A super-hard high entropy boride containing Hf, Mo, Ti, V, and W

Super-hard (Hf,Mo,Ti,V,W)B2 was synthesized by boro-carbothermal reduction and densified by spark plasma sintering. This composition was produced for the first time as a single-phase ceramic in the present research. The optimized ceramic had a single hexagonal AlB2-type crystalline phase with a grain size of 3.8 mu m and homogeneous distribution of the constituent metals. The Vickers hardness exhibited the indentation size effect, increasing from 27 GPa at a load of 9.8 N to as high as 66 GPa at a load of 0.49 N. This is the highest hardness reported to date for high entropy boride ceramics.