HfSi₂-HfB₂-SiC coating prepared at low temperature to protect SiC-coated C/C composites against oxidation at 1473–1973 K

To improve the oxidation resistance of carbon/carbon (C/C) composites in 1473–1973 K, a HfSi₂-HfB₂-SiC coating was prepared on the surface of SiC-coated C/C composites by a combined method of slurry dipping and oxidation sintering at low temperature (below 1473 K). The Hf-Si-O multiphase layer generated by the oxidation of the coating acted as oxygen barriers at 1473–1973 K in air. The in-situ formed HfSiO₄ particles enhanced the thermal stability of the SiO₂ glass, terminated propagation of the crack, and relieved CTE mismatch between the inner and outer coating. Besides, B₂O₃ filled pores in the early stage of oxidation and then SiO₂ took the place of the B₂O₃ as a new filler with the increase time of oxidation. As a result, the coating can protect C/C composites for 516 h at 1473 K, 744 h at 1773 K, and 64 h at 1973 K with a mass gain of 0.63 wt%, 1.41 wt% and −1.91 wt%, respectively.