Ceramic (Si3 N4 /SiC) composites have been produced by sintering. From the sintered block, three point bend specimens were cut out. A semi‐circular crack was made on the centre of the tension surface of the test specimen with the aid of a Vicker’s indenter. The diameter of the semi‐circular crack was about 60–70 μm. The specimens were subsequently heat treated at 1300°C for 1 h in vacuum, nitrogen or air. The bending strength was measured at room temperature, 800 and 1000°C. The specimens heat treated in air recovered considerably their bending strength at 1000°C, several specimens failed at a location different from the healed crack and their average strength showed the same value of that for smooth specimens. From these results, it is concluded that the recovery in bending strength of a cracked specimen was caused by crack healing.

Si3 N4 /SiC composite ceramics were sintered and subjected to three‐point bending on specimens made according to the appropriate JIS standard. A semi‐circular surface crack of 110 μm in diameter was made on each specimen. By using three kinds of specimen (smooth, cracked and crack healed), crack‐healing behaviour, cyclic and static fatigue strengths were determined systematically at room temperature and 1000 °C. The main conclusions are as follows: (i) Si3 N4 /SiC composite ceramics have the ability to heal after cracking; (ii) crack‐healed specimens showed similar cyclic and static fatigue strengths as smooth specimens, this being caused by crack healing; (iii) crack‐healed zones had a sufficient fatigue strength and most fractures occurred outside the pre‐cracked zone in those crack‐healed specimens.

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