Grain size influence on residual stresses in alumina/zirconia composites

The grain size (GS) and volume fraction of alumina have been systematically varied in composites with a zirconia matrix and the corresponding residual stresses have been assessed by means of piezo-spectroscopy. The compressive stress in alumina depends on the volume fraction and it is well predicted by a stochastic model based on information theory. No dependence with GS has been detected, except at the highest volume content (20% vol. alumina). Conversely the stress distribution is independent from the volume fraction and depends on GS: intermediate values of GS exhibit the wider stress distribution. The tensile stress in zirconia shows no clear correlation with the volume fraction and increases with increasing zirconia GS. This latter behaviour has been compared with a model based on diffusional relaxation of stresses. The model reproduces correctly the stress change due to different alumina contents, but it diverges from the experimental data at smaller GSs, overestimating the residual stress. It is suggested that grain boundary sliding may also contribute to the relaxation of stresses.