High temperature fracture behavior of ZrB₂-SiC-graphite composite in vacuum and air

Hot pressed ZrB₂-SiC-graphite ultra high temperature ceramic composites were prepared for the fracture toughness tests in vacuum up to 1800 °C and in air up to 1300 °C. Measured results show that a brittle-ductile transition temperature of ZrB₂-SiC-graphite composite is deduced to be 1300 °C, and ZrB₂-SiC-graphite composite exhibits a linear behavior below 1300 °C. Fracture toughness in vacuum linearly decreases with the increase of temperature below 1300 °C, increases from 1300 °C to 1600 °C, and has a rapid reduction at 1800 °C. The phenomenon can be attributed to the relaxation of thermal residual stress under 1300 °C, the existence of plastic flow from 1300 °C to 1600 °C, and the degeneration of graphite flakes and cavitation formation at 1800 °C. In comparison to vacuum, the fracture toughness in air improves by the healing effect of pre-oxidation below 1300 °C. The microstructure observations further reveal the great effects of temperature and pre-oxidation on the fracture behavior of ZrB₂-SiC-graphite composite.