A novel ZrB₂–C₃N₄ composite with improved mechanical properties

In this research, a novel application for graphitic carbon nitride as the sintering aid and reinforcement was investigated. In this regard, g-C₃N₄ doped ZrB₂-based ultrahigh temperature ceramic was fabricated by route of spark plasma sintering at 1850 °C under an applied pressure of 40 MPa for 6 min. Microstructure development, sinterability, and mechanical properties were studied and compared to the additive-free ZrB₂. The sinterability was remarkably enhanced, and a fully-dense ceramic was obtained by addition of 5 wt% g-C₃N₄, in comparison with the monolithic ZrB₂ with a relative density of 76.5%. The elimination of oxide surface impurities of ZrB₂ (B₂O₃ and ZrO₂) through chemical reactions with g-C₃N₄ and the formation of in-situ phases like ZrC and BN were confirmed by microstructural and phase analyses. Mechanical properties were enhanced with a small amount of g-C₃N₄ additive due to the densification improvement and prevention of grain growth. The values of hardness, indentation fracture toughness and flexural strength increased from 10.1 GPa, 1.9 MPa m^½ and 187.6 MPa for monolithic ZrB₂ to 16.2 GPa, 5.4 MPa m^½ and 516.4 MPa for g-C₃N₄ doped ZrB₂ ceramic.