THERMAL-SHOCK RESISTANCE OF NEXTEL SILICA-ZIRCONIA CERAMIC-MATRIX COMPOSITES MANUFACTURED BY FREEZE-GELATION/

Ceramic-matrix composites consisting of a silica-zirconia matrix reinf orced by 25% by volume continuous Nextel alumino-silicate fibres have been manufactured by a sol-gel route incorporating freeze-gelation and subjected to thermal shock by water quenching from temperatures up to 800 degrees C. Although it was demonstrated that the Nextel fibres ex hibited significant degradation of properties during composite process ing, they still had a significant reinforcing effect. The material sho wed a critical temperature of approximately 550 degrees C, below which it was unaffected by water quenching. Above this level there was a 40 and 50% step reduction in modulus and strength respectively. Acoustic emission monitoring showed a significant difference in materials subj ect to quenching from beyond the critical temperature. There was a low er initial yet higher final AE event rate for a monotonically increasi ng flexural load, and a bimodal amplitude distribution. This indicated that the material responds in general terms in the manner correspondi ng to the well-known Hasselmann model of thermal shock of monolithic c eramics, but the presence of fibres appears to improve the thermal sho ck resistance by raising the predicted critical temperature somewhat.