Orthogonal optimization of oxidation resistance and diffusion behavior in non-equimolar (HfZrTaCr)B₂ high-entropy ceramics with SiC-Y₂O₃ co-addition

The oxidation behavior of non-equimolar (HfZrTaCr)B₂ high-entropy ceramics synergistically modified by SiC and Y₂O₃ was investigated using an orthogonal experimental design. Isothermal oxidation at 1500 °C in air followed parabolic kinetics for all compositions, consistent with diffusion-controlled oxidation. Statistical analysis indicates that SiC dominates the oxidation resistance (62.32%), followed by Y₂O₃ (28.26%), whereas the effect of CrB₂ fraction is comparatively minor (9.42%). The optimized composition exhibits a markedly reduced parabolic rate constant (k_p) of 7.32 mg²·cm⁻⁴·h⁻¹. Microstructural characterization reveals the formation of a dense multilayer oxide scale, consisting of an outer SiO₂-rich glassy layer and an inner Y-stabilized (Hf, Zr)O₂ framework. The elimination of interconnected porosity suppresses short-circuit diffusion pathways, resulting in a transition of the rate-controlling step from defect-assisted transport toward predominantly lattice diffusion through the compact scale. The enhanced oxidation resistance is attributed to a glass–ceramic coupled barrier mechanism that effectively limits oxygen transport.