Effect of tensile stress during crack healing on the remaining strength of silicon nitride ceramics at high temperatures

Crack healing by high temperature oxidation in fine ceramics is an effective method to overcome their low toughness. The crack healing behavior of conventional Si₃N₄ was examined. The healing temperature chosen for this study was 1100°C, a temperature at which cracks were imperfectly healed. The effect of stress during crack healing was experimentally investigated based on the strength of the imperfectly healed crack. The primary focus of this study was on the investigation of healing behavior during a tensile stress, under which the oxidization of cracks was accelerated and the Si₃N₄ oxidized significantly because of opened cracks. The following results were obtained: (1) Healing without stress successfully enhanced both remaining strength and apparent fracture toughness of the cracked specimen at room temperature and temperatures as high as 1000°C. (2) The remaining strength when healing in air was higher than that when healing in a low pressure atmosphere. However, the difference in remaining strength between the two was relatively small. (3) The remaining strength when healing with compressive stress was as well as that without stress. (4) Both remaining strength and apparent fracture toughness of the healed specimen, with tensile healing, hardly increased. It was also concluded that the amount of oxidation did not significantly affect the healing behavior in this study.