High-entropy (Hf_0.25Zr_0.25Ti_0.25Cr_0.25)B₂ ceramic incorporated SiC-Si composite coating to protect C/C composites against ablation above 2400 K

The ablation behavior and protection mechanism of high-entropy (Hf_0.25Zr_0.25Ti_0.25Cr_0.25)B₂ ceramic incorporated SiC–Si (HETMB₂-SiC-Si) composite coating on C/C composite were investigated under ultra-high temperature (up to 2404 K) dynamic ablation test using oxyacetylene torch (OAT). Thermodynamic computations, multicomponent oxide phase diagrams, schematic diagram of the oxidation protection effect of multi-component ceramics, as well as microstructure and composition evolution analyses were built and carried out. Compared with the monolithic or conventional solid-solution diborides incorporated SiC-based composite coatings, the HETMB₂-SiC-Si composite coatings presented the minor mass and thickness changes after ablation for 60 s. The oxidation-dominated ablation procedure with good resistance to mechanical scouring mainly contains the selective oxidation of each elements in the coating materials, the phase separation of the Zr, Hf and Ti (group IVB) oxides, the selective melting of Si (group IVA) and Cr (group VIB) oxides, as well as the oxygen diffusion. This resulted in differentiated oxidation scale with enhanced high-temperature thermally-stable and defect-sealing ability under different ablation environments (regions). This work provides precious and insightful information for introducing high-entropy materials to improve the heat resistance of high-temperature thermal protection systems applied in increasingly severe environments.