Influence of graphite nano-flakes on densification and mechanical properties of hot-pressed ZrB₂-SiC composite

Hot pressed monolithic ZrB₂ ceramic (Z), ZrB₂-20 vol% SiC composite (ZS₂₀) and ZrB₂-20 vol% SiC-10 vol% nano-graphite composite (ZS₂₀G_n10) were investigated to determine the influence of graphite nano-flakes on the sintering process, microstructure, and mechanical properties (Vickers hardness and fracture toughness) of ZrB₂-SiC composites. Hot pressing at 1850 °C for 60 min under 20 MPa resulted in a fully dense ZS₂₀G_n10 composite (relative density: 99.6%). The results disclosed that the grain growth of ZrB₂ matrix was efficiently hindered by SiC particles as well as graphite nano-flakes. The fracture toughness of ZS₂₀G_n10 composite (7.1 MPa m^1/2) was essentially improved by incorporating the reinforcements into the ZrB₂ matrix, which was greater than that of Z ceramic (1.8 MPa m^1/2) and ZS₂₀ composite (3.8 MPa m^1/2). The fractographical observations revealed that some graphite nano-flakes were kept in the ZS₂₀G_n10 microstructure, besides SiC grains, which led to toughening of the composite through graphite nano-flakes pull out. Other toughening mechanisms such as crack deflection and branching as well as crack bridging, due to the thermal residual stresses in the interfaces, were also observed in the polished surface.