Influence of nano-graphite addition on hardness and fracture toughness of ZrB2-SiC composites fabricated by spark plasma sintering

ZrB2­­-SiC-based ultrahigh temperature composites were fabricated by spark plasma sintering and reinforced with different amount of graphite nano-flakes (0, 2.5, 5, 7.5 and 10 wt%). Sintering process was performed at a temperature of 1900 °C for a dwell time of 7 min under an applied pressure of 40 MPa in a vacuum atmosphere. The effect of graphite nano-flake on the hardness and fracture toughness of samples were studied. The results showed that with increasing the nano-graphite content, the hardness of composites linearly decreased from 19.5 GPa for the graphite-free sample to 12.1 GPa for the composite reinforced with 10 wt% nano-graphite. An inverse trend was observed for the fracture toughness of ceramics, as it was increased from 4.3 MPa.m1/2 for the graphite-free sample to 8.2 MPa.m1/2 for the 7.5 wt% nano-graphite doped composite. The drop in hardness values, with increasing the nano-graphite content, was related to the intrinsic softness of graphite phase. However, an obvious enhancement in the fracture toughness values due to the addition of more graphite nano-flakes was attributed to the activation of several toughening mechanisms in such materials.