Surface Transformation of Ultrahigh-Temperature ZrB2–HfB2–SiC–CCNT Ceramics Under Exposure to Subsonic N2-CH4 Plasma Flow

The chemical and microstructural transformation of the surface of a 31.5 vol.% ZrB2-31.5 vol.% HfB2-27 vol.% SiC-10 vol.% CCNT ultrahigh-temperature ceramic sample (where CCNT refers to carbon nanotubes) was studied under the influence of a subsonic N2-plasma flow with the addition of 5 mol% methane, simulating aerodynamic heating in the atmosphere of Titan. As in the case of pure nitrogen flow, it was found that silicon carbide is removed from the surface. Zirconium and hafnium diborides are partially transformed into a Zr-Hf-B-C-N solid solution in the experiment conducted. XRD, Raman spectroscopy, and SEM-EDX analysis show that the presence of C2 in the N2-CH4 plasma flow leads to surface carbonization (formation of a graphite- and diamond-like coating with a high proportion of amorphous carbon), resulting in significant changes in the microstructure and emissivity, potentially affecting the catalytic properties of the surface.