The ability of ceramic materials to withstand high temperature and hostile environments offers great prospect for potential major improvements in the design performance of high temperature components in chemical processing, power generation and industrial waste recovery applications. Their use as structural materials, however, has been limited primarily because of their poor fracture toughness and lack of damage tolerance. Although research in the past has resulted in major advances in the low temperature toughening of ceramics especially be second phase reinforcements, the majority of these low temperature toughening mechanisms become ineffective at high temperatures. In this research the fundamental micromechanisms of high temperature subcritical crack growth in three classes of ceramic-matrix composites, chosen to reflect different primary room temperature toughening mechanisms, namely crack deflection, crack trapping and crack bridging, will be examined. The approach will be 1) to develop a viable high temperature crack growth monitoring technique in semiconducting ceramics, ii) to identify salient mechanisms impeding crack advances in these composites at high temperatures, iii) to examine the microstructural origins of these mechanisms in terms of the interaction between the crack and the constituents of the composite, iv) to document the toughening from these mechanisms as a function of the microstructural parameters, and v) to establish physical models describing the microstructural dependence of the toughening mechanism.
