Indentation induced crack nucleation and propagation in single crystal MgO

Abstract

A study of indentation induced cracks in single crystal Magnesium Oxide ([001] orientation) was conducted to understand the micromechanisms controlling crack nucleation (and subsequent propagation) in semibrittle solids. Contrary to the expectations based on generally accepted crack nucleation models, no evidence of dislocation pile-ups was observed. However, SEM and TEM study of the deformation structure within the indent impression did reveal the presence of shear fault induced microcracking. This phenomenon was confirmed by examination of cross-sections through the indents using SEM. When the indentation temperature was varied from ambient to 800°C, a direct correlation was observed between the degree of subsurface damage, and the absence or presence of surface radial cracks. Based on the afore-mentioned results, an alternate crack nucleation mechanism is proposed, whereby shear fault induced microcracks act as nuclei for subsequent radial cracking. Further, microdiffraction analysis of the misorientations induced outside the indent suggest that crack propagation parallel to {110} is the result of severe plastic incompatibility across this plane.