Toughened bulk high-entropy diborides with high hardness and enhanced oxidation resistance via SiC whiskers

The fracture toughness and oxidation resistance are crucial factors affecting the practical applications of high-entropy diboride ceramics (HEBs). In this study, silicon carbide whiskers (SiC w ) were introduced as a reinforcement for HEBs for the first time. To achieve a uniform dispersion of SiC w within the HEB powders, a combination of simultaneous ultrasonic dispersion, mechanical agitation, and freeze-drying technology was employed. Consequently, a series of composites, (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2 )B 2 -nSiC w ( n  = 0–20 vol%), were densified by spark plasma sintering. The effect of SiC w on the microstructure and mechanical properties of the HEBs were studied, as well as the toughening mechanism involved. The results revealed that the introduction of SiC w led to grain-size refinement in (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2 )B 2 , along with significant improvements in relative density, hardness, fracture toughness, and oxidation resistance. The relative density, hardness and fracture-toughness of (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2 )B 2 -SiC w composites exhibited an initial increase followed by a decrease with increasing SiC w content. Specifically, the (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2 )B 2 –15 vol% SiC w composite simultaneously obtained the highest hardness of 25.6 GPa and fracture-toughness of 4.8 MPa·m 1/2 , representing a 26% improvement compared to (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2 )B 2 ceramics.