Repeated thermal shock behavior of ZrB2-SiC-graphite composite under prestress

The thermal shock resistance of ZrB2-SiC-graphite composite under nominal prestress of 0, 20, 30, 40 or 50 MPa after subjected to 10 and 30 cycles of thermal shock was evaluated by measuring the residual flexural strength of the tested specimen. In each test the applied prestress kept constant and in each cycle the specimen center was heated to 2000 °C within 5 s through electrical resistance heating method and cooled down naturally to room temperature. A lot of broken SiO2 bubbles in the tested specimens were observed with a SEM. For the specimen subjected to 10 cycles of thermal shock, the residual flexural strength does not show big change under different levels of prestress, although the thickness of oxide layer increases at larger prestress, which is presumably attributed to the effect of the oxide layer that heals the cracks and the pores and enhances the strength. For the specimen subjected to 30 cycles of thermal shock, the residual strength decreases, in general, with the increase of prestress level. The thermal shock fatigue under different levels of prestress was also tested, and it was found that the increase of prestress may speed the failure of the specimen, indicating that the level of prestress may fatally affect the failure of the material.