Creep Damage Mechanisms in Structural Ceramics

The mechanisms responsible for fracture in structural ceramics at elevated temperatures are reviewed. It is seen that some features of the process are common to a wide range of materials, with quite different microstructures. These include a sensitivity to flaws or microstructural inhomogeneities which nucleate microcracks. However, it is often observed that the flaws which control failure under creep conditions are different from those responsible for fast fracture at ambient temperatures. Another common feature is the development of cracks through a process of gradual damage accumulation. There are however, differences which depend on the nature of microstructure. In pure materials and some multi-phase alloys with high volume fractions of the binder phase, the heterogeneities responsible for fracture initiation are far apart, and extensive crack growth occurs prior to failure. Such materials are crack growth dependent. In multiphase alloys in which the binder is a minor phase, the high degree of constraint produces many sites for damage to nucleate. However, the low ductility and damage tolerance of such materials leads to failure after only a modest amount of crack propagation. In such materials, modified Monkman-Grant behaviour is observed.