Improved densification and hardness of high-entropy diboride ceramics from fine powders synthesized via borothermal reduction process

High-entropy boride ceramics of (Zr(0.2)Ta(0.2)Ti(0.2)Nb(0.2H)f(0.2))B-2 and (Zr0.2W0.2Ti0.2Mo0.2Hf0.2)B-2 were prepared by combining powder synthesis via borothermal reduction process at 1600 degrees C and subsequent ceramic densification by spark plasma sintering at 2000 degrees C in argon. High-entropy diboride powdered products exhibited ultra-fine particle size (< 0.5 mu m) after borothermal reduction. (Zr0.2Ta0.2Ti0.2Nb0.2Hf0.2)B-2 ceramic with the single phase was obtained after sintering. However, the minor phase with the nominal WB composition was detected in (Zr0.2W0.2Ti0.2Mo0.2Hf0.2)B-2 ceramic. (Zr0.2W0.2Ti0.2Mo0.2Hf0.2)B-2 ceramic exhibited a high relative density associated with the high hardness of 27.7 GPa, which were higher than those measured for (Zr0.2Ta0.2Ti0.2Nb0.2Hf0.2)B-2 ceramics (94.0% in relative density and 16.4 GPa in hardness) or the reported values (similar to 92% in relative densities and 17.5-23.7 GPa in hardness) of other high-entropy boride ceramics.