Crack Healing Behaviour and High Temperature Strength of Silicon Nitride Ceramics.

Crack healing behaviours of silicon nitride have been investigated as a function of sintering additives and crack healing condition, systematically. The semicircular crack (average diameter is 110 μm) was made in the center of specimen using Vickers hardness indenter. Some of the cracked specimens were heat-treated at 800°C∼1 400°C, 1∼150 hour in air atmosphere. Bending strength was measured at room and elevated temperature up to 1 400°C. Bending strength of cracked specimen reduced to about 48% of that of smooth specimen. However, the bending strength of pre-cracked specimens recovered completely up to that of smooth specimen by the heat treatment. And in several specimens, failure occured from the outside of pre-cracked area even at 1 300°C. This strength recovery was caused by crack healing. By analysing the test results, the following conclusion was obtained. The silicon nitride which has the highest crack healing ability within this experiment is silicon nitride with 20 wt.% SiC and 8 wt.% Y2O3. The best healing condition for the specimen is 1 200°C∼1 300°C, 1 h in air. The crack healed member at the condition has enough bending strength and low sensitivity to static fatigue up to 1 400°C.