Densification mechanism and microstructural evolution of Si₃N₄ composites with CeO₂ as additives during spark plasma sintering

The densification mechanism of CeO 2 doped Si 3 N 4 ceramics during spark plasma sintering (SPS) was investigated. The sintering process was observed to undergo significant densification at the intermediate stage of sintering (1435 °C–1816 °C), and the density and grain size of CeO 2 doped Si 3 N 4 ceramic were found to be greater than those of pure Si 3 N 4 ceramic under the same sintering conditions. To determine the densification mechanism, a creep model was utilized to determine the densification mechanism which could be interpreted based on the values of the stress exponent ( n ) and the apparent activation energy ( Q d ). The results revealed that the Q d of pure Si 3 N 4 (381.15 kJ mol −1 ) is higher than that of Si 3 N 4 -CeO 2 powder (265.61 kJ mol −1 ). Moreover, the stress exponent n of Si 3 N 4 -CeO 2 powder (1.12–1.33) was higher than that of pure Si 3 N 4 powder (1.00–1.18). This can be attributed to the fact that the addition of CeO 2 resulted in the formation of a liquid phase at the grain boundary which leading to a shift in the controlling mechanism from grain boundary diffusion to grain boundary sliding.