A. Morales-Rodriguez er al. Journal of the European Ceramic Society 29(2009)1625-1630 (ii)The values of the apparent diffusion coefficient Dgbdeduced from Eqs. (2)and (4)( Fig. 6)are, in fact, an upper limit of the actual values because the alumina layers support an addi- from the con by the rigid interface bonding for the strain to be the same in the soft and hard layers. This additional stress, not taken into account in the simple composite creep model(Eq (3)), should be superimposed on the applied stress(Eq (4)), thus giving lower apparent diffusivities than those plotted in Fig. 6. 5. Conclusions Fully dense AlO3/ZTA(60 vol %o Al2O3 +40 vol. %03 mol% Y203-stabilized tetragonal ZrO2) layered composites with uni- form layers and strong interfaces have been produced starting from sheets obtained by tape casting Mechanical tests were performed in compression in air at constant strain rate and constant load between 1400 and 1500oc with the loading axis parallel to the layer interfaces (isostrain configuration). After testing, the layer interfaces maintain their structural integrity. The creep parameters, stress exponent n and activation energy @, are close to l and 700 kJ/mol, respectively Except for the most severe deformation conditions, the laminates exhibit a damage-tolerant regime, characterized by an extensive grain boundary cavitation without coalescence into microcracks, reaching strains of up to 30%o without failure An analysis based on a duplex creep model demonstrates 0255075100125150 that the alumina phase dominates the overall creep process of Position (um the laminates, accumulating more of the applied load. It is shown that diffusional creep controlled by oxygen grain boundary diffu Fig.7.EDX/SEM linescan across the alumina layer in the laminate, showing sion is consistent with mechanical data. An unintentional doping the presence of yttrium and zirconium in the layer. of the nominally high-purity alumina layers with yttrium and zi conium has been detected by EDX/SEM measurements, caused modes have been reported 5-At high stresses, th by the elevated temperatures of the laminate fabrication proces is dictated by the growth of several cracks by coa- lescence of two-grain boundary cavities, with very small Acknowledgments failure strains(typically <1%). At low stresses, the final failure occurs by the coalescence of creep damage, con- The authors would like to thank the Ministerio de Ciencia y sisting of a large density of two-grain boundary cavities, at Tecnologia(Spain) for the financial support through the Project large strains(typically >20%) In the laminates, the stress No. MAT2000-1117. CNR and CSIC are also acknowledged carried by the alumina layers oAI is not very far from the for the financial support provided to ISTEC-CNR and Depar maximum stress omax supported by high-purity monolithic tamento de Fisica de la Materia Condensada(Universidad de lumina; for example, at 1500C and Eo=2 x 10-5s-1, Sevilla), in the framework of bilateral agreement oAl= 2.5, 0c 2 35 MPa and omax=55 MPa(Fig. 2), and lus a premature failure of the laminate would be expected. References Because of the unintentional doping, however, such a com- parison must be done with cation-doped alumina, not with alLDB. d pristine one. Yoshida et al. have reported a decrease in Ceram Soc. BulL. 1992. 71, 969 creeprate higher than one order of magnitude in 0.045 mol% 2. Moya, J.S., Layered ceramics. Adv: Mater, 1995. 7. 185-189 han. H. liller, G.A., Unique opportunities for Y2O3-doped Al2O3 owing to segregation of yttrium at the microstructural engineering with duplex and laminar ceramic composites. alumina grain boundaries. A similar effect has been found J.Am. Ceram.Soc.,1992,75,1715-1728 with other dopant cations 22-24 Therefore, the stress level 4. Marshall, D. B, Ratto, J.J. and Lange, E. E, Enhanced fract at which the alumina layers are submitted in the lami- in layered microcomposites of Ce-ZrO2 and Al2 03. JAm nate corresponds to a low-stress damage-tolerant regime as experimentally observed 5. Oeschner, M., Hillman, C and Lange, F. F, Crack bifurcation in laminar ceramic composites. J. Am. Ceram Soc., 1996, 79, 1834-1838A. Morales-Rodríguez et al. / Journal of the European Ceramic Society 29 (2009) 1625–1630 1629 Fig. 7. EDX/SEM linescan across the alumina layer in the laminate, showing the presence of yttrium and zirconium in the layer. failures modes have been reported.15–17 At high stresses, the fracture is dictated by the growth of several cracks by coa￾lescence of two-grain boundary cavities, with very small failure strains (typically <1%). At low stresses, the final failure occurs by the coalescence of creep damage, con￾sisting of a large density of two-grain boundary cavities, at large strains (typically >20%). In the laminates, the stress carried by the alumina layers σAl is not very far from the maximum stress σmax supported by high-purity monolithic alumina; for example, at 1500 ◦C and ε˙o = 2 × 10−5 s−1, σAl = 2.5, σc  35 MPa and σmax = 55 MPa (Fig. 2), and thus a premature failure of the laminate would be expected. Because of the unintentional doping, however, such a com￾parison must be done with cation-doped alumina, not with pristine one. Yoshida et al.25 have reported a decrease in creep rate higher than one order of magnitude in 0.045 mol% Y2O3-doped Al2O3 owing to segregation of yttrium at the alumina grain boundaries. A similar effect has been found with other dopant cations.22–24 Therefore, the stress level at which the alumina layers are submitted in the lami￾nate corresponds to a low-stress damage-tolerant regime, as experimentally observed. (ii) The values of the apparent diffusion coefficient Dgb deduced from Eqs.(2) and (4)(Fig. 6) are, in fact, an upper limit of the actual values because the alumina layers support an addi￾tional in-plane stress arising from the constraint imposed by the rigid interface bonding for the strain to be the same in the soft and hard layers.36 This additional stress, not taken into account in the simple composite creep model (Eq. (3)), should be superimposed on the applied stress (Eq. (4)), thus giving lower apparent diffusivities than those plotted in Fig. 6. 5. Conclusions Fully dense Al2O3/ZTA (60 vol.% Al2O3 + 40 vol.% 3 mol% Y2O3-stabilized tetragonal ZrO2) layered composites with uni￾form layers and strong interfaces have been produced starting from sheets obtained by tape casting. Mechanical tests were performed in compression in air at constant strain rate and constant load between 1400 and 1500 ◦C with the loading axis parallel to the layer interfaces (isostrain configuration). After testing, the layer interfaces maintain their structural integrity. The creep parameters, stress exponent n and activation energy Q, are close to 1 and 700 kJ/mol, respectively. Except for the most severe deformation conditions, the laminates exhibit a damage-tolerant regime, characterized by an extensive grain boundary cavitation without coalescence into microcracks, reaching strains of up to 30% without failure. An analysis based on a duplex creep model demonstrates that the alumina phase dominates the overall creep process of the laminates, accumulating more of the applied load. It is shown that diffusional creep controlled by oxygen grain boundary diffu￾sion is consistent with mechanical data. An unintentional doping of the nominally high-purity alumina layers with yttrium and zir￾conium has been detected by EDX/SEM measurements, caused by the elevated temperatures of the laminate fabrication process. Acknowledgments The authors would like to thank the Ministerio de Ciencia y Tecnología (Spain) for the financial support through the Project No. MAT2000-1117. CNR and CSIC are also acknowledged for the financial support provided to ISTEC-CNR and Depar￾tamento de Física de la Materia Condensada (Universidad de Sevilla), in the framework of bilateral agreement. References 1. Marshall, D. B., Design of high-toughness laminar zirconia composites. Am. Ceram. Soc. Bull., 1992, 71, 969–973. 2. Moya, J. S., Layered ceramics. Adv. Mater., 1995, 7, 185–189. 3. Harmer, M. P., Chan, H. M. and Miller, G. A., Unique opportunities for microstructural engineering with duplex and laminar ceramic composites. J. Am. Ceram. Soc., 1992, 75, 1715–1728. 4. Marshall, D. B., Ratto, J. J. and Lange, F. F., Enhanced fracture toughness in layered microcomposites of Ce–ZrO2 and Al2O3. J. Am. Ceram. Soc., 1991, 74, 2979–2987. 5. Oeschner, M., Hillman, C. and Lange, F. F., Crack bifurcation in laminar ceramic composites. J. Am. Ceram. Soc., 1996, 79, 1834–1838