Effect of LaB₆ content on the gas evolution and structure of ZrC coating for carbon/carbon composites during ablation

ZrC coatings with different contents of LaB₆ for SiC-coated carbon/carbon composites were fabricated by supersonic atmosphere plasma spraying. The mechanism of gas evolution during oxidation at ultra-high temperatures and the oxide scale structure evolution of the coating after addition of LaB₆ with different contents were studied. Results showed that rare earth borate rather than rare earth oxide mainly reduced the melting resistance of the oxide scale. The oxide scale for ZrC coating with 10% LaB₆ was unfavorable to the gas escape. After addition of 20% LaB₆, the oxide scale was favorable to the gas escape but disadvantageous to the coating applied in the environment containing scouring because of the reduced melting resistance. High content of LaB₆ also could cause evident boiling of B₂O₃, leading to producing big pores in the oxide scale during ablation. Appropriate content of LaB₆ for ZrC coating was found to be 15%.