Rare earth tantalate coating materials for thermal and oxidation protection of carbon-carbon composites

To protect carbon/carbon composites against thermal corrosion and high-temperature oxidation, the thermal/oxygen barrier properties and static oxidation behavior of rare earth tantalates (YbTaO4 and LuTaO4) are investigated here. Both samples with the m' phase are successfully fabricated. The formation of oxygen vacancies benefits from atomic substitution and valence variation and the widths of the domain boundaries (18–23 nm) are close to the phonon mean free path. Low thermal conductivity is caused by Umklapp phonon-phonon scattering, oxygen vacancies and domain walls, with LuTaO4 having a value of 1.903 W∙m−1∙K−1 at 900 °C. Furthermore, LuTaO4 has decreased oxygen-ion conductivity due to a high activation energy and immobile vacancies. A LuTaO4/Si coating can efficiently prevent the oxidation of C/C composites at 1100 °C for 15 h. The coating failure results from oxygen penetrating along the microscopic cracks and mismatch of the thermal expansion coefficients.