Oxidation of Multi-layer Graphite Using an Atomistic Multi-lattice Kinetic Monte Carlo Model

Thermal protection systems (TPSs) in hypersonic vehicles are primarily made from carbon-based materials and are used to protect against damage in re-entry conditions. These conditions include exposure to high temperature and highly reactive atomic oxygen, which causes defects to form and grow, eventually resulting in erosion of the TPS. In this work, a multi-lattice atomic scale Kinetic Monte Carlo (KMC) model is used to understand defect formation in carbon under varying temperatures and as a function of the energy barrier for oxygen diffusion between graphene layers as a model for HOPG. Our KMC model directly models adsorption and diffusion of epoxy groups on the carbon surface, and it allows defects to form on an initially pristine surface as a result of interactions between surface groups such as lactone-ethers and ether-lactone-ethers and is therefore novel because it does not assume a priori the presence of defects. The KMC simulations reveal differences in defect shape and etch rates as the temperature changes. The shape of defects changes from circular to branched as the temperature increases from 1300 to 2200 K at 10,000 Pa. We also use the KMC model to understand how an increased energy barrier of epoxy diffusion between carbon layers affects defect formation. For the two conditions, we compute and compare the in-depth etch rate and discuss its implications. These analyses demonstrate the consequences of atomic-scale influences on pitting and erosion at larger scales.