Enhancement mechanism of high temperature stability for in-situ oxide scale on high-entropy carbide coatings induced by rare earth

For ultra-high temperature ceramic (UHTC) coatings used in aerospace thermal protection systems, enhancing the oxygen barrier capability and thermal stability of in-situ oxide scale during ablation is crucial for stable service. Herein, a high-entropy design strategy is proposed to boost the compactness and phase stability of oxide scales by leveraging multi-component synergistic effects. Three high-entropy carbide coatings, (Hf-Zr-Ti-Sc-Y-Sm)C (HEC-Sm), (Hf-Zr-Ti-Sc-Y-Er)C (HEC-Er) and (Hf-Zr-Ti-Sc-Y-Lu)C (HEC-Lu), with better ablation resistance than ZrC, (Hf-Zr-Ti)C, and (Hf-Zr-Ti-Sc-Y)C coatings,were designed. HEC-Lu delivers optimal ablation resistance, featuring a linear ablation rate of -0.173 ± 0.012 μm·s^-1, superior to HEC-Sm (-0.111 ± 0.032 μm·s^-1) and HEC-Er (-0.144 ± 0.022 μm·s^-1). This superior performance is correlated with higher Ti retention and stronger calculated Ti-O bonding, increasing Ti content in the oxide scale by 53.85  % and 33.33  % compared to HEC-Sm and HEC-Er, thereby inhibiting volatilization. Rare-earth (Sm, Er, Lu) incorporation can reduce the diffusion coefficients of Ti and O atoms in oxide scales by one order of magnitude at 2573 K compared with TiO₂. Notably, Lu introducing yields a higher Mulliken population (0.279) for Ti-O bonds than that of Sm (0.268) and Er (0.274), which strengthens Ti-O bonds inside the oxide scale. This work presents a novel material selection scheme for anti-ablation coatings in extreme environments.