330 Modern Physical Metallurgy and Materials Engineering The search for greater effie gines, petrol and diesel, has focused attention o egions of the engine that are subjected to the mos evere conditions of heat and wear. Sialons have been adopted for pre-combustion bers in indi- ect diesel engines. Replacement of metal with ceramic also improves the power/weight ratio. It is still possible that the original goal of researches on silicon nitride m-grains and sialons, the ceramic gas turbine, will eventually be 10.4.3 Zirconia Figure 10.6 Crack propagating into grains of i-zirconia ausing them to transform into mm-zirconi Zirconium oxide(zrO2) has a very high melting point (2680C), chemical durability and is hard and strong because of these properties, it has long been used for fractory containers and as an abrasive medium. at temperatures above 1200"C, it becomes electrically conductive and is used for heating elements in furnace operating with oxidizing atmospheres. Zirconia-based materials have similar thermal expansion characteris- tics to metallic alloys and can be usefully integrated conia, thereby reducing notch-sensitivity and radiar .,'x with metallic components in heat engines. In addition to these established applications, it has been found fracture toughness values into the 15-20 mn m band, thus providing a new class of toughened ceram ics. This approach is an alternative to increasing the toughness of a ceramic by either(1)adding filaments or (2)introducing microcracks that will blunt the tip Zirconia is polymorphic, existing in three crystalline forms; their interrelation, in order of decreasing tem- perature, is as follows Melt Cubic- Tetragonal Monoclinic zirconia caronia polycrystal, PSZ= partially-stabilized =cubic-stabilized zircon The technique of transformation-toughening hinges ilizin form so that it is metastable at room temperature. diagram for the zirconia-rich end of the ZrO2-Y203 Stabilization, partial or whole, is achieved by adding system(Figure 10.7). The same principles apply im ertain oxides (Y2O3, MgO, CaO) to zirconia. In a very general sense to the other two binary sys- the metastable condition, the surrounding structur tems, ZrO2-MgO and ZrO2-Cao. Y tria is partic opposes the expansive transition from t- to m-forms ularly effective as a stabilizer. Three zirconia-based the event of a propagating crack passing into or types of ceramic have been superimposed upon the ear metastable regions, the concentrated stress field diagram; CSZ, TZP and PSZ. The term CSZ refers at the crack tip enables t-crystals of zirconia-rich to material with a fully-stabilized cubic(not tetrag solid solution to transform into stable, but less dense, onal)crystal structure which cannot take advantage m-ZrO2(Figure 10.6). The transformation is marten- of the toughening transformation. It is used for fur sitic in character. The associated volumetric expan- nace refractories and crucibles. The version known sion(3-5%v/v) tends to close the crack and relieve as tetragonal zirconia polycrystal (TZP) contains the stresses at its tip This transformation mechanism is least amount of oxide additive( e.g. 2-4 mol%Y 2O3) primarily responsible for the beneficial toughening and is produced in a fine-grained form by sinter- ffect of a metastable phase within the microstructure. ing and densifying ultra-fine powder in the tem The relative stability of zirconia-rich solid solutions perature range 1350-1500.C; such temperatures are can be conveniently expressed in terms of the phase well within the phase field for the tetragonal solid
330 Modern Physical Metallurgy and Materials Engineering The search for greater efficiency in automotive engines, petrol and diesel, has focused attention on regions of the engine that are subjected to the most severe conditions of heat and wear. Sialons have been adopted for pre-combustion chambers in indirect diesel engines. Replacement of metal with ceramic also improves the power/weight ratio. It is still possible that the original goal of researches on silicon nitride and sialons, the ceramic gas turbine, will eventually be achieved. 10.4.3 Zirconia Zirconium oxide (ZrO2) has a very high melting point (2680~ chemical durability and is hard and strong; because of these properties, it has long been used for refractory containers and as an abrasive medium. At temperatures above 1200~ it becomes electrically conductive and is used for heating elements in furnaces operating with oxidizing atmospheres. Zirconia-based materials have similar thermal expansion characteristics to metallic alloys and can be usefully integrated with metallic components in heat engines. In addition to these established applications, it has been found practicable to harness the structural transitions of zirconia, thereby reducing notch-sensitivity and raising fracture toughness values into the 15-20 MN m -3/2 band, thus providing a new class of toughened ceramics. This approach is an alternative to increasing the toughness of a ceramic by either (1) adding filaments or (2) introducing microcracks that will blunt the tip of a propagating crack. Zirconia is polymorphic, existing in three crystalline forms; their interrelation, in order of decreasing temperature, is as follows: Melt -Cubic ----~ Tetragonal 2680~ c 2370~ t 950~ Monoclinic 1150~ m The technique of transformation-toughening hinges upon stabilizing the high-temperature tetragonal (t) form so that it is metastable at room temperature. Stabilization, partial or whole, is achieved by adding certain oxides (Y203, MgO, CaO) to zirconia. In the metastable condition, the surrounding structure opposes the expansive transition from t- to m-forms. In the event of a propagating crack passing into or near metastable regions, the concentrated stress field at the crack tip enables t-crystals of zirconia-rich solid solution to transform into stable, but less dense, m-ZrO2 (Figure 10.6). The transformation is martensitic in character. The associated volumetric expansion (3-5% v/v) tends to close the crack and relieve stresses at its tip. This transformation mechanism is primarily responsible for the beneficial toughening effect of a metastable phase within the microstructure. The relative stability of zirconia-rich solid solutions can be conveniently expressed in terms of the phase t-grains ~ i Crack ~ m-grains j Figure 10.6 Crack propagating into grains of t-zirconia, causing them to transform into m-zirconia. 9 9 ' ,,. o\ i\ I !,0oo\\,,\ \, s~ I I I I m'c I I I I I- ! I -- Ya ~ t y 0 5 10 1.5 1 O0 ZrO Y~O~/% w/w I 20 Figure 10.7 Schematic phase diagram for Zr02 -Y2 03 system." all phases depicted are solid solutions. TZP = tetragonal zirconia polycrystal, PSZ = partially-stabilized zirconia, CSZ = cubic-stabilized zirconia. diagram for the zirconia-rich end of the ZrO2-Y203 system (Figure 10.7). The same principles apply in a very general sense to the other two binary systems, ZrO2-MgO and ZrO2-CaO. Yttria is particularly effective as a stabilizer. Three zirconia-based types of ceramic have been superimposed upon the diagram; CSZ, TZP and PSZ. The term CSZ refers to material with a fully-stabilized cubic (not tetragonal) crystal structure which cannot take advantage of the toughening transformation. It is used for furnace refractories and crucibles. The version known as tetragonal zirconia polycrystal (TZP) contains the least amount of oxide additive (e.g. 2-4 tool% Y203) and is produced in a fine-grained form by sintering and densifying ultra-fine powder in the temperature range 1350-1500~ such temperatures are well within the phase field for the tetragonal solid
Ceramics and glasses 331 olution( Figure 10.7). After cooling to room temper ature, the structure is essentially single-phase, consi ains(0. 2-1 um) of t-zrO2 which make this material several times stronger than other types of zirconia-toughened ceramics. A typical TZP microstructure, as revealed by electron microscopy is shown in Figure 10.8. Added oxide(s)and sili- cate impurities form an intergranular phase which can liquid-phase sintering during consolidation similar effect is utilized in the production of silicon In partially-stabilized zirconia(PSZ), small t-crys tals are dispersed as a pre f coarser cubic grains. Zirconia is mixed with 8-10 mol% additive (MgO, Cao or Y2O3)and heat treated in two stages(Figure 10.7). Sintering in the mperature range 1650-1850.C produces a parent olution with a cubic structure which is then modified by heating in the range 1100-1450C. This second treatment induces a precipitation of coherent t-crystals(200 nm in size) within the c-grains. The morphology of the precipitate depends upon the nature of the added solute(e.g. ZrO2-MgO, ZrO2-CaO and ZrO2-Y2O, solid solutions produce lenticular, cuboid and platey crystals, respectively). The average size of electro precipitate crystals is determined by the conditions of (from Green, 1984, p. 84; by permission of marcel temperature and time adopted during heat-treatment in the crucial t +cfield of the phase diagram In the third example of transformation-toughening So far, we have concentrated upon t-zirconia grains are dispersed in a dissimilar ceramic behaviour at or below ambient temperature matrix, for example, in ZT(A) or ZT(Al203) they perature of a zirconia-toughened material is raised are dispersed among alumina grains(Figure 10.9). 900-1000 C, which is close to the t-m transition tem- results when conventional processing methods are used ineffective. In addition, thermal cycling in service but it has also been found possible to produce an tends to induce the t-m transition at temperatures in the ntragranular distribution. As with PSZ materials th size of metastable particles and matrix grains must be range 800-900 C and the toughening property is grad- carefully controlled and balanced ually lost. This tendency for fracture toughness to fall the investigation of alternative forms of stabilization in systems which have much higher transformation tem- peratures(e. g ZT(HfO2). Intergranular residues(e.g in TZP), despite their beneficial effect during sintering, become easier to deform as the temperature rises and the material then suffers loss of strength and resistance to creep. 10.4.4 Glass-ceramics 10. 4.4.1 Controlled devitrification of a glass It has long been appreciated that crystallization can ake place in conventional glassy structures, particu arly when they are heated. However, such crystalliza 200mm tion is initiated at relatively few sites and there is a tendency for crystals to grow perpendicular to the free Figure 10.8 Electron micrograph of tetragonal-zirconia urface of the glass in a preferred manner. The result ystal stabilized with 3 mol. yttria(with ng structure, being coarsely crystalline and strongly knowledgement to M. G. Cain, Centre for Advanced oriented, is mechanically weak and finds no practical Materials Technology, University of Warwick, UK) cation
solution (Figure 10.7). After cooling to room temperature, the structure is essentially single-phase, consisting of very fine grains (--~0.2-1 ~tm) of t-ZrO2 which make this material several times stronger than other types of zirconia-toughened ceramics. A typical TZP microstructure, as revealed by electron microscopy, is shown in Figure 10.8. Added oxide(s) and silicate impurities form an intergranular phase which can promote liquid-phase sintering during consolidation. (A similar effect is utilized in the production of silicon nitride.) In partially-stabilized zirconia (PSZ), small t-crystals are dispersed as a precipitate throughout a matrix of coarser cubic grains. Zirconia is mixed with 8-10 mol% additive (MgO, CaO or Y203) and heattreated in two stages (Figure 10.7). Sintering in the temperature range 1650-1850~ produces a parent solid solution with a cubic structure which is then modified by heating in the range 1100-1450~ This second treatment induces a precipitation of coherent t-crystals (--~200 nm in size) within the c-grains. The morphology of the precipitate depends upon the nature of the added solute (e.g. ZrO2-MgO, ZrO2-CaO and ZrO2-Y203 solid solutions produce lenticular, cuboid and platey crystals, respectively). The average size of precipitate crystals is determined by the conditions of temperature and time adopted during heat-treatment in the crucial 't + c' field of the phase diagram. In the third example of transformation-toughening, t-zirconia grains are dispersed in a dissimilar ceramic matrix; for example, in ZT(A) or ZT(AI203) they are dispersed among alumina grains (Figure 10.9). An intergranular distribution of the metastable phase results when conventional processing methods are used but it has also been found possible to produce an intragranular distribution. As with PSZ materials, the size of metastable particles and matrix grains must be carefully controlled and balanced. Figure 10.8 Electron micrograph of tetragonal-zirconia polycrystal stabilized with 3 tool. % yttria (with acknowledgement to M. G. Cain, Centre for Advanced Materials Technology, Universi~ of Warwick, UK). Ceramics and glasses 331 Figure 10.9 Duplex structure of ZT (Al2 03 ) consisting of alumina and t-zirconia grains (back-scattered electron image). (from Green, 1984, p. 84; by permission of Marcel Dekker Inc.). So far, we have concentrated upon mechanical behaviour at or below ambient temperature. If the temperature of a zirconia-toughened material is raised to 900-1000~ which is close to the t-m transition temperature, the toughening mechanism tends to become ineffective. In addition, thermal cycling in service tends to induce the t-m transition at temperatures in the range 800-900~ and the toughening property is gradually lost. This tendency for fracture toughness to fall as the service temperature increases has naturally led to the investigation of alternative forms of stabilization in systems which have much higher transformation temperatures (e.g. ZT(HfO2)). Intergranular residues (e.g. in TZP), despite their beneficial effect during sintering, become easier to deform as the temperature rises and the material then suffers loss of strength and resistance to creep. 10.4.4 Glass-ceramics 10.4.4.1 Controlled devitrification of a glass It has long been appreciated that crystallization can take place in conventional glassy structures, particularly when they are heated. However, such crystallization is initiated at relatively few sites and there is a tendency for crystals to grow perpendicular to the free surface of the glass in a preferred manner. The resulting structure, being coarsely crystalline and strongly oriented, is mechanically weak and finds no practical application