Relation between Grain Growth and Phase Transformation of Silicon Nitride

Several silicon nitride pre-sintered bodies with different α-phase contents were prepared by hot-pressing. The grain growth behavior was investigated by annealing pre-sintered bodies for 1 to 4h at 1800°C or 2h at 1900°C. The microstructure of annealed body became finer with the increase of α content in pre-sintered body. The α/β phase transformation was completed at the early stage of annealing. The materials developed “in-situ composite” microstructures consisting of large grains and fine matrix grains. The β nuclei were formed or grown to the large grains with phase transformation. On the other hand, the matrix grains were elongated with phase transformation. In the following stage of the annealing, grain growth behavior was dominated by the grain size distribution which formed at the early stage of annealing. In the case of material from α powder, the microstructure becomes fine and a few β nuclei form during phase transformation. So, the material developed an “in-situ composite” microstructure because the driving force of grain growth is high. On the other hand, the microstructure of material from β powder becomes homogeneous because the driving force of grain growth decreased by presence of large number of nuclei.