558 G. de Portu et al. /Composites: Part B 37(2006)556-567 35 Fig. 1. Effect of residual stresses on crack propagation in laminated structure (a) Layers containing compressive stress. Note that crack propagation perpendicular to the stress is hindered.( b) Layer containing tensile residual stress In this case, only cracks perpendicular to the interface(i.e to the direction of the stress)are visible to stimulate residual stresses in laminated structures it is In this paper, we take into consideration hybrid laminated necessary to use materials with different coefficients of composites of alternate layers of pure alumina and of a thermal expansion and shrinkage. In the range 25-1500C composite formed by 60 vol% Al2O3+40 vol% tetragonal alumina has a coefficient of thermal expansion (CTA) ZrO2 stabilized with 3 mol% Y203(3Y-TZP) fabricated by a≈9×10-6-, while zirconia has a≈11×10-6°-1. warm pressing and sintering of layers produced by tape casting In addition, alumina has different sintering rate and lower shrinkage than zirconia. On the other hand to control the 3.1. Tape casting residual stresses in the structure and avoid defects durin sintering, such as edge cracking [9, 11 tunnelling cracks This study entailed the use of a high purity (99.7%)alumina [52,53(Fig. 2)or delamination [54](Fig 3), an appropriate powder(Alcoa A16-SG, Alcoa Aluminum Co., New York, design and controlled mismatch in CTE among different USA)and a zirconia powder (TZ3Y-S, Tosoh Corp. Japan) laminae is necessary. To reach this goal usually, at least one doped with 3 mol% of Y203(usually referred to as 3Y-TZP) of the layers of the laminate is made of an alumina-zirconia both with an average particle size of 0.3 um. laminate. The reason for choosing alumina and zirconia as According to previous experiences[49,55,56],the different the constituent materials of ceramic laminates can be powders were mixed with organic binders, dispersant, generally traced back to the excellent bonding between plasticizers and solvents to obtain suitable slips for tape the layers in the absence of excessive diffusion between casting Slurry compositions were the same for both Al2O3 and omponents, their good thermo-mechanical properties and Al2O3-ZrO2 composite powders. In all slurry compositions the their relatively ease of processing organic components/inorganic powder weight ratio was fixed These characteristics make the two materials interesting at 1.6 and the binders were present at the same percentag idates for the production of ceramic multilayers. the plasticizer.to stimulate residual stresses in laminated structures it is necessary to use materials with different coefficients of thermal expansion and shrinkage. In the range 25–1500 8C alumina has a coefficient of thermal expansion (CTA) az9!10K6 8CK1 , while zirconia has az11!10K6 8CK1 . In addition, alumina has different sintering rate and lower shrinkage than zirconia. On the other hand to control the residual stresses in the structure and avoid defects during sintering, such as edge cracking [9,11], tunnelling cracks [52,53] (Fig. 2) or delamination [54] (Fig. 3), an appropriate design and controlled mismatch in CTE among different laminae is necessary. To reach this goal usually, at least one of the layers of the laminate is made of an alumina–zirconia laminate. The reason for choosing alumina and zirconia as the constituent materials of ceramic laminates can be generally traced back to the excellent bonding between the layers in the absence of excessive diffusion between components, their good thermo-mechanical properties and their relatively ease of processing. These characteristics make the two materials interesting candidates for the production of ceramic multilayers. In this paper, we take into consideration hybrid laminated composites of alternate layers of pure alumina and of a composite formed by 60 vol% Al2O3C40 vol% tetragonal ZrO2 stabilized with 3 mol% Y2O3 (3Y-TZP) fabricated by warm pressing and sintering of layers produced by tape casting. 3.1. Tape casting This study entailed the use of a high purity (99.7%) alumina powder (Alcoa A16 -SG, Alcoa Aluminum Co., New York, USA) and a zirconia powder (TZ3Y-S, Tosoh Corp. Japan) doped with 3 mol% of Y2O3 (usually referred to as 3Y-TZP) both with an average particle size of 0.3 mm. According to previous experiences [49,55,56], the different powders were mixed with organic binders, dispersant, plasticizers and solvents to obtain suitable slips for tape casting. Slurry compositions were the same for both Al2O3 and Al2O3–ZrO2 composite powders. In all slurry compositions the organic components/inorganic powder weight ratio was fixed at 1.6 and the binders were present at the same percentage of the plasticizer. Fig. 1. Effect of residual stresses on crack propagation in laminated structure. (a) Layers containing compressive stress. Note that crack propagation perpendicular to the stress is hindered. (b) Layer containing tensile residual stress. In this case, only cracks perpendicular to the interface (i.e. to the direction of the stress) are visible. 558 G. de Portu et al. / Composites: Part B 37 (2006) 556–567