Improving Physical, Mechanical and Tribological Properties of High–Entropy Ti–Cr–Zr–Nb–Hf Coatings by Nitrogen Modification

High-entropy alloy (HEA) coatings based on the Cr–Ti–Zr–Nb–Hf system represent a promising class of functional materials intended for application under extreme operation conditions. This study presents a comprehensive investigation of Cr–Ti–Zr–Nb–Hf HEA coatings, both in their as–deposited state and following nitrogen modification. Ion–plasma sputtering of the coatings was conducted in Ar and Ar + N2 atmospheres, respectively. Synchrotron X–ray absorption spectroscopy (XAS) revealed that nitrogen incorporation facilitates the formation of a single–phase high–entropy nitride, designated (Cr, Ti, Zr, Nb, Hf)N, characterized by strong covalent metal–nitrogen bonds and substantial lattice distortion. Nitrogen modification was found to yield more than a twofold increase in hardness and an approximately twofold increase in Young’s modulus in contrast to nitrogen–free sputtering. Tribological tests conducted over a temperature range of –60°C to +150°C demonstrated high thermal stability of the nitride coating. Nitrogen–assisted modification further enhanced wear resistance by nearly an order of magnitude, while the coefficient of friction decreased by a factor of three to four. A model–based description of the structural and mechanical characteristics employing the Miedema model and the Mazhnik–Oganov framework along with an analysis of tribological properties grounded in the Bowden–Tabor and Archard theories, exhibited good agreement with experimental observations. The findings of the study indicate that the (Cr, Ti, Zr, Nb, Hf)N coating represents a stable, high–strength high–entropy compound dominated by covalent interatomic interactions.