Nonequilibrium Oxidation Dynamics of SiC: From Epitaxial Passivation to Kinetically Driven Ablation and Oxygen-Transport Crossover

Silicon carbide (SiC) oxidation in dissociated hypervelocity flows is governed by complex nonequilibrium gas-surface processes, yet the effects of atomic oxygen flux, incident velocity, and surface temperature on oxidation pathways remain poorly understood. In this work, we employ reactive molecular dynamics simulations on a model 3C-SiC(001) surface to investigate these effects and clarify the resulting oxidation dynamics of SiC. The results show that under mild conditions, the surface forms an ordered, quasi-two-dimensional epitaxial oxide monolayer with a strained cristobalite-like registry, indicating effective passivation. This passivating regime breaks down at elevated fluxes, which promote amorphous growth through reaction-induced interfacial superheating and kinetic crowding. Hypervelocity impact further destabilizes this ordered structure, driving a crossover to kinetically activated chemical ablation and material consumption near 3.0 km/s. Beyond these collisional effects, increasing surface temperature reveals a transport crossover near 1600 K. Across this crossover, the diffusion activation energy for oxygen transport increases from 11.3 kJ/mol, consistent with transport through a porous, defect-rich oxide layer, to 161.0 kJ/mol, characteristic of cooperative bond-switching in a softened carbon-rich network. This high-temperature softening is accompanied by faster oxide growth and enhanced carbon phase segregation. These results delineate a mechanistic framework of passivation, chemical erosion, sputtering-like removal, and high-temperature transport mediated by network restructuring, providing atomistic insight into nonequilibrium degradation processes relevant to SiC-based thermal protection materials in extreme aerospace environments.