Tape-cast alumina-zirconia laminates: processing and mechanical properties Table 2. Composition of the retained tape casting slurry Table 3. Physical properties of the various grades tested. E and G are measured normal to the direction of the layering Function Volv Poissons ratio is equal to 0- 24 for all the grades Alumina t Zirconia Ceramic powder 306 Material E(GPa) G(GPa Solvent Dispersant PVB A 143 PEG 300 Plasticizer Plasticizer AZo 880 AZS/AZ4 9.06 AZ 342 AZIO/AZ5 between them. The shrinkage rate increases when 7/5/7 362 the binder/plasticizer ratio increases because the 3/15/3 high binder content increases the links between 2/2/2102/2/2 particles and then the shrinkage rate. In order to avoid cracking during drying, the formulation of the slurry has to be defined to achieve a low way that compressive residual stresses develop cizer-rich organIc phase 6 igh amount of plasti- the outer layers. These residual stresses were shrinkage rate i.e. with a n the of the azio induced upon cooling from high temperature system, the shrinkage rate does not exceed a value (1400C), as a result of different thermal expan of I um s sion coefficients between the layers. The classical The apparent densities of dried tapes, excluding plate theory(plane stress hypothesis) was used to the organic phase, were determined for various calculate the normal stresses in each layer of the volume solids and binder/plasticizer ratios. When two-phase composite. 4 In its main lines, the cal- the volume solids ratio decreases, the organic phase culation is based on the assumption that a cross- prevent particles from packing together, therefore section before deformation remains a cross-section he density decreases. A low binder/plasticizer after deformation and that layers remain plane ratio leads to green tapes with higher densiti and parallel to each other throughout deformation When the binder/plasticizer ratio decreases, the This results in the following form for the elastic low-viscosity plasticizer-rich organic phase allows displacement vector at any point (x, x2, x3) a good packing of ceramic particles l{(x1,x2) 3. 1.3 Thermocompression ability u=2=2(x1,x2 The thermocompression behaviour of green sheets is sensitive to x and Y. delaminations were numerous in samples with low quantities of organic where (1, 2) subscripts refer to the in-plane axis phases (X4 to X7)(Table 1). a lot of delamina- and (3)is normal to the layering The studied lam tions were observed in samples with a high quan- inates are constructed such that they have com tity of plasticizer (Y1). Plasticizers are low plete symmetry of individual lamina thickness and molecular weight species which can act as a lubri- properties about the middle plane of the laminate cant between the individual layers, limiting the Furthermore, no texture was introduced by the bond strength during thermocompression tape casting process, so that the studied laminates The final formulation of tapc casting slurries can be considered as perfectly orthotropic. Hence, was defined in order to tape cast non-cracked in the case of pure linear elasticity, the thermal tapes with a high green density, which do not lead induced normal(N, and N2) and tangential (T12 to delamination during thermocompression of forces, per unit length, are given by: 4 laminated composites. This formulation (Table 2) was applied to the tape casting of pure alumina N1「A1A1201e and of AlO3+5 vol%o ZrO, with similar green tape A21A220 characteristics and a good thermocompression 00 AbLE ability As= 2Gd 3. 2 Laminar composites I du: du 3. 2. I Design and e sical f monoliths and com posites are given in Table 3. These physical prop- One recalls that eqns(4)only stand for the elastic erties were used to design the composites in such a part of the strain components(elastic gtotal-gthermal),Tape-cast alumina-zirconia laminates: processing and mechanical properties 303 Table 2. Composition of the retained tape casting slurry Component Function Vol’% _ Alumina + Zirconia MEKlethanol Phosphate ester PVB PEG 300 DBP Ceramic powder 30.6 Solvent 57.6 Dispersant 0.8 Binder 4.6 Plasticizer 2.9 Plasticizer 3.5 between them. The shrinkage rate increases when the binder/plasticizer ratio increases because the high binder content increases the links between particles and then the shrinkage rate. In order to avoid cracking during drying, the formulation of the slurry has to be defined to achieve a low shrinkage rate, i.e. with a high amount of plasti￾cizer-rich organic phase. In the case of the AZ10 system, the shrinkage rate does not exceed a value of 1 pm ss’. The apparent densities of dried tapes, excluding the organic phase, were determined for various volume solids and binder/plasticizer ratios. When the volume solids ratio decreases, the organic phase prevent particles from packing together, therefore the density decreases. A low binder/plasticizer ratio leads to green tapes with higher densities. When the binder/plasticizer ratio decreases, the low-viscosity plasticizer-rich organic phase allows a good packing of ceramic particles. 3.1.3 Thermocompression ability The thermocompression behaviour of green sheets is sensitive to X and Y. Delaminations were numerous in samples with low quantities of organic phases (X4 to X7) (Table 1). A lot of delamina￾tions were observed in samples with a high quan￾tity of plasticizer (Yl). Plasticizers are low molecular weight species which can act as a lubri￾cant between the individual layers, limiting the bond strength during thermocompression. The final formulation of tape casting slurries was defined in order to tape cast non-cracked tapes with a high green density, which do not lead to delamination during thermocompression of laminated composites. This formulation (Table 2) was applied to the tape casting of pure alumina and of A&0,+5 vol% ZrO, with similar green tape characteristics and a good thermocompression ability. 3.2 Laminar composites 3.2. I Design Some physical properties of monoliths and com￾posites are given in Table 3. These physical prop￾erties were used to design the composites in such a Table 3. Physical properties of the various grades tested. E and G are measured normal to the direction of the layering. Poisson’s ratio is equal to 0.24 for all the grades Material E (GPa) G (GPa) (YXJ IJllV~ (IO” “C 1) A 355 143 8.69 Al A 363 146 AZ10 341 137 8.80 AZSIAZS 349 141 9.06 AZlOlAZlO 342 138 AZIOIAZS 71517 362 146 311513 349 141 212121 Io/21212 377 152 way that compressive residual stresses develop in the outer layers. These residual stresses were induced upon cooling from high temperature (14OO”C), as a result of different thermal expan￾sion coefficients between the layers. The classical plate theory (plane stress hypothesis) was used to calculate the normal stresses in each layer of the two-phase composite.24 In its main lines, the cal￾culation is based on the assumption that a cross￾section before deformation remains a cross-section after deformation and that layers remain plane and parallel to each other throughout deformation. This results in the following form for the elastic displacement vector at any point (x,, x2, x3): [ Ul = r&x,, x2) ii = u2 = l&x,, x2) (3) u3 = &X3> where (1,2) subscripts refer to the in-plane axis and (3) is normal to the layering. The studied lam￾inates are constructed such that they have com￾plete symmetry of individual lamina thickness and properties about the middle plane of the laminate. Furthermore, no texture was introduced by the tape casting process, so that the studied laminates can be considered as perfectly orthotropic. Hence, in the case of pure linear elasticity, the thermally induced normal (N, and N,) and tangential (T,?) forces, per unit length, are given by:24 Ed with A,, = - vEd 1 - v2’ A,, = A,, = ~ 1 --I and E.. = ! %! + ?!! ” 2 [ aXj aXi 1 ’ (4) A,, = 2Gd One recalls that eqns (4) only stand for the elastic part of the strain components (.?lastic = E”‘~’ -E~~~““~‘)