Mechanical properties and thermal stability of reactively sputtered multi-principal-metal Hf-Ta-Ti-V-Zr nitrides

• Published in: Surface & coatings technology 2020-05, Vol.389, p.125674, Article 125674
• Main Authors: Kirnbauer, A., Kretschmer, A., Koller, C.M., Wojcik, T., Paneta, V., Hans, M., Schneider, J.M., Polcik, P., Mayrhofer, P.H.
• Format: Article
• Language: English
• Subjects: Annealing; Chemical composition; Chemically complex alloys; Coatings; Electron diffraction; Entropy; Grain size; Hafnium; High-entropy alloys (HEAs); High-entropy nitrides; Indentation; Magnetron sputtering; Mechanical properties; Multi-principal element nitrides; Nitrides; Nitrogen; Tantalum compounds; Thermal stability; Thermogravimetric analysis; Thin films; Titanium compounds; Vanadium; X-ray diffraction; Zirconium compounds
• ISSN: 0257-8972; 1879-3347; 1879-3347
• DOI: 10.1016/j.surfcoat.2020.125674

Crystalline (Hf,Ta,Ti,V,Zr)N nitride thin films, with a high-entropy metal-sublattice, were synthesized at 440 °C by reactive magnetron sputtering using an equimolar Hf-Ta-Ti-V-Zr-compound target. The coatings are single-phase fcc structured mono-nitrides for N2/(Ar + N2) flow-rate-ratios (fN2) between 30 and 45%. For higher fN2 a small fraction of a second phase (next to the fcc matrix) can be detected by X-ray diffraction (XRD) and selected area electron diffraction (SAED). All coatings studied (prepared with fN2 between 30 and 60%) show similar chemical compositions and hardness (H) values between 30.0 and 34.0 GPa with indentation moduli of ~460 GPa. Atom probe tomography (APT) indicates a homogenous distribution of all elements within our fcc-(Hf,Ta,Ti,V,Zr)N even after vacuum-annealing at 1300 °C. While H decreased from 32.5 to 28.1 GPa by this annealing treatment, the coating is still single-phase fcc structured with a defect density (expressed by XRD and SAED features, transmission electron microscopy contrast, and grain sizes) comparable to the as-deposited state. Only after vacuum-annealing at 1500 °C, XRD and APT reveal the formation of hexagonal structured (Ta,V)2N. The onset of nitrogen-loss – detected by thermogravimetric analysis – is ~1350 °C. Based on our results we can conclude that the sluggish diffusion within our fcc-(Hf,Ta,Ti,V,Zr)N warrants the single-phase fcc structure up to 1300 °C, although ab initio based calculations would suggest the lower-entropy products [fcc-(Hf,Zr)N, fcc-(Ta,V)N, and fcc-TiN] and [fcc-(Hf,Zr)N and fcc-(Ta,Ti,V)N] to be energetically more stable up to 1302 K. [Display omitted] •Single-phase fcc-(Hf,Ta,Ti,V,Zr) nitride coatings•Random distribution of elements up to Ta = 1300 °C•Sluggish diffusion allows for postponed decomposition.•Nitrogen loss causes decomposition of fcc-solid-solution at Ta ≥ 1350 °C.