Dense high-entropy boride ceramics with ultra-high hardness

Highly dense high-entropy boride ceramics with ultra-high hardness were prepared by spark plasma sintering of the self-synthesized high-entropy boride powders. High-entropy boride compounds of (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2, (Hf0.2Mo0.2Zr0.2Nb0.2Ti0.2)B2 and (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 with some oxide impurities were obtained by borothermal reduction at 1600 °C. The yield of the samples after sintering was improved to be 97.2 wt%, 92.8 wt% and 97.6 wt%. The densification and hardness of the high-entropy boride ceramics were remarkably enhanced after sintering the as-obtained powders at 2000 °C. The relative density of Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2 ceramics was 99.2% and the hardness achieved a value up to 28.3 ± 1.6 GPa, which was significantly higher than reported values.