strength being retained. The modulus also dropped off as a function of temperature(Fig. 6). Examination of Plate n°4 the fracture surfaces showed a significant reduction in pu ed to rt and little evidence of pull-out at 1400 C(Fig. 7), which is con- sistent with reduced fibre strength at high tempera tures.20,2 Scratching of the zirconia off the fibre surfaces was more evident at 1400%C. and an accumu lation of zirconia was also seen on some fibres. one explanation may be that zirconia is exhibiting some plasticity at the testing temperature, modifying the 50101 ball-bearing "mechanisI Samples from plate 4 were aged at 1400 C for 100 and Temperature (C) 1000 h in air and tested at rt to measure residual strength(Fig. 8). Samples aged at 1400C, 100 h, had Fig.5. UTS as a function of temperature for [0/90k, Al 03/Al203 UTS values above 100 MPa(134 and 107 MPa) similar to the as-received sample (110 MPa) and the Youngs modulus had increased slightly (to 202 and 187 GPa, respectively). No dramatic loss of properties was Plate n°4 observed after ageing for 1000 h, one specimen even retained the initial UTS. The pull-out lengths were smaller compared to the as-received samples but still about 10 mm in length. This confirms previous bend test results5.7 that composite behaviour is retained after ageing at 1400oC. However, coarsening of the zirconia interphase grains had clearly taken place during the 50 ageing. 15 Grain size grew from below a micron to a few microns during the 1000-h heat-treatment. This coarsening effect could be detrimental in the long run. It has been shown that a porous structure, which is pre- vented from densifying, will evolve via a de-sintering mechanism(i.e the breaking of sintered necks leading Youngs modulus as a function of temperature for [0/90)s s to pore coalescence). Eventually a gap may form between fibre and matrix. The coarsening effect would therefore. alter the load transfer mechanism but embrittlement should not occur roughness features like sapphire surface facetting and One sample was subjected to thermal cycling and tes cusps from initially sintered zirconia grains. However, ted after 1367 cycles(corresponding to an accumulated these features were not thought to be too detrimental to time of 230 h at 1200oC)were reached. An UtS of 148 the fibre strength. I5 MPa and an elastic modulus of 150 GPa was noted. The No significant differences between plates 4 and 5 were UTS is plotted in Fig. 8 for comparison with the other seen in the high temperature tensile tests(Fig. 5). At thermally aged samples. The results demonstrates very 800C, the rate of property drop-off was 40% compared good thermal shock resistance for the composite mate to the initial UTS. The Uts did not change between rial, a property that monolithic oxides otherwise do not 1200 and 1400C with 50% of room temperature(RT) exhibit 800°c 1200°C Fig. 7. Reduced fibre pull-out lengths at elevated temperatures (all samples are from plate 5)roughness features like sapphire surface facetting and cusps from initially sintered zirconia grains. However, these features were not thought to be too detrimental to the ®bre strength.15 No signi®cant di€erences between plates 4 and 5 were seen in the high temperature tensile tests (Fig. 5). At 800C, the rate of property drop-o€ was 40% compared to the initial UTS. The UTS did not change between 1200 and 1400C with 50% of room temperature (RT) strength being retained. The modulus also dropped o€ as a function of temperature (Fig. 6). Examination of the fracture surfaces showed a signi®cant reduction in ®bre pull-out at 1200C compared to RT and very little evidence of pull-out at 1400C (Fig. 7), which is con￾sistent with reduced ®bre strength at high tempera￾tures.20,21 Scratching of the zirconia o€ the ®bre surfaces was more evident at 1400C, and an accumu￾lation of zirconia was also seen on some ®bres. One explanation may be that zirconia is exhibiting some plasticity at the testing temperature, modifying the ``ball-bearing'' mechanism. Samples from plate 4 were aged at 1400C for 100 and 1000 h in air and tested at RT to measure residual strength (Fig. 8). Samples aged at 1400C, 100 h, had UTS values above 100 MPa (134 and 107 MPa) similar to the as-received sample (110 MPa) and the Young's modulus had increased slightly (to 202 and 187 GPa, respectively). No dramatic loss of properties was observed after ageing for 1000 h, one specimen even retained the initial UTS. The pull-out lengths were smaller compared to the as-received samples but still about 10 mm in length. This con®rms previous bend test results15,17 that composite behaviour is retained after ageing at 1400C. However, coarsening of the zirconia interphase grains had clearly taken place during the ageing.15 Grain size grew from below a micron to a few microns during the 1000-h heat-treatment. This coarsening e€ect could be detrimental in the long run. It has been shown that a porous structure, which is pre￾vented from densifying, will evolve via a de-sintering mechanism18 (i.e. the breaking of sintered necks leading to pore coalescence). Eventually a gap may form between ®bre and matrix. The coarsening e€ect would, therefore, alter the load transfer mechanism but embrittlement should not occur. One sample was subjected to thermal cycling and tes￾ted after 1367 cycles (corresponding to an accumulated time of 230 h at 1200C) were reached. An UTS of 148 MPa and an elastic modulus of 150 GPa was noted. The UTS is plotted in Fig. 8 for comparison with the other thermally aged samples. The results demonstrates very good thermal shock resistance for the composite mate￾rial, a property that monolithic oxides otherwise do not exhibit. Fig. 5. UTS as a function of temperature for [0/90]8,s Al2O3/Al2O3 composites. Fig. 6. Young's modulus as a function of temperature for [0/90]8,s Al2O3/Al2O3 composites. Fig. 7. Reduced ®bre pull-out lengths at elevated temperatures (all samples are from plate 5). M. Holmquist et al. / Journal of the European Ceramic Society 20 (2000) 599±606 603