A water-quenching technique was used to evaluate the thermal-shock strength behaviour of silicon nitride ceramics in an air atmosphere. When the tensile surface was shielded from air during the heating and soaking process, the quenched specimens showed a gradual decrease in strength at temperatures above 600 deg C. However, the specimens with the air-exposed surface exhibited a ~16% and ~29% increase in strength after quenching from 800 and 1000 deg C, respectively. This is because of the occurrence of surface oxidation, which may cause the healing of surface cracks and the generation of surface compressive stresses. As a result, some pre-oxidation of Si3N4 components before exposure to a thermal-shock environment is recommended in practical applications.
