Carbon content regulation on phase ratio of (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂-SiC ceramics: New interpretation of ablation mechanism based on morphological evolution

This study fabricated (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂-SiC multi-phase ceramics by tailoring the initial carbon content (5–20 wt%) to systematically elucidate the decisive influence of phase ratio on ultra-high-temperature ablation behavior. High-HEB systems (S1-S2), despite forming a (Zr,Hf)O₂-A₆B₂O₁₇ nano-twinned skeleton, exhibited aggravated porosity after 120 s due to an insufficient SiO₂ glass phase. Excessive SiC (S4) degraded performance owing to an over-thick glass layer and t→m phase transformation cracks in (Zr,Hf)O₂ induced by Ti depletion of (Zr,Hf)TiO₄. In contrast, the optimized three-phase composition (S3, HEB:SiC:Si ≈ 4:5:1) constructed a triple-layer "skeleton–sealing–pinning" architecture, consisting of a surface (Zr,Hf)O₂ layer for erosion resistance, a (Zr,Hf)TiO₄ interlayer blocking oxygen ingress, and A₆B₂O₁₇ nano-twins pinning the molten oxide, complemented by Ti-enriched films that retarded oxidation. This synergistic structure achieved the lowest ablation rates (1.26±0.21 mg·s⁻¹, 2.00±0.23 µm·s⁻¹). This work provides quantitative guidance for the composition–structure–performance design of high-entropy thermal protection materials.