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We investigate the synthesis of the high entropy boride (HEB), MoNbTaVWB10, in a highly reactive environment facilitated by hydrogen feedgas in microwave plasma (MW plasma). Dissociation of molecular hydrogen to form copious amounts of atomic hydrogen allows efficient reduction of the metal oxide precursors with less excess boron needed for HEB formation than if an Ar-rich feedgas is used. This study demonstrates that hydrogen plasma promotes the hexagonal AlB2-type structure at temperatures as low as 1500C, achieving a predominantly single -phase structure at 1750C. In both environments, hardness and surface topography of the HEBs are measured, highlighting the enhanced effectiveness of the reactive feedgas in the synthesis process. XRD analysis confirms the enhanced reduction efficiency and phase purity facilitated by atomic hydrogen, while SEM/EDX reveals improved elemental uniformity and minimized vanadium sublimation. Compared to argon-rich plasma, hydrogen plasma results in larger grain sizes and reduced microstrain, underscoring its role in optimizing the microstructure and synthesis of HEBs. The findings show the advantages of a reactive synthesis environment for sustainable and efficient metallurgical process, enabling advanced material applications. 

Metal oxide thermal reduction, enabled by microwave-induced plasma, was used to synthesize high entropy borides (HEBs). This approach capitalized on the ability of a microwave (MW) plasma source to efficiently transfer thermal energy to drive chemical reactions in an argon-rich plasma. A predominantly single-phase hexagonal AlB2-type structural characteristic of HEBs was obtained by boro/carbothermal reduction as well as by borothermal reduction. We compare the microstructural, mechanical, and oxidation resistance properties using the two different thermal reduction approaches (i.e., with and without carbon as a reducing agent). The plasma-annealed HEB (Hf0.2, Zr0.2, Ti0.2, Ta0.2, Mo0.2)B2 made via boro/carbothermal reduction resulted in a higher measured hardness (38 ± 4 GPa) compared to the same HEB made via borothermal reduction (28 ± 3 GPa). These hardness values were consistent with the theoretical value of ~33 GPa obtained by first-principles simulations using special quasi-random structures. Sample cross-sections were evaluated to examine the effects of the plasma on structural, compositional, and mechanical homogeneity throughout the HEB thickness. MW-plasma-produced HEBs synthesized with carbon exhibit a reduced porosity, higher density, and higher average hardness when compared to HEBs made without carbon.