Production and characterization of ultra refractory HfB2 SiC composites

Two ultra refractory HfB2-based composites were fabricated by hot pressing, both containing about 19 vol% of ?-SiC particles. Conversely, two different additive systems were chosen, namely 6vol% of Si3N4 and 3vol% of HfN. The first composition was hot pressed at 1850°C for 20 minutes, the second at 1900°C for 40 minutes. The materials achieved near full density. The microstructure showed diboride grains with a regular shape and SiC particles mostly located intergranularly. In both cases, limited levels of secondary phases were identified, for instance BN, HfO2 and a Hf(C,N) solid solution. Interesting mechanical properties were measured. The composite containing Si3N4 exhibited 560?100 MPa and 150?5 MPa of flexural strength at 25°C and 1500°C, respectively. The composite containing HfN showed 650?50 MPa and 465?45 MPa of flexural strength at 25°C and 1500°C, respectively. Its load-displacement curve at 1500°C obeyed a linear law, whilst the other composite substantially deviated from a linear elastic behaviour. The introduction of proper amount of sintering aids favoured considerably densification of these strongly covalent compounds. Reduced amounts of refractory secondary phases after sintering were crucial in retaining good mechanical properties at high temperatures. Furthermore the presence of SiC strengthened the composites and enhanced their resistance to oxidation (in ambient air).