Abstract
Ceramic-matrix composites consisting of a silica—zirconia matrix reinforced 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 °C. Although it was demonstrated that the Nextel fibres exhibited significant degradation of properties during composite processing, they still had a significant reinforcing effect. The material showed a critical temperature of approximately 550 °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 subject to quenching from beyond the critical temperature. There was a lower initial yet higher final AE event rate for a monotonically increasing flexural load, and a bimodal amplitude distribution. This indicated that the material responds in general terms in the manner corresponding to the well-known Hasselmann model of thermal shock of monolithic ceramics, but the presence of fibres appears to improve the thermal shock resistance by raising the predicted critical temperature somewhat.
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