MICROSTRUCTURAL DEVELOPMENT DURING PHASE- TRANSFORMATION OF SILICON-NITRIDE CERAMICS

The influence of phase transformation on grain growth behavior was inv estigated by annealing dense, fine-grained alpha- or beta-silicon nitr ide ceramics with or without large alpha- or beta-nuclei at 1100 degre es C for 0.5 to 2h. In the case of alpha-materials, alpha-phase conten t decreased with annealing time, which resulted in the development of in-situ composite microstructures having a small number of large elong ated grains in the fine-grained matrix. The large grains in alpha-mate rials were based on the grain growth from both formed beta-nuclei thro ugh the phase transformation and added beta-nuclei. In the case of bet a-materials, only the selective abnormal grain growth of added beta-nu clei was observed. Aspect ratios of both matrix and large grains in al pha-materials were larger than those in beta-materials. The grain-grow th rate of added beta-nuclei in alpha-materials was also higher than t hose in beta-materials. The grain growth of silicon nitride should be accelerated by alpha/beta phase transformation, which enhanced supersa turation of liquid phase due to the higher solubility of alpha-grains than that of beta-grains and the formation of fine beta-grains.