Microstructure and gas-surface interaction studies of a low-density carbon-bonded carbon fiber composite in atmospheric entry plasmas

Carbon-bonded carbon fiber (CBCF) composites are a cost-effective solution for the production of low-density carbon-phenolic Thermal Protection Systems (TPS). This new TPS for spacecraft requires new experimental data for model development and validation. Ablation experiments of a CBCF composite were carried out in an inductively-coupled plasma generator to assess the performance in high-enthalpy flows. Surface temperatures up to 2900 K led to strong surface ablation and test samples of hemispherical shape responded with constant surface temperatures and recession rates. Cylindrical samples experienced a continuous surface temperature increase. Emission spectra of the cyano radical CN were indicative of a 4-5 mm reactive boundary layer. Deviation from thermal equilibrium was found by comparison to simulated spectra. Micrographs revealed an oxidation zone in the order of 0.2 mm at the surface, suggesting a gas phase diffusion controlled ablation regime. Strong corrosion of the fibers in nitrogen plasma is attributed to wall nitridation.