Si-modified high-entropy (Hf, Ta, Ti, V, Zr) carbide coatings: Oxidation resistance and scale integrity

Abstract

High-entropy metal-sublattice carbide thin films are promising for high-temperature applications due to their exceptional mechanical and thermal stability, but their oxidation resistance remains limited. We synthesized (Hf, Ta, Ti, V, Zr)C coatings without and with ∼5 and 11 at.% Si (relative to the metal sublattice) via reactive and non-reactive magnetron sputtering to investigate the effects of Si addition and deposition route on oxidation behavior. All coatings crystallized in a single-phase face-centered cubic (fcc) structure. Simultaneous thermal analysis revealed oxidation onset at ∼665–667 °C, with Si raising this temperature by up to 35 °C. Reactively deposited coatings exhibited greater mass loss, likely due to ∼60 % volatilization of V₂O₅, which may be related to their lower Hf and Ta content but higher Ti and V content. In-situ XRD showed nearly simultaneous formation of ZrO₂, TiO₂, and (Ti, Zr)O₂ around 650 °C, while Si-containing coatings retained the fcc phase to higher temperatures, indicating delayed oxidation. SEM analysis of coatings oxidized at 700 °C revealed severe spallation and porosity in Si-free and low-Si samples, whereas the 11 at.% Si coating formed a continuous, adherent oxide scale. The measured oxide thickness exceeded predictions for dense scales by ∼25 %, consistent with internal porosity and swelling. These results demonstrate that Si additions significantly improve oxidation resistance and oxide-scale integrity, establishing Si as a key design parameter for high-temperature protective coatings.