Effects of Frequency on the Crack-Healing Behavior of Si3N4/SiC Composite under Cyclic Stress

Si3N4/SiC ceramics were hot-pressed to investigate the effect of stress frequency on the crack-healing behavior under cyclic stress. The specimens having pre-crack of 100µ m were subjected to crack-healing under constant or cyclic bending stresses of 300MPa at temperatures between 800 and 1200°C. The resultant bending strength of the crack-healed specimen was investigated. At a healing temperature of 900°C, the pre-cracks had been completely healed both under constant stress and cyclic stress of 0.5Hz. However, the pre-cracks had not completely healed under a cyclic stress of 5 and 10Hz. Thus, the stress frequency affects crack-healing behaviors at 900°C. On the other hand, when the healing temperature was higher than 1000°C, the pre-cracks were healed completely under both constant stress and cyclic stress up to 10Hz. The effects of stress frequency were dependent on the healing temperature. The reasons for this were discussed from the viewpoint of competitive mechanism of fatigue crack growth and crack-healing.