The evaluation of the influence of machining residual stresses on the bending strength of Al2O3 ceramics

Even after near net-shape processing, machining is often necessary to obtain the required surface quality and precision of dimensions. In industrial applications, machining is in most cases performed by grinding which leads to complex changes of the material properties in the near-surface layers. Since the effects of grinding-induced damage and residual stresses may have competing influences on the strength, the knowledge of these effects is of principal interest. Therefore, Al2O3 samples were machined with different grinding parameters. The near-surface distributions of grinding residual stresses were determined non-destructively by means of depth-resolved X-ray residual stress analyses. It was found that the compressive surface residual stresses increase with increasing depth of cut of the individual grains of the grinding wheels. In parallel, an increase of the 4-point bending strength of the samples was observed. Its magnitude was compatible with the effects of the grinding residual stresses on the bending strength which are expected according to fracture mechanics calculations. It was found that near-surface compressive residual stresses increase the bending strength even if failure from critical defects initiates at depths larger than the thickness-of the compressively stressed surface layer.