S.Y. Park et al. Journal of the European Ceramic Society 20(2000)2463-2468 period, rapid drying rate or high content of binder can In order to investigate crack formations in the lami be contributed to residual stress in the laminate. In this nates, dilatometer measurements of each layer were study, however, such a differential stress due to drying carried out up to 1300C for 15 min with a constant can be eliminated by the careful control of the drying heating rate (10 C/min). Dilatometer measurements ite. When the layers have different densification rates showed that mullite exhibited a high volume shrinkage d thermal expansion coefficients, thermal mismatch (66%)compared to monoclinic and cubic ZrO2(50 and stresses are generated during the densification process or 56%, respectively). Thus, the crack formation in the cooling process. Hillman et al. reported that two classes mullite layer is expected rather than in the Zro2 layer of cracks were observed in Al2O3/ZrO2 laminate com- during the densification of laminates. However, the posites. Cracks with a large opening displacement are crack formation in the mullite layer was observed only originated from drying and subsequent densification in the mM specimen, as shown in Fig. 2. Therefore, it is period, whereas cracks with a small opening displacement believed that the different densification rate is not the originated from thermal expansion mismatch during the main reason for the crack formations in the laminates cooling period. However, it is somewhat difficult to con- during densification clude which factor is more attributable to the crack forma- Therefore, we explored other possibilities resulting in tion either in the densification period or the cooling period. crack formations, such as thermal expansion mismatch during cooling. Thermal expansion coefficients(a)were measured individually on the samples that were pre- sintered at 1300C. These experiments demonstrate that mullite shows the lowest a(5x10-6/C) at 1300.C, m-zro2 while the a values of the tetragonal and cubic ZrO2 are 9×10-6C,10.5×10-6/°C, respectively, as shown in Fig. 3. However, the thermal expansion behavior of monoclinic ZrO2 showing a rapid shrinkage at 1150C was quite different compared to other zirconia. This different thermal behavior in monoclinic ZrO2 is closely related to the phase transformation of ZrO2, i.e. mono tetra transformation (volume contraction) at 1150.C during heating, and tetra-mono transformation(volume expansion) at 930C during cooling, as shown in Fig 4 Mullite Because the thermal expansion coefficient of ZrO2 is higher than that of mullite, it is believed that Zro2 lay ers contain tensile stress and form channel cracks during t-Zro cooling from the hot pressing temperature, as schema tically shown in Fig. 5. The higher density of channel 12.0 t-Zro2 3Y7 0.2mm c-7rO Mullite Mullite c-7rO 2 020040060080010001200 Fig. 3. Thermal expansion coefficient of specimens: mullite, mono- b)tetra-ZrO, and (c)cubiczrO2 ZrO,. tetra-ZrO, and cubic-ZrO,period, rapid drying rate or high content of binder can be contributed to residual stress in the laminate. In this study, however, such a di€erential stress due to drying can be eliminated by the careful control of the drying rate. When the layers have di€erent densi®cation rates and thermal expansion coecients, thermal mismatch stresses are generated during the densi®cation process or cooling process. Hillman et al.12 reported that two classes of cracks were observed in Al2O3/ZrO2 laminate com￾posites. Cracks with a large opening displacement are originated from drying and subsequent densi®cation period, whereas cracks with a small opening displacement originated from thermal expansion mismatch during the cooling period. However, it is somewhat dicult to con￾clude which factor is more attributable to the crack forma￾tion either in the densi®cation period or the cooling period. In order to investigate crack formations in the lami￾nates, dilatometer measurements of each layer were carried out up to 1300C for 15 min with a constant heating rate (10C/min). Dilatometer measurements showed that mullite exhibited a high volume shrinkage (66%) compared to monoclinic and cubic ZrO2 (50 and 56%, respectively). Thus, the crack formation in the mullite layer is expected rather than in the ZrO2 layer during the densi®cation of laminates. However, the crack formation in the mullite layer was observed only in the mM specimen, as shown in Fig. 2. Therefore, it is believed that the di€erent densi®cation rate is not the main reason for the crack formations in the laminates during densi®cation. Therefore, we explored other possibilities resulting in crack formations, such as thermal expansion mismatch during cooling. Thermal expansion coecients () were measured individually on the samples that were pre￾sintered at 1300C. These experiments demonstrate that mullite shows the lowest  (510ÿ6 / C) at 1300C, while the  values of the tetragonal and cubic ZrO2 are 910ÿ6 / C, 10.510ÿ6 / C, respectively, as shown in Fig. 3. However, the thermal expansion behavior of monoclinic ZrO2 showing a rapid shrinkage at 1150C was quite di€erent compared to other zirconia. This di€erent thermal behavior in monoclinic ZrO2 is closely related to the phase transformation of ZrO2, i.e. mono￾tetra transformation (volume contraction) at 1150C during heating, and tetra-mono transformation (volume expansion) at 930C during cooling, as shown in Fig. 4. Because the thermal expansion coecient of ZrO2 is higher than that of mullite, it is believed that ZrO2 lay￾ers contain tensile stress and form channel cracks during cooling from the hot pressing temperature, as schema￾tically shown in Fig. 5. The higher density of channel Fig. 3. Thermal expansion coecient of specimens: mullite, mono￾ZrO2, tetra-ZrO2, and cubic-ZrO2. Fig. 2. SEM micrographs of laminates composites: (a) mono-ZrO2, (b) tetra-ZrO2, and (c) cubic-ZrO2. S.-Y. Park et al. / Journal of the European Ceramic Society 20 (2000) 2463±2468 2465