Fabrication and microstructures of MoSi2 reinforced Si3N4 matrix composites

Details of the fabrication and microstructures of hot-pressed MoSi2 reinforced-Si3N4 matrix composites were investigated as a function of MoSi2 phase size and volume fraction, and amount of MgO densification aid. No reactions were observed between MoSi2, and Si3N4 at the fabrication temperature of 1750 degrees C. Composite microstructures varied from particle-matrix to cermet morphologies with increasing MoSi2 phase content. The MgO densification aid was present only in the Si3N4 phase. An amorphous glassy phase was observed at the MoSi2-Si3N4 phase boundaries, the extent of which decreased with decreased MgO level. No general microcracking was observed in the MoSi2-Si3N4 composites, despite the presence of a substantial thermal expansion mismatch between the MoSi2 and Si3N4 phases. The critical MoSi2 particle diameter for microcracking was calculated to be 3 mu m. MoSi2 particles as large as 20 mu m resulted in no composite microcracking; this indicated that significant stress relief occurred in these composites, probably because of plastic deformation of the MoSi2 phase.