Predesigned Carbon Vacancies Unlock Superior Oxidation Resistance of High-Entropy Carbides by Stabilizing a Protective M-C-O Interphase.

The initial oxidation stage of high-entropy carbides (HECs) is a decisive yet poorly understood "black box" that governs their ultimate performance for ultra-high-temperature applications. Here, we unveil a powerful strategy-predesigning carbon vacancies-to fundamentally enhance HECs oxidation resistance. By integrating in situ x-ray diffraction (XRD) and in situ transmission electron microscopy coupled with electron energy loss spectroscopy (TEM-EELS), we provide the first direct, atomic-scale visualization of oxygen atoms preferentially occupying these predesigned vacancies during the incipient oxidation stage, leading to the dynamic formation of a metastable metal-carbon-oxygen (M-C-O) interfacial layer. Our combined experimental and theoretical analyses reveal a dual enhancement mechanism: kinetically, M-C-O suppresses both oxygen adsorption on the (111) surface and its subsequent inward diffusion; thermodynamically, they stabilize the HEC lattice at elevated temperatures, thereby increasing the energy barrier for M─C bond cleavage. This work not only deciphers the atomistic origin of enhanced oxidation resistance but also establishes a simple, general, and efficient design principle for next-generation ultra-high-temperature ceramics.