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
Ceramics and glasses 325 rld.2 In the Bayer process, prepared bauxitic ore It has been mentioned that fluxing oxides are added digested under pre eous solution to lower-grade aluminas in order to form an intergranu f sodium hydroxide and then'seeded'to induce pre- lar phase(s). Although this fluid inter-granular material cipitation of Al(OH)3 crystals, usually referred to by facilitates densification during firing, its presence in the mineral term 'gibbsite'.(The conditions of time, the final product can have a detrimental effect upon temperature, agitation, etc. during this stage greatly strength and resistance to chemical attack. As a con is chemically decomposed by heating(calcined) at a for demanding applications. In general, an increase in temperature of 1200.C. Bayer calcine, which consists alumina content from 88% to 99.8% requires a corre- of a-alumina(>99% Al2O3), is graded according to sponding increase in firing erature from 1450 C to he nature and amount of ies. Sodium oxide, 1750.C 'Harder'firing incurs heavier energy costs and Na2O, ranges up to 0.6% and is al signifi- has led to the development of reactive alumina which cance because it affects sintering behaviour and elec- has an extremely small particle size(1 um)and a large trical resistance. The calcine consists of agglomerates specific surface. 'Softer'firing temperatures became of a-alumina crystallites which can be varied in aver- possible with this grade of alumina and the need to age size from 0.5 to 100 um by careful selection of debase the alumina with relatively large amounts of calcining conditions dditives was challenged. Bayer calcine is commonly used by manufactur Shrinkage is the most apparent physical chang ers to produce high-purity alumina components as to take place when a green ceramic compact is well as numerous varieties of lower-grade ponents fired, The linear shrinkage of alumina is about 20% containing 85-95% Al2O3. For the latter the and dimensions may vary by up to 1%,Diamond composition of the calcine is debased by additions machining is used when greater precision is needed of oxides such as SiO2, Cao and Mgo which act as but requires care as it may damage the surface and fluxes,forming a fluid glassy phase between the grains introduce weakening flaws of a-alumina during sintering The chosen grade of alumina, together with an ecessary additives, is ground in wet ball-mills to 10. 4.2 From silicon nitride to sialons ing the aqueous suspension into a flow of hot gas 10.4. 2. I Reaction-bonded silicon nitride (spray-drying)and separating the alumina in a cyclone (rbSN) unit.The free-flowing powder can be shaped by a Silicon nitride, which can be produced in several ways variety of methods(e.g. dry, isostatic-or hot-pressing, has found application under a variety of difficult condi slip-or tape-casting, roll-forming, extrusion, injection- tions(e.g. cutting tools, bearings, heat engines, foundry moulding). Extremely high production rates are often equipment, furnace parts, welding jigs, metal-working es, etc. ) Its original development was largely stimu for ce are incorporated with the powder; for ins and it was difficult to produce complex ceramic shapes thermoplastic can be hot-mixed with alumin to facilitate injection-moulding and later bur close dimensional tolerances, The properties avail- able from existing materials were variable and specifi tape-casting, which produces thin substrates for micro- service requirements, such as good resistance to ther electronic circuits, alumina powder is suspended in an mal shock and attack by molten metal and/or slag, organic liquid could not be met. The development of silicon nitride minimized these problems; it has also had a profound 10.4.1.3 Densification by sintering effect upon engineering thought and practice The fragile and porous shapes are finally fired Silicon nitride exists in two crystalline forms(a, B) in kilns(continuous or both belong to the hexagonal system. Bonding is pre- process and, wherever here has been a natu- dominantly covalent. Silicon nitride was first produced ral tendency to reduce the length of the time cycle for by an innovative form of pre less sintering. First, a small components. Faster rates of cooling after fragile pre-form of silicon powder(mainly a-Si3N4)is ng at the maximum temperature have been found to repared, using one of a wide variety of forming meth- give a finer, more desirable grain structure. ods (e.g die-pressing, isostatic-pressing, slip-casting, lame-spraying, polymer-assisted injection-moul Long-distance transportation c costs have extrusion). In the first stage of a reaction-bonding pro- cess, this pre-form is heated in a nitrogen atmosphere kaolinite can be leached in concentrated and the following chemical reaction takes place ulphuric acid, then precipitated as an alt salt which is calcined to form alumina. 3Si+ 2N2= Si3 N4
Ceramics and glasses 325 world. 2 In the Bayer process, prepared bauxitic ore is digested under pressure in a hot aqueous solution of sodium hydroxide and then 'seeded' to induce precipitation of AI(OH) 3 crystals, usually referred to by the mineral term 'gibbsite'. (The conditions of time, temperature, agitation, etc. during this stage greatly influence the quality of the Bayer product.) Gibbsite is chemically decomposed by heating (calcined) at a temperature of 1200~ Bayer calcine, which consists of c~-alumina (>99% A1203), is graded according to the nature and amount of impurities. Sodium oxide, Na20, ranges up to 0.6% and is of special significance because it affects sintering behaviour and electrical resistance. The calcine consists of agglomerates of c~-alumina crystallites which can be varied in average size from 0.5 to 100 ~tm by careful selection of calcining conditions. Bayer calcine is commonly used by manufacturers to produce high-purity alumina components as well as numerous varieties of lower-grade components containing 85-95% A1203. For the latter group, the composition of the calcine is debased by additions of oxides such as SiO2, CaO and MgO which act as fluxes, forming a fluid glassy phase between the grains of c~-alumina during sintering. The chosen grade of alumina, together with any necessary additives, is ground in wet ball-mills to a specified size range. Water is removed by spraying the aqueous suspension into a flow of hot gas (spray-drying) and separating the alumina in a cyclone unit. The free-flowing powder can be shaped by a variety of methods (e.g. dry, isostatic-or hot-pressing, slip- or tape-casting, roll-forming, extrusion, injectionmoulding). Extremely high production rates are often possible; for instance, a machine using air pressure to compress dry powder isostatically in flexible rubber moulds ('bags') can produce 300-400 spark plug bodies per hour. In some processes, binders are incorporated with the powder; for instance, a thermoplastic can be hot-mixed with alumina powder to facilitate injection-moulding and later burned off. In tape-casting, which produces thin substrates for microelectronic circuits, alumina powder is suspended in an organic liquid. 10.4.1.3 Densification by sintering The fragile and porous 'green' shapes are finally fired in kilns (continuous or intermittent). Firing is a costly process and, wherever possible, there has been a natural tendency to reduce the length of the time cycle for small components. Faster rates of cooling after 'soaking' at the maximum temperature have been found to give a finer, more desirable grain structure. 2Long-distance transportation costs have prompted investigation of alternative sources. For instance, roasted kaolinite can be leached in concentrated hydrochloric or sulphuric acid, then precipitated as an aluminium salt which is calcined to form alumina. It has been mentioned that fluxing oxides are added to lower-grade aluminas in order to form an intergranular phase(s). Although this fluid inter-granular material facilitates densification during firing, its presence in the final product can have a detrimental effect upon strength and resistance to chemical attack. As a consequence, powders of high alumina content are chosen for demanding applications. In general, an increase in alumina content from 88% to 99.8% requires a corresponding increase in firing temperature from 1450~ to 1750~ 'Harder' firing incurs heavier energy costs and has led to the development of reactive alumina which has an extremely small particle size (1 ~tm) and a large specific surface. 'Softer' firing temperatures became possible with this grade of alumina and the need to debase the alumina with relatively large amounts of additives was challenged. Shrinkage is the most apparent physical change to take place when a 'green' ceramic compact is fired. The linear shrinkage of alumina is about 20% and dimensions may vary by up to +1%. Diamond machining is used when greater precision is needed but requires care as it may damage the surface and introduce weakening flaws. 10.4.2 From silicon nitride to sialons 10.4.2.1 Reaction-bonded silicon nitride (RBSN) Silicon nitride, which can be produced in several ways, has found application under a variety of difficult conditions (e.g. cutting tools, bearings, heat engines, foundry equipment, furnace parts, welding jigs, metal-working dies, etc.). Its original development was largely stimulated by the search for improved materials for gas turbines. Prior to its development in the 1950s, the choice of fabrication techniques for ceramics was restricted and it was difficult to produce complex ceramic shapes to close dimensional tolerances. The properties available from existing materials were variable and specific service requirements, such as good resistance to thermal shock and attack by molten metal and/or slag, could not be met. The development of silicon nitride minimized these problems; it has also had a profound effect upon engineering thought and practice. Silicon nitride exists in two crystalline forms (c~,/3): both belong to the hexagonal system. Bonding is predominantly covalent. Silicon nitride was first produced by an innovative form of pressureless sintering. First, a fragile pre-form of silicon powder (mainly ct-Si3N4) is prepared, using one of a wide variety of forming methods (e.g die-pressing, isostatic-pressing, slip-casting, flame-spraying, polymer-assisted injection-moulding, extrusion). In the first stage of a reaction-bonding process, this pre-form is heated in a nitrogen atmosphere and the following chemical reaction takes place: 3Si + 2N2 -- Si3N4
326 Modern Physical Metallurgy and Materials Engineering A reticular network of reaction product forms through- point of intergranular phase significantly More specifi out the mass, bonding the particles together with cally, it yields crystalline oxynitrides(e. g. Y2Si3O3N4 out liquefaction. Single crystal'whiskers'of a-silicon which dissolve impurities(e. g. CaO)and form refrac nitride also nucleate and grow into pore space. Reac- tory solid solutions (mixed crystals). Unfortu tion is strongly exothermic and close temperature cor at high temperatures, yttria-containing silicon trol is necessary in order to prevent degradation of has a tendency to oxidize in a catastrophic and he silicon. The resultant nitrided compact is strong tive manner enough to withstand conventional machining. In the Ithough the use of dies places a restriction upor second and final stage of nitridation, the component component shape, hot-pressing increases the bulk den is heated in nitrogen at a temperature of 1400"C, sity and improves strength and corrosion resistance forming more silicon nitride in situ and producing a The combination of strength and a low coefficient slight additional change in dimensions of less than 1%. of thermal expansion(approximately 3. 2 x 10-bC (Alumina articles can change by nearly 10% during over the range 25-1000C)in hot-pressed silicor ring. The final microstructure consists of a-Si3N4 nitride confer excellent resistance to thermal shock (60-90%),B-Si3N4(10-40%),unreacted silicon and Small samples of HPSN are capable of surviving 100 porosity(15-30%). As with most ceramics, firing is thermal cycles in which immersion in molten steel e most costly stage of production (1600.C)alternates with quenching into water The final product, reaction-bonded silicon nitride In a later phase of development, other researchers (RBSN), has a bulk density of 2400-2600 kg m used hot isostatic-pressing(HIPing)to increase densit It is strong, hard and has excellent resistance to wear, further and to produce much more consistent proper thermal shock and attack by many destructive fluid itride powder, again used as the starti (molten salts, slags, aluminium, lead, tin, zinc, etc. ) material, together with a relatively small amount of the Its modulus of elasticity is high oxide additive(s )that promote liquid-phase sintering, is 10.4.2.2 Hot-pressed forms of silicon nitride in glass(silica or borosilicate). The capsule is evac (HPSN, HIPSN uated at a high temperature sealed and then hiped In the early 1960s, a greater degree of densification was with gas as the pressurizing medium, at pressures up chieved with the successful production of hot-pressed to 300 MN m"2 for a period of I h. Finally, the glass silicon nitride(HPSN)by G. G. Deeley and co-workers envelope is removed from the isotropic HIPSN compo at the Plessey Co. UK. Silicon nitride powder, which nent by sand-blasting. Like HPSN its microstructure cannot be consolidated by solid-state sintering alone is mixed with one or more fluxing oxides(magne- nterg residue(mainly siliceous glass). sia, yttria, alumina) and compressed at a pressure of Production routes involving deformation at very higl I h. The thin film of silica that is usually present on sil- inevitably cause a very substantial amount of shrinkage forms a molten phase. Densification and mass trans- undergoes negligible shrinkage during sintering at the ort then take place at the high temperature in a typical lower process temperature of 1400'C and accordingl liquid-phase sintering process. As this intergranular contains much weakening porosity, say 15-30% v/v ase cools, it forms a siliceous glass which can be By the early 1970s, considerable progress had been nade in producing silicon nitride by reaction-bonding encouraged to crystallize(devitrify) by slow cooling hot-pressing and other routes. Howe\es by then it had ately produces a limited amount of second phase(up to become evident that further significant improvements 3%v/v)as a means of bonding the refractory particles; in the quality and capabilities of silicon nitride were however, this bonding phase has different properties to unlikely. At this juncture, attention shifted to the sialons silicon nitride and can have a weakening effect particu larly if service temperatures are high. Thus, with 3-5% 10.4.2.3 Scientific basis of sialons added magnesia, at temperatures below the softening Although silicon nitride possesses extremely usefu int of the residual glassy phase, say 1000C, silicon properties, its engineering exploitation has been ham- nitride behaves as a brittle and stiff material; at higher pered by the difficulty of producing it in a fully dense temperatures, there is a fairly abrupt loss in strength, form to precise dimensional tolerances. Hot-pressing as expressed by modulus of rupture(MoR) values, and offers one way to surmount the problem but it is a slow deformation under stress(creep)becomes evident. costly process and necessarily limited to simple shapes For these reasons, controlled modification of the struc- The development of sialons provided an attractive and ture of the inter-granular residual phase is of particular feasible solution to these problems scientific concern Sialons are derivatives of silicon nitride and are Yttria has been used as an alternative densifier to accordingly also classified as nitrogen ceramics. The magnesia. Its general effect is to raise the softening acronym sialon signifies that the material is base
326 Modern Physical Metallurgy and Materials Engineering A reticular network of reaction product forms throughout the mass, bonding the particles together without liquefaction. Single crystal 'whiskers' of or-silicon nitride also nucleate and grow into pore space. Reaction is strongly exothermic and close temperature control is necessary in order to prevent degradation of the silicon. The resultant nitrided compact is strong enough to withstand conventional machining. In the second and final stage of nitridation, the component is heated in nitrogen at a temperature of 1400~ forming more silicon nitride in situ and producing a slight additional change in dimensions of less than 1%. (Alumina articles can change by nearly 10% during firing.) The final microstructure consists of ot-Si3N4 (60-90%), /3-Si3N4 (10-40%), unreacted silicon and porosity (15-30%). As with most ceramics, firing is the most costly stage of production. The final product, reaction-bonded silicon nitride (RBSN), has a bulk density of 2400-2600 kg m -3. It is strong, hard and has excellent resistance to wear, thermal shock and attack by many destructive fluids (molten salts, slags, aluminium, lead, tin, zinc, etc.). Its modulus of elasticity is high. 10.4.2.2 Hot-pressed forms of silicon nitride (HPSN, HIPSN) In the early 1960s, a greater degree of densification was achieved with the successful production of hot-pressed silicon nitride (HPSN) by G. G. Deeley and co-workers at the Plessey Co. UK. Silicon nitride powder, which cannot be consolidated by solid-state sintering alone, is mixed with one or more fluxing oxides (magnesia, yttria, alumina) and compressed at a pressure of 23 MN m -2 within radio-frequency induction-heated graphite dies at temperatures up to 1850~ for about 1 h. The thin film of silica that is usually present on silicon nitride particles combines with the additive(s) and forms a molten phase. Densification and mass transport then take place at the high temperature in a typical 'liquid-phase' sintering process. As this intergranular phase cools, it forms a siliceous glass which can be encouraged to crystallize (devitrify) by slow cooling or by separate heat-treatment. This HP route deliberately produces a limited amount of second phase (up to 3% v/v) as a means of bonding the refractory particles; however, this bonding phase has different properties to silicon nitride and can have a weakening effect, particularly if service temperatures are high. Thus, with 3-5% added magnesia, at temperatures below the softening point of the residual glassy phase, say 1000~ silicon nitride behaves as a brittle and stiff material; at higher temperatures, there is a fairly abrupt loss in strength, as expressed by modulus of rupture (MoR) values, and slow deformation under stress (creep) becomes evident. For these reasons, controlled modification of the structure of the inter-granular residual phase is of particular scientific concern. Yttria has been used as an alternative densifier to magnesia. Its general effect is to raise the softening point of intergranular phase significantly. More specifically, it yields crystalline oxynitrides (e.g. Y2Si303N4) which dissolve impurities (e.g. CaO) and form refractory solid solutions ('mixed crystals'). Unfortunately, at high temperatures, yttria-containing silicon nitride has a tendency to oxidize in a catastrophic and disruptive manner. Although the use of dies places a restriction upon component shape, hot-pressing increases the bulk density and improves strength and corrosion resistance. The combination of strength and a low coefficient of thermal expansion (approximately 3.2 x 10 -6 ~ over the range 25-1000~ in hot-pressed silicon nitride confer excellent resistance to thermal shock. Small samples of HPSN are capable of surviving 100 thermal cycles in which immersion in molten steel (1600~ alternates with quenching into water. In a later phase of development, other researchers used hot isostatic-pressing (HIPing) to increase density further and to produce much more consistent properties. Silicon nitride powder, again used as the starting material, together with a relatively small amount of the oxide additive(s) that promote liquid-phase sintering, is formed into a compact. This compact is encapsulated in glass (silica or borosilicate). The capsule is evacuated at a high temperature, sealed and then HIPed, with gas as the pressurizing medium, at pressures up to 300 MN m -2 for a period of 1 h. Finally, the glass envelope is removed from the isotropic HIPSN component by sand-blasting. Like HPSN, its microstructure consists of fl-Si3N4 (>90%) and a small amount of intergranular residue (mainly siliceous glass). Production routes involving deformation at very high temperatures and pressures, as used for HPSN and HIPSN, bring about a desirable closure of pores but inevitably cause a very substantial amount of shrinkage (20-30%). (In contrast to HPSN and HIPSN, RBSN undergoes negligible shrinkage during sintering at the lower process temperature of 1400~ and accordingly contains much weakening porosity, say 15-30% v/v.) By the early 1970s, considerable progress had been made in producing silicon nitride by reaction-bonding, hot-pressing and other routes. However, by then it had become evident that further significant improvements in the quality and capabilities of silicon nitride were unlikely. At this juncture, attention shifted to the sialons. 10.4.2.3 Scientific basis of sialons Although silicon nitride possesses extremely useful properties, its engineering exploitation has been hampered by the difficulty of producing it in a fully dense form to precise dimensional tolerances. Hot-pressing offers one way to surmount the problem but it is a costly process and necessarily limited to simple shapes. The development of sialons provided an attractive and feasible solution to these problems. Sialons are derivatives of silicon nitride and are accordingly also classified as nitrogen ceramics. The acronym 'sialon' signifies that the material is based
Ceramics and glasses 327 upon the Si-Al-O-N system. In 1968, on the basis of three tetrahedra. In the unit cell. six Si4+ ions balance structural analyses of silicon nitrides, it was predicted the electrical charge of eight N3-, giving a starting for tetrahedral network could be replaced by aluminium is customarily represented by the chemical formula than silicon. Furthermore, it was also predicted that replaced by oxygen atoms. The term z ranges in value systematic replacement of silicon by aluminium would from 0 to 4. Although considerable solid solution in allow other types of metallic cation to be accommo- silicon nitride is possible, the degree of replacement dated in the structure. Such replacement within the sought in practice is often quite small. with replace SiNa structural units of silicon nitride would make ment, the formula for the tetrahedral unit changes from it possible to simulate the highly versatile manner in SiN4 to(Si, Al)(O, N)4 and the dimensions of the unit which SiOa and AlOa tetrahedra arrange themselves in el increas aluminosilicates. A similarly wide range of structures and properties was anticipated for this new family of sition to shift towards that of alumina. the structural ceramic'alloys. About two years after the vital pre- coordination in the solid solution is fourfold (AIO) diction, British and Japanese groups, acting indepen- whereas in alumina it is sixfold(AlO ) The strength dently, produced B-silicon nitride, the solid solution of the al-o bond in a sialon is therefore about 50%0 In B-silicon nitride, the precursor, SiNa tetrahedra stronger than its counterpart in alumina; this concentra form a network structure, Each tetrahedron has a cen tral Si"+ which is surrounded by four equidistant N3- ions makes a sialon intrinsically stronger than alumina. The problem of representing complex phase rela- (Figure 10.2). Each of these corner n- is common to tionships in a convenient form was solved by adopt ing the 'double reciprocal'diagram, a type of phase diagram originally developed for inorganic salt sys- tems by German physical chemists many years ago Figure 10.3 shows how a tetrahedron for the four ele- ments Si, Al,O and n provides a symmetrical frame of reference for four compounds. By using linear scales calibrated in equivalent %(rather than the usual No), each way on a tetrahedral edge and the resulting section is square. An isothermal version of this type of diagram SitE ALO Si figure 10.2 The crystal structur B'-(Si,AD)3(O N)4O metal atom, O non-metal atom By K. H. Jack and colleagues at the University of equivalent Al writings of K. H. Jack on silicon nitride and sialons provide ight into the complexities of developing a new Figure 10.3 Relation between Si-Al-O-N tetrahedron and
Ceramics and glasses 327 upon the Si-AI-O-N system. In 1968, on the basis of structural analyses of silicon nitrides, it was predicted ~ that replacement of nitrogen (N 3-) by oxygen (O 2-) was a promising possibility if silicon (Si 4+) in the tetrahedral network could be replaced by aluminium (AI3+), or by some other substituent of valency lower than silicon. Furthermore, it was also predicted that systematic replacement of silicon by aluminium would allow other types of metallic cation to be accommodated in the structure. Such replacement within the SiN4 structural units of silicon nitride would make it possible to simulate the highly versatile manner in which SiO4 and A104 tetrahedra arrange themselves in aluminosilicates. A similarly wide range of structures and properties was anticipated for this new family of ceramic 'alloys'. About two years after the vital prediction, British and Japanese groups, acting independently, produced if-silicon nitride, the solid solution which was to be the prototype of the sialon family. In /3-silicon nitride, the precursor, SiN4 tetrahedra form a network structure. Each tetrahedron has a central Si 4+ which is surrounded by four equidistant N 3- (Figure 10.2). Each of these corner N 3- is common to Q Figure 10.2 The crystal structure of ~-Si3N4 and ff-(Si, Al)3 (O,N) 4 0 metal atom, 0 non-metal atom (from Jack, 1987, pp. 259-88; reprinted by permission of the American Ceramic Society). N AI I By K. H. Jack and colleagues at the University of Newcastle-upon-Tyne; separate British and Japanese groups filed patents for producing sialons in the early 1970s. The writings of K. H. Jack on silicon nitride and sialons provide an insight into the complexities of developing a new engineering material. three tetrahedra. In the unit cell, six Si 4+ ions balance the electrical charge of eight N 3-, giving a starting formula Si6Ns. Replacement of Si 4+ and N 3- by AI 3+ and O 2-, respectively, forms a ff-sialon structure which is customarily represented by the chemical formula Si6-zAlzOzNs-z, where z = number of nitrogen atoms replaced by oxygen atoms. The term z ranges in value from 0 to 4. Although considerable solid solution in silicon nitride is possible, the degree of replacement sought in practice is often quite small. With replacement, the formula for the tetrahedral unit changes from SiN4 to (Si, AI) (O, N)4 and the dimensions of the unit cell increase. Although replacement causes the chemical composition to shift towards that of alumina, the structural coordination in the solid solution is fourfold (A104) whereas in alumina it is sixfold (A106). The strength of the A1-O bond in a sialon is therefore about 50% stronger than its counterpart in alumina; this concentration of bonding forces between aluminium and oxygen ions makes a sialon intrinsically stronger than alumina. The problem of representing complex phase relationships in a convenient form was solved by adopting the 'double reciprocal' diagram, a type of phase diagram originally developed for inorganic salt systems by German physical chemists many years ago. Figure 10.3 shows how a tetrahedron for the four elements Si, AI, O and N provides a symmetrical frame of reference for four compounds. By using linear scales calibrated in equivalent % (rather than the usual weight, or atomic %), each compound appears midway on a tetrahedral edge and the resulting section is square. An isothermal version of this type of diagram AI4N4 % equivalent AI % equivalent N Figure 10.3 Relation between Si-AI-O-N tetrahedron and square Si3 06 -Al4 06 -Al4 N4 -Si3 N4 plane
328 Modern Physical Metallurgy and Materials Engineering MAN(3AI,O 104.2. 4 Production of sialons The start point for sialon production from silicon 3A,O AINI r at a temperature of 1800C will lie bottom left-hand corner of Four Figure and Si with Al produces the desired B-phase which is represented by the narrow diagonal zone project ing towards the AlO6 corner. Such alloying of the ceramic structure produces progressive and subtle changes in the structure of silicon nitride by altering MaIS, N,o The resultant properties can be exceptional. Impor- at temperatures above 1000"C are greatly superior to those of conventional silicon nitride. Relatively sin Euv %. 5 ple fabrication procedures, similar to those used for oxide ceramics, can be adopted. Pressureless-sintering Figure 10. 4 Si-Al-O-N behaviour diagram at 1800C enables dense complex shapes of moderate size to be (from Jack, 1987, pp. 259-88: reprinted by pe produced the American Ceramic Society) B-Si3N4 powder is the principal constituent of the tarting mixture for alloying,(As mentioned previ is shown in Figure 10.4. Thedouble reciprocal char: silica.) Although fine aluminium nitride would appear Si/Al along the vertical and horizontal axes, respe to be an appropriate source of replacement aluminiun tively. It is necessarily assumed that the valency of the fabrication routes which involve aqueous solutions or four elements is fixed (i.e. Si4+, A1+,o2-and N3-) As the formula for the component Si3N4 contains 12 binders. One patented method for producing a B-sialon cations and 12 anions, the formulae for the other three nitride (and its associated silica) with a specially prepared polytypoid. The phase relations for thi along the axes are expressed in the forms which give method are shown in figure 10.4 The equivalent of a given element in these formulae An addition of yttrium oxide to the mixture can be derived from the following equations causes an intergranular liquid phase to form during pressureless-sintering and encourage densification. By Equivalent %o oxygen it is possible to 100( atomic%O×2) (denitrify). In sialons. as in many other ceramics, atomIc%O×2)+( atomIc%N×3) the final character of the intergranular phase has Equivalent nitrogen structure of B grains glass is strong and resists thermal shock at temperatures approaching 1000C Equivalent aluminium However, at higher temperatures the glassy phase deforms in a viscous manner and strength suffers 100( atomic%Al×3 Improved stability and strength can be achieved by a atomIc%Al×3)+( atomic%Si×4) losely-controlled heat-treatment which transforms the Equivalent %o silicon (YAG, phase into crystals of yttrium-aluminium-garnet as represented in the following equation Sis AION, Y-Si-AI-O-N Thus the intermediate phase labelled 3/2(Si2N2O β- sialon Oxynitride ontains 25 equivalent o oxygen and is located one glass quarter of the distance up the left-hand vertical scale Sis+rAl- O,- N7+r Y3AlsO12 An interesting feature of the diagram is the parallel Modified YAG sequence of phases near the aluminium nitride corner B-sialon to as aluminium nitride ' polytypoids, or polytypes'. The two-phase structure of p grains YAG is They have crystal structures that follow the pattern extremely stable. It does not degrade in the presence of of wurtzite(hexagonal ZnS)and are generally stable, molten metals and maintains strength and creep resis refractory and oxidation-resistant ance up to a temperature of 1400C
328 Modern Physical Metallurgy and Materials Engineering Sl30,~ e/, .3(3AI20) 2STO2) AI,O~ ~ 4/t13AIzO3 AIN) Eaulv % N 4/~(AI;,O~ AIN) 3,,~,1 :io N S,,N. i800 C-- AI.N, I Equtv 9'0 S~ Equ=v % AI Figure 10.4 Si-AI-O-N behaviour diagram at 1800~ (from Jack, 1987, pp. 259-88; reprinted by permission of the American Ceramic Society). is shown in Figure 10.4. The 'double reciprocal' characteristic refers to the equivalent interplay of N/O and Si/AI along the vertical and horizontal axes, respectively. It is necessarily assumed that the valency of the four elements is fixed (i.e. Si 4+, AI 3+, 02- and N3-). As the formula for the component Si3N4 contains 12 cations and 12 anions, the formulae for the other three components and for the various intermediate phases along the axes are expressed in the forms which give a similar charge balance (e.g. 5i306 rather than SiO2). The equivalent % of a given element in these formulae can be derived from the following equations: Equivalent % oxygen 100(atomic %0 • 2) (atomic %0 • 2)+ (atomic %N x 3) Equivalent % nitrogen = 100%- equivalent % oxygen Equivalent % aluminium 100(atomic %AI • 3) (atomic %AI • 3)+ (atomic %Si • 4) Equivalent % silicon = 100% - equivalent %AI Thus the intermediate phase labelled 3/2 (Si2N20) contains 25 equivalent % oxygen and is located one quarter of the distance up the left-hand vertical scale. An interesting feature of the diagram is the parallel sequence of phases near the aluminium nitride corner (i.e. 27R, 21R, 12H, 15R and 8H). They are referred to as aluminium nitride 'polytypoids', or 'polytypes'. They have crystal structures that follow the pattern of wurtzite (hexagonal ZnS) and are generally stable, refractory and oxidation-resistant. 10.4.2.4 Production of sialons The start point for sialon production from silicon nitride powder at a temperature of 1800~ will lie in the vicinity of the bottom left-hand corner of Figure 10.4. Simultaneous replacement of N with O and Si with A1 produces the desired /3'-phase which is represented by the narrow diagonal zone projecting towards the A1406 comer. Such 'alloying' of the ceramic structure produces progressive and subtle changes in the structure of silicon nitride by altering the balance between covalent and ionic bonding forces. The resultant properties can be exceptional. Importantly, the oxidation resistance and strength of sialons at temperatures above 1000~ are greatly superior to those of conventional silicon nitride. Relatively simple fabrication procedures, similar to those used for oxide ceramics, can be adopted. Pressureless-sintering enables dense complex shapes of moderate size to be produced. /3-5i3N4 powder is the principal constituent of the starting mixture for 'alloying'. (As mentioned previously, these particles usually carry a thin layer of silica.) Although fine aluminium nitride would appear to be an appropriate source of replacement aluminium, it readily hydrolyses, making it impracticable to use fabrication routes which involve aqueous solutions or binders. One patented method for producing a/3'-sialon (z = 1) solves this problem by reacting the silicon nitride (and its associated silica) with a speciallyprepared 'polytypoid'. The phase relations for this method are shown in Figure 10.4. An addition of yttrium oxide to the mixture causes an intergranular liquid phase to form during pressureless-sintering and encourage densification. By controlling conditions, it is possible to induce this phase either to form a glass or to crystallize (devitrify). In sialons, as in many other ceramics, the final character of the intergranular phase has a great influence upon high-temperature strength. A structure of fl' grains + glass is strong and resists thermal shock at temperatures approaching 1000~ However, at higher temperatures the glassy phase deforms in a viscous manner and strength suffers. Improved stability and strength can be achieved by a closely-controlled heat-treatment which transforms the glassy phase into crystals of yttrium-aluminium-garnet (YAG), as represented in the following equation: SisA1ON7 + Y-Si-A1-O-N /3'-sialon Oxynitride (z = 1) glass Sis+xAll_xOl_xN7+x + Y3AI5012 Modified YAG /3'-sialon The two-phase structure of fl' grains + YAG is extremely stable. It does not degrade in the presence of molten metals and maintains strength and creep resistance up to a temperature of 1400~
Ceramics and glasses 329 More recent work has led to the production of sialons from precursors other than B-silicon nitride (e. g. a-sialons from a-silicon nitride and O-sialons 2000 from oxynitrides). K. H. Jack proposed that a-silicon nitride, unlike the B-form, is not a binary compoun and should be regarded as an oxynitride, a defect struc ture showing limited replacement of nitrogen by oxy- gen. The formula for its structural unit approximates to SiN3. O, 1. Dual-phase or composite structures have also been developed in which paired combinations of Sialon B, a-and O- phases provide enhancement of engi neering properties. Sometimes, as in a/p composites, there is no glassy or crystalline intergranular phase The sialon principle can be extended to some unusual Alumina natural waste materials. For instance, two siliceous materials, volcanic ash and burnt rice husks have each been used in sinter mixes to produce sialons, Although such products are low grade, it has been proposed that WC/Co they could find use as melt-resistant refract 10.4.2.5 Engineering applications of sialons The relative ease with which sialons can be shaped is one of their outstanding characteristics. Viable shap Temp. ( C) alumina and wC/Co extrusion,slip-casting and injection-moulding; their cutting tool tips (from Jack, 1987, pp. 259-88; reprinted by variety has been a great stimulus to the search for permission of the American Ceramic Society) ovel engineering applications. Similarly, their ability order of 1800 C, without need of pressure application, The strength and wear resistance of sialons led favours the production of complex shapes. However, their use in the metal-working operations of extrusion due allowance must be made for the large amount of (hot- and cold-)and tube-drawing. In each process, linear shrinkage (20-25%)which occurs as a result the relative movement of the metal stock through the of liquid phase formation during sintering. Although die aperture should be fast with low friction and mini final machining with diamond grit, ultrasonic energy mal die wear, producing closely dimensioned bar/tube or laser beam energy is possible, the very high hard- with a smooth and sound surface texture. Sialon die ness of sialons encourages adoption of a near-net-shape serts have been successfully used fo ceramics, sialon components are extremely sensitive the long-established use of tungsten carbide inserts in curvature or section can frequently improve service Sialons have also been used for the plugs(captive floating)which control bore size during certain performance. The structure of a sialon is, of course, the tube-drawing operations. It appears that the absence main determinant of its properties. Fortunately, sialons of metallic microconstituents in sialons obviates the risk of attributes such as strength, stability at high tempera and/or plugs and the metal being shaped. Sialon tools molten metals can be developed in order to withstand have made it possible to reduce the problems normally During metal-machining, tool tips are subjected stainless steels to highly destructive and complex conditions which The endurance of sialons at high temperatures and include high local temperatures and thermal shock, in the presence of invasive molten metal or slag has high stresses and impact loading, and degradation by led to their use as furnace and crucible refractories hardness of p-sialon (+ glass)is much greater than ponents in electrical machines for welding(e. g. gas bide( Figure 10.5). The introduction of tool tips made cations can demand resistance to thermal shock and from this sialon was a notable success. They wer wear, electrical insulation, great strength as well as found to have a longer edge life than conventional immunity to attack by molten metal spatter. sialons tungsten carbide inserts, could remove metal at high have proved superior to previous materials(alumina, speed with large depths of cut and could tolerate the hardened steel) and have greatly extended the service shocks, mechanical and thermal, of interrupted cutting. life of these small but vital machine components
Ceramics and glasses 329 More recent work has led to the production of sialons from precursors other than fl-silicon nitride (e.g. a'-sialons from a-silicon nitride and O'-sialons from oxynitrides). K. H. Jack proposed that a-silicon nitride, unlike the /%form, is not a binary compound and should be regarded as an oxynitride, a defect structure showing limited replacement of nitrogen by oxygen. The formula for its structural unit approximates to SiN3.900.1. Dual-phase or composite structures have also been developed in which paired combinations of if-, a'- and O'- phases provide enhancement of engineering properties. Sometimes, as in a'/ff composites, there is no glassy or crystalline intergranular phase. The sialon principle can be extended to some unusual natural waste materials. For instance, two siliceous materials, volcanic ash and burnt rice husks, have each been used in sinter mixes to produce sialons. Although such products are low grade, it has been proposed that they could find use as melt-resistant refractories. 10.4.2.5 Engineering applications of sialons The relative ease with which sialons can be shaped is one of their outstanding characteristics. Viable shaping techniques include pressing (uniaxial, isostatic), extrusion, slip-casting and injection-moulding; their variety has been a great stimulus to the search for novel engineering applications. Similarly, their ability to densify fully during sintering at temperatures in the order of 1800~ without need of pressure application, favours the production of complex shapes. However, due allowance must be made for the large amount of linear shrinkage (20-25%) which occurs as a result of liquid phase formation during sintering. Although final machining with diamond grit, ultrasonic energy or laser beam energy is possible, the very high hardness of sialons encourages adoption of a near-net-shape approach to design. As with many other engineering ceramics, sialon components are extremely sensitive to shape and it is generally appreciated that a change in curvature or section can frequently improve service performance. The structure of a sialon is, of course, the main determinant of its properties. Fortunately, sialons are very responsive to 'alloying' and combinations of attributes such as strength, stability at high temperatures, resistance to thermal shock, mechanical wear and molten metals can be developed in order to withstand onerous working conditions. During metal-machining, tool tips are subjected to highly destructive and complex conditions which include high local temperatures and thermal shock, high stresses and impact loading, and degradation by wear. At a test temperature of 1000~ the indentation hardness of ff-sialon (+ glass) is much greater than that of either alumina or cobalt-bonded tungsten carbide (Figure 10.5). The introduction of tool tips made from this sialon was a notable success. They were found to have a longer edge life than conventional tungsten carbide inserts, could remove metal at high speed with large depths of cut and could tolerate the shocks, mechanical and thermal, of interrupted cutting. t E E I1 kg load 2000 1500 "'m ~~%~ Sialon 1000 Alumina 500 ~~ i WC/Co I .... II 0 500 1000 Temp. (~ Figure 10.5 Hot hardness of sialon, alumina and WC/Co cutting tool tips (from Jack, 1987, pp. 259-88; reprinted by permission of the American Ceramic Society). The strength and wear resistance of sialons led to their use in the metal-working operations of extrusion (hot- and cold-) and tube-drawing. In each process, the relative movement of the metal stock through the die aperture should be fast with low friction and minimal die wear, producing closely dimensioned bar/tube with a smooth and sound surface texture. Sialon die inserts have been successfully used for both ferrous and non-ferrous metals and alloys, challenging the long-established use of tungsten carbide inserts. Sialons have also been used for the plugs (captive or floating) which control bore size during certain tube-drawing operations. It appears that the absence of metallic microconstituents in sialons obviates the risk of momentary adhesion or 'pick-up' between dies and/or plugs and the metal being shaped. Sialon tools have made it possible to reduce the problems normally associated with the drawing of difficult alloys such as stainless steels. The endurance of sialons at high temperatures and in the presence of invasive molten metal or slag has led to their use as furnace and crucible refractories. On a smaller scale, sialons have been used for components in electrical machines for welding (e.g. gas shrouds, locating pins for the workpiece). These applications can demand resistance to thermal shock and wear, electrical insulation, great strength as well as immunity to attack by molten metal spatter. Sialons have proved superior to previous materials (alumina, hardened steel) and have greatly extended the service life of these small but vital machine components