Fabrication and Microstructures of MoSi ₂ Reinforced–Si ₃ N ₄ Matrix Composites

Abstract

Details of the fabrication and microstructures of hot‐pressed MoSi 2 reinforced–Si 3 N 4 matrix composites were investigated as a function of MoSi 2 phase size and volume fraction, and amount of MgO densification aid. No reactions were observed between MoSi 2 and Si 3 N 4 at the fabrication temperature of 1750°C. Composite microstructures varied from particle–matrix to cermet morphologies with increasing MoSi 2 phase content. The MgO densification aid was present only in the Si 3 N 4 phase. An amorphous glassy phase was observed at the MoSi 2 –Si 3 N 4 phase boundaries, the extent of which decreased with decreased MgO level. No general microcracking was observed in the MoSi 2 –Si 3 N 4 composites, despite the presence of a substantial thermal expansion mismatch between the MoSi 2 and Si 3 N 4 phases. The critical MoSi 2 particle diameter for microcracking was calculated to be 3 μm. MoSi 2 particles as large as 20 μm resulted in no composite microcracking; this indicated that significant stress relief occurred in these composites, probably because of plastic deformation of the MoSi 2 phase.