Composition design and oxidation mechanism of quaternary single-phase (HfZrTaCr)B₂: Phase evolution and synergistic CrTaO₄@(Hf, Zr)O₂/B₂O₃ dual diffusion barrier at elevated temperatures

In this study, a series of equimolar quaternary single-phase high-entropy diborides, (HfZrTaTm)B₂ (Tm = Ti, V, Cr, Nb), were systematically designed and synthesized, with a focus on investigating their oxidation behavior at intermediate to high temperatures. Their performance was compared with that of the conventional quinary system (HfZrTaTiNb)B₂. The results revealed that the quaternary systems exhibit excellent oxidation resistance, with (HfZrTaCr)B₂ showing the best performance—demonstrating a weight gain of only 2.573 % after thermogravimetry oxidation at 1400 ℃, representing an 81.09 % reduction compared to the quinary (HfZrTaTiNb)B₂. Oxidation mechanism studies reveal that (HfZrTaCr)B₂ forms a dense dual-layer protective structure, where CrTaO₄ precipitates fill the pores within the (Hf, Zr)O₂ oxide skeleton and synergize with the molten B₂O₃ phase to effectively block oxygen diffusion. In (HfZrTaV)B₂, high-field-strength cations V⁵⁺ help suppress B₂O₃ volatilization by stabilizing the glass phase. In contrast, the Ti/Nb-containing (HfZrTaTiNb)B₂ system forms porous and isolated oxides, which compromise the continuity of the protective layer and lead to accelerated oxidation. This work provides a new compositional optimization strategy for designing high-entropy ceramics with superior ultra-high-temperature oxidation resistance.