Hideyuki Emoto

Control and characterization of abnormally grown grains in silicon nitride ceramics

Abstract The intrinsic grain growth behavior of β-Si 3 N 4 was investigated by annealing fine-grained β-Si 3 N 4 ceramics with 0–30 wt% β-nuclei. The development of bimodal microstructures was observed in annealed materials with 0.1–10 wt% nuclei, caused by the abnormal grain growth of nuclei due to the large driving force. This driving force was related to the difference in nuclei and matrix grain solubility in the liquid phase. The driving force for abnormal grain growth was also related to the stable morphology of β-grains of about 4 in aspect ratio. The fact that the driving force was constant in materials with 0.1–3 wt% nuclei is explained by the constant diffusion space for nuclei. Nuclei addition exceeding 10 wt% decreased the driving force because of nuclei interaction. A unimodal microstructure was developed with 30 wt% nuclei addition.

Microstructure of liquid phase sintered superplastic silicon carbide ceramics

Superplastic silicon carbide ceramics were fabricated at low temperatures by a liquid phase sintering very fine β–SiC powder. The microstructural features of this material, of both before and after the superplastic deformation, have been investigated by transmission electron microscopy. Evaluated from the point of view of phase transformation, dislocation motion, and dynamic grain growth, the materials show very stable microstructures, indicating that the superplastic deformation process was dominated by the liquid phase assisted grain boundary sliding. In addition, the material is also characterized by the formation of clusters of fine SiC particles (5–20 nm) encapsulated in layered graphitized carbon.