Effects of nitrogen concentration on the microstructure and mechanical properties of nanocrystalline (TiZrNbTaMo)N high-entropy nitride coatings: Experimental investigations and first-principles calculations

Novel (TiZrTaNbMo)N high-entropy alloy nitride (HEAN) coatings were prepared by double cathode glow discharge method, at various nitrogen flow ratios (RN = N2/Ar), on to Ti-6Al-4V substrates. The effects of different nitrogen pressures, RN, on the microstructure and mechanical properties of these coatings were studied by X-ray diffraction, electron microscopy and nanoindentation. Over the range of RN values investigated, from 5% to 30%, these coatings exhibit a face centered cubic (FCC) structure and are composed of equiaxed nano-crystals between 8 and 12 nm in diameter. The thicknesses of the coatings range between 14.4 and 23.6 mu m, far exceeding that of typical HEAN coatings prepared by magnetron sputtering. The (TiZrTaNbMo)N coating prepared at RN = 20% exhibits the highest values for both hardness and Young's modulus (39.7 GPa and 310.2 GPa, respectively), thus approaching the level of superhard coatings. Combined with the results of first-principles calculations, one of the most important factors governing the high hardness of these coatings is the existence of both covalent and ionic bonds between the N atom and the surrounding metal atoms. Moreover, compared with most HEAN coatings, the coatings prepared by this method exhibit an improved strength-toughness balance, which is essential for surface protection.