Temperature-Dependent Toughening in Whisker-Reinforced Ceramics

The mechanical response of whisker-reinforced ceramics, which is dominated by interfacial chemistry-arid stress, is expected to vary as thermal mismatch stresses diminish and grain boundary phases soften with increasing temperature. To examine the temperature dependence of the toughening process two silicon carbide whisker-reinforced systems have been studied: alumina and mullite. The systems represent a range of thermal mismatch conditions and potential crack-whisker interactions including crack bridging, whisker pull-out and crack deflection. For both systems, fracture toughness and hardness are measured as a function of temperature. The fracture toughness of both materials shows little change to 1100°C. However, post-mortem fractography indicates that limited pullout occurs in the mullite/SiC_w system at elevated temperatures. Only whiskers oriented normal to the fracture plane are pulled out without fracture due to the additional bending stresses on the whiskers. In the alumina/SiC_w system, the mode of fracture remains invariant with temperature because of high whisker compressive stresses. These observations indicate that for randomly-oriented whiskers substantial toughening by pull-out is contingent upon low interfacial stresses coupled with high strength whiskers which can sustain large bending stresses.