Feasibility and ablation behavior of on-site HfSi₂and HfSi₂–HfO₂repair coatings on C/C–SiC–ZrC composites

Protective coatings are indispensable for thermal protection systems, but damage during installation and service is often unavoidable. To restore the protective function of C/C–SiC–ZrC composites, an on-site repair strategy was developed using a non-sintered repair coating composed of polysilazane and HfSi₂ particles. According to GJB323A-96 Standard with heat fluxes of 2.38 MW/m², the ablation tests were investigated. Short-term exposure (10 s) induced the rapid oxidation of HfSi₂, forming a dense HfO₂ layer that sealed the surface and effectively hindered oxygen ingress. After 120 s, the repaired samples showed negative linear and mass ablation rates (−0.99 ± 0.07 μm/s and −0.50 ± 0.05 mg/s, respectively), indicating that the repaired coating still provided effective protection for the substrate. In contrast, uncoated composites suffered severe ablation with rates of 0.80 ± 0.17 μm/s and 1.00 ± 0.08 mg/s. Notably, the ablation rate of the repair coating was comparable to that of the in-situ coating, confirming the effectiveness of the proposed on-site repair strategy. Meanwhile, the HfSi₂–HfO₂ repair coating prepared through compositional and structural optimization further reduced the discrepancy between linear and mass ablation rates, confirming the effectiveness and reproducibility of the repair strategy.