Synthesis of high entropy boride in reactive MW-plasma environments: Enhanced reducing capability

Abstract

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.