High-Temperature Oxidation of SiC-Based Composite: Rate Constant Calculation from ReaxFF MD Simulations, Part II

Space vehicles often encounter very high temperature and harsh oxidative environments. To ensure proper thermal protection, layers composed of SiC and EPDM polymer are placed on the outer surface of the space vehicle. The O-2 and H2O molecules are able to oxidize the SiC network, creating SiO2-type structures that may form a protective layer, while also pyrolyzing and burning the EPDM polymer, causing ablation. Reactive molecular dynamics simulations nicely complement experiment, as they provide direct observation and information to calculate physical parameters such as transport diffusivities and reaction constants. In this study, rate models were developed and molecular dynamics simulated trajectories were used to extract Arrhenius parameters that describe the initial stages of transport and kinetics of SiC oxidation by O-2 and H2O and the combustion and pyrolysis of EPDM. The simulations showed that O-2 was able to oxidize SiC more efficiently than H2O, with the transport activation barrier of O-2 in the range of 40-70 kJ/mol, and for H2O at 125-150 kJ/mol. The oxidizer molecules created a continuous surface of SiO2 that grew on top of the SiC layer. The O atoms insert themselves in between the Si-C bonds, causing the C atoms to migrate into a carbonaceous phase in the center of the simulation box. A model of the EPDM polymer was used for the combustion and pyrolysis simulations. An Arrhenius analysis gave activation barriers of 183 kJ/mol for combustion and 213 kJ/mol for pyrolysis. By comparison, experimental observations of EPDM and similar polymers conclude a range of similar to 100-250 kJ/mol for both combustion and pyrolysis, indicating that ReaxFF can give reasonable quantitative predictions for these thermal studies, demonstrating the possibility of direct calculation of Arrhenius parameters from reactive molecular dynamics simulations.