Strength of Hot‐Pressed Silicon Nitride After High‐Temperature Exposure

Short‐term exposure of hot‐pressed silicon nitride to temperatures greater than 800°C in an oxidizing atmosphere causes an increase in the room‐temperature strength and eliminates the truncated strength distribution produced by room‐temperature proof‐testing. Acid‐etching the proof‐tested samples restores the original truncated distribution. These strength changes are shown to be related to the formation of a glassy phase on the surface that smooths out the preexisting machining flaws. More extensive, long‐term oxidation produces surface pits that lead to an irreversible change in strength