Composition design of oxidation resistant non-equimolar high-entropy ceramic materials: An example of (Zr–Hf–Ta–Ti)B₂ ultra-high temperature ceramics

High-entropy borides (HEBs) are unable to serve in environments above 1800 °C because of their poor oxidation resistance, which severely limits the application of these materials in ultra-high temperature environments. To solve this problem, a series of HEBs with different ratios of metal elements were designed and prepared in this work, and their oxidation behavior above 1800 °C was investigated. The results showed that non-equimolar HEBs possessed excellent oxidation ablation resistance relative to equimolar HEBs. The oxidized surface of (Zr_1/4Hf_1/4Ta_1/4Ti_1/4)B₂ formed craters due to excessive liquid products and violent volatilization, while (Hf_4/5Zr_1/15Ta_1/15Ti_1/15)B₂ formed a dense oxide layer after oxidation, which had the best antioxidant performance. The content and type of different metal elements significantly affect the oxidative behavior and products, and the ratio of liquid oxidation products plays a critical role in the antioxidant ability. An appropriate amount of liquid that fills the pores of the solid not only better blocks the diffusion channels of oxygen but also promotes the densification of the oxide layer through flow mass transfer. The oxidation of HEBs to generate corresponding high-entropy oxides avoids thermal mismatch between different oxides, reduces cracks and thermal stresses caused by phase transitions or grain growth, and further promotes the formation of a dense scale. This work provides a first look at the oxidation behaviors of non-equimolar HEBs in an ultra-high-temperature environment and proposes guiding rules for the design of HEB components (limiting the ratio of liquid oxidation products to the range of 10–27 mol%).