Effect of Brief High-Enthalpy Exposure on Subsequent High-Temperature Mass Loss in a 3D Carbon–Carbon Composite

Significant discrepancies between oxidation rates measured in laboratory-scale tests and those inferred under in-service hypersonic conditions have been reported for carbon–carbon (C/C) composite materials, owing to differences in heat transfer, flow environment, and specimen geometry. This work examines how brief exposure to high-enthalpy wind tunnel flow conditions alters subsequent high-temperature mass-loss behavior under inert conditions, measured via thermogravimetric analysis (TGA) in a nitrogen environment, as a proxy for changes in degradation and oxidation susceptibility. Two nominally identical 3D high-density C/C wedges were tested in the HyperReact facility to achieve stagnation temperatures of roughly 1700 K and stagnation pressures of 7.24 MPa in a high-blockage, subsonic configuration. Small specimens extracted from the CC wedges were then subjected to TGA at 1623 K for 5 h under flowing high-purity nitrogen. Total mass loss during the isothermal hold increased steadily with pre-exposure duration: 0.78% (0 s), 0.96% (2 s), 1.05% (5 s), and 2.04% (9 s), corresponding to an increase in average mass-loss rate from 2.61×10⁻³ %/min (virgin) to 6.80×10⁻³ %/min (9 s), a 161% increase. Because the TGA environment is nominally oxygen-free, this enhanced mass loss is interpreted as evidence that brief high-enthalpy oxidation in HyperReact creates a modified near-surface region which is characterized by increased gas-accessible surface area and altered carbon microstructure that is intrinsically more susceptible to high-temperature degradation and, by extension, to oxidation under test conditions. Future work will use SEM/EDS on virgin and pre-exposed specimens, in conjunction with additional TGA tests, to quantify correlations between surface damage, local composition, and mass-loss behavior, providing data-driven guidance for incorporating transient damage effects into hypersonic material screening and qualification frameworks.