Reinforcing effects of SiC whiskers and carbon nanoparticles in spark plasma sintered ZrB₂ matrix composites

ZrB₂-based ceramics, reinforced with 25 vol% SiC whiskers (SiC_w) as well as 0, 2.5, 5 and 7.5 wt% carbon nanoparticles (C_np), were prepared by spark plasma sintering (SPS) at 1900 ºC under 40 MPa for 7 min in a vacuum environment. The influences of C_np content on densification behavior, microstructure evolution, hardness and fracture toughness of ZrB₂–SiC_w ceramics were investigated. Compared to the carbon-free sample, the grain growth of ZrB₂ matrix was moderately decreased (~ 20%) after the addition of C_np. The in-situ formation of B₄C and ZrC phases was attributed to the elimination of surface oxide impurities through their chemical reactions with the C_np additive. All composite samples approached their theoretical densities. A hardness of 21.9 GPa was obtained for ZrB₂–SiC_w sample, but the hardness values linearly decreased by the addition of soft carbon additives and reached 14.6 GPa for the composite doped with 7.5 wt% C_np. The fracture toughness showed another trend and increased from 4.7 MPa m^½ for the carbon-free sample to 7.1 MPa m^½ for 5 wt% C_np-reinforced composite. The formation of new carbides and the presence of unreacted C_np resulted in toughness improvement. Various toughening mechanisms such as crack branching, bridging, and deflection were detected and discussed.