Erosion caused by high-speed incoming atomic oxygen (AO) on the surfaces of Very Low Earth Orbit (VLEO) vehicles creates micro-roughness, which significantly affects gas-surface interactions such as gas-molecule momentum/energy accommodation and catalytic reactions, thereby posing challenges to accurate aerodynamic design of such vehicles. At present, the catalytic coeffcients and corresponding chemical energy accommodation coeffcients obtained by different researchers using different experimental equipments or simulation methods exhibit considerable discrepancies. This is partly due to the lack of understanding of the mechanisms underlying AO-induced erosion of aerospace materials and the evolution of surface micro-roughness.
In this paper, the molecular dynamics method, combining the previously-introduced continual and single scattering approaches, is employed to simulate the dynamic evolution process in which a SiC surface undergoes sustained erosion by high-speed AO until a stable micro-roughness morphology is formed. The influence of the nanoscale roughness on the catalytic reaction mechanism and chemical energy accommodation behavior is explored.
The results indicate that, from the pristine state to the roughness-stabilized state, the surface undergoes seven stages of evolution, progressively forming a four-layer structure, in which the hopping diffusion of AO within the surface oxide layer is an important mechanism for the formation of the micro-rough surface structure. The dominant chemical reaction mechanisms and the principal influencing factors differ across various stages, which could lead to different reaction coeffcients in specific studies if these details of surface evolution are not properly accounted for. On this basis, the typical physicochemical phenomena occurring in gas-surface reactions are statistically analyzed, quantitatively reasonable catalytic coeffcients and chemical energy accommodation coeffcients are calculated, and the patterns by which incident molecular velocity, surface temperature, and micro-roughness affect them are investigated. At high surface temperatures, the obtained catalytic coeffcients range from 0.3 to 0.4, consistent with experimental results reported in the literature. At low surface temperatures, however, the catalytic coeffcient (0.68) obtained under high-speed incidence markedly exceeds typical literature values reported for low-speed conditions. The chemical energy accommodation coeffcient increases from 0.67 to 0.81 with increasing surface temperature, slightly higher than literature results for smooth surfaces, consistent with the related physical analysis. The results underline the influence of nanoscale roughness on chemical energy accommodation, and help explain possible reasons for the considerable discrepancies in catalytic data reported in some of the literature.
This study deepens the understanding of complex gas-surface interactions in high-speed AO environments, and provides a reference for the aerodynamic design of VLEO vehicles operating near the Kármán line. Owing to the limitations imposed by computational cost, this study has obtained only limited quantitative data, and some conclusions tend to be qualitative. In this research direction, there remain a number of issues worthy of further exploration in future work.