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