Thermal conductivity and hardness of three single-phase high-entropy metal diborides fabricated by borocarbothermal reduction and spark plasma sintering

Abstract Four high-entropy metal diborides have been synthesized and densified by borocarbothermal reduction of metal oxides with boron carbide and graphite and subsequent spark plasma sintering. Three of them, (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)B2, (Hf0.2Zr0.2Ti0.2Ta0.2Mo0.2)B2, and (Hf0.2Zr0.2Ti0.2Ta0.2Cr0.2)B2, possess single high-entropy phases and have been sintered to >99% of the theoretical densities. The fourth (Hf0.2Zr0.2Ti0.2Mo0.2W0.2)B2 specimen contained a secondary Ti–Mo–W rich secondary phase in addition to the primary metal diboride phase. The specimens made by borocarbothermal reduction exhibit improved hardnesses in comparison with those samples previously fabricated via high energy ball milling and spark plasma sintering. Interestingly, the single-phase (Hf0.2Zr0.2Ti0.2Ta0.2Mo0.2)B2 and (Hf0.2Zr0.2Ti0.2Ta0.2Cr0.2)B2 (both of which have Vickers hardness values of ∼25 GPa) are substantially harder than (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)B2 (20.5 GPa), despite MoB2 and CrB2 being typically considered as softer components. These single-phase high-entropy metal diborides were found to have low thermal conductivities of 12-25 W/mK, which are ∼1/10 to ∼1/5 of the reported values of HfB2 and ZrB2.