FABRICATION AND MICROSTRUCTURES OF MOSI2 REINFORCED SI3N4 MATRIX COMPOSITES

Details of the fabrication and microstructures of hot-pressed MoSi2 re inforced-Si3N4 matrix composites were investigated as a function of Mo Si2 phase size and volume fraction, and amount of MgO densification ai d. No reactions were observed between MoSi2, and Si3N4 at the fabricat ion temperature of 1750 degrees C. Composite microstructures varied fr om 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 boun daries, the extent of which decreased with decreased MgO level. No gen eral 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 micro cracking was calculated to be 3 mu m. MoSi2 particles as large as 20 m u m resulted in no composite microcracking; this indicated that signif icant stress relief occurred in these composites, probably because of plastic deformation of the MoSi2 phase.