Microstructural Development during Phase Transformation of Silicon Nitride Ceramics

Abstract

The influence of phase transformation on grain growth behavior was investigated by annealing dense, fine-grained α- or β-silicon nitride ceramics with or without large α- or β-nuclei at 1700°C for 0.5 to 2h. In the case of α-materials, α-phase content decreased with annealing time, which resulted in the development of in-situ composite microstructures having a small number of large elongated grains in the fine-grained matrix. The large grains in α-materials were based on the grain growth from both formed β-nuclei through the phase transformation and added β-nuclei. In the case of β-materials, only the selective abnormal grain growth of added β-nuclei was observed. Aspect ratios of both matrix and large grains in α-materials were larger than those in β-materials. The grain-growth rate of added β-nuclei in α-materials was also higher than those in β-materials. The grain growth of silicon nitride should be accelerated by α/β phase transformation, which enhanced supersaturation of liquid phase due to the higher solubility of α-grains than that of β-grains and the formation of fine β-grains.