Composition-optimized Hf–Zr–Ti–C–N coatings for exceptional ablation resistance

Multi-element carbonitrides have emerged as promising candidates for ultra-high temperature thermal protection, yet the influence of compositional variation on their ablation behavior remains insufficiently understood. Here, a series of Hf-Zr-Ti-C-N bulk ceramics with systematically varied metallic atomic ratios are fabricated by spark plasma sintering. Comprehensive evaluation of fracture toughness, flexural strength and oxyacetylene flame ablation performance identifies Hf4/6Zr1/6Ti1/6CN as the optimal ceramic matrix, which is subsequently applied as a coating on graphite with a SiC-graded functional transition layer. After ablation for 120 s at 3,000 °C, the coating exhibits excellent ablation resistance, with mass and linear ablation rates of -1.23 ± 0.25 mg/s and -2.07 ± 0.37 μm/s. Its superior ablation resistance originates from a synergistic interplay of phase transformation into a dense HfO2-dominated oxide barrier, the self-healing effect of the SiO2-rich phase coupled with stress relaxation by the Si-C-O nanowires network, and interfacial strengthening induced by the Si-enriched diffusion layer. Combined with molecular dynamics simulations, the oxidation evolution pathway during the initial stage of ablation is further elucidated and validated at the atomic scale. This work establishes a composition-optimized carbonitride coating with outstanding ablation resistance and provides a mechanistic framework for the rational design of next-generation thermal protection systems intended for extreme aerospace environments.