Effects of nano-graphite content on the characteristics of spark plasma sintered ZrB₂–SiC composites

In this study, ZrB₂–25 vol% SiC composite containing 0, 2.5, 5, 7.5 and 10 wt% graphite nano-flakes were prepared by spark plasma sintering (SPS) process at 1900 °C for 7 min under 40 MPa. The fabricated composite samples were compared to examine the influences of nano-graphite content on the densification, microstructure and mechanical properties of ZrB₂–SiC-based ultrahigh temperature ceramics. Fully dense composites were obtained by adding 0–5 wt% nano-graphite, but higher amounts of additive led to a small drop in the sintered density. The growth of ZrB₂ grains was moderately hindered by adding nano-graphite but independent of its content. The hardness linearly decreased from 19.5 for the graphite-free ceramic to 12.1 GPa for the sample doped with 10 wt% nano-graphite. Addition of graphite nano-flakes increased the fracture toughness of composites as a value of 8.2 MPa m^½ was achieved by adding 7.5 wt% nano-graphite, twice higher than that measured for the graphite-free sample (4.3 MPa m^½). The in-situ formation of ZrC and B₄C nano-particles as well as the presence of unreacted graphite nano-flakes led to a remarkable enhancement in fracture toughness through activating several toughening mechanisms such as crack deflection, crack bridging, crack branching and graphite pullout.