Direct measurement of residual stresses and their effects on the microstructure and mechanical properties of heat-treated Si3N4 ceramics

For the first time, the residual stresses induced during crystallization of the amorphous grain boundary phase in heat-treated Si3N4 with Yb2O3 as a single additive have been directly measured by X-ray residual stress analyses. The magnitude of the room-temperature strength reduction is comparable to that of the corresponding residual stress. Most of the large residual stress is introduced by large volume changes during the phase transformation. Microscopically, the large residual stress essentially results in the formation of flaws including strain contrast and transgranular cracks in elongated β-Si3N4 grains. Consequently, the large residual stress is a controlling factor for the reduction of the room-temperature strength in the heat-treated material. However, the influence of residual stresses on mechanical properties would be less at higher temperatures.