Oxide-scale evolution and dynamic oxidation mechanism of ZrB₂-SiC in high-enthalpy plasma wind tunnel

In this work, a 1 MW Plasma wind tunnel was employed to study the dynamic oxidation mechanisms of ZrB₂-SiC at heat flux ranging 0.5∼5.0 MW/m² in reduced pressure. The results show a transition in the uppermost oxide layer from SiO₂-rich glass to SiO₂ when increasing heat flux from ≈0.55 and ≈1.70 MW/m² (≈1100 °C and ≈1340 °C) to ≈2.50 MW/m² (≈1550 °C). A temperature fluctuation was highlighted at ≈2.50 MW/m². At higher heat flux (≥3.5 MW/m²), a ‘thermal instability’ occurred at ≈1650 °C. In addition, the ZrO₂ grain structure was temperature dependent, particularly at ≈1.70 MW/m² where the ZrO₂ grain evolved from tiny grain, to equiaxed grain and to columnar grain. At higher heat flux (≥2.50 MW/m²), columnar grains were observed prior to the ‘thermal instability’. Finally, as oxidation of ZrB₂-SiC was a diffusion-control process, the oxidation rate was dependent not only on temperature, but on grain structure of ZrO₂.