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

TL;DR

This work tackles how multi-principal-element design, via entropy and element-specific interactions, affects the stability and performance of Cr–Hf–Mo–Ta–W–N nitrides. By uniting ab initio DFT predictions with reactive magnetron sputtering experiments, it shows that nitrogen vacancies stabilise the fcc nitride phase and that hafnium and tantalum strongly promote chemical stability, whereas tungsten tends to destabilise the lattice. High-temperature deposition yields denser microstructures with higher hardness and modulus, though nitrogen retention remains a critical factor for thermal endurance; coatings that retain $\gtrsim$ $10$ at.% N after early anneals survive more extreme heating. TEM reveals tungsten segregation and HfO$_2$ formation during oxidation, while Ta enrichment proves essential for superior thermal and oxidation stability. Overall, enthalpic factors and kinetic realities modulate stability beyond configurational entropy, underscoring the need to balance entropy with element-specific nitride chemistry in high-temperature ceramic coatings.

Abstract

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 approx. 50C (AT) and approx. 580C (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 1200C on Si and oxidation at 1400C on sapphire. AT coatings failed early, while most HT coatings endured. Nitrogen loss less than 10 at.% at 1000C was critical for survival. TEM revealed tungsten segregation and HfO2 formation, while fcc nitride remained dominant. Ta enrichment proved essential for superior thermal and oxidation stability.

Figures (14)

• Figure 1: DFT-predicted chemical stability (top row, a-*) and structural parameters (bottom row, b-*) of high-entropy nitrides, (W, Ta, Hf, Mo, Cr)N$_x$, with various elemental compositions, together with reference data for the parent binary nitrides. The content of each element is visualised by coloured bars, with gray for N, green for W, yellow for Ta, blue for Hf, cyan for Mo, red for Cr, and white denoting vacancies on the N sublattice. Panels (a) and (b) depict formation energy, $E_\mathrm{f}$, and lattice parameter, $a$ for HENs with fully occupied N sublattice (a-1; b-1); 25% of N vacancies (a-2; b-2); 50% of N vacancies (a-3; b-3); and the binary nitrides (a-4; b-4).
• Figure 2: SEM micrographs showing typical morphology of the deposited coatings.
• Figure 3: TEM micrographs of coatings a) AT4 and b) HT4 showing typical microstructure of the deposited coatings. SAED patterns with rings identified as fcc structure are plotted as an inset in each micrograph.
• Figure 4: STEM HAADF micrographs of the upper part of samples a) AT4 and c) HT4. corresponding EDX oxygen maps are shown in b) and d).
• Figure 5: X-ray diffractograms of the a) AT and b) HT coatings. Miller indices corresponding to an fcc phase are provided at the top of the image.