Coupling DSMC and AKMC to simulate graphite erosion due to hyperthermal oxidation at the nanoscale

A one-way coupling is introduced between the particle-based gas flow method, direct simulation Monte Carlo (DSMC), and the material evolution method, atomistic-kinetic Monte Carlo (AKMC), at the nanoscale. The coupled DSMC → AKMC method is used to simulate hyperthermal oxidative pitting in graphite exposed at various tilt angles, with relevance to ablative pitting in carbon fibers used for Thermal Protection System (TPS) materials during reentry. Simulated DSMC particles have a one-to-one mapping with real atoms or molecules, and the two methods are spatially coupled such that particles from DSMC can directly trigger adsorption events in AKMC. The coupled method more accurately captures gas behavior on pit geometries compared to pure uncoupled AKMC that typically uses simplified assumptions to approximate the adsorption behavior from the gas phase. Comparison between results from DSMC → AKMC and pure uncoupled AKMC reveals a discrepancy between their predicted pit geometries, especially at lower angles of graphite tilt. That is, pure uncoupled AKMC falsely overpredicts the depth and narrowness of the evolving pits due to the simplified assumption of vertical oxygen incidence rather than a realistic oblique incidence as in DSMC → AKMC. This discrepancy in pit geometry is essential to acknowledge when evaluating the influence of pit geometry on the structural stability of carbon fibers during reentry ablation. In addition, the physical insights on pit evolution under realistic gas behavior from the coupled DSMC → AKMC simulations can be used to improve uncoupled AKMC simulations by applying appropriate boundary conditions. Lastly, an exploratory analysis is performed in DSMC to determine if CO molecules emitted from the oxidizing graphite affect the number density of oxygen local to the graphite, assessing the need for a two-way coupling between DSMC and AKMC. • A one-way coupling between DSMC and AKMC is presented to simulate graphite oxidation. • Challenges in coupling the two methods at the nanoscale are discussed. • Coupling captures realistic gas-surface effects such as oblique incidence of gas particles. • Pit evolution in graphite exposed to oxygen gas at various orientations is simulated. • Realistic oblique incidence of gas particles causes defects to grow wider than deeper.