Atomic-Level Insights into the HfB₂ Oxidation Mechanism by the Combination of Density Functional Theory and Ab Initio...

High-temperature oxidation of HfB₂ governs its long-term stability in extreme environments, yet the atomistic dynamics of surface oxidation remain incompletely elucidated. Herein, the oxidation process of the HfB₂(0001) surface was systematically investigated by combining density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. The results show that O₂ undergoes spontaneous dissociation on the Hf-terminated surface, forming a HfO₂ layer that effectively suppresses further oxygen adsorption. In contrast, O₂ dissociation on the B-terminated surface requires overcoming an energy barrier of 0.28 eV, and the resulting [BO₃] units lead to surface structural degradation. AIMD simulations at 973 and 1673 K further demonstrate distinct reaction pathways: on the Hf-terminated surface, O atoms react exclusively with the surface Hf atoms; on the B-terminated surface, O atoms initially form B₂O₃ units, subsequently diffuse into the subsurface layer, and react with Hf atoms beneath the surface. This process involves reaction intermediates evolving sequentially from [HfB₂O] to [Hf₂B₂O], then to [Hf₃BO], and finally transforming into [Hf₃O]. Notably, an elevated temperature accelerates oxygen diffusion and intermediate transformation, increasing the oxidation rate. These findings provide atomic-scale theoretical insights into HfB₂ oxidation and establish a basis for optimizing the oxidation resistance.