Novel (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2 high entropy diborides with superb hardness sintered by SPS under a mild condition

The composition and synthesis approach of high entropy ceramics have significant influences on their microstructures and mechanical properties. A new (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2 high entropy diboride ceramic with excellent mechanical properties was successfully prepared by spark plasma sintering (SPS) at a relatively “low temperature” (1800 °C) in this work. The effects of two mixing strategies, grinding and ball milling, on the solid solubility of the calcined powders were studied. Furthermore, the effects of these two mixing strategies on the phase and morphology of the calcined powders and the mechanical properties of the SPS sintered ceramics were analyzed. Experimental results indicated that, compared with a single solid solution phase within the high entropy boride powders obtained by ball milling, incomplete solid-solution presented in the high entropy boride powders obtained using grinding treatment. It promoted the solid-state diffusion and in-situ reaction of the diborides in the ceramics during the sintering process, thereby improving the densification and the hardness of the high entropy diboride ceramics. By grinding treatment, (Hf0.2Zr0.2Ta0.2V0.2Nb0.2)B2 high entropy ceramic with a relative density of 94% and a hardness of up to 25.34 ± 1.5 GPa at an indentation load of 9.8 N was obtained at 1800 °C. This work not only expanded the family of high-entropy diboride ceramics but also proved the advantage of grinding treatment in preparation of high entropy diboride ceramics.