Phase transformation in spark plasma sintered ZrB₂–V–C composites at different temperatures

Spark plasma sintering was employed for the fabrication of ZrB₂–V–C composites at different consolidation temperatures (1800, 1900 and 2000 °C) using an external pressure of 40 MPa for 7 min under vacuum atmosphere. Vanadium powders and graphite nano-flakes were applied as metallic and carbonaceous sinter additives, respectively, in the ZrB₂ matrix ultrahigh temperature ceramic composites. All composites approached their theoretical densities as the relative density values were >99%. The in-situ synthesis of VB₂ and ZrC compounds, during the reactive sintering process at all temperatures, was verified by phase and microstructural analyses. It was found that vanadium and graphite were entirely consumed via reaction with the matrix by sintering at 1900 °C, while the majority of them were left intact at the temperature of 1800 °C. However, the compounds of VB₂ and ZrC were partially reacted together, at 2000 °C, which led to the synthesis of extra complex phases of V_1.54B₅₀ and V₃Zr₃C. The hardness of composites slightly decreased with increasing the sintering temperature, while the fracture toughness was remarkably enhanced.