S.Y. Park et al. Journal of the European Ceramic Society 20(2000)2463-2468 2467 composites whereas ZrO, refers to the coatin layer in as shown in Fig. 6(b). XRD analysis of the Zro2 layer the mullite fiber/mullite matrix system. Cracks, coming in the tM laminate composite shows mainly tetragonal from the mullite layer, in the mM system are related to and a small content of monoclinic phase at the hot- crack bifurcation in ZrO, layers [Fig. 6(a). In this case, pressing condition. It might be expected that the tetra the cracks are deflected along the edge crack path in gonal Zro, located on the crack processing zone would ZrOz layer. Considering the stress distribution in the be transformed into monoclinic ZrO2 during the crack laminates, it can be expected that the indentation cracks propagation. In the case of the cM sample, however at the mullite layer will propagate straight through the cracks propagate through the mullite layer into the mullite layer, since the mullite layer locates in biaxial ZrO2 layer without any deflection behavior, as shown in tensile stress. However, the cracks which enter to the Fig. 6(c). It means that the interface of mullite/cubic ZrO, layer from the mullite layer propagate differently. ZrO, was not effective for crack deflection Recently Lange et al. 3 observed the crack bifurcation in Thermal shock resistance of mM, tM, and cM speci Al2O3: laminate composites when cracks entered mens were investigated by cyclic heating and cooling into thin Al2O3 layers sandwiched between Zr(12Ce)O2 between room temperature and 1300oC. During the layers. Because the Al2O3 layer is subjected to a com- cyclic test, 10 cycles were applied and specimens pressive stress due to thermal contraction upon cooling, annealed I h at 1300C. After cyclic test, tM, CM spec cracks entering the Al,O3 layers showed a deflection mens were remained without any changes, while mM behavior along the interface without passing through specimen was catastrophically degraded due to the the matrix. This kind of crack propagation observed in repeated tetra- mono phase transformation including a ZrO, layers of the mM laminate composites, due to the large volume expansion of mono-ZrO, layer, as shown formation during cooling. d compressive stress, developed from tetra- mono trans- in Fig. 7 Interestingly, crack propagation was different in the tM laminate composites. The cracks were deflected and 4. Conclusions arrested at the interface between mullite and zro, layers, This study has demonstrated that thermal mismatch stresses are closely related to the formation of cracks and the crack deflection behavior in the mullite/zirconia (a) Mullite/mono-zrO2 laminate composites. Two kinds of cracks were observed in zirconia layers; one is the channel cracks in layers subjected to tensile stress, another is the edge cracks in layers subjected to compressive stress. Channel cracks were formed during cooling due to high thermal expansion coefficients of zirconia, and edge crack was 1 cm formed due to the tetra- mono transformation of zirco nia during cooling. Among three forms of zirconia, partially stabilized type(tetragonal phase)was effective (b) Mullite/tetra-ZrO in deflecting the cracks at the interface due to the stress induced tetra-mono phase transformation. According to the Al2O3-SiOr-ZrOz equilibrium phase diagram mul- lite-ZrO2 are compatibles in solid state, therefore from a thermodynamic point of view no reaction is possible at the interface. The results obtained in this study sug- gest that partially stabilized zirconia coating on mullite fiber may provide damage tolerant for mullite/mullite tes (c) Mullite/cubic-Zro2 Acknowledgements This work was supported by the Korean Science Engineering Foundation(KOsEF) and the Alexander von humbolt foundation the authors would like to Fig. 7. Mullite/ZrO2 laminate composites after thermal cycles: (a) thank Professor W.A. Kaysser for useful discussion, B mullite/mono-ZrO2,,(b) mullite/ tetra-Zro2, and (c) mullite/cubic. Kanka for technical support, and J H. Song for the preparation of the manuscriptcomposites whereas ZrO2 refers to the coatin layer in the mullite ®ber/mullite matrix system. Cracks, coming from the mullite layer, in the mM system are related to crack bifurcation in ZrO2 layers [Fig. 6(a)]. In this case, the cracks are de¯ected along the edge crack path in ZrO2 layer. Considering the stress distribution in the laminates, it can be expected that the indentation cracks at the mullite layer will propagate straight through the mullite layer, since the mullite layer locates in biaxial tensile stress. However, the cracks which enter to the ZrO2 layer from the mullite layer propagate di€erently. Recently Lange et al.13 observed the crack bifurcation in Al2O3/ZrO2 laminate composites when cracks entered into thin Al2O3 layers sandwiched between Zr(12Ce)O2 layers. Because the Al2O3 layer is subjected to a com￾pressive stress due to thermal contraction upon cooling, cracks entering the Al2O3 layers showed a de¯ection behavior along the interface without passing through the matrix. This kind of crack propagation observed in ZrO2 layers of the mM laminate composites, due to the compressive stress, developed from tetra-mono trans￾formation during cooling. Interestingly, crack propagation was di€erent in the tM laminate composites. The cracks were de¯ected and arrested at the interface between mullite and ZrO2 layers, as shown in Fig. 6(b). XRD analysis of the ZrO2 layer in the tM laminate composite shows mainly tetragonal and a small content of monoclinic phase at the hot￾pressing condition. It might be expected that the tetra￾gonal ZrO2 located on the crack processing zone would be transformed into monoclinic ZrO2 during the crack propagation. In the case of the cM sample, however, cracks propagate through the mullite layer into the ZrO2 layer without any de¯ection behavior, as shown in Fig. 6 (c). It means that the interface of mullite/cubic ZrO2 was not e€ective for crack de¯ection. Thermal shock resistance of mM, tM, and cM speci￾mens were investigated by cyclic heating and cooling between room temperature and 1300C. During the cyclic test, 10 cycles were applied and specimens annealed 1 h at 1300C. After cyclic test, tM, cM speci￾mens were remained without any changes, while mM specimen was catastrophically degraded due to the repeated tetra-mono phase transformation including a large volume expansion of mono-ZrO2 layer, as shown in Fig. 7. 4. Conclusions This study has demonstrated that thermal mismatch stresses are closely related to the formation of cracks and the crack de¯ection behavior in the mullite/zirconia laminate composites. Two kinds of cracks were observed in zirconia layers; one is the channel cracks in layers subjected to tensile stress, another is the edge cracks in layers subjected to compressive stress. Channel cracks were formed during cooling due to high thermal expansion coecients of zirconia, and edge crack was formed due to the tetra-mono transformation of zirco￾nia during cooling. Among three forms of zirconia, partially stabilized type (tetragonal phase) was e€ective in de¯ecting the cracks at the interface due to the stress induced tetra-mono phase transformation. According to the Al2O3±SiO2±ZrO2 equilibrium phase diagram mul￾lite±ZrO2 are compatibles in solid state, therefore from a thermodynamic point of view no reaction is possible at the interface. The results obtained in this study sug￾gest that partially stabilized zirconia coating on mullite ®ber may provide damage tolerant for mullite/mullite composites. Acknowledgements This work was supported by the Korean Science Engineering Foundation (KOSEF) and the Alexander von Humbolt Foundation. The authors would like to thank Professor W.A. Kaysser for useful discussion, B. Kanka for technical support, and J.H. Song for the preparation of the manuscript. Fig. 7. Mullite/ZrO2 laminate composites after thermal cycles: (a) mullite/mono-ZrO2, (b) mullite/tetra-ZrO2, and (c) mullite/cubic￾ZrO2. S.-Y. Park et al. / Journal of the European Ceramic Society 20 (2000) 2463±2468 2467