Effect of testing temperatures and thermal treatments on flexural strength of Si3N4-MoSi2-BN Composites

Two (Si3N4+ 5 vol% MoSi2) +20 vol% BN composites differing for BN grain size and aspect ratio were sintered by hot-pressing using Y2O3 and Al2O3 as sintering aids. The flexural strength degradation of the Si3N4–MoSi2–BN composites due to high-temperature effects was evaluated by testing the composites at room temperature after thermal treatments in air and at high temperatures. After an exposition of 100 h at 1500°C in air, the retained room-temperature strength was about 2/3 of the pristine value. This good strength retention was due to the formation of a surface borosilicatic glassy phase and of a partially oxidized sub-surface region which prevented extensive bulk damage. After thermal cycling tests at 1500°C, strength degradation ranges between 30% and 45% and depends on the characteristics of the glassy layer formed on the surfaces. For both composites, the high-temperature strength progressively decreases above 1300°C due to sub-critical crack growth