies, includ-ing dielectrics for super-capacitors, electrolytes for electrochemicaldevices, and sealants for high-temperature solid oxide fuel cells(SOFC). In [20], glass–ceramic sealants free from barium oxide(BaO) and sodium oxide (Na2O) have been designed and investi-gated in the crystallization field of diopside (CaMgSi2O6). Further,the influence of Bi2O3 (different amounts 1, 3 and 5 wt.%) additionon the flow properties, sintering, and crystallization behavior alongwith electrical conductivity and long-term thermal stability ofsealants has been investigated. The investigated glass–ceramiccompositions were highly effective in performing metal-to-metalsealing with smooth interface and negligible interfacial reactions,thus proving them to be potential sealants for applications in SOFC.The amount of amorphous character in the glass–ceramics in-creases during prolonged heat treatments at SOFC operating tem-peratures, which can be beneficial to provide self-healing abilityto the sealant.

Glass–ceramics are suitable for use in super-capacitors, andsome glasses are investigated as solid electrolytes in lithium

Fig. 13. Comparison charts of mechanical properties GMT vs Hybrid (adapted from [16]).

E. Axinte / Materials and Design 32 (2011) 1717–1732 1727

batteries. A recent study of controlled crystallization of lithiumaluminum silicate (LAS) glass–ceramic shows that with the opti-mum nucleating agents (2%ZrO2 + 2.36%TiO2), LAS glass–ceramicswith fine grain, high transparency and good mechanical propertieswere obtained, due to the b-quartz solid solution formed after thecrystallization process [21]. A simple and low cost manufacturingtechnique for obtaining the glass–ceramic matrix composites isdescribed as follows [22]: sintering with simultaneous crystalliza-tion of fine glass powders allowed the preparation of denseglass–ceramics based on unusual feldspar crystals, at a very lowtemperature (750 �C) and with limited processing times. Al2O3

platelets were added to the glass powders. The observed effectsare improvement in bending strength (exceeding 100 MPa),microhardness (approaching 9 GPa), and fracture toughness – evenfor limited concentrations (which varied from 5 to 15 vol.%). Theelastic modulus and coefficients of thermal expansion (CTE) aremajor contributors to the stresses that can lead to cracking of theseal. The model presented by Milhans et al. [23] is useful fordesigners of glass–ceramic SOFC seal materials : the effective elas-tic properties and CTE of a glass–ceramic were predicted usinghomogenization techniques. Using G18 (a glass–ceramic SOFCsealant) as an initial reference material, the effectiveness of differ-ent homogenization models was investigated for a two-phaseglass–ceramic. The predictive model offers accurate macroscopicvalues on both the elastic modulus and the CTE of glass–ceramicmaterials, provided the estimated amorphous values arereasonable.

Hollow glass bubble technology was developed by 3M in the1960s, and today this material is used in aerospace and militarysystems, paints and coatings, marine hulls, oil and gas explorationand production and other industrial uses. Glass bubbles are madefrom a chemically stable soda–lime borosilicate glass and pro-duced in a range of properties, including nominal diameters from9 lm to 70 lm, densities from 0.15 to 0.60 g/cm3, and compressivestrength ratings from 1.7 MPa to 20 MPa. Resin compounder NoblePolymers (Grand Rapids, Michigan, subsidiary of Cascade Engineer-ing) has developed a low-density polyolefin resin formulation thatreduces the weight of thermal plastic olefin (TPO) parts by up totwenty percent. This masterbatch bulk resin additive incorporateshollow glass bubbles from 3M Company to displace resin and re-duce part density in injection molded, thermoformed, and ex-truded thermoplastic parts.

Recently, hollow glass micro-spheres (HGMS) are considerate asa viable alternative for hydrogen storage [12]. Some advantages forusing HGMS as hydrogen storage and transportation containers are(a) the materials are cheap and most of the batch consists of recy-cled cullet; (b) the micro-spheres are non-explosive and each indi-

vidual sphere contains a small volume of hydrogen to be hazardousif the sphere break; (c) HGMS are strong due to their small size andare able to contain hydrogen at high pressures – up to 100 MPa; (d)no additional transportation vessel is needed, so they serve aslightweight containers for relatively high hydrogen mass density;(e) HGMS are reusable and recyclable. For loading with hydrogen,the glass spheres (produced by different methods, as flame spraypyrolysis of glass frit) are inserted into a high-pressure hydrogenenvironment and are heated enough for diffusion of hydrogen intospheres. The loaded spheres are extracted from hydrogen environ-ment and cooled at ambient temperature.

4.2. Advances in bulk metallic glasses (BMG) design and manufacture

Metallic glasses exhibit unique softening behavior above theirglass transition, and this softening has been increasingly exploredfor thermoplastic forming of metallic glasses. Metallic glasses canbe patterned on extremely small-length scales ranging from10 nm to millimeters. It is possible that this may solve the prob-lems of nanoimprint lithography where nanomolds made of siliconbreak easily. Nanomolds made from metallic glasses are easy tofabricate and more durable than silicon molds [24]. The processof making nanoscale devices by simple stamping or molding mayrevolutionize the manufacture of nanodevices from computermemory to biomedical sensors.

Metallic glass (Ti40Cu36Pd14Zr10) is considered non-carcino-genic, is about three times stronger than titanium, and its Young’sModulus matches bones. It has a high wear resistance and does notproduce abrasion powder. The alloy (Mg60Zn35Ca5) rapidly cooledto achieve amorphous structure (BMG) is being investigated as abiomaterial for implantation into bones as screws, pins, or plates,to fix fractures [25].

The first reported metallic glass was an alloy (Au75Si25) pro-duced at Caltech by Klement Jr., Willens, and Duwez in 1960. Thisand other early glass-forming alloys had to be cooled extremelyrapidly (on the order of 1 � 106 K/s) to avoid crystallization. Animportant consequence of this was that metallic glasses could onlybe produced in a limited number of forms (typically ribbons, foils,or wires) in which one dimension was small so that heat could beextracted quickly enough to achieve the necessary cooling rate. Asa result, metallic glass specimens (with a few exceptions) werelimited to thicknesses of less than 100 lm. Bulk metallic glasseshave been paid great attentions for its theoretical and practical rea-sons since the bulk amorphous Pd–Cu–Si and Pd–Ni–P alloys wasfirst synthesized by water quenching method in 1974.

Ref. [26] describes how copper was inserted into hard magneticbulk metallic glass Nd–Fe–Al alloys. The effects on the magnetic

1728 E. Axinte / Materials and Design 32 (2011) 1717–1732

properties and glass-forming ability (GFA) were assessed usingvibrating sample magnetometer and thermal expansion method.The as-cast alloys exhibited higher GFA ability and hard magneticproperties at room temperature. The enhancement of GFA is attrib-uted to more negative mixing heat between Cu and Nd, andaccordingly, less crystallite and more Fe-rich clusters result inhigher coercivity up to 313 kA/m. Adding more Cu, however, willcause an abrupt reduction in coercivity for the precipitation of softmagnetic and non-magnetic crystalline phase.

The effects of alloy addition on the glass-forming ability, ther-mal stability, and mechanical properties of Mg–Cu–Tb-based bulkmetallic glasses were investigated in [27]. The Mg-based BMG havereceived intensive attention due to their high specific strength andrelatively low cost. Mg–Cu-based BMG could be fabricated by con-ventional copper mold casting method in air atmosphere withoutobviously decreasing the GFA. These BMGs exhibit higher specificstrength comparing to their crystalline counterparts. In [27],authors investigated the effects of alloy additions on the GFA andmechanical properties of a typical glass-forming composition,Mg65Cu25Tb10, which approaches the ideal brittle behavior associ-ated with silicate glasses The authors demonstrate that appropri-ate additions of Ag, Zn or Be in Mg65Cu25Tb10 could not onlyimprove the glass-forming ability but also enhance the strengthand plasticity of these amorphous metallic alloys within a certaincomposition range.

Zirconium-based metallic glasses (Zr-based BMG) present goodmechanical properties that combines the high fracture stress, elas-tic strain (up to 2%), significant fracture toughness, and good corro-sion resistance. In [28] is studied the influence of aluminumcontent on the GFA and on the mechanical properties of theZr–Ni–Cu–Ti alloys. Multicomponent Zr75�xAlxNi10Cu10Ti5

(x = 15–20 at.%) alloys were produced by melt spinning methodobtaining ribbons, and by casting technique into a copper mold,manufacturing rod-shaped samples with a maximum diameter of2 mm. Supercooled liquid region depends on chemical compositionand exceeds 45 �C. Vickers microhardness of studied alloys iscomparable to the highest ones for other Zr-based BMG.

The glass-forming ability (GFA) is one of the most importantfactors in the study of metallic glasses. No standard definitionhas been made for this parameter up to now, and many indicatorshave been developed. A review of advances in characterization ofGFA is given by Bing et al. [29]. In this work, four types of criteriafor the glass-forming ability are categorized and reviewed: (1)indicators with characteristic temperatures; (2) indicators involv-ing structural factors; (3) indicators based on Miedema’s model;and (4) indictors based on phase diagram. Authors underline thata single indicator cannot be used to predict GFA of all the metallic

Fig. 14. The illustration of ab

glass systems. Some techniques as CALPHAD predict and describewell the glass-forming and new glass-forming composition regionsin some multicomponent alloy systems. The authors final conclu-sion are that the formation of metallic glass is a non-equilibriumprocess, and studies in view of equilibrium theories may lead toincorrect results and that is a long way to develop proper indica-tors for GFA, which are theoretically strict and composed of verysimple and fundamental parameters.

4.3. Advanced techniques of manufacturing for glass and glass–ceramics

Sometimes, glass pieces require some supplementary opera-tions as drilling, cutting, grinding, and stamping for meeting theirfunctional or aesthetic role.

The mechanical and physical properties of glasses (especiallybrittleness and low ductility) make them very difficult or impossi-ble to manufacture them by conventional mechanical processes.

Drilling of holes in glasses is possible to non-tempered glassesand ceramics. The glass bits have a spade-shaped point and aremade of tungsten carbide to withstand the friction of drilling inglass. Diamond drillers are also used for holes with diameter smal-ler that 10 mm.

Conventional glass cutting is done by scoring and breakingwhich produces micro-cracks and splinters and leaves cutting oilresidues. These influences lead to lowered strength and pollutionof the glass sheets. If the glass has to be tempered, further grinding,polishing, and washing processes are needed.

Cutting still remains one of the most common operations forglasses. Old-fashioned hand-cut glass remains indispensable forany glass-processing workshop but today’s technologies allow usto cut glass with machines.

Waterjet cutting technology can be described as a controlledaccelerated erosion process. This system consists of high-pressurewater, mixed with abrasives, that passes through a gauge orifice atthree times the speed of sound. Such pressure produces a pureworking power able to cut any shape of glass or other materials.In comparison with the traditional diamond cutting system, ithas the following advantages: flexibility, high accuracy, works onany material, low cost, lack of heat generation. Short communica-tion [30] reports a particular application of abrasive waterjet (AVJ)turning that proved its technological and economical capability, i.e.profiling and dressing of grinding wheels with ceramic bonds. Theschematic process and the experimental setup and is described byFig. 14 (adapted from [30]).

The results are very suggestive illustrated by Fig. 15. This tech-nology seems to be attractive for those manufacturing sectors for

rasive waterjet turning.

Fig. 15. Some results of waterjet turning.

E. Axinte / Materials and Design 32 (2011) 1717–1732 1729

roughing and semi-finishing off-the-shelf grinding wheels for theproduction lines that require mass utilization of specially profiledgrinding wheels.

Laser cutting of flat glass is a subject of a handful of specialists.In practice use for daily production, today there are approximately50 installations for flat glass laser cutting worldwide and the samenumber in research laboratories. The main problem is that the

Fig. 16. Laser-induced scoring by tension – schematic method (from [31]).

Fig. 17. Microscopic views of brut (a) and fi

glass tolerates only a certain amount of energy per square per time.If this value is too large, the glass breaks or gets a grainy structurein the heated zone. To meet this requirements, many patents areknown, all of them try, by forming a special laser spot, to optimizethe energy placement. The invention called LiST (laser-inducedscoring by tension – Grenzebach Maschinenbau GmbH) has to con-trol not only the laser energy and spot size but also its timing in away that more heat on the glass surface can flow to deeper areasinto the glass, where finally a multiplicative energy rate can createmany times higher tension in the glass (see Fig. 16 – from [31]).

The technical limit in cutting speed is the mechanical possibilityto move a small cooling head along the score trace. There is an eco-nomical limit too: double speed requires double laser power,which is very expensive to buy.

Development of advanced ceramics has gained significantimportance because of their desirable properties. Their engineeringapplications are still limited owing to the limitations in developingoptimized (minimum damages, high rate of material removal, min-imum roughness, etc.) machining techniques.

In [32], a cylinder from magnesium–silicon-vitrified ceramic(Steatite 63–63 HRC with initial surface roughness Ra = 3.25 lm)was plunge grinded with a ceramic bonded abrasive wheel (66A16K 5 V217 – pink Al2O3). At constant work piece velocity(vw = 14 m/min) and constant plunge feed (ft = 0.03 mm/min), dif-ferent cutting speeds were used (vd = 30; 50 and 70 m/s). Good re-sults was revealed at cutting speed 70 m/s by obtaining a finalsurface roughness Ra = 0.8 lm and a finished surface as in Fig. 17b.

Experimental study [33] investigates the grinding characteris-tics and surface and subsurface integrity of ground silicon carbide

nished (b) ceramic surfaces (from [32]).

1730 E. Axinte / Materials and Design 32 (2011) 1717–1732

to explore the material removal and damage formation mecha-nisms involved during high removal rate grinding. The experimen-tal investigations revealed that an increase in material removalrate did not affect surface finish and surface morphology.

Imprinting technology is an ancient technique for the reproduc-tion of writings on appropriate supports. Since 1990s, one of theimprinting techniques, i.e., injection molding, has been used forcompact disk (CD) production. More recently, the semiconductorindustry is interested in imprint related techniques because ofthe mass production requirement of future microelectronic circuitswith a possible critical dimension down to a few nanometers [33].Up to now, the process has found its way into scientific researchdomains and production lines in micro-optics (Fig. 18) and intoprototyping for nanophotonics and biotechnology and is also ex-pected to have additional impact in fabrication of memory devicesand in display technology.

Nanoimprint lithography (NIL) was introduced in 1995 byStephen Chou. He demonstrated results from an experiment wherea laboratory press was used to press together a patterned stamp,made from a SiO2-coated Si-wafer, with a silicon substrate coatedwith a thermoplastic polymer [34]. The NIL process is described

Fig. 18. 300-mm soft stamp replicated lens wafer and 200-mm stacked wafer levelcamera module (from [33]).

Fig. 19. The thermal nanoimprint li

very well by Fig. 19 (adapted from [35]). It is based on the use ofthermoplastic polymer spin coated on the substrate. The thermo-plastic polymer is heated above the glass transition point of thepolymer, and the heated template is brought into contact withthe polymer. Once the polymer has filled all the cavities of the tem-plate, the substrate and the template are cooled down and the tem-plate is separated from the substrate. A negative replica of thetemplate is created on the polymer. In order to use imprinted poly-mer for pattern transfer to other layers on the substrate, polymerleft on the indented areas has to be removed. Because the flow ofthe polymers is not free of resistance, a residual layer appears(Fig. 19d).

5. Conclusions and strategic lines in glass development

Glass as a material will always exist, but many new applicationsand manufacturing processes will involve glass in combinationwith other materials. Optical fibers are currently manufacturedwith one or more different coating, which are often plastics. Withthe increasing sophistication of optoelectronic devices, there is anincreasing need to combine optical and electronic devices for manyapplications such as transmission of audio, video, and data infor-mation. Glasses and ceramics (stand-alone or composite with other

thography (adapted from [33]).

Fig. 20. Foil of flexible ceramic ZircoFlex™.

Table 9Criteria for material selection.

Criteria Ratings

1 Supply property (depletion time, years) A < 25; B – 25 to 50; C – 51 to 100; D > 1002 Recycled supply property (% of recycled material used) A – 100; B – >50; C – 25 to 50; D < 253 Embedded energy property, MJ/kg of material A < 50; B – 50 to 99; C – 100 to 200; D > 2004 Environmental impact property, scorecard hazard rating A < 25; B – 25 to 50; C – 51 to 75; D > 755 Legal property (legal restrictions) A – no problems; B – minor problems; C – serious legal problems; D – under severe legal restriction6 Longevity (grade of degradation in environment) A – quickly degrades; D – unlimited lifetime7 Recyclable property (% material recyclable) A – 100; B – 50 to 100; C < 50; D = 08 Social and political property A – no social or political problems; D – social malign; political restrictions

E. Axinte / Materials and Design 32 (2011) 1717–1732 1731

materials) will find increasing application in biological and medicalareas. Materials such as photochromic, electrochromic, and ther-mochromic glasses, which respond to external stimuli, are beingdeveloped with various, sometimes unusual, applications. Revolu-tionary materials, as the flexible ceramic heat shield material (Zir-coFlex), are recently developed and fabricated by using a newtechnology in which the ceramic material is sprayed in the formof thousands of individual ‘platelets’ onto the surface of the alumi-num backing foil (see Fig. 20). Early applications for ZircoFlex™ foilare coming from the automotive industry, where the foil can beused to protect sensitive components from heat in increasinglycrowded engine bays.

Al Gore (former US vice president) became the publicly recog-nizable face of the environmental issue after his Oscar-winningdocumentary movie An Inconvenient Truth (2006). This documen-tary helped enormous to make the issue of global warming a recog-nized problem worldwide. By winning of Nobel Peace Prize in 2007,Al Gore brought once more to the forefront the problem of globalwarming. The first impact on glass industry was produced byswitching on large scale to compact fluorescent light bulbs (bythe replacing of incandescent bulbs 4200 + tons of carbon were off-set). Glass industry has successfully reacted to environmentalissues by offering a variety of applications to make buildings moreenergy efficient and ecologically friendly. Today’s glass can be prac-tically custom-made to fit into any environmental conditions andoffer specific appearances and performance. The latest develop-ment in the industry has been the introduction of self-cleaningglass. While progress in the glass industry continues, we can expectglass in the near future that will react to external stimuli, the so-called ‘‘smart glasses’’, offering maximum comfort and excellentenergy efficiency inside buildings. Some glass applications alsouse alternative natural resources to preserve the environment inwhich we live. Some states have begun to regulate or even stimu-late the introduction of such energy-preserving applications [36].

The modern glass industry have a huge contribution throughdevelopment and the introduction of new energy-efficient glassproducts and applications, some issues in glass manufacturingand processing still must be addressed in the future.

A great challenge for the actual and the future glass industry isto increase the contribution to environment preservation. Accord-ing to the criteria of environment-friendly selection of materials(Table 9 [37]), the weakness of glass is the ‘‘embedded energy’’.The embedded energy for glass mass unit can be rated at D(>200), the worst as possible.

The challenge for researchers and glass industry is to act for sig-nificant decreasing of embedded energy in glass mass unit. Thereare two possible ways to solve this desiderate: (a) continuousdeveloping of high-efficiency furnaces; (b) continuously develop-ing of new glass compositions, with lower glass transition temper-atures. The synergetic effect of simultaneous applying of both wayscould offer a good future for glass industry. Most recent tendenciesare to use the green energy (wind and solar) in producing andmanufacturing of glasses.

The modern glass industry must increase contribution throughdevelopment and the introduction of new energy-efficient glassproducts and applications, some issues in glass manufacturingand processing still must be addressed in the future.

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