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 (ZS20Gn10) 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 ZS20Gn10 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 ZS20Gn10 composite (7.1 MPa m1/2) was essentially improved by incorporating the reinforcements into the ZrB2 matrix, which was greater than that of Z ceramic (1.8 MPa m1/2) and ZS20 composite (3.8 MPa m1/2). The fractographical observations revealed that some graphite nano-flakes were kept in the ZS20Gn10 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.