V. Garnieret al /Journal of the European Ceramic Sociery 25(2005)3485-3493 ▲Sicw"L·1300°c 0. 6].SICw'L'-1200C sicw"H"-1200°C △△Δ△ Time(Hours Fig 8. Creep curves, at 1200 and 1300"C under 100 MPa, for alumina-SiC whisker composites. creep rate is 7x 10-s for theL composite after 60h of the composite with 'L SiC whiskers has been significantly of exposure under load 100 MPa and the corresponding final reduced by 2 orders of magnitude(=1.5 x 10-8s-)On the strain is 0.15%, same results have been observed earlier by other hand, a short stage of tertiary creep is observed before several authors 28-30 breaking at a strain of about 0.6% and after 40 h Observations of Al2O3/SiCw composites after creep test Concerning the composites with 'H whiskers(Fig 8)the ng have revealed that the Sic whiskers on the surface as specimen has been broken after only 2 h of testing. This creep well as in the composite core have been oxidized. Precise behaviour degradation and the increase of the fracture tough examination of a sample fracture surface, broken at room ness under air, at high temperatures, may be explained by the temperature after creep testing at 1200C in air, has been SiC oxidation and the amount of glassy phase in the grain performed. The SEM micrographs(Fig. 7)show the occur- boundaries rence of the glassy phase on the surface fracture. The I As it has been discussed earlier, toughening by deviation uid phase formed on exposed surfaces could migrate near and crack bridging occur in whisker reinforced materials the surface grain boundaries of alumina composite and ac- Consequently, the process zone size is not negligible and celerate creep deformation process. At higher temperatures non-linear macroscopic fracture behaviour must be observed, (1200C), the volume fraction of liquid phase increases and producing R-curve effect in which toughness (KR)varies with the viscosity of liquid phase decreases. Consequently, grain crack growth. So, the existence of a R-curve behaviour has boundaries sliding are more important and induce the for- been investigated by SENB method At room temperature and mation of grain boundary cavities, the creep behaviour may 1000C, no stable crack propagation could be obtained for be deeply modified. The observed creep deformation for"L' the composites prepared using 'L SiCw, suggesting that there composite at a stress of 100 MPa and 1300C confirms the are no extended R-curve behaviour But hereafter 1200C precedent hypothesis(Fig 8). At 1300C, the creep strength stable crack propagation was observed showing a significant 876 aAaaaa ▲▲A▲▲ ▲Sicw"L a SiCW'H Fig 9. Rising crack-growth resistance(R-curve)of alumina-SiC whisker composites at 1200C3490 V. Garnier et al. / Journal of the European Ceramic Society 25 (2005) 3485–3493 Fig. 8. Creep curves, at 1200 and 1300 ◦C under 100 MPa, for alumina–SiC whisker composites. creep rate is 7 × 10−10 s−1 for the ‘L’ composite after 60 h of exposure under load 100 MPa and the corresponding final strain is 0.15%, same results have been observed earlier by several authors.28–30 Observations of Al2O3/SiCw composites after creep test￾ing have revealed that the SiC whiskers on the surface as well as in the composite core have been oxidized. Precise examination of a sample fracture surface, broken at room temperature after creep testing at 1200 ◦C in air, has been performed. The SEM micrographs (Fig. 7) show the occur￾rence of the glassy phase on the surface fracture. The liq￾uid phase formed on exposed surfaces could migrate near the surface grain boundaries of alumina composite and ac￾celerate creep deformation process. At higher temperatures (>1200 ◦C), the volume fraction of liquid phase increases and the viscosity of liquid phase decreases. Consequently, grain boundaries sliding are more important and induce the for￾mation of grain boundary cavities, the creep behaviour may be deeply modified. The observed creep deformation for ‘L’ composite at a stress of 100 MPa and 1300 ◦C confirms the precedent hypothesis (Fig. 8). At 1300 ◦C, the creep strength of the composite with ‘L’ SiC whiskers has been significantly reduced by 2 orders of magnitude (=1.5 × 10−8 s−1). On the other hand, a short stage of tertiary creep is observed before breaking at a strain of about 0.6% and after 40 h. Concerning the composites with ‘H’ whiskers (Fig. 8) the specimen has been broken after only 2 h of testing. This creep behaviour degradation and the increase of the fracture tough￾ness under air, at high temperatures, may be explained by the SiC oxidation and the amount of glassy phase in the grain boundaries. As it has been discussed earlier, toughening by deviation and crack bridging occur in whisker reinforced materials. Consequently, the process zone size is not negligible and non-linear macroscopic fracture behaviour must be observed, producing R-curve effect in which toughness (KR) varies with crack growth. So, the existence of a R-curve behaviour has been investigated by SENB method. At room temperature and 1000 ◦C, no stable crack propagation could be obtained for the composites prepared using ‘L’ SiCw, suggesting that there are no extended R-curve behaviour. But hereafter 1200 ◦C stable crack propagation was observed showing a significant Fig. 9. Rising crack-growth resistance (R-curve) of alumina–SiC whisker composites at 1200 ◦C