Effects of Preoxidation on the Strength and Fatigue of Hot-Pressed Silicon Nitride

The strength of as-machined and pre-oxidized hot-pressed silicon nitride was measured in three-point bending at temperatures of 23, 800, 1000, and 1100°C as a function of stressing rate. Pre-oxidation at 980°C for 50 h significantly increased the strength for the test temperatures of 23 and 800°C; however, at 1000°and 1100°C the strengths of pre-oxidized and as-machined samples were statistically indistinguishable and significantly less than the room temperature strengths. The fatigue resistance of the pre-oxidized specimens was found to be similar to that of the as-machined specimens at all temperatures with both exhibiting a high fatigue resistance (N=100) at 23 and 800°C and a significant decrease in fatigue resistance (N=20) at 1000 and 1100°C. Additional results showed that flaw generation and the growth of existing flaws are competing processes at high temperatures and fracture strength is determined by the most dominant of these processes.