High- and medium-entropy nitride coatings from the Cr–Hf–Mo–Ta–W–N system: Properties and high-temperature stability

High- and medium-entropy nitride coatings from the Cr–Hf–Mo–Ta–W–N system were studied using ab initio calculations and experiments to clarify the role of entropy and individual elements in phase stability, microstructure, and high-temperature behaviour. Formation energy calculations indicated that nitrogen vacancies stabilise the cubic (fcc) phase, with hafnium and tantalum acting as strong stabilisers, while tungsten destabilises the lattice. Coatings were deposited by reactive magnetron sputtering at ∼ 50 °C (AT) and ∼ 580 °C (HT). All exhibited columnar fcc structures; high-temperature deposition produced denser coatings, lower nitrogen content, and larger crystallites, resulting in higher hardness and elastic modulus. Thermal stability was tested up to 1200 °C on Si and oxidation at 1400 °C on sapphire. AT coatings failed early, while most HT coatings endured. Nitrogen loss ≲ 10 at.% at 1000 °C was critical for survival. TEM revealed tungsten segregation and HfO 2 formation, while fcc nitride remained dominant. Ta enrichment proved essential for superior thermal and oxidation stability. • Ab initio calculations indicate stabilizing (destabilizing) effect of Hf and Ta (W). • Properties of the as-deposited coatings are governed by their microstructure. • Nitrogen release during annealing leads to a high concentration of vacancies. • Tungsten segregates and hafnium forms oxides during oxidation. • Not entropy, but tantalum content is critical for achieving the best stability.