Shinji Saito

Crack-Healing under Cyclic Stress and Improvement of the Resultant Fatigue Strength of Si3N4/SiC

Si3N4/SiC composite ceramics were hot-pressed in order to investigate their crack-healing behavior under cyclic stress and the resultant static fatigue strength. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under a cyclic bending stress of 210MPa in air at 900, 1000, 1100, and 1200 °C. The bending strength and static fatigue strength of the crack-healed specimens were systematically investigated at each healing temperature. The specimens which has been crack-healed and static fatigue-tested at 900 and 1000 °C showed lower static fatigue strength than those tested at 1100 and 1200 °C. Detailed investigation on the fracture surface of static fatigue-tested specimens showed that oxidation of the base material had strong effects on the static fatigue strength. It was found that when the specimens were pre-oxidized in air at 1300 °C, the surface was covered by a protective oxide layer, leading to a significant improvement of static fatigue strength at 900 and 1000 °C.

Improvement of Static Fatigue Strength of Silicon Nitride Composite

Si3 N4 /SiC composite ceramics were hot-pressed in order to investigate their crack-healing behavior under cyclic stress and the resultant static fatigue strength. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under a cyclic bending stress of 210MPa in air at 900, 1000, 1100, and 1200 °C. The bending strength and static fatigue strength of the crack-healed specimens were systematically investigated at each healing temperature. The specimens which has been crack-healed and static fatigue-tested at 900 and 1000°C showed lower static fatigue strength than those tested at 1100 and 1200 °C. Detailed investigation on the fracture surface of static fatigue-tested specimens showed that oxidation of the base material had strong effects on the static fatigue strength. It was found that if the specimens were pre-oxidized in air at 1300 °C, the surface was covered by a protective oxide layer, leading to a significant improvement of static fatigue strength at 900 and 1000 °C.Copyright

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.

Threshold stress for crack-healing of silicon nitride ceramics

Si3 N4 /SiC composite ceramics were hot-pressed in order to investigate the crack-healing behavior under stress. Semi-elliptical surface cracks of 100 μm in surface length were introduced on each specimen. The pre-cracked specimens were crack-healed under cyclic or constant bending stress of 5 Hz at healing temperatures of 1000° and 1200°C and the resultant bending strength and fatigue strength were studied. The threshold stress for crack-healing, below which the pre-cracked specimens recovered their bending strength, was investigated. The mechanisms of crack-healing in Si3 N4 /SiC under stress were also discussed.Copyright