Microstructural mechanisms of cyclic fatigue-crack propagation in grain-bridging ceramics

The microstructural basis of cyclic fatigue-crack propagation in grain-bridging ceramics is investigated for monolithic, hot-pressed alumina and silicon nitride. In both materials, rates of subcritical crack growth under cyclic loads are observed to be many orders of magnitude faster than corresponding growth rates under monotonic loading at equivalent stress-intensity levels. This behaviour is attributed to diminished crack-tip shielding under cyclic loads caused by a degradation of the grain-bridging zone in the wake of the crack tip associated with frictional wear at the grain/matrix interface; the reduced shielding acts locally to enhance the crack-tip driving force in cyclic fatigue. Micromechanical modelling of this process is shown to be consistent with fractographic and in situ crack-profile analyses. The effect on crack-growth rates of microstructural and mechanical variables are examined in light of this mechanism.