S.Y. Park et al. Journal of the European Ceramic Society 20(2000)2463-2468 crac In specimen, In companise to those in he mM specimen are constrained from the tM specimen, is due to the larger thermal mismatch expansion at 933C, producing residual compressive stress. Obviously, residual mismatch stresses in the mM stress in Zro2 layers and tensile stress in mullite layers specimen are closely related to mono-tetra phase trans- The polished and the fractured surfaces of the laminate formation of ZrO?. Due to the volume increase during composites were investigated by SEM and as expected tetragonal to monoclinic phase transformation of no reaction zone was observed at the interface between unstabilized ZrO2 during cooling, the monoclinic ZrO2 ZrO2 and mullite. Only a sharp interface between ZrO and mullite layers was observed in the whole samples Indentation was performed into the mullite layer only. This appears straightforward since in our model ZrO, (Mono) system, mullite layers to the matrix of ceramic matrix Mullite (b) ZrO,(Tetra) lOoum taro T IF Mullite Fig. 4. Schematic demonstration of cracks in mullite/ZrO2 laminat mullite/mono-ZrO2,(b) mullite/tetra-ZrO2, and bic-ZrO,(or, Tensile stress: oc, compressive stress; IF. Zro 02004006008001000120014 Fig. 6. SEM micrographs of crack propagation at the mullite/Zro2 laminate composites:(a)mullite/mono-ZrO2, (b)mullite/ Fig. 5. Dimension change in mono-ZrO2 during thermal cycle tetra-ZrO2, and(c)mullite/cubic-ZrO2cracks in the cM specimen, in comparison to those in the tM specimen, is due to the larger thermal mismatch stress. Obviously, residual mismatch stresses in the mM specimen are closely related to mono-tetra phase trans￾formation of ZrO2. Due to the volume increase during tetragonal to monoclinic phase transformation of unstabilized ZrO2 during cooling, the monoclinic ZrO2 layers in the mM specimen are constrained from expansion at 933C, producing residual compressive stress in ZrO2 layers and tensile stress in mullite layers. The polished and the fractured surfaces of the laminate composites were investigated by SEM and as expected no reaction zone was observed at the interface between ZrO2 and mullite. Only a sharp interface between ZrO2 and mullite layers was observed in the whole samples. Indentation was performed into the mullite layer only. This appears straightforward since in our model system, mullite layers to the matrix of ceramic matrix Fig. 4. Schematic demonstration of cracks in mullite/ZrO2 laminate composites: (a) mullite/mono-ZrO2, (b) mullite/tetra-ZrO2, and (c) mullite/cubic-ZrO2. (T, Tensile stress; C, compressive stress; IF, interface.) Fig. 5. Dimension change in mono-ZrO2 during thermal cycle. Fig. 6. SEM micrographs of crack propagation at the interface of mullite/ZrO2 laminate composites: (a) mullite/mono-ZrO2, (b) mullite/ tetra-ZrO2, and (c) mullite/cubic-ZrO2. 2466 S.-Y. Park et al. / Journal of the European Ceramic Society 20 (2000) 2463±2468