Comparative study of reactively and non-reactively sputtered high-entropy metal sublattice carbides

Abstract

Non-reactive (nr) as well as reactive (r) magnetron sputtering was utilised to compare the phase formation and mechanical properties of (Hf,Ta,Ti,V,Zr)-carbide coatings. The nr-synthesised coatings form a single-phase fcc-structured solid solution, whereas the r-synthesised ones show a strong dependence on the used acetylene flow (fC2H2). Thereby, only the coating deposited with fC2H2 = 20 % (the total flow is Ar +C2H2) exhibit a single-phase structure, whereas coatings deposited with lower acetylene flow rate ratio show partly XRD-amorphous patterns. The hardness comparison of the two single-phased coatings shows that the nr-sputtered one exhibits 41.6 ± 1.6 GPa, while the r-sputtered one is noticeably softer with 34.8 ± 1.6 GPa. Upon vacuum annealing of free-standing powdered coating materials, both show no structural change up to 1300 °C, only a reduction in their X-ray diffraction peak widths (indicative for structural relaxation effects). However, vacuum annealing of coated sapphire substrates at 1000–1300 °C caused spallation of the r-(Hf,Ta,Ti,V,Zr)C, while the nr-(Hf,Ta,Ti,V,Zr)C stayed intact, with an additional phase formed only when annealed at 1300 °C supposedly due to an interface reaction with the sapphire substrate. This nr-(Hf,Ta,Ti,V,Zr)C only slightly softens to 40.2 ± 2.1 GPa upon annealing to 1200 °C, while a further increase to 1300 °C causes a reduction to 34.0 ± 2.4 GPa, due to this interface reaction and recovery effects. These findings suggest that nr-(Hf,Ta,Ti,V,Zr)C offers superior phase stability, mechanical robustness, and resistance to high-temperature degradation over their reactively-prepared counterparts, making them better suited for applications demanding thermal stability and hardness retention.