Pyrolytic graphite and vitreous carbon Most of the graphitic carbons produced for industry

10.4.6.7 Pyrolytic graphite and vitreous carbon Most of the graphitic carbons produced for industry

are polycrystalline and, as indicated previously, often exhibit bulk anisotropy, usually as a result of the shap- ing process. Anisotropy is the outstanding character- istic of individual graphite crystals; in fact, graphite can exhibit more anisotropy in certain properties than most other types of crystal. In polycrystalline prod- ucts, it has often been necessary to cancel out dan- gerous anisotropic effects by deliberately randomizing the orientation of the grains (e.g. moderator graphite for nuclear reactors). A converse approach has led to the successful production of graphite with enhanced anisotropy (i.e. pyrolytic graphite (PG)). Compared to

conventional graphite, PG is less chemically active and mechanically stronger. It can also be machined to close dimensional tolerances. In the 1960s, for the first time, it became possible to produce pieces of near-perfect graphite that could be used to explore the physics and chemistry of anisotropy in greater detail. 1

In the basic method of production, pyrolytic graphite is deposited by bringing a mixture of hydrocarbon gas (methane, CH 4 , or propane, C 3 H 8 ) and an inert car- rier gas into contact with a substrate material which is heated to a temperature of approximately 2000 ° C. Molecules of the hydrocarbon are thermally ‘cracked’ within the boundary layer of flow at the substrate

surface. (e.g. CH 4 D C gr C 2H 2 . As indicated previ- ously, the physical character of the deposit is deter- mined by the process conditions, such as temperature, partial pressure of hydrocarbon gas, substrate mate- rial and area. These conditions, which are extremely critical, decide whether the deposit-forming carbon reaches the substrate surface as individual atoms, planar fragments or as three-dimensional clusters. Thus, planar fragments form laminar deposits which are highly anisotropic whereas larger clusters form isotropic deposits with little or no overall anisotropy. If the substrate takes the form of a resistance-heated carbon mandrel, a tubular deposit can be produced.

Pyrolytic graphite, as-deposited, consists of crystal- lites which exhibit some degree of disorder as well as stratification. In order to explain the nature of its struc- ture, it is helpful to compare it with that of graphite in its most stable form. The perfect crystal has a reg- ular ABABAB. . . stacking of hexagonal f0002g layers in which the distance of closest approach between layers is approximately 0.335 nm. This exact registry of hexagonal planes, with its ABABAB. . . mode of stacking, is not found in pyrolytic graphite. Instead, synthesis by chemical vapour deposition (CVD) pro- duces a ‘turbostratic’ arrangement of layers in which the hexagonal networks, although still parallel, do not register precisely with each other (Figure 10.15). This disregistry has been likened to a pack of play- ing cards in which the long and short edges are each misaligned. Such structures are both disordered and stratified, hence the term ‘turbostratic’. Each crystallite comprises a number of hexagonal layers and the inter- layer distance is greater than the ideal of 0.335 nm. It is useful to assign a c-axis to the stack of layers form- ing each crystallite and to determine average values

for this dimension ⊲L c ⊳ by X-ray diffraction analysis. In general terms, a turbostratic structure in graphite is envisaged as a conglomerate of crystallites sepa- rated by fairly stable transition zones of carbon atoms. If annealed at a very high temperature, say 3200 ° C,

1 Professor A. R. J. P. Ubbelohde, FRS (1907–1988) and co-workers at Imperial College, London, produced valuable

prototypes and made many important contributions to graphite science (e.g. stress-recrystallization, intercalation compounds).

342 Modern Physical Metallurgy and Materials Engineering

Figure 10.15 Structures of (a) crystalline graphite and (b) turbostratic carbon (from Cahn and Harris, 11 Jan, 1969, pp. 132–41; by permission of Macmillan Magazines Ltd) .

a turbostratic structure will ‘heal’ and approach the as high as 200 can be produced by encasing a PG stable ABABAB. . . configuration.

deposit in an envelope of commercial polycrystalline Various other methods are available for producing

graphite and hot-pressing at temperatures in the order pyrolytic graphite. Composite structures for aerospace

C. The envelope deforms plastically and the applications have been produced by depositing

of 3000 °

compression forces align the crystallites. In laminar pyrolytic graphite on mats of carbon fibre. In the

forms of pyrolytic graphite, the anisotropy is such field of nuclear reactors, fluidized bed techniques have

that the material acts as an excellent conductor in been used to coat fuel particles of uranium dioxide

directions parallel to the hexagonal networks and as with separate PG layers of different anisotropy. In a

an equally efficient insulator in the c-direction. Thus, fluidized bed, a rising flow of hydrocarbon gas and

the temperature gradient in the direction perpendic- inert gas levitates the charge of particles; the bed

ular to the hexagonal networks can be very steep temperature is very uniform and close temperature

C mm 1 ). One practical consequence of this control is possible. Fluidized beds operating at a

(say, 300 °

gradient occurs during the growth of a PG deposit relatively low temperature of 1500 °

on a heated mandrel, when the temperature at the used to coat prosthetic devices such as dental implants

C have also been

gas/deposit interface gradually decreases, causing the and disc occluders for heart valves with a 1 mm layer

structure of the deposited carbon to change as well. of carbon. It is necessary to polish these coatings

(This is one reason PG deposits are often very thin: before use in order to remove any weakening flaws.

the thickest deposits are usually about 30–40 mm.) On Their wear resistance and strength have been enhanced

the other hand, engineering advantage can be taken of by including silane in the bed atmosphere. Carbon is

the anisotropy of laminar PG. For example, pyrolytic one of the dominant chemical elements in the human

graphite was advocated by Ubbelohde for small cham- body and it is perhaps not surprising that it is a

bers or crucibles in induction furnaces operating at valuable prosthetic material, being compatible with

C. Chamber walls with living tissue and with blood and other body fluids.

temperatures of 2000–3000 °

their internal surfaces parallel to the a-layers favour Structural disorder, in its various aspects, is largely

lateral heat transfer and give temperature uniformity determined by the conditions of synthesis; thus the

within the working space, a feature usually sought by relative orientation of the crystallites to each other,

furnace designers. 1 At the same time, the external wall as developed by synthesis, may be parallel (laminar),

surfaces operate at much lower temperatures; radiation random (isotropic), etc. Both laminar and isotropic

losses in accordance with Stefan’s law (T 4 ) are min- deposits of pyrolytic graphite are hard and strong

imized, to the benefit of the thermal efficiency of the

because L c is small. (Typically, L c values for PG are

furnace unit.

about 3–10 nm.) Although highly anisotropic deposits are possible, further enhancement can be achieved

1 A tobacco pipe with a PG-lined bowl has been marketed; by ‘stress-recrystallization’. For instance, near-ideal

one would expect the high thermal conductance of its graphite with anisotropy ratios of thermal conductivity

interior to sustain slow combustion.

Ceramics and glasses 343 The glassy, low-porosity material known as vitreous

proved fruitful. As the interlayer distance in graphite is carbon is derived from the pyrolysis of a thermosetting

quite large, with relatively weak forces acting between polymer, rather than from a hydrocarbon gas (e.g.

layers, it is chemically convenient to regard each layer phenol formaldehyde resin). In a typical production

as a very large macroaromatic molecule. (This assump- procedure, a specially prepared resin is shaped by

tion is reasonable because the covalent C–C bond moulding, carbonized in an inert atmosphere at a

length of 0.1415 nm in the layers is similar to that found temperature of 900 °

C and finally fired in a vacuum in the aromatic organic molecule of benzene, C 6 H 6 .) at a temperature of 1800 °

Within individual layers of most forms of graphite, there gas are evolved; because of their disruptive effect,

C. Very large amounts of

are usually clusters of vacant sites which form ‘holes’. the thickness of the product is necessarily restricted

Although chemical reactions with foreign atoms which to about 5–8 mm. Shrinkage figures in the order of

penetrate between layers can take place at these ‘holes’, 20–50% have been quoted. These features demand

they are more likely to occur at the layer edges where close control of firing conditions.

free valency bonds project. For instance, attack by oxy- The structure of vitreous carbon inherits certain

gen is favoured when the edges of layers impinge on bonding characteristics from the complex organic pre-

free surfaces, voids or cracks. If reaction with for- cursor. Thus graphitization and ordering are inhibited.

eign atoms does occur within the layers, newly-formed The crystallites are much smaller than those developed

bonds can cause the layers to buckle. in pyrolytic graphite. They are only about 5 nm across.

In contrast to these rather random forms of The crystallites have a larger-than-ideal interlayer dis-

attack and reaction, it is possible to interpose tance of about 0.35 nm. It appears that cross-links

(intercalate) substantial numbers of foreign atoms or persist between the layers and that these bonds have

molecules between the parallel macromolecules to

a stabilizing and strengthening effect. Furthermore, form stoichiometric graphite compounds, as indicated because the crystallites are randomly oriented, vitreous

in Figure 10.16. (Ubbelohde coined the term ‘synthetic carbon is isotropic. Like silica glass, vitreous carbon

metals’ for this new class of material.) These is hard, brittle and fractures in a conchoidal manner.

intercalation compounds are produced by exposing Vitreous carbon is valued for its ability to with-

graphite to a vapour phase or chemical solution or by stand penetration and attack by aggressive liquids such

electrolysis. By using potassium as the intercalate, a as concentrated hydrofluoric acid, caustic alkalis and

family of potassium graphites has been produced (e.g. molten metals (e.g. Al, Cu, Pb, Sn. Zn). Its very low

C 60 K, C 48 K, C 36 K, C 24 K, C 8 K). This regular sequence porosity and high purity contribute to this inertness.

Generally, it has good resistance to attack by metals which do not form carbides. However, like graphite, it is attacked by alkali metals. In the chemical labo- ratory, vitreous carbon is used for crucibles, furnace boats, filters, non-stick burette taps, etc. in chemical analysis, crystal growing and zone-refining activities, often proving superior to traditional materials such as platinum, nickel, glass and quartz. Serving the elec- tronics industry, vitreous carbon ware has been used in the manufacture of gallium phosphide and arsenide for optical/microwave devices, germanium for transistors and special glasses for fibre optics. In fact, it was the urgent need for an inert container for gallium phos- phide which stimulated the development of vitreous carbon. In the chemical industry, it is used for rotating seals in pumps handling difficult liquids. In addition to being harder and stronger than all other forms of carbon, with the exception of diamond, it is conduc- tive and refractory at temperatures up to 3000 °

C and

can be used to make resistance- or induction-heated reaction vessels. Ultrasonic machining of vitreous car- bon is feasible, producing sound and clean-cut edges. (Graphite has a more layered structure and machining tends to round or break up edges.)

10.4.6.8 Chemical reactivity and intercalation of

Figure 10.16 Potassium graphite, C 8 K . Hexagon layers of

graphite

graphite separate and slip into the unstaggered sequence to permit intercalation of potassium atoms (from Ubbelohde,

Study of the manner in which foreign atoms interact 1964; by permission of BCURA Ltd., Coal Res. Establ., with and penetrate the layered structure of graphite has

Stoke Orchard, Cheltenham) .

344 Modern Physical Metallurgy and Materials Engineering of stoichiometric ratios shows that as intercalation

proceeds, the distance between the potassium-filled regions decreases stepwise in a systematic manner. The intercalates force the macromolecules apart and sometimes cause them to shear past each other, changing the ABABAB . . . stacking sequence to AAAA . . . Intercalation species include alkali metals, halogens, caesium and rubidium, nitrate, bisulphate and phosphate radicals.

Intercalation produces interesting electrical effects involving two forms of charge transfer. Graphite itself has moderate electrical conductivity at low tempera-

tures with about one charge carrier per 10 4 atoms of

carbon. (In copper, the ratio is 1:1.) Intercalation with

potassium to form potassium graphite ⊲C 8 K⊳ raises the

electron-donating alkali metal injecting electrons into the ‘empty’ band of energy levels in graphite. Con- versely, when bromine intercalates to form the com-

pound C 8 Br, the electrical conductivity again increases significantly as a result of the halogen atoms acting as electron acceptors and removing electrons from the

Figure 10.17 Spherical molecule of buckminsterfullerene ‘full’ electron band of graphite. In this case, the num-

(courtesy of Professor H. W. Kroto, University of Sussex, ber of carriers rises to about one in ten (Cve). The

UK) .

electron transfer associated with intercalation can pro- duce striking colour changes: the potassium graphites

of a truncated icosahedron to give a C 60 shell struc- golden bronze, respectively. These unique processes

C 24 K (intermediate) and C 8 K (saturated) are blue and

ture that is reminiscent of geodesic buildings and of charge transfer have provided valuable insights into

of a soccer ball (Figure 10.17). The C 60 molecule the chemistry of bonding.

was named buckminsterfullerene. 2 Larger fullerene As a general rule, near-ideal graphite intercalates

molecules have since been produced which have more more readily than less perfect forms; that is, the inter-

than 200 carbon atoms per shell. calate can penetrate more easily between the macro-

molecules without hindrance from interlayer bonds. The discovery of this third form of carbon, with Certain substances have the ability to ‘unpin’ the

symmetrical characteristics completely different to layers, acting as intercalation catalysts (e.g. iodine

those of diamond and graphite, prompted attempts monochloride). These catalysts have been used in

to evolve synthetic routes capable of producing solid large-scale chemical processing. Intercalation with

buckminsterfullerene in macroscopical quantities large halogens (bromine or chlorine) has been used to

enough for more detailed scientific study and for com- strengthen the vital bond between filaments and the

mercial exploitation. Powder and thin-film samples of resin matrix in carbon fibre ‘reinforced’ polymers.

a solid crystalline form, fullerite, were subsequently Intercalation forms the basis of rapid proving tests

produced in gram quantities. 3 In the basic method, a that have been used by the nuclear industry to sim-

sooty mixture of graphitic carbon, C 60 and C 70 is pro- ulate the action of neutron bombardment upon pro-

duced by striking an arc between carbon electrodes totype graphites. The intercalating bromine atoms

in an atmosphere of helium. The other fullerene, C 70 , force the layers of graphite to open up in the c-

which forms in smaller quantities than C 60 , appar- direction.

ently has elongated molecules. The soot contains about 5–10% of the two fullerenes. After extraction with

10.4.6.9 From buckminsterfullerene to fullerite benzene, micrometre-sized crystals of C 60 molecules form. Some disordering of stacked hexagonal arrays of

Circa 1985, examination of vaporized carbon led to the exciting discovery of a new and unexpected molecular

form. 1 Mass spectrometry revealed that stable clus- 2 Named after the American architect R. Buckminster Fuller ters of carbon atoms, relative molecular mass 720,

(1895–1983) who pioneered the geodesic building, a were present in significant quantity. It was postu-

revolutionary design form which encloses the maximum of lated that these molecules were spherical and cage- space with the minimum of materials. Many spherical viruses also have icosahedral symmetry.

shaped, with 60 carbon atoms forming the vertices 3 Solid C 60 was produced by collaboration between D. R. Huffman and W. Kratschmer, and their co-workers,

1 Discovered by Professor Harold W. Kroto and colleagues at the University of Arizona, USA, and the Max-Planck- at Sussex University.

Institut f¨ur Kernphysik, Heidelberg, Germany, respectively.

Ceramics and glasses 345

C 60 molecules is evident. STEM micrographs of mono- measuring the slope of the line. Although the above layers have been produced which clearly show hexag-

two expressions are valid for many types of glass, onally close-packed arrays of the spherical molecules

for certain glasses there are conditions of temperature (i.e. six-fold symmetry)

and stress which result in non-Newtonian and/or non- In parallel with its impact on organic and poly-

Arrhenian behaviour.

mer chemistry, C 60 stimulated many proposals in As indicated in Section 2.6, the viscosity of oxide the materials field. Proposed ‘buckyball’ products

glasses depends upon composition and temperature. have included lubricants, semiconductors and fila-

Generally, an increase in the concentration of modi- mentary reinforcement for composites. It has been

fying cations and/or a rise in temperature will cause suggested that the cage-like structure of individual

the viscosity to fall. Figure 10.18 shows the rela- molecules makes them suitable for encapsulating metal

tion between viscosity and temperature for a typical or radioisotope atoms, a form of molecular-scale pack-

SiO 2 –Na 2 O–CaO glass. Two key values of viscosity aging (e.g. La 3C ). There is also interest in bonding ions

are marked; they correspond to two practical temper- to their exterior surfaces.

atures which are known as (1) the Littleton softening point ⊲viscosity D 10 6.6 Nsm 2 ⊳ and (2) the annealing point ⊲viscosity D 10 12.4 Nsm 2 ⊳ .

10.5 Aspects of glass technology

The softening point is determined by a standard- ized procedure and gives the maximum temperature at

10.5.1 Viscous deformation of glass

which the glass can be handled without serious change in dimensions. For ordinary silica glass, it is about

Viscosity 4 is a prime property of glass. Although the 1000 K. At the annealing temperature, the ions are suf- following account deals mainly with the working and

ficiently mobile to allow residual stresses to be relieved annealing of glass, it is broadly relevant to glazes and

in about 15 min. The point in the curve at which the to the glassy intergranular phase found in many ceram-

slope is a maximum corresponds to the inflection (fic- ics. Two mathematical expressions help to describe the

tive or glass transition temperature) in the specific vol- nature of viscous flow in glasses. First, there is the

ume/temperature curve for the particular glass-forming formula for Newtonian flow in an ideal fluid:

system. The working range for commercial silica glass corresponds to a viscosity range of 10 3 –10 7 Nsm 2 . The curve for this glass is quite steep, indicating that temperature control to within, say, š10 °

C is necessary during working (i.e. drawing, blowing, rolling, etc.). Figure 10.19 provides a comparison of the viscosity

the viscosity coefficient. In its melting range, a curves for different types of glass. The difficulties of typical SiO 2 –Na 2 O–CaO glass has a viscosity of

working pure silica glass (‘fused quartz’) are immedi- 5–50 N s m 2 . (The viscosity of liquid metals is

ately apparent. Even at a temperature of 1300 °

C this

12 roughly 1 mN s m 2 .) As glass is hot-drawn, its cross- glass has a viscosity of about 10 Nsm , which is sectional area decreases at a rate which depends solely

still too high for working. Glasses with special chemi- upon the drawing force and viscosity, not upon area.

cal and physical properties often have a steep viscosity For this reason, the glass extends uniformly and does

curve and tend to devitrify, presenting difficulties dur- not ‘neck’.

ing drawing at traditional working temperatures. One The second expression, which is of the Arrhenius-

type, concerns the temperature-dependency of visco- sity, as follows:

exp ⊲Q/kT⊳ where A is a constant, Q is the activation energy,

k is the Boltzmann constant and T is the absolute temperature. The activation energy for viscous flow is

4 Dynamic (absolute) viscosity is the force required to

move 1 m 2 of plane surface at a velocity of 1 m s 1 to a

second plane surface which is parallel to the first and separated 1 m from it by a layer of the fluid phase. Kinematic viscosity D absolute viscosity/density. Unit of

Figure 10.18 Viscosity versus temperature curve for a reciprocal of viscosity.

viscosity D 1 N s m 2 D 10 P (poise). Fluidity is the

typical SiO 2 –Na 2 O–CaO glass .

346 Modern Physical Metallurgy and Materials Engineering Pyrex glass, 81SiO 2 –13B 2 O 3 –4Na 2 O–2Al 2 O 3 ,

combines two network-formers and is well known for its chemical durability. Because of its relatively low

expansivity (˛ D 3.3 ð 10 ° C ; 0–300 ° C), it is tol- erant to thermal shock and is widely used for chemi- cal glassware in laboratories and industrial processing plant. Sometimes it is referred to as a ‘borosilicate’

glass, unfortunately implying crystallinity. In certain glasses, being an intermediate, PbO can actually form a network. This oxide is a flux and, unlike Na 2 O, does not reduce the electrical resistiv- ity; glasses in the SiO 2 –PbO–Na 2 O–Al 2 O 3 system are widely used for electronic and electrical components. Their high lead content favours absorption of X-rays and -rays, making them suitable for radio-logical

Figure 10.19 Viscosity curves for typical glasses . shielding (65% PbO) and colour television tubes (23% PbO). Lead also increases the refractive index and dis-

persion of SiO 2 -based glasses. These effects are used to advantage in optical lenses and in the decorative

solution has been to hot-extrude such glasses through glass known confusingly as ‘lead crystal’. In the latter

case, 20–30% PbO provides craftsmen with a ‘long’ high pressures (up to 10 GN m ), it is possible to

graphite dies into rod or tube form. 1 By using very

glass, that is, one with a large temperature interval for

overcome a high viscosity of 10 7 Nsm and to use

working/manipulation, within which viscosity changes

a lower extrusion temperature (e.g. 950 ° C). Such tem-

very slightly.

Alumina ⊲ Al 2 O 3 ⊳ , an intermediate, frequently the temperature required for traditional methods, pre-

peratures, which can be as much as 200 °

C lower than

features in the composition of glasses. In small vent devitrification and, by suppressing surface tension

amounts, say 1–2% w/w, it improves the resistance effects, help to form rods and tubes with sharp and

of SiO 2 -based glasses to attack by water. However, its accurate profiles.

presence increases melt viscosity and makes melting and fining more difficult; for instance, gas bubbles

10.5.2 Some special glasses

separate less readily. The dependence of viscosity upon composition is always an important matter in glass

Most glasses produced are based upon silica and fluxed technology. Al 3C cations enter the tetrahedral holes in with alkali metal oxides, with their chemical formu-

the network, replacing tetravalent silicon cations. As lation adjusted to suit the application (e.g. bottles,

a result, the number of non-bridging O anions is plate glass, lamps, etc.). In its purest form, vitreous

reduced and the strength and rigidity of the network silica has a very low linear coefficient of thermal

expansion ⊲˛ D 0.55 ð 10 °

is enhanced. By substantially raising the alumina

content to 15–25% w/w, it is possible to develop temperature applications. ‘Fused quartz’ is made by

C ⊳ and is used in high-

valuable stability at high temperatures. For exam- melting sand; high-purity ‘fused silica’ is made by

ple, 62SiO 2 –17Al 2 O 3 –8CaO–7MgO–5B 2 O 3 –1Na 2 O

C, which temperatures above 1500

vapour deposition after reacting SiCl 4 with oxygen at

glass has a softening point in excess of 900

C higher than that of Pyrex glass. This tentative.)

C. (These two definitions are

is about 100 °

‘aluminosilicate’ glass is used for combustion tubes.

Window (‘soda-lime’) glass, typically 72SiO 2 – 14Na 2 O–10CaO–2MgO–1Al 2 O 3 , owes its conve-

nient range of working temperature ⊲700–1000 ° C⊳ to

10.5.3 Toughened and laminated glasses

Glasses, in general, are weak when placed in ten- tiveness of their fluxing action can be gauged by the fact that the softening point of this glass ⊲700 °

the two network-modifiers, Na 2 O and CaO. The effec-

sion during bending and do not withstand impact

blows. However, in compression, glasses can exhibit at least 800

C⊳ is

C lower than that of high-purity silica exceptional strength. Accordingly, sheet glass is often glass. The softening point, as defined in the ASTM

strengthened and toughened by developing high resid- test method, is the temperature at which a glass fibre of

ual compressive stresses in the surface layers. They uniform diameter elongates at a specific rate under its

oppose any applied tensile stresses (say, on the convex own weight. The chemical formulation given uses the

face of a bent sheet of glass), and must be overcome ‘system’ nomenclature recommended by F. V. Tooley

before this vulnerable surface begins to develop dan- (i.e. oxide components are stated in order of decreasing

gerous tensile stresses. Various physical and chemical % w/w.)

processes, applicable on a mass-production scale, are available to improve the load-bearing capacity and

1 Philips Research Laboratories, Aachen, Germany. behaviour under impact conditions.

Ceramics and glasses 347 from molten salt displace smaller ions from the sur-

faces of immersed glass, causing them to expand (e.g. ‘K-for-Na’, ‘Na-for-Li’). On cooling from a relatively low temperature, the core is rigid and the presence of absorbed, larger ions in the surface layers develops compressive stresses. Chemical strengthening methods often use glasses of special composition.

The bottle-manufacturing industry uses chemical surface treatments to strengthen returnable bottles (improving trippage) and to provide a useful lubricity as they pass through bottle-filling plant at high speed. (Rubbing two glass bottles together reveals whether a lubricating coating is present.)

The alternative to the above toughening methods, based upon a composite approach, is to interleave two matched sheets of ‘float’ glass with an interlayer of

Figure 10.20 Distribution of residual stress in toughened plastic, nearly 1 mm thick. Polyvinyl butyrate (PVB) glass plate (stress values in MN m ) .

is commonly used for the interlayer. In the mid-1960s, this more costly method gained favour for car wind- screens as a result of accident statistics which indi-

The term ‘toughening’ generally refers to both heat- cated that a laminated windscreen was less likely to treatment and chemical methods. In thermal temper-

cause fatal injury than a toughened one. 1 Cracking ing, a toughening process used extensively for side

is confined to the area of impact. The PVB inter- and rear glazing in cars, glass sheets are cut and

layer deforms elastically, absorbs impact energy and shaped, heated uniformly to a temperature above T g holds glass fragments in place, reducing the risk of

(say, 700 ° C), soaked for 2–4 min and then quenched facial lacerations. Visibility is retained after impact. with jets of air. Glass has a low thermal conductiv-

De-icing windscreens have fine heating filaments of ity and the eventual contraction of the core regions is

tungsten embedded between the glass layers (Triplex restrained by the rigid outer layers, producing the bal-

Hotscreen ). Laminated glass with a thicker plastic interlayer is used for aircraft windscreens; these must

shown in Figure 10.20a. The glass sheet must be free withstand extreme thermal conditions, ranging from from flaws in order to withstand the severe quench

° C⊳ to kinetic heating ⊲150 ° C⊳, stresses and must be at least 2–3 mm thick so that the

and the possible high-speed ‘strikes’ of birds weighing desired stress gradients develop. Surface stresses up

about 2 kg.

to 200 MN m are produced by this method. Heat- The manufacture of laminated glass demands clean- ing toughened glass to a temperature of 200 °

liness so that the PVB will bond strongly to the glass. relieve these stresses. Small blunt-edged fragments are

C will

Curved windscreens are made by separately bending produced when toughened glass fractures underimpact.

matched pairs of glass sheet at a temperature just above Toughened products cannot be cut or shaped; notching,

C. After cooling, PVB is applied to one sheet of drilling, etc. must be done before treatment.

each pair in an air-conditioned room and the matching Chemical strengthening/toughening is also used to

sheet added. Air can be removed from the PVB/glass develop protective compressive stresses. These treat-

interfaces of the curved assembly by applying a vac- ments are more versatile, being suitable for thin and

uum to its edges. The glazing is finally autoclaved at thick sections of glass and for unusual shapes (e.g.

a pressure of 1.2–1.4 MN m and a temperature of Chemcor .) However, because ion exchange and diffu-

C in order to complete the bonding. sion are involved, they require longer process times.

The oil crisis of the mid-1970s underlined the Their purpose is to alter the surface composition and

need for weight saving and fuel efficiency in car design. As a direct consequence, the thickness of lam-

reduce its coefficient of thermal expansion. On cool- inated windscreens decreased from 3.0 mm/3.0 mm ing from a high temperature, the greater contraction

to 2.1 mm/2.1 mm. (Toughened glass decreased in of the hot core is restrained and very high compres-

thickness from 5–6 mm to 3–4 mm.) Advances in sive stresses develop in the modified surface layers

glass technology have made it possible for modern (Figure 10.20b). In one version of chemical strength-

car designs to increase the area of glazing, use flush, ening, sodium-containing glass is heated in an atmo-

sphere of water vapour and sulphur dioxide and an C C 1

exchange between H ions and Na ions takes place Strong sunlight striking a quench-toughened window glass in the glass surface (i.e. ‘H-for-Na’). In another high-

is polarized by reflection; the stress-birefringent patterns produced by the quench jets are often apparent, particularly

temperature version, a ‘Li-for-Na’ exchange occurs when viewed through Polaroid sunglasses. A pastime for when the glass is immersed in a melt of lithium salt.

traffic jams is to note the choice made by car manufacturers In low-temperature chemical strengthening, large ions

between quench-toughened and laminated glass.

348 Modern Physical Metallurgy and Materials Engineering polyurethane adhesive-mounted windows (of low drag

coefficient) and achieve rounded, aerodynamic pro- files. 1