S.Y. Park et al. Journal of the European Ceramic Society 20(2000)2463-2468 with 3 mol%Y2O3(TZ-3Y, Tosho, Japan), cubic zir- to 1300C and 15 min holding time with controlled conia with 8 mol%Y2O3(TZ-8Y, Tosho, Japan). The heating and cooling rate. Thermal expansion coefficients of mullite precursor was calcined at 1 100C for 5 h in air. each layer were measured on a dilatometer( Bahr-thermo- Each slip was prepared by planetary ball milling with 70 analyse, Germany) for temperatures up to 1300C wt% solid loading. For the preparation of stable aqu eous slip, total amount of 2.5 wt. polyglycerin, cellu- lose. and silicon emulsion were added in order to 3. Results and discussion enhance the powder dispers reen dimensions of 60x60 mm were obtained by tape casting Phase composition and microstructure of as-pro- After tape casting, tapes were dried for 24 h at ambient cessed laminate reveal that the tapes produced from the condition. For the binder burn-out, dried tapes were calcined mullite precursor are fully crystallized into placed in air at 700C for 2 h with a slow heating rate of mullite after hot-pressing at 1300 C for 15 min under 15 MPa in air, as shown in Fig. 1. The ZrO2 tapes pro- Three combinations of laminate composites (mM: duced from unstabilized ZrO2 powder yield purely monoclinic ZrO2/mullite, tM: tetragonal ZrO2/mullite, monoclinic phase, whereas partially stabilized ZrO CM: cubic ZrO2/mullite)were formed by alternating layers with 3 mol% Y2O3 tapes retained mainly tetragonal (7 layers of ZrO, and 8 layers of mullite). Alternating phase and partial content of monoclinic phase. Fully layers of mullite precursor/zirconia were hot-pressed at stabilized ZrO, with 8 mol%Y2O3 tapes retained only 1300 C, 15 MPa, for 15 min in air. After hot-pressing, cubic phase under the given condition. pecimens were cooled down with & C/min to room Fig. 2 shows SEM micrographs of the laminate com temperature without pressure. Hot-pressed specimens posites, fabricated with an alternating layer of mullite were polished with a diamond paste of 3 um, and then and monoclinic ZrO2(a), tetragonal ZrO2(b), cubic Vickers indentation was carried out. Indention load ZrO2(c). After hot-pressing, all samples display channel with 10 kg was enough to produce an observable crack. cracks parallel to the hot pressing direction. In the X-ray diffractometer (Siemens D-5000, Germany) was samples tM and cM, the channel cracks are in the Zro used for the phase identification of specimen. Micro- layer, while in the sample mM the channel cracks are in structure characterization of layered composites has the mullite layer. Additionally, in the sample mM, edge performed by SEM(Philips, 525M, Netherlands) on the cracks perpendicular to hot-pressing direction are polished and the fracture surfaces of the specimens. observed in the ro2 layers Dilatometer measurements have been carried out to Typical cracks originated from mismatch stress are study the densification behavior of each layer including dependent on the several factors, such as differential mullite precursor, mono-zirconia, tetra-zirconia, and drying, differential densification, and different thermal cubic zirconia. These measurements were carried out up expansion coefficients during cooling. During the drying 6000 2000 50.0060.0070.0080.00 Fig. I. X-ray diffraction patterns for(a) the sample hot-pressed at 1300C, 15 MPa for 15 min in air and (b) mullite precursor powderwith 3 mol% Y2O3 (TZ-3Y, Tosho, Japan), cubic zir￾conia with 8 mol% Y2O3 (TZ-8Y, Tosho, Japan). The mullite precursor was calcined at 1100C for 5 h in air. Each slip was prepared by planetary ball milling with 70 wt.% solid loading. For the preparation of stable aqu￾eous slip, total amount of 2.5 wt.% polyglycerin, cellu￾lose, and silicon emulsion were added in order to enhance the powder dispersion. Green tapes with dimensions of 6060 mm were obtained by tape casting. After tape casting, tapes were dried for 24 h at ambient condition. For the binder burn-out, dried tapes were placed in air at 700C for 2 h with a slow heating rate of 2C/min. Three combinations of laminate composites (mM: monoclinic ZrO2/mullite, tM: tetragonal ZrO2/mullite, cM: cubic ZrO2/mullite) were formed by alternating layers (7 layers of ZrO2 and 8 layers of mullite). Alternating layers of mullite precursor/zirconia were hot-pressed at 1300C, 15 MPa, for 15 min in air. After hot-pressing, specimens were cooled down with 8C/min to room temperature without pressure. Hot-pressed specimens were polished with a diamond paste of 3 mm, and then Vickers indentation was carried out. Indention load with 10 kg was enough to produce an observable crack. X-ray di€ractormeter (Siemens D-5000, Germany) was used for the phase identi®cation of specimen. Micro￾structure characterization of layered composites has performed by SEM (Philips, 525M, Netherlands) on the polished and the fracture surfaces of the specimens. Dilatometer measurements have been carried out to study the densi®cation behavior of each layer including mullite precursor, mono-zirconia, tetra-zirconia, and cubic zirconia. These measurements were carried out up to 1300C and 15 min holding time with controlled heating and cooling rate. Thermal expansion coecients of each layer were measured on a dilatometer (Bahr-thermo￾analyse, Germany) for temperatures up to 1300C. 3. Results and discussion Phase composition and microstructure of as-pro￾cessed laminate reveal that the tapes produced from the calcined mullite precursor are fully crystallized into mullite after hot-pressing at 1300C for 15 min under 15 MPa in air, as shown in Fig. 1. The ZrO2 tapes pro￾duced from unstabilized ZrO2 powder yield purely monoclinic phase, whereas partially stabilized ZrO2 with 3 mol% Y2O3 tapes retained mainly tetragonal phase and partial content of monoclinic phase. Fully stabilized ZrO2 with 8 mol% Y2O3 tapes retained only cubic phase under the given condition. Fig. 2 shows SEM micrographs of the laminate com￾posites, fabricated with an alternating layer of mullite and monoclinic ZrO2 (a), tetragonal ZrO2 (b), cubic ZrO2 (c). After hot-pressing, all samples display channel cracks parallel to the hot pressing direction. In the samples tM and cM, the channel cracks are in the ZrO2 layer, while in the sample mM the channel cracks are in the mullite layer. Additionally, in the sample mM, edge cracks perpendicular to hot-pressing direction are observed in the ZrO2 layers. Typical cracks originated from mismatch stress are dependent on the several factors, such as di€erential drying, di€erential densi®cation, and di€erent thermal expansion coecients during cooling. During the drying Fig. 1. X-ray di€raction patterns for (a) the sample hot-pressed at 1300oC, 15 MPa for 15 min in air and (b) mullite precursor powder. 2464 S.-Y. Park et al. / Journal of the European Ceramic Society 20 (2000) 2463±2468