Synergistic Effects of SiCw and Ni Addition on the Densification and Mechanical Properties of (M0.2Ti0.2Ta0.2V0.2Nb0.2)B2 (M=Hf, Zr, or Cr) High-Entropy Ceramics

The improvement of densification and fracture toughness in high-entropy ceramics is important to realizing their practical applications. In this study, SiC whiskers and metal Ni additions were incorporated to solve these problems of high-entropy boride ceramics. The influence of sintering temperatures (1450–1650 °C) on the densification, microstructure, hardness, fracture toughness, and bending strength of (M0.2Ti0.2Ta0.2V0.2Nb0.2)B2-SiCw-Ni (M=Hf, Zr, or Cr) composites prepared by hot-pressing technology were studied. Results showed that when SiC whiskers and metal Ni additions were used as additives, increasing sintering temperatures from 1450 to 1600 °C promoted the densification of high-entropy boride ceramics. This was mainly attributed to the high sintering driving force. However, when the temperature further increased to 1650 °C, their densification behavior decreased. At a sintering temperature of 1600 °C, these high-entropy borides ceramics all had the highest densification behavior, leading to their high hardness and fracture toughness. The highest relative density was 96.3%, the highest hardness was 22.02 GPa, and the highest fracture toughness was 13.25 MPa·m1/2, which was improved by the co-function of SiC whiskers and plastic metal Ni. Meanwhile, in the adopted sintering temperature range of 1450 to 1650 °C, the highest bending strength at room temperature of these high-entropy boride ceramics could reach 320.8 MPa. Therefore, this research offers an effective densification, strengthening, and toughening method for high-entropy boride composites at a low sintering temperature.