shown in Fig. 2. The fibre spacing is relatively well controlled. Zirconia inclusions in the matrix appear to have limited the alumina grain size as they were mostly Plate n5-grip failure located at grain boundaries and triple grain junctions Some porous regions with lower density could be observed between fibres in the plane of plies. In addi- Plate n4- gauge failure tion, the zirconia fibre coating has been deformed into ""Mickey Mouse ears"on either side of the fibres and in some cases even detached from the fibre. This is a result of the hot-pressing conditions that will enhance the axial compressive deformation (which is vertical in Fig. 2). The zirconia interphase had an average thick ness of 5-10 Hm. No microstructural differences were seen between plates 4 and 5. 4.2. Mechanical testing rature tensile stress/st curves of as-received [0/90]ss Al2O3/Al2O3 composite. Tensile testing at room temperature of as-received samples from plates 4 and 5 showed similar UTS (ulti- mate tensile strength) values, although the modulus for outside the gauge length. Smaller pull-out lengths( 5mm) he samples from plate 5 was higher(193 GPa instead of were noted. This indicates a stronger fibre /matrix 152 GPa)(Fig 3). Both sets of samples showed classical bonding increasing the interfacial shear stress between stress/strain curves with a knee at w100 MPa and a low fibre and matrix, which also was the purpose of the modulus section to the point of failure. For plate 4, very changes made to the interphase process long(up to 50 mm) pull-out lengths were observed and Remains of the porous zirconia interphase layer were only a few matrix cracks appeared after the matrix visible on both the fibre and the internal surfaces of the cracking stress had been reached. This behaviour was matrix hole, Fig 4. Round shape, loose individual zir hought to originate from the low load transfer to raise conia grains were also observed. It has been suggested the stress on the composite around a main crack and that a damage zone propagates in the porous zirconia eventually initiate more cracks within the matrix. The interphase. Relative sliding between fibre and matrix UTS, 110 MPa was approximately halved compared to will then further crush the porous structure by breaking similar material with unidirectional fibre lay-up. 15, 16 sintered necks between the grains. Eventually the por This was attributed partly to a reduction in fibre volume ous sintered structure is transformed into individual fraction in the tensile direction to M15%. The strain to round powder grains that are rolled between the fibre failure was 0.45% which is comparable with the cur- and matrix forming small" ball-bearings"that promote rently commercially available CMCs. For samples from sliding. 15 Close examination of the fibre surface revealed plate 5 slightly higher UTS were observed. The low indicated strain to failures were caused by rupture 999228KV 58818FmlD17 99928KU8198198琴mW Fig. 4. Fracture surface of as-received [0/90]8.s Al2O3/Al2O3 composite fibre with remains of porous zirconia interphaseshown in Fig. 2. The ®bre spacing is relatively well controlled. Zirconia inclusions in the matrix appear to have limited the alumina grain size as they were mostly located at grain boundaries and triple grain junctions. Some porous regions with lower density could be observed between ®bres in the plane of plies. In addi￾tion, the zirconia ®bre coating has been deformed into ``Mickey Mouse ears'' on either side of the ®bres and in some cases even detached from the ®bre. This is a result of the hot-pressing conditions that will enhance the axial compressive deformation (which is vertical in Fig. 2). The zirconia interphase had an average thick￾ness of 5±10 mm. No microstructural di€erences were seen between plates 4 and 5. 4.2. Mechanical testing Tensile testing at room temperature of as-received samples from plates 4 and 5 showed similar UTS (ulti￾mate tensile strength) values, although the modulus for the samples from plate 5 was higher (193 GPa instead of 152 GPa) (Fig. 3). Both sets of samples showed classical stress/strain curves with a knee at 100 MPa and a low modulus section to the point of failure. For plate 4, very long (up to 50 mm) pull-out lengths were observed and only a few matrix cracks appeared after the matrix cracking stress had been reached. This behaviour was thought to originate from the low load transfer to raise the stress on the composite around a main crack and eventually initiate more cracks within the matrix. The UTS, 110 MPa was approximately halved compared to similar material with unidirectional ®bre lay-up.15,16 This was attributed partly to a reduction in ®bre volume fraction in the tensile direction to 15%. The strain to failure was 0.45% which is comparable with the cur￾rently commercially available CMCs. For samples from plate 5 slightly higher UTS were observed. The low indicated strain to failures were caused by rupture outside the gauge length. Smaller pull-out lengths (5 mm) were noted. This indicates a stronger ®bre/matrix bonding increasing the interfacial shear stress between ®bre and matrix, which also was the purpose of the changes made to the interphase process. Remains of the porous zirconia interphase layer were visible on both the ®bre and the internal surfaces of the matrix hole, Fig. 4. Round shape, loose individual zir￾conia grains were also observed. It has been suggested that a damage zone propagates in the porous zirconia interphase. Relative sliding between ®bre and matrix will then further crush the porous structure by breaking sintered necks between the grains. Eventually the por￾ous sintered structure is transformed into individual round powder grains that are rolled between the ®bre and matrix forming small ``ball-bearings'' that promote sliding.15 Close examination of the ®bre surface revealed Fig. 2. Microstructure of hot pressed [0/90]8,s composite. Fig. 3. Room temperature tensile stress/strain curves of as-received [0/90]8,s Al2O3/Al2O3 composite. Fig. 4. Fracture surface of as-received [0/90]8,s Al2O3/Al2O3 composite showing pulled-out ®bre with remains of porous zirconia interphase layer. 602 M. Holmquist et al. / Journal of the European Ceramic Society 20 (2000) 599±606