A detailed study characterizing the strength and nature of crack propagation has been made in several high-temperature structural ceramics, namely, hot-pressed silicon nitride (Si₃N₄), Refel silicon carbide (SiC), sialons, and lithium-aluminum-silicate (LAS) glass ceramic. Fracture phenomenology was studied fractographically as a function of flaw size, temperature, and loading rate. The temperature dependence of the fracture behavior was evaluated using uncracked and precracked specimens.

All the materials showed the same general characteristics of temperature dependence of both the fracture stress and crack-growth phenomena. The only variation between the different materials was the temperature regime in which the differing phenomena occurred. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A region of fast fracture (catastrophic crack extension) existed up to a certain temperature, in which the mode of crack propagation was primarily transgranular. Above this temperature, the strength (fracture stress) decreased considerably due to the presence of subcritical or slow crack growth (SCG) prior to the final catastrophic fracture, and the extent of SCG increased with increasing temperature. Slow crack growth is a result of microplastic deformation, which occurred by grain-boundary sliding and grain separation, and the mode of crack propagation is exclusively intergranular.

The influence of the loading rate on SCG and fracture stress was investigated. Generally, at a given temperature, the extent of SCG decreased with increasing loading rate until, at a sufficiently high rate, catastrophic fracture occurred directly from the initial crack with no slow crack extension. Directly analogous observations were made in stress-rupture studies in which the extent of SCG decreased with increasing stress level.