Microstructure and Mechanical Properties of B₄C-HfB₂-SiC Ceramic Composites Prepared by In Situ Reaction

B₄C-HfB₂-SiC (BCHS) ceramic composites were fabricated by in situ reaction with B₄C and HfSi₂ mixed powders through spark plasma sintering in this work. Effects of sintering temperature and sintering additive (HfSi₂) content on the density, microstructure, and mechanical properties of the BCHS specimens were studied. The BCHS composite containing 15-30 vol.% HfSi₂ reached over 98% theoretical density in the temperature range of 1600-1750 °C under the pressure of 50 MPa. The Vickers hardness, fracture toughness, and room temperature flexural strength of BCHS sintered at 1650 °C with 25 vol.% HfSi₂ reached a maximum value of 31.3 GPa, 5.6 MPa·m^1/2, and 573.9 MPa, respectively. The flexural strength was not decreased at elevated temperatures, and the flexural strength at temperatures of 400 °C, 600 °C, and 800 °C was evaluated to be 540.7 MPa, 518.8 MPa, and 586.9 MPa, respectively. Dense B₄C-HfB₂-SiC composites were fabricated by the spark plasma sintering process. The introduction of HfSi₂ can significantly enhance the density and mechanical properties of B₄C ceramic.