Effect of graphite flake on microstructure as well as mechanical properties and thermal shock resistance of ZrB2–SiC matrix ultrahigh temperature ceramics

ZrB2–20vol.%SiC containing the various volume fractions of graphite flake (ZSG) composites were investigated to determine the effect of graphite content on the microstructure as well as the mechanical properties and thermal shock resistance. The results revealed that the flexural strength generally decreased as the graphite volume fractions increased. Compared with the fracture toughness of about 4.5 MPa m1/2 for the ZrB2–SiC composites, the toughness of the ZSG composites is essentially higher than that of the ZrB2–SiC composites due to their lower strength relative to the ZrB2–SiC composites. The toughening mechanisms, such as the crack deflection and bridging, were related to the thermal residual stresses, and the thermal residual stresses in interfaces were calculated using Hsueh's formula. Moreover, the critical crack size that represents thermal shock resistance was calculated using the Griffith criterion. It was found that the thermal shock resistance was improved with the increasing graphite volume fractions.