Abstract: To withstand the thermal gradients experienced by ablation-resistant coatings in extreme environments, we integrate a functionally graded design with the compositional engineering of multicomponent borides to enhance the ablation resistance and cyclic stability of a (Hf,Zr,Ti)B₂-SiC composite coating. The coating, with compositional gradients in Si and Ti across its thickness, was deposited onto carbon/carbon composites using atmospheric plasma spraying. After three 120-s oxyacetylene flame cyclic tests above 2200 °C, the coating exhibited remarkable ablation resistance, with a linear recession rate of -0.15 µm s^-1. The resulting oxide scale comprises an outer porous, fine-grained, and lattice-distorted (Hf,Zr,Ti)O₂ layer and an inner (Hf,Zr)O₂-SiO₂ composite layer. This hierarchical architecture merges high-temperature stability, thermal insulation, and oxidation resistance. The coating remained effective even after prolonged exposure (6 × 120 s); however, extended cycles led to (Hf,Zr,Ti)O₂ grain coarsening and SiO₂ volatilization, resulting in performance degradation. This study proposes an effective strategy to enhance the long-term ablation resistance of coatings, contributing to the development of advanced thermal protection systems.

Key words: Functionally graded coatings, Ultra-high temperature ceramics, Ablation resistance, Multicomponent borides, C/C composites