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

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.