Influence of Si on structure, mechanical properties, and thermal stability of reactively and non-reactively sputtered high-entropy (Hf,Ta,Ti,V,Zr) carbides

Abstract

Designing carbides that combine high thermal stability and mechanical strength for extreme environments remains challenging. Here, high-entropy metal-sublattice carbide thin films, (Hf,Ta,Ti,V,Zr)C, with and without silicon additions (up to 21 at.% of the metal fraction), were synthesized by reactive and non-reactive magnetron sputtering. Structural analysis confirms a stable single-phase face-centered cubic (fcc) structure for Si-free and low-Si coatings, both as-deposited and after annealing at 900 °C (vacuum) and 1400 °C (He-atmosphere). Transmission electron microscopy (for Si-free and moderate Si containing ones) and X-ray diffraction (all compositions) reveal increased crystallite size and/or reduced microstrain with higher Si content. Mechanical tests show hardness of 31–36 GPa and elastic modulus of 397–427 GPa, with minimal dependence on Si. Silicon addition increases deposition rate while preserving a dense, fine-fibrous morphology. Thermal analysis in He-atmosphere up to 1400 °C indicates minor carbon loss in Si-free samples above 850 °C (reactive) and 1000 °C (non-reactive), delayed to above 1100 °C with 5 at.% Si. Despite this, most coatings retain their single-phase fcc structure. The Si-free (Hf₀.₁₉Ta₀.₂₄Ti₀.₂₅V₀.₂₁Zr₀.₁₁)C film softens only slightly from 33.1 ± 3.2 GPa to 29.1 ± 2.6 GPa when vacuum-annealing at 900 °C. These findings demonstrate exceptional thermal and mechanical stability, highlighting these films as promising candidates for high-temperature applications.