Erosion of refractory carbides in high-temperature hydrogen from ab initio computations

Advanced concepts for in-space propulsion require coatings that are resistant to erosion in high temperature and pressure hydrogen. The erosion of refractory carbides of interest for this application (ZrC, NbC, HfC, and TaC) is investigated using combined ab initio thermodynamic computations and equilibrium product analyses. The carbides are shown to erode through a combination of four governing reactions, the relative extent of which depend on environmental conditions. The product profiles from these reactions are complex but exhibit lower hydrogen saturation at higher temperatures and lower pressures. A metric is derived to determine the applicability of equilibrium analyses for erosion rates, based on experimental conditions. Heritage mass loss experiments on ZrC in hydrogen satisfy the equilibrium criteria, and, correspondingly, the computed equilibrium erosion rate agrees quantitatively. The results suggest that previously postulated non-equilibrium effects, namely the prolonged incongruent vaporization originating from high carbon mobility, do not drive erosion over the hours-long timescales of the experiments. For specific in-space propulsion designs, comparisons of carbide performance show TaC and HfC outperform other carbides and meet the criteria needed to close designs.