Toughening of ZrB₂-based composites with in-situ synthesized ZrC from ZrO₂ and graphite precursors

ZrB₂ matrix composites toughened with in-situ formed ZrC were fabricated by spark plasma sintering (SPS) utilizing ZrB₂/ZrO₂/graphite powder mixtures at 1900 °C for 7 min under 40 MPa load. Different amounts of graphite nano-flakes (3, 6, and 9 wt%) were added to ZrB₂–20 vol% ZrO₂ aiming in-situ formation of ZrC reinforcement. Clean ZrB₂/ZrC interfaces were observed in the as-sintered microstructure, especially in the composite containing 6 wt% graphite. The reduction of ZrO₂ with graphite, which formed ZrC, was progressed over a zirconium oxycarbide (ZrC_xO_y) intermediate. The hardness of the SPSed ZrB₂-based composites was decreased with increasing the graphite content. However, the fracture toughness of the composites showed a reverse trend. By adjusting the graphite content to 6 wt%, excellent control of microstructure could be achieved for the prepared composites with a higher amount of ZrC and lower content of other unfavorable phases. It was finally found that achieving the optimum microstructure of the composite could improve its mechanical performance, and provides a hardness of 17.3 GPa and fracture toughness of 5.0 MPa m^1/2.