Direct measurement of residual stresses and their effects on the microstructure and mechanical properties of heat-treated Si3N4 ceramics II: With CeO2 as a single additive

The heat treatment of silicon nitride ceramics with only CeO2 as a sintering additive was carried out at different temperatures. Large residual stress was induced only by the non-relaxed volume changes and not by the relaxed volume changes (i.e. the crystallized phases with cavities or microcracks). The fact that the liquid formation temperature of the Si-Ce-O-N system (∼1470 °C) is much lower than that of the Si-Yb-O-N system (∼1650 °C) is the reason why the residual stress is comparable almost to each other in CeO2-doped Si3N4, but increases steadily with heat-treatment temperature in Yb2O3-doped Si3N4. The large residual stress and the induced cavities and microcracks are the dominant factors for the reduction in room-temperature strength of the heat-treated samples. The defects in β-Si3N4 grains of heat-treated samples were caused by the large residual stress, and may lead to the reduction of both room- and high-temperature strengths. These results significantly extends our previous study (Guo GF, Li JB, Yang XZ, Lin H, Liang L, He MS, Tong XG, Yang J. Acta Mater 2006;54:2311) on heat-treated Si3N4 ceramics with only Yb2O3 (one of the heavier lanthanide oxides) as a sintering additive.