The effect of - seeding on microstructural development of silicon nitride based materials has been investigated. In particular, to observe more distinctly the abnormal grain growth in pressureless sintering, fine -(mean particle size: 0.26 ) powder classified by sedimentation method was used. It was possible to prepare silicon nitride with abnormally grown grains under low nitrogen pressure of 1 atm thanks to the heterogeneous nucleation on seed particles. The size and morphology of silicon nitride grains were strongly influenced by the presence of - seed and overall chemical composition. For specimens with initially low -content, the large grains grew without a significant impingement by other large grains. On the contrary, for specimens with initially high -content, steric hindrance was effective. The resulting microstructure was less inhomogeneous and characterized by unimodal grain size distribution.

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