The effect of sintering aids on microstructure and mechanical properties of HfB₂-SiC composites

Three kinds of HfB₂ based ultra-high temperature ceramics, HfB₂-20vol%SiC (HS), HfB₂-20vol%SiC-5vol%Si₃ N₄ (HSS) and HfB₂-20vol%SiC-5vol%AlN (HSA) were fabricated from commercially available powders by hot-pressing in a graphite element furnace at high temperature under 30 MPa. The mechanical properties, microstructures and phase constitutions of HS, HSS and HSA were investigated. The results show that the introduction of Si₃N₄ and A1N as sintering aids, can markedly decrease the sintering temperature from 2200°C to 1850°C. The relative densities of the three kinds of ultra high temperature ceramics also increased from 95% to 99%. The microstructure analysis of the three composites all revealed homogeneous microstructures with regularly dispersion of SiC particles, and some secondary grain-boundary phases were found mainly at the triple junctions of the diboride grains of HSS and HSA. The introduction of Si₃N₄ or A1N into HfB₂-SiC composite resulted in the change of fracture mode from the trans-granular to the inter/trans-granular fracture mode.