Subsurface and surface cracks under contact loading in transformation-toughened ceramics

Abstract

The wearing surface in rolling and sliding wear is approximated by a half-plane subjected to frictional contact loading. Under such loading inherent flaws, such as microcracks, voids and inhomogeneities, eventually develop into macrocracks with a resultant deterioration in wear performance. Two macrocrack configurations are studied: a straight subsurface crack parallel to the surface and a straight edge crack perpendicular to the surface. The cracks are modelled using the dislocation formalism, and the contact load is assumed to be Hertzian. In the vicinity of the crack tips, where a high tensile stress field is anticipated, the material is allowed to undergo irreversible phase transformation in accordance with an appropriate transformation criterion. The transformation zone around the crack tip (or tips) is modelled as a collection of minute circular regions. Such a model is a good representation of the transformable precipitates, for example in partially stabilized zirconia. The elastic field from the transformed circular regions is obtained by applying the Eshelby formalism. The phase transformation exerts a strong influence on the behaviour of the cracked material compared to the purely elastic material through a significant closing effect on the crack faces. This results in the mode I stress intensity factor being almost zero. However, as the transformation zone is highly asymmetric relative to the crack faces, because of the non-uniform stress field due to the applied contact load, the mode II loading of the crack tip(s) can be severely enhanced. This results in high residual near tip deformations after unloading, i.e. a high residual mode II stress intensity factor.