Effects of testing temperatures and thermal treatments on flexural strength of Si3N4-MoSi2-BN composites

The effects of testing temperatures and thermal treatments on the flexural strength of the Si3N4-MoSi2-Bn composites were investigated. The testing temperature was taken 1500° C to assess the performances of Si3N4-MoSi2-Bn composites. The relationships between the microstructural and morphological changes at the surface and the sub-surface area due to oxidation and mechanical strength degradation were analyzed. The retained room temperature strength of the composites was same and the microstructural changes induced by the thermal treatments introduced the same type of defects in the composites. This strength retention was due to the formation of a surface borosilicatic glassy phase and the partially oxidized sub-surface region which prevented extensive bulk damage. It was observed that the high temperature strength decreased with the increment of the testing temperature due to softening of the grain boundary phase.