Oxidation induced emissivity evolution of silicon carbide based thermal protection materials in hypersonic environments

The emissivity of typical SiC-based thermal protection materials was measured in-situ at a wide temperature range (800 ~ 2300°C) inside a plasma wind tunnel that was capable of simulating hypersonic environments on-ground. Based on it, the evolution mechanism dominated by dynamic oxidation was discussed. The results suggest an emissivity of C_f/SiC 0.84 ~ 0.88 at 858 ~ 1502°C, prior to “temperature jump”. If “temperature jump” emerged, the emissivity was decreased rapidly to ≈0.76. The emissivity drop was explained by the microstructural transition of the oxidized surfaces that were triggered by the dissipation of SiO₂ oxide scale at 1600 ~ 1900°C. Similar emissivity evolution was observed in SiC_f/SiC after “temperature jump”. The effect of temperature on the emissivity of ZrB₂-SiC was more pronounced. It was increased from ≈0.73 to ≈0.98 at 1009 ~ 1297°C, and was plateaued at 1298 ~ 1497°C, ≈0.98. This was a consequence of the formation of higher percentage SiO₂-rich layers. However, due to the dissipation of SiO₂ and B₂O₃, the emissivity of ZrB₂-SiC was declined at higher temperatures, from ≈0.98 (≈1497°C) to ≈0.85 (≈1768°C).