Chapter 10 Ceramics and glasses 10.1 Classification of ceramics availabili ructural control The term ceramic, in its modern context, covers during processing.Ceramics an extremely broad range of inorganic materials; in their properties and statistical concepts often need they contain non-metallic and metallic elements and to be incorporated into design procedures for stressed are produced by a wide variety of manufacturing components. Design must recognize the inherent brit techniques. Traditionally, ceramics are moulded from tleness, or low resistance to crack propagation, and silicate minerals, such as clays, dried and fired at tem- modify, if necessary, the mode of failure. Ceramics peratures of 1200-1800'C to give a hard finish. Thus because of their unique properties, show great promise we can readily see that the original Greek word ker- as engineering materials but, in practice, their prodt amos, meaning burned stuff or kiln-fired material,, tion on a commercial scale in specified forms with has long been directly appropriate Modem ceramics, repeatable properties is often beset with many pro- however, are often made by processes that do not blems involve a kiln-fining step(e.g. hot-pressing, reaction Using chemical composition as a basis, it is possible sintering, glass-devitrification, etc. ) Although ceram- to classify ceramics into five main categories this simple distinction from metals and alloys has 1. Oxides -alumina, Al203(spark plug insulators, become increasingly inadequate and arbitrary as new grinding wheel grits), magnesia, Mgo(refrac ceramics with unusual properties are developed and ry linings of furnaces, crucibles), zirconia come into use generally classified, according to type or function, in various ways. In industrial panels,M2+oM3*O3(ferrites, magnets,transis- terms, they may be listed as pottery, heavy clay prod- tors, recording tape),'fused silica glass (laboratory bricks, silica, alumina, basic, neutral), cement and 2. Carbides- silicon carbide, Sic (chemical plant concrete, glasses and vitreous enamels, and engineer- ing( technical, fine) ceramics. Members of the final uts for molten aluminium, hi group are capable of very high strength and hardness, bearings), boron nitride bn (crucibles, m wheels for high-strength steels to very close dimensional tolerances. These will be 3. silicates pa porcelain (electricalcomponents), steat our prime concern. Their introduction as engineering 4. Sialons -based on Si-al-o-n and M-Si-AI siderable scientific effort and has revolution O-N where M= Li, Be, Mg, Ca, Sc, Y, rare earth (tool inserts for high-speed cutting, extrusion dies, neering design practice. In general, the development turbine blades). of engineering ceramics has been stimulated by th 5. Glass-ceramics Pyroceram, Cercor, Pyrosil(re drive towards higher, more energy-efficient, process cuperator discs for heat exchangers) temperatures and foreseeable shortages of strategic minerals. In contrast to traditional ceramics, which The preceding two methe ramics use naturally-occurring and, inevitably, rather variable industrial and chemical, ar minerals, the new generation of engineering ceramics materials scientist and tech =
Chapter 10 Ceramics and glasses 10.1 Classification of ceramics The term ceramic, in its modern context, covers an extremely broad range of inorganic materials; they contain non-metallic and metallic elements and are produced by a wide variety of manufacturing techniques. Traditionally, ceramics are moulded from silicate minerals, such as clays, dried and fired at temperatures of 1200-1800~ to give a hard finish. Thus we can readily see that the original Greek word keramos, meaning 'burned stuff' or 'kiln-fired material', has long been directly appropriate. Modem ceramics, however, are often made by processes that do not involve a kiln-firing step (e.g. hot-pressing, reactionsintering, glass-devitrification, etc.). Although ceramics are sometimes said to be non-metallic in character, this simple distinction from metals and alloys has become increasingly inadequate and arbitrary as new ceramics with unusual properties are developed and come into use. Ceramics may be generally classified, according to type or function, in various ways. In industrial terms, they may be listed as pottery, heavy clay products (bricks, earthenware pipes, etc.), refractories (firebricks, silica, alumina, basic, neutral), cement and concrete, glasses and vitreous enamels, and engineering (technical, fine) ceramics. Members of the final group are capable of very high strength and hardness, exceptional chemical stability and can be manufactured to very close dimensional tolerances. These will be our prime concern. Their introduction as engineering components in recent years has been based upon considerable scientific effort and has revolutionized engineering design practice. In general, the development of engineering ceramics has been stimulated by the drive towards higher, more energy-efficient, process temperatures and foreseeable shortages of strategic minerals. In contrast to traditional ceramics, which use naturally-occurring and, inevitably, rather variable minerals, the new generation of engineering ceramics depends upon the availability of purified and synthesized materials and upon close microstructural control during processing. Ceramics are subject to variability in their properties and statistical concepts often need to be incorporated into design procedures for stressed components. Design must recognize the inherent brittleness, or low resistance to crack propagation, and modify, if necessary, the mode of failure. Ceramics, because of their unique properties, show great promise as engineering materials but, in practice, their production on a commercial scale in specified forms with repeatable properties is often beset with many problems. Using chemical composition as a basis, it is possible to classify ceramics into five main categories: 1. Oxides ~ alumina, A1203 (spark plug insulators, grinding wheel grits), magnesia, MgO (refractory linings of furnaces, crucibles), zirconia, ZrO2 (piston caps, refractory lining of glass tank furnaces), zirconia/alumina (grinding media), spinels, M2+O.M~+O3 (ferrites, magnets, transistors, recording tape), 'fused' silica glass (laboratory ware), 2. Carbides ~ silicon carbide, SiC (chemical plant, crucibles, ceramic armour), silicon nitride, Si3N4 (spouts for molten aluminium, high-temperature bearings), boron nitride, BN (crucibles, grinding wheels for high-strength steels). 3. Silicates ~ porcelain (electrical components), steatites (insulators), mullite (refractories). 4. Sialons ~ based on Si-AI-O-N and M-Si-A1- O-N where M -- Li, Be, Mg, Ca, Sc, Y, rare earths (tool inserts for high-speed cutting, extrusion dies, turbine blades). 5. Glass-ceramics ~ Pyroceram, Cercor, Pyrosil (recuperator discs for heat exchangers). The preceding two methods of classifying ceramics, industrial and chemical, are of very little use to the materials scientist and technologist, who is primarily
Ceramics and glasses 321 concerned with structure/property relations. One can 2050.C. The type of inter-atomic bonding is res& predict that a ceramic structure with a fine grain(crys- sible for the relatively low electrical conductivity I)size and low porosity is likely to offer advantages ceramics. For general applications they are usually of mechanical strength and impermeability to contact regarded as excellent electrical insulators, having no ing fuids. It is therefore scientifically appropriate free electrons. However, ion mobility becomes signif- classify ceramic materials in microstructural terms, in icant at temperatures above 500-600C and they then become progressively more conductive. This property 1. Single crystals of appreciable size (e.g. ruby laser can prove a problem in electric furnaces The strength of ceramics under compressive crystal) ing is excellent; accordingly, designers of 2. Glass(non-crystalline)of appreciable size ( e.g. artefacts as different as arches in buildings and sheets of 'foamglass) cutting tool tips ensure that the forces during d metal. 3. Crystalline or glassy filaments (e.g. E-glass for are essentially compressive. In contrast, the tensile ss-reinforced polymers, single-ci strength of ceramics is not exceptional, sometimes silica glass in Space Shuttle tiles oor,largely because of the weakening effect of sur- 4. Polycrystalline aggregates bonded by a glassy ce flaws. Thus, in some cases, glazing with a thin matrix(e.g. porcelain pottery, silica refractories, vitreous layer can seal surface cracks and improve the tensile strength. The strength of ceramics is com- 5. Glass-free polycrystalline aggregates (e.g. ultra- monly expressed as a modulus of rupture( MoR)value, pure, fine-grained, zero-porosity'forms of alumina, obtained from three-point bend tests, because in the more conventional type of test with uniaxial loading,as 6. Polycrystalline aggregates produced by heat sed for metals, is difficult to apply with perfect uniax treating glasses of special composition (e.g. glass- iality; a slight misalignment of the machine grips will ceramics induce unwanted bending stresses. Ceramics are ge 7. Composites(e.g. silicon carbide or carbon filaments erally regarded as brittle non-ductile materials, with n a matrix of glass or glass-ceramic, magnesia- little or no plastic deformation of the microstructure graphite refractories, concrete) either before or at fracture. For this reason, which rules out the types of production processes involv This approach to classifying ceramics ing deformation that are so readily applied to metals and polymers, ceramic production frequently centres crystalline(glassy)attributes of the ceramic on the manipulation and ultimate bonding together of significance of introducing grain boundary surfaces fine powders. The inherent lack of ductility implies that and the scope for deliberately mixing two phases witl eramics are likely to have a better resistance to slow ery different properties plastic deformation at very high temperatures (creep) The modulus of elasticity of ceramics can be excep- 10.2 General properties of ceramics tionally high (Tab 1). This modulus expresses stiffness, or the amount of stress necessary to pro- The constituent atoms in a ceramic are held togethe duce unit elastic strain, and, like strength, is a primary by very strong bonding forces which may be ionic, design consideration. However, it is the combination covalent or a mixture of the two. as a direct conse- f low density with this stiffness that makes ceramics quence, their melting points are often very high, mak- particularly attractive for structures in which weight ing them eminently suited for use in energy-intensive reduction is a prime consideration stems such as industrial furnaces and gas turbines ve long For instance, alumina primarily owes its importance been an interesting proposition because, apart from as a furnace refractory material to its melting point of reducing the total mass that has to be levitated, they are Table 10.1 Specific moduli of various materials Modulus of elasticity Bulk density Specific modulus E/GN m-4) (a/kg m -3J Alumina Glass(crown) Aluminiun 0.026 0.027 Oak (with grain) 0019 Concrete 0.006 0.003
Ceramics and glasses 321 concerned with structure/property relations. One can predict that a ceramic structure with a fine grain (crystal) size and low porosity is likely to offer advantages of mechanical strength and impermeability to contacting fluids. It is therefore scientifically appropriate to classify ceramic materials in microstructural terms, in the following manner: 1. Single crystals of appreciable size (e.g. ruby laser crystal) 2. Glass (non-crystalline) of appreciable size (e.g. sheets of 'float' glass) 3. Crystalline or glassy filaments (e.g. E-glass for glass-reinforced polymers, single-crystal 'whiskers', silica glass in Space Shuttle tiles) 4. Polycrystalline aggregates bonded by a glassy matrix (e.g. porcelain pottery, silica refractories, hot-pressed silicon nitride) 5. Glass-free polycrystalline aggregates (e.g. ultrapure, fine-grained, 'zero-porosity' forms of alumina, magnesia and beryllia) 6. Polycrystalline aggregates produced by heattreating glasses of special composition (e.g. glassceramics) 7. Composites (e.g. silicon carbide or carbon filaments in a matrix of glass or glass-ceramic, magnesiagraphite refractories, concrete). This approach to classifying ceramics places the necessary emphasis upon the crystalline and noncrystalline (glassy) attributes of the ceramic body, the significance of introducing grain boundary surfaces and the scope for deliberately mixing two phases with very different properties. 10.2 General properties of ceramics The constituent atoms in a ceramic are held together by very strong bonding forces which may be ionic, covalent or a mixture of the two. As a direct consequence, their melting points are often very high, making them eminently suited for use in energy-intensive systems such as industrial furnaces and gas turbines. For instance, alumina primarily owes its importance as a furnace refractory material to its melting point of 2050~ The type of inter-atomic bonding is responsible for the relatively low electrical conductivity of ceramics. For general applications they are usually regarded as excellent electrical insulators, having no free electrons. However, ion mobility becomes significant at temperatures above 500-600~ and they then become progressively more conductive. This property can prove a problem in electric furnaces. The strength of ceramics under compressive stressing is excellent; accordingly, designers of ceramic artefacts as different as arches in buildings and metalcutting tool tips ensure that the forces during service are essentially compressive. In contrast, the tensile strength of ceramics is not exceptional, sometimes poor, largely because of the weakening effect of surface flaws. Thus, in some cases, glazing with a thin vitreous layer can seal surface cracks and improve the tensile strength. The strength of ceramics is commonly expressed as a modulus of rupture (MoR) value, obtained from three-point bend tests, because in the more conventional type of test with uniaxial loading, as used for metals, is difficult to apply with perfect uniaxiality; a slight misalignment of the machine grips will induce unwanted bending stresses. Ceramics are generally regarded as brittle, non-ductile materials, with little or no plastic deformation of the microstructure either before or at fracture. For this reason, which rules out the types of production processes involving deformation that are so readily applied to metals and polymers, ceramic production frequently centres on the manipulation and ultimate bonding together of fine powders. The inherent lack of ductility implies that ceramics are likely to have a better resistance to slow plastic deformation at very high temperatures (creep) than metals. The modulus of elasticity of ceramics can be exceptionally high (Table 10.1). This modulus expresses stiffness, or the amount of stress necessary to produce unit elastic strain, and, like strength, is a primary design consideration. However, it is the combination of low density with this stiffness that makes ceramics particularly attractive for structures in which weight reduction is a prime consideration. In aircraft gas turbines, ceramic blades have long been an interesting proposition because, apart from reducing the total mass that has to be levitated, they are Table 10.1 Specific moduli of various materials Modulus of elasticity Bulk density Specific modulus (E/GN m -2 ) (p/kg m -3 ) (E/p) Alumina 345 3800 Glass (crown) 71 2600 Aluminium 71 2710 Steel (mild) 210 7860 Oak (with grain) 12 650 Concrete 14 2400 Perspex 3 1190 0.091 0.027 0.026 0.027 0.019 0.006 0.003
322 Modern Physical Metallurgy and Materials Engineerin subject to lower centrifugal forces than metallic ver- The ability of certain ceramic oxides to exist in sions. It is therefore common practice to appraise com- either crystalline or non-crystalline forms has been petitor materials for aircraft in terms of their specific commented upon previously. Silica and boric oxide moduli, in which the modulus of elasticity is divided possess this ability. In glass-ceramics, a metastable by density. Ceramics consist largely of elements of low glass of special composition is shaped while in the atomic mass, hence their bulk density is usually low, viscous condition, then heat-treated in order to induce typically about 2000-4000 kg m". Ceramics such as nucleation and growth of a fine, completely crystalline dense alumina accordingly tend to become pre-eminent structure. (This manipulation and exploitation of the in listings of specific moduli (Table 10.1) crystalline and glassy states is also practised with The strong interatomic bonding means that ceram- metals and polymers. This glass-forming potential is ics are hard as well as strong. That is, they resist an important aspect of ceramic science. The property penetration by scratching or indentation and are poten- of transparency to light is normally associated with tially suited for use as wear-resistant bearings and glasses. notably with the varieties based upon silica abrasive particles for metal removal. Generally, impact However, transparency is not confined to glasses and onditions should be avoided. Interestingly, shape can single crystals. It is possible to produce some oxides influence performance; thus, the curved edges of din- normally regarded as opaque, in transparent, polycrys- ner plates are carefully designed to maximize resis- talline forms (e. g hot-pressed magnesia) tance to chipping. Although the strength and hardness So far as sources in the earths crust of materials are often related in a relatively simpl ramIcs are outstandingly abundant, it must also be recognized albeit costly. Strength can be enhanced in this way that the processes for producing the new ceramics can but great care is necessary as there is a risk that the be very costly, demanding resort to highly specialized machining operation will damage, rather than improve, equipment and exacting process control During the consolidation and densification of a green powder compact in a typical firing operation, sintering of the particles gradually reduces the amount 10.3 Production of ceramic powders porosity,by volume, of the fired material ranges from The wide-ranging properties and versatility of mod a direct influence upon the modulus of rupture; thus, which they are manufactured. A fine powder is usually because of its finer texture. is twice as the starting material, or precursor; advanced ceram strong as fired earthenware. Pore spaces. particularly if mainly produced from powders with a size interconnected, also lower the resistance of a ceramic range of 1-10 um Electrical properties are extremely the electronics industry for even finer particles(in th ment of porosity, say 25-30% by volume. is used nanometre range). The basic purpose of the manufac to lower the thermal conductivity of insulating re turing process is to bring particle surfaces together and to develop strong interparticle bonds. It follows that Ceramics are often already in their highest state specific surface area, expressed per unit mass is of par- ticular significance. Characterization of the powder in chemical reactivity when exposed to hot oxidizing terms of its physical and chemical properties, such as environments. Their refractoriness, or resistance to re distribution, shape, surface topography, purity and degradation and collapse during service at high tem reactivity, is an essential preliminary to the actual man- ns from the strong interatomic bonding. ufacturing process. Tolerances and limits are becoming However, operational temperatures are subject to su den excursions and the resulting steep gradients of tem- The three principal routes for producing high-grade erature within the ceramic body can give rise to stress powders are based upon solid-state reactions, solution imbalances. As the ceramic is essentially non -ducti and vaporization. The solid-state reaction route, long stresses are not relieved by plastic deformation and exemplified by the acheson process for silicon carbide racking may occur in planes roughly perpendicular (Section 10.4.5.2) invOlves high temperatures. It is to the temperature gradient, with portions of ceramic used in more refined forms for the production of becoming detached from the hottest face. The sever- other carbides (TiC, wC), super-conductive oxides ty of this disintegration, known as spalling, depends and silicon nitride, An aggregate is produced and the mainly upon thermal expansivity (a)and conductivity necessary size reduction(commi (k). Silica has a poor resistance to spalling whereas sil sk of contamination. Furthermore, as has long been C 1000C and then quenched in cold water nitride can withstand being heated to a temperature known in mineral-dressing industries, fine grinding is energy-intensive and costly
322 Modern Physical Metallurgy and Materials Engineering subject to lower centrifugal forces than metallic versions. It is therefore common practice to appraise competitor materials for aircraft in terms of their specific moduli, in which the modulus of elasticity is divided by density. Ceramics consist largely of elements of low atomic mass, hence their bulk density is usually low, typically about 2000-4000 kg m -3. Ceramics such as dense alumina accordingly tend to become pre-eminent in listings of specific moduli (Table 10.1). The strong interatomic bonding means that ceramics are hard as well as strong. That is, they resist penetration by scratching or indentation and are potentially suited for use as wear-resistant bearings and as abrasive particles for metal removal. Generally, impact conditions should be avoided. Interestingly, shape can influence performance; thus, the curved edges of dinner plates are carefully designed to maximize resistance to chipping. Although the strength and hardness of materials are often related in a relatively simple manner, it is unwise to assume that a hard material, whether metallic or ceramic, will necessarily prove to be wear-resistant. Grinding of ceramics is possible, albeit costly. Strength can be enhanced in this way but great care is necessary as there is a risk that the machining operation will damage, rather than improve, the critical surface texture. During the consolidation and densification of a 'green' powder compact in a typical firing operation, sintering of the particles gradually reduces the amount of pore space between contiguous grains. The final porosity, by volume, of the fired material ranges from 30% to nearly zero. Together with grain size, it has a direct influence upon the modulus of rupture; thus, bone china, because of its finer texture, is twice as strong as fired earthenware. Pore spaces, particularly if interconnected, also lower the resistance of a ceramic structure to penetration by a pervasive fluid such as molten slag. On the other hand, deliberate encouragement of porosity, say 25-30% by volume, is used to lower the thermal conductivity of insulating refractories. Ceramics are often already in their highest state of oxidation. Not surprisingly, they often exhibit low chemical reactivity when exposed to hot oxidizing environments. Their refractoriness, or resistance to degradation and collapse during service at high temperatures, stems from the strong interatomic bonding. However, operational temperatures are subject to sudden excursions and the resulting steep gradients of temperature within the ceramic body can give rise to stress imbalances. As the ceramic is essentially non-ductile, stresses are not relieved by plastic deformation and cracking may occur in planes roughly perpendicular to the temperature gradient, with portions of ceramic becoming detached from the hottest face. The severity of this disintegration, known as spalling, depends mainly upon thermal expansivity (c~) and conductivity (k). Silica has a poor resistance to spalling whereas silicon nitride can withstand being heated to a temperature of 1000~ and then quenched in cold water. The ability of certain ceramic oxides to exist in either crystalline or non-crystalline forms has been commented upon previously. Silica and boric oxide possess this ability. In glass-ceramics, a metastable glass of special composition is shaped while in the viscous condition, then heat-treated in order to induce nucleation and growth of a fine, completely crystalline structure. (This manipulation and exploitation of the crystalline and glassy states is also practised with metals and polymers.) This glass-forming potential is an important aspect of ceramic science. The property of transparency to light is normally associated with glasses, notably with the varieties based upon silica. However, transparency is not confined to glasses and single crystals. It is possible to produce some oxides, normally regarded as opaque, in transparent, polycrystalline forms (e.g. hot-pressed magnesia). So far as sources in the earth's crust are concerned, mineral reserves for ceramic production are relatively plentiful. While one might observe that important constituent elements such as silicon, oxygen and nitrogen are outstandingly abundant, it must also be recognized that the processes for producing the new ceramics can be very costly, demanding resort to highly specialized equipment and exacting process control. 10.3 Production of ceramic powders The wide-ranging properties and versatility of modern engineering ceramics owe much to the ways in which they are manufactured. A fine powder is usually the starting material, or precursor; advanced ceramics are mainly produced from powders with a size range of 1-10 ~m. Electrical properties are extremely structure-sensitive and there is a strong demand from the electronics industry for even finer particles (in the nanometre range). The basic purpose of the manufacturing process is to bring particle surfaces together and to develop strong interparticle bonds. It follows that specific surface area, expressed per unit mass, is of particular significance. Characterization of the powder in terms of its physical and chemical properties, such as size distribution, shape, surface topography, purity and reactivity, is an essential preliminary to the actual manufacturing process. Tolerances and limits are becoming more and more exacting. The three principal routes for producing high-grade powders are based upon solid-state reactions, solution and vaporization. The solid-state reaction route, long exemplified by the Acheson process for silicon carbide (Section 10.4.5.2), involves high temperatures. It is used in more refined forms for the production of other carbides (TIC, WC), super-conductive oxides and silicon nitride. An aggregate is produced and the necessary size reduction (comminution) introduces the risk of contamination. Furthermore, as has long been known in mineral-dressing industries, fine grinding is energy-intensive and costly
e. The Bayer process for converting bauxite into alu stance, the ability of an austenitic stainle na is a solution-treatment method. In this be cold-drawn to the dimensions of a fine hy tant process, which will be examined in deta needle tube is strong evidence of structural m tated from a caustic solution and then converted to non-deformable; consequently, manufacturing routes alumina by heating. Unfortunately, this calcination has usually avoid plastic deformation and there is a greater agglomerate is necessary. In the more recent spray- becoming visible or causing actual disintegration. The drying and spray-roasting techniques, which are widely final properties of an advanced ceramic are extremely used to produce oxide powders, sprayed droplets of sensitive to any form of structural heterogeneity. The concentrated solutions of appropriate salts are rapidly development of special ceramics and highly-innovative heated by a stream of hot gas. Again, there is a risk of production techniques has encouraged greater use of non-destructive evaluation(NDE)techniques at key These difficulties, which stem from the inherent points in the manufacturing programme. At the desigr physical problem of removing all traces of solvent in a stage, guidelines of the following type are advisedly atisfactory manner, have encouraged development of applied to the overall plan of production: methods based upon a'solution-to-gelation'(sol-gel approach. The three key stages of a typical sol-gel I. Precursor materials, particularly ultra-fine powders, process should be scientifically characterized 2. Each and every unit operation should be closely 1. Production of a colloidal suspension or solution studied and controlled (sol)(e.g. concentrated solution of metallic salt in 3. NDE techniques should be carefully integrated dilute acid) within the overall scheme of operati 2. Adjustment of pH. addition of a gelling agent, evaporation of liquid to produce a gel 3. Carefully controlled calcination to produce fine 10.4 Selected engineering ceramic particles of ceramic 10.4.1 Alumina Sol-gel methods are applicable to both ceramics and 10.4.1. 1 General propert es and applications of vell as powders. One variant involves hydrolysis alumina f distillation-purified alkoxides(formed by reacting Alumina is used of the twenty or so metal oxides with alcohol). The hydroxide particles oxide cerar regarded as the historic precipitated from the sol are spherical, uniform in forerunner ring ceramics The actual shape and sub-micron sized. Sintering does not drasti ntent of as Al2O,, ranges from ally change these desirable characteristics. Although 85% to 99.9%, depending upon the demands of the tend to be high and processing times are lengthy, application sol-gel methods offer an attractive way to produce Alumina-based refractories of coarse grain size are oxide powders, such as alumina, zirconia and titania, used in relatively massive forms such as slabs, shapes that will flow, form and sinter readily and give a pr and bricks for furnace construction. Alumina has a uct with superior properties. Currently, there is great high melting point (2050oC)and its heat resistance interest in vapour phase methods that enable pow- or refractoriness. has long been appreciated by fur lers with a particle size as small as 10-20 nm to nace designers. In fact, there has been a trend for be produced(e.g. oxides, carbides, nitrides, silicide borides). The high-energy input required for vaporiza. replaced by more costly high-alumina materials and beams. The powder is condensed within a carrier gas ionic purity alumina Interatomic bonding forces,partly rystal structure of alumina is physically stable up to nent filters or electrostatic precipitators. Sometimes, temperatures of 1500-1700oC. It is used for pre a chemical vapour deposition process(CVD), a thin tive sheaths for temperature-measuring ther film is condensed directly upon a substrate which have to withstand hot and aggressive environ- The manufacture of an advanced ceramic usuall ments and for filters which remove foreign particles involves a number of steps, or unit operations. Each and oxide dross from fast-moving streams of molter peration is subject to a number of interactin ving cast from fued a, o casting. Large refractory blocks bles(time, temperature, pressure, etc )and, by ha a very specific effect upon the developing structure melting glass. However, although alumina is a heat- macro-and micro-), makes its individual contribution resisting material with useful chemical stability, it is to the final quality of the product. when ductile met- more sensitive to thermal shock than silicon carbide als are shaped by plastic deformation, each operation and silicon nitride. A contributory factor is its rela- stresses the material and is likely to reveal flaws. (For tively high linear coefficient of thermal expansion(a)
Ceramics and glasses 323 The Bayer process for converting bauxite into alumina is a solution-treatment method. In this important process, which will be examined in detail later (Section 10.4.1.2), aluminium hydroxide is precipitated from a caustic solution and then converted to alumina by heating. Unfortunately, this calcination has a sintering effect and fine grinding of the resultant agglomerate is necessary. In the more recent spraydrying and spray-roasting techniques, which are widely used to produce oxide powders, sprayed droplets of concentrated solutions of appropriate salts are rapidly heated by a stream of hot gas. Again, there is a risk of agglomeration and grinding is often necessary. These difficulties, which stem from the inherent physical problem of removing all traces of solvent in a satisfactory manner, have encouraged development of methods based upon a 'solution-to-gelation' (sol-gel) approach. The three key stages of a typical sol-gel process are: 1. Production of a colloidal suspension or solution (sol) (e.g. concentrated solution of metallic salt in dilute acid) 2. Adjustment of pH, addition of a gelling agent, evaporation of liquid to produce a gel 3. Carefully controlled calcination to produce fine particles of ceramic. Sol-gel methods are applicable to both ceramics and glasses and are capable of producing filaments as well as powders. One variant involves hydrolysis of distillation-purified alkoxides (formed by reacting metal oxides with alcohol). The hydroxide particles precipitated from the sol are spherical, uniform in shape and sub-micron sized. Sintering does not drastically change these desirable characteristics. Although costs tend to be high and processing times are lengthy, sol-gel methods offer an attractive way to produce oxide powders, such as alumina, zirconia and titania, that will flow, form and sinter readily and give a product with superior properties. Currently, there is great interest in vapour phase methods that enable powders with a particle size as small as 10-20 nm to be produced (e.g. oxides, carbides, nitrides, silicides, borides). The high-energy input required for vaporization is provided by electric arcs, plasma jets or laser beams. The powder is condensed within a carrier gas and then separated from the gas stream by impingement filters or electrostatic precipitators. Sometimes, in a chemical vapour deposition process (CVD), a thin film is condensed directly upon a substrate. The manufacture of an advanced ceramic usually involves a number of steps, or unit operations. Each operation is subject to a number of interacting variables (time, temperature, pressure, etc.) and, by having a very specific effect upon the developing structure (macro- and micro-), makes its individual contribution to the final quality of the product. When ductile metals are shaped by plastic deformation, each operation stresses the material and is likely to reveal flaws. (For instance, the ability of an austenitic stainless steel to be cold-drawn to the dimensions of a fine hypodermic needle tube is strong evidence of structural integrity.) Individual ceramic particles are commonly brittle and non-deformable; consequently, manufacturing routes usually avoid plastic deformation and there is a greater inherent risk that flaws will survive processing without becoming visible or causing actual disintegration. The final properties of an advanced ceramic are extremely sensitive to any form of structural heterogeneity. The development of special ceramics and highly-innovative production techniques has encouraged greater use of non-destructive evaluation (NDE) techniques at key points in the manufacturing programme. At the design stage, guidelines of the following type are advisedly applied to the overall plan of production: 1. Precursor materials, particularly ultra-fine powders, should be scientifically characterized. 2. Each and every unit operation should be closely studied and controlled. 3. NDE techniques should be carefully integrated within the overall scheme of operations. 10.4 Selected engineering ceramics 10.4.1 Alumina 10.4.1.1 General properties and applications of alumina Alumina is the most widely used of the twenty or so oxide ceramics and is often regarded as the historic forerunner of modern engineering ceramics. The actual content of alumina, reported as A1203, ranges from 85% to 99.9%, depending upon the demands of the application. Alumina-based refractories of coarse grain size are used in relatively massive forms such as slabs, shapes and bricks for furnace construction. Alumina has a high melting point (2050~ and its heat resistance, or refractoriness, has long been appreciated by furnace designers. In fact, there has been a trend for aluminosilicate refractories (based upon clays) to be replaced by more costly high-alumina materials and high-purity alumina. Interatomic bonding forces, partly ionic and partly covalent, are extremely strong and the crystal structure of alumina is physically stable up to temperatures of 1500-1700~ It is used for protective sheaths for temperature-measuring thermocouples which have to withstand hot and aggressive environments and for filters which remove foreign particles and oxide dross from fast-moving streams of molten aluminium prior to casting. Large refractory blocks cast from fused alumina are used to line furnaces for melting glass. However, although alumina is a heatresisting material with useful chemical stability, it is more sensitive to thermal shock than silicon carbide and silicon nitride. A contributory factor is its relatively high linear coefficient of thermal expansion (c~)
324 Modern Physical Metallurgy and Materials Engineering The respective a-values/x 10-k- for silicon car. bide, silicon nitride and alumina are 8, 4.5 and 3.5 Termina when intended for use as engineering components at lower temperatures alumina ceramics usually fine grain size (0.5-20 um)and virtually zero porosity Development of alumina to meet increasingly stringent demands has taken place continuously over many years and has focused mainly upon control of chemical com- osition and grain structure. The chemical inertnes of alumina and its biocompatibility with human tissue have led to its use for hip prostheses. An oft-quoted xample of the capabilities of alumina is the insula ing body of the spark -ignition plug for petrol-fuell ngines(Figure 10. 1). Its design and fabrication meth ods have been steadily evolving since the early 1900 In modern engines, trouble-free functioning of a plug depends primarily upon the insulating capability of Seals its isostatically-pressed alumina body. Each plug is expected to withstand temperatures up to 1000'C, sud den mechanical pressures, corrosive exhaust gases and a potential difference of about 30 kV while ' pre cisely 50-100 times per second over long periods of time. Plugs are provided with a smooth glazed(glassy) surface so that any electrically-conductive film of con tamination can be easily removed. The exceptional insulating properties and range of alumina ceramics have long been recognized in th electrical and electronics industries(e. g substrates for circuitry, sealed packaging for onductor micro circuits). Unlike metals, there are no 'free electrons available in the structure to form a fow of current The dielectric strength, which is a measure of the abil- potential without breakdown or discharge, is very high. acknowledgements to champion spark Plug division of trical charge, the resistivity is still significantly high Electrical properties usually benefit when the purity of Spark-plug insulators'and water-pump sealing rings in tak nternal combustion engines are striking examples of this principle at work advantage of the excellent compressive strength, hard- ness and wear resistance of alumina (e.g. rotating 10.4.1.2 Preparation and shaping of alumina seals in washing machines and in water pumps for automobile engines, machine jigs and cutting tools, powders soil-penetrating coulters on agricultural equipment, Examination of the general form of the production shaft bearings in watches and tape-recording machines, route for alumina ceramics from ore to finished shape guides for fast-moving fibres and yarns, grinding provides an insight into some of the important factors abrasives).(Emery, the well-known abrasive, is an and working principles which guide the ceramics tech impure anhydrous form of alumina which contains nologist and an indication of the specialized shaping as much as 20% SiO2+Fe203; pretreatment is often methods that are available for ceramics. As mentione unnecessary. The constituent atoms in alumina, alu- earlier, each stage of the production sequence makes minium and oxygen, are of relatively low mass and the s and the its own individual and vital contribution to the final correspondingly low density (3800 kg m")is often uct and must be carefully contro advantageous. However, like most ceramics, alumina The principal raw material for alumina production s brittle and should not be subjected to either impac is bauxite Al2O(OH)4, an abundant hydrated rock blows or excessive tensile stresses during service. occurring as large deposits in various parts of the their functioning can vitally affect the performance and Over the period 1902-1977 Robert Bosch Ltd developed verall efficiency of a much larger engineering system. more than 20000 different types of spark plug
324 Modem Physical Metallurgy and Materials Engineering The respective a-values/x 10 -6 K -l for silicon carbide, silicon nitride and alumina are 8, 4.5 and 3.5. When intended for use as engineering components at lower temperatures, alumina ceramics usually have a fine grain size (0.5-20 ~tm) and virtually zero porosity. Development of alumina to meet increasingly stringent demands has taken place continuously over many years and has focused mainly upon control of chemical composition and grain structure. The chemical inertness of alumina and its biocompatibility with human tissue have led to its use for hip prostheses. An oft-quoted example of the capabilities of alumina is the insulating body of the spark-ignition plug for petrol-fuelled engines (Figure 10.1). Its design and fabrication methods have been steadily evolving since the early 1900s. In modern engines, trouble-free functioning of a plug depends primarily upon the insulating capability of its isostatically-pressed alumina body. Each plug is expected to withstand temperatures up to 1000~ sudden mechanical pressures, corrosive exhaust gases and a potential difference of about 30 kV while 'firing' precisely 50-100 times per second over long periods of time. Plugs are provided with a smooth glazed (glassy) surface so that any electrically-conductive film of contamination can be easily removed. The exceptional insulating properties and range of alumina ceramics have long been recognized in the electrical and electronics industries (e.g. substrates for circuitry, sealed packaging for semiconductor microcircuits). Unlike metals, there are no 'free' electrons available in the structure to form a flow of current. The dielectric strength, which is a measure of the ability of a material to withstand a gradient of electric potential without breakdown or discharge, is very high. Even at temperatures approaching 1000~ when the atoms tend to become mobile and transport some electrical charge, the resistivity is still significantly high. Electrical properties usually benefit when the purity of alumina is improved. Many mass-produced engineering components take advantage of the excellent compressive strength, hardness and wear resistance of alumina (e.g. rotating seals in washing machines and in water pumps for automobile engines, machine jigs and cutting tools, soil-penetrating coulters on agricultural equipment, shaft bearings in watches and tape-recording machines, guides for fast-moving fibres and yarns, grinding abrasives). (Emery, the well-known abrasive, is an impure anhydrous form of alumina which contains as much as 20% SiO2 + Fe203; pretreatment is often unnecessary.) The constituent atoms in alumina, aluminium and oxygen, are of relatively low mass and the correspondingly low density (3800 kg m -3) is often adwintageous. However, like most ceramics, alumina is brittle and should not be subjected to either impact blows or excessive tensile stresses during service. Alumina components are frequently quite small but their functioning can vitally affect the performance and overall efficiency of a much larger engineering system. Terminal Ce,am,c Resistor ~ ,.- ~ insulator Steel shell Copper-cored nickel electrodes Figure 10.1 Spark plug for petrol engine (with acknowledgements to Champion Spark Plug Division of Cooper GB Ltd). Spark-plug insulators 1 and water-pump sealing rings in internal combustion engines are striking examples of this principle at work. 10.4.1.2 Preparation and shaping of alumina powders Examination of the general form of the production route for alumina ceramics from ore to finished shape provides an insight into some of the important factors and working principles which guide the ceramics technologist and an indication of the specialized shaping methods that are available for ceramics. As mentioned earlier, each stage of the production sequence makes its own individual and vital contribution to the final quality of the product and must be carefully controlled. The principal raw material for alumina production is bauxite A120(OH)4, an abundant hydrated rock occurring as large deposits in various parts of the I Over the period 1902-1977 Robert Bosch Ltd developed more than 20000 different types of spark plug