Residual stress and subsurface damage in machined alumina and alumina/silicon carbide nanocomposite ceramics

We have used TEM and Hertzian indentation to study the interrelation between subsurface damage and residual stress introduced by grinding and diamond polishing surfaces of polycrystalline alumina and 5%SiC/alumina nanocomposites. In all cases a layer of high dislocation density was found near the surface. This varied in thickness from about 300 nm for alumina polished with 1 μm diamond grit to greater than 6 μm for a nanocomposite surface wheel-ground with 150 μm diamond grit. For a given finishing process the nanocomposites showed a greater depth of dislocation activity than alumina. In alumina, extensive basal twinning was found beneath the ground surfaces. Hertzian indentation data indicates a residual compressive stress of about 1500 MPa confined to the dislocation-containing region. Mechanisms for the enhanced dislocation activity in the nanocomposites are discussed.