Silicon Carbide (SiC) serves as one of the most important thermal protection materials for hypersonic (Ma> 5) vehicles because of their excellent oxidation resistance, thermal stability, and mechanical properties. In particular, the oxidation resistance of SiC plays a critical role in determining the reliability of the thermal protection system (TPS), and thereby the safety of vehicles, under the extreme aerothermal environments. However, the oxidation of SiC is highly sensitive to its surrounding environments.

With the ever-increasing speed of vehicles, active thermal protection means are necessary for easing the more severe heating load. Transpiration cooling using water (H₂O) as the coolant is a promising technology. However, the introduction of H₂O to the high temperature oxidizing environment surrounding a SiC surface will complicate the reaction mechanism of SiC, which is largely unexplored.

In this work, reactive force field (ReaxFF) molecular dynamics (MD) simulations are employed to systematically investigate the oxidation behavior of the 6H-SiC surface (0001) exposed to water/oxygen (H₂O/O₂) mixtures at 1500 K. The simulations have shown a two-stage reaction feature. The first stage dominated by chemical adsorption of gas molecules on the SiC surface with a relatively low reaction rate, while the second stage is dominated by the oxidation reaction on the SiC surface with a significantly increased reaction rate. H₂O plays an important role in affecting the reaction rate, but in opposite directions in different stages. In the first stage, increasing the H₂O/O₂ ratio markedly shortens the duration of the first stage; in the subsequent oxidation stage, the oxidation rate decreases gradually with the increasing H₂O/O₂ ratio.

An in-depth trajectory analysis reveals that the effects of H₂O on the oxidation rate observed in different reaction stages originate from its influence on the dissociation behavior of O₂ molecules and the activity of surface sites. In the first stage, H₂O reacts with adjacent adsorbed O₂ molecules on the SiC surface to form hydroperoxyl intermediates. Compared to the direct dissociation of adsorbed O₂ under the pure O₂ environment, the formation of the hydroperoxyl intermediate significantly lowers the activation energy barrier for O₂ dissociation. As the H₂O/O₂ ratio increases, O₂ begins to dissociate on the SiC surface earlier, resulting in earlier transition to the surface oxidation stage. As the reaction proceeds, oxygen atoms and hydroxyl groups gradually cover the SiC surface. Owing to the higher adsorption energy of hydroxyl groups on Si dangling bonds compared to O₂, the adsorbed hydroxyl groups saturate these active sites and inhibit subsequent O₂ adsorption on occupied sites. In addition, the concentration of O₂, which has a stronger capability of oxidizing SiC, is lower in the H₂O/O₂ mixture system. As a result of these two effects, the presence of H₂O lowers the rate of oxidizing SiC in the oxidation stage.

Overall, this work reveals the microscopic mechanism by which H₂O accelerates the oxidation of SiC during the initial stage and gradually transitions to an inhibitory effect in subsequent stages, thereby explaining the different mechanistic influences of H₂O on the two-stage oxidation process of SiC. This work provides a theoretical insight into the oxidation mechanisms of SiC under high-temperature humid environments and offers useful guidance to the design and environmental adaptability evaluation of TPS for hypersonic vehicles operating under complex service conditions.