Self-crack-healing behavior under cyclic stress of silicon nitride composite at elevated temperature

Si₃N₄/SiC composite ceramics were hot-pressed to investigate the crack-healing behavior under stress. Semi-elliptical surface cracks of 0.1 mm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under cyclic or constant bending stress, and the resultant bending strength and cyclic fatigue strength were studied. The threshold stress for crack-healing was investigated at healing temperatures of 1000° and 1200°C. The cyclic fatigue strengths of crack-healed specimens were also investigated at healing temperatures of 900° and 1000°C. The main conclusions are as follows: (1) The threshold cyclic and constant stresses for crack-healing, below which pre-cracked specimens recovered their bending strength, were 300 MPa which was 75% of the bending strength of the pre-cracked specimens, and (2) The crack-healed specimens exhibited quite high cyclic fatigue strength at crack-healing temperatures of 900° and 1000°C.