Effect of surface brittle-to-ductile transition on high-temperature thermal shock resistance of Al₂O₃ ceramics

The effect of surface brittle-to-ductile transition (BDT) on the high-temperature thermal shock resistance (TSR) of Al2O3 ceramics was investigated by quenching in a cold compressed-air flow. The brittle-to-ductile transition (BDT) and transition temperature were determined from the transformation in fracture mechanism via measuring flexural strength at elevated temperatures. Effects of cooling rate on the residual strength and microstructural evolution of Al2O3 ceramics were systematically studied. Results showed that the BDT temperature in Al2O3 ceramics was around 1000 degrees C. The BDT mechanism is associated with the softening of the grain boundary glass phase, which permits a certain degree of crack passivation to reduce thermal stress concentration. Compared with the original strength of 375 MPa, the residual strength of the specimen decreased rapidly to 190 MPa at a final temperature of 700 degrees C, while the residual strength of the specimen was maintained to 352 MPa at a final temperature of 1000 degrees C. The appearance of surface BDT in Al2O3 ceramics during the high-temperature thermal shock process produces a favorable impact on improving the residual strength of specimens by blunting the crack tip and reducing the surface thermal stress. Therefore, the surface BDT in Al2O3 ceramics is beneficial to the improvement of the high-temperature TSR of Al2O3 ceramics.