Role of mixed oxides in designing the composition of multicomponent transition metal diborides with improved oxidation resistance

Ultra-High Temperature Ceramics involving the co-presence of multiple transition metals have recently attracted significant attention owing to the potential for generating solid solutions with improved oxidation resistance compared to their individual constituents. Along this line, the compositions of binary and ternary Zr–Hf–Ta diborides are engineered a priori in this work to enhance their oxidation performance. This is achieved by promoting the formation of thermally stable A6Ta2O17 (A = Zr, Hf) within the oxide scale upon exposure to oxidizing environments. Systems crafted with a (Zr,Hf):Ta ratio equal to 3:1 are observed to display better oxidation resistance compared to the equiatomic counterparts. Correspondingly, while A6Ta2O17 is the exclusive or dominant oxidation product generated in the former case, detrimental individual oxides (TaO2, Ta2O5, and ZrO2) are also formed in the latter. Thus, compositional tailoring is key to enhancing the oxidation resistance of multicomponent diborides, shifting away from the standard equimolar paradigm.