Multi-anion sublattice engineering enables exceptional ablation-resistant performance in Hf_0.8Zr_0.2B_0.1C_0.5N_0.4–SiC ultra-high temperature ceramics

High-entropy ultrahigh temperature ceramics (UHTCs) have garnered significant attention for their outstanding designability and performance, yet existing strategies remain largely confined to cationic sublattice engineering, leaving the potential of anionic site manipulation unexplored. Herein, we extend the entropy-stabilization paradigm to the anion sublattice by designing a multianion Hf_0.8Zr_0.2B_0.1C_0.5N_0.4 solid solution. The resulting Hf_0.8Zr_0.2B_0.1C_0.5N_0.4–SiC ceramic achieves negative ablation rates (−0.049 and −0.287 μm·s⁻¹) under 2600 °C plasma flame exposure, markedly outperforming Hf_0.8Zr_0.2C–SiC. This exceptional ablation-resistant performance originates from the synergistic effects enabled by multianion sublattice engineering. The incorporation of B–C–N intrinsically enhances fracture toughness, while in situ precipitation of hexagonal graphite during ablation extrinsically arrests cracks through interfacial shear, preventing catastrophic disintegration. Furthermore, the multianion matrix undergoes a sequential oxidation process, forming an HfZrBCNO interlayer that acts as an oxygen scavenger. Concurrently, h-BN precipitates at grain boundaries, serving as compliant diffusion barriers that impede oxygen ingress into SiC. This dual-layer protection mechanism suppresses the active oxidation of SiC (SiC + O₂ → SiO + CO) and promotes the formation of a dense, scouring-resistant HfZrO₂–SiO₂ composite barrier. By demonstrating simultaneous microstructural toughening and mesoscale oxidation management, this work establishes multianion sublattice engineering as a transformative platform for designing next-generation thermal protection materials beyond the limits of conventional entropy-stabilized ceramics.