Ablative and catalytic behavior of carbon-based porous thermal protection materials in nitrogen plasmas

Heat shields used to protect space capsules during very high-speed atmospheric entry incorporate lightweight insulating refractories based on carbon-fiber preforms. These ablators, with up to 80% porosity, present exceptional thermal and chemical properties. A joint experimental and modeling approach to study such materials is presented, which contributes to improving their design in relevant operating conditions. Samples were tested in an inductively coupled plasma at 1.5 kPa and 25 kPa pressures, with surface temperatures ranging from 1500 to 2800 K. The recession rate was measured in-situ and the local flow conditions were reconstructed numerically from experimental data. The porous medium was imaged by X-ray Computerized Micro-Tomography (CMT), and the depth affected by the gas-solid interaction phenomena due to the plasma exposure was extracted from these data. A simple analytical model has been derived to relate observable and reconstructed quantities and the fiber-scale reaction constants. Image-based numerical simulations of ablation by nitridation, with simultaneous catalytic recombination of atomic nitrogen, were compared to the analytical model and used to extract the intrinsic, fiber-scale reaction rate constants from test data. Results show, among others, that the fiber-scale nitridation rate constant is close to the rate of graphite nitridation, and that it decreases strongly with pressure.