Crack-healing behavior of structural ceramics under constant and cyclic stress at elevated temperature

Three types of structural ceramics, Al2O3/SiC, mullite/SiC and Si3N4/SiC having excellent crack-healing ability were sintered. A semi-elliptical surface crack of 75-110 mum in surface length was made on each specimen with a Vickers indenter. The basic crack-healing behavior was studied systematically on Al2O3/SiC as a function of crack-healing temperature and time. Three point bending tests were carried out to investigate the strength of crack-healed samples. The specimens containing the semi-elliptical surface crack were healed under static or cyclic bending stress on mullite/SiC and Si3N4/SiC. It was shown that a crack could be healed completely either under a static or a cyclic stress. The competition between self crack-healing rate (S-CHR) and crack growth rate (C-GR = da/dt) determines the crack-healing behavior under stress: (i) In case of S-CHR > C-GR, a crack can be healed completely even under stress and recover of strength is complete. (ii) In case of S-CHR = C-GR, a crack cannot be healed completely and recover of strength is insufficient. (iii) In case of S-CHR < C-GR, the specimen fails due to crack growth under stress.