Determination of active nitridation reaction efficiency of graphite in inductively coupled plasma flows

Accurate prediction of the gas-surface interaction during spacecraft reentry remains a challenging problem for thermal protection system design. Attempts to model the surface chemistry of ablating materials focus on oxidation and sublimation of the carbon surface, but usually neglect nitridation processes. Although nitridation may only lead to a minor increase in overall mass loss through ablation, it can highly impact other surface chemical processes through consumption of available atomic nitrogen, affecting the energy balance. We present experiments on graphite ablation in nitrogen plasmas, aiming at the determination of carbon nitridation reaction efficiencies in a relevant reentry environment reproduced in an inductively coupled plasma facility. The actual carbon nitridation reaction efficiencies are extracted using a numerical model with ablative boundary condition coupled to the flowfield. Emission spectroscopy of the CN radiation provides local CN species densities used for validation of the numerical model. The data are in-line with values reported in literature and extend the investigated temperature range. Our computed CN species densities agree well with the experimental values, providing confidence in our method. Based on our findings and additional literature data, we propose an updated Arrhenius form for evaluation of nitridation reaction efficiencies as a function of the wall temperature.

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