Synergistic hardening in a dual phase high‐entropy (Hf,Nb,Ta,Ti,Zr)C–(Hf,Nb,Ta,Ti,Zr)B2 ultra‐high temperature ceramic

Abstract A dual phase high‐entropy (Hf,Nb,Ta,Ti,Zr)C–(Hf,Nb,Ta,Ti,Zr)B 2 ultra‐high temperature ceramic was synthesized using a single step boro‐carbothermal reduction route. The synthesized powder was densified by spark plasma sintering at 2000°C, resulting in complete solid solution formation and a relative density of ≈99%. The dual phase ceramic was 43 vol% high‐entropy carbide and 57 vol% high‐entropy boride. The grain sizes were 0.85 ± 0.34 µm for the carbide and 0.87 ± 0.33 µm for the boride with minimal residual oxide (0.2 vol%) detected in the microstructure. The resulting composition had a higher microhardness than the individual boride and carbide ceramics across the range of testing loads with maximum hardness of 47.5 ± 4 GPa at a load of 0.49N. The high hardness is attributed to the minimum residual oxide, submicron grains, favorable carbide‐to‐boride ratio, homogeneous metal distribution within the phases, uniform microstructure, and synergistic dual phase hardening.