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.
KeywordsSilicon Nitride Subcritical Crack Growth Flaw Population Room Temperature Strength Proof Test
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