High entropy nitride (HEN) thin films of AlCoCrCu0.5FeNi deposited by reactive magnetron sputtering

Thin films of high entropy alloys (HEAs) are of great interest for surface engineering applications due to their exceptional properties including superior hardness, resistance to oxidation and corrosion, high temperature stability and high hydrophobicity. The microstructural and the physical properties of high entropy nitride (HEN) thin films, that are chemically inert and possess superior properties, can be effectively tunned using magnetron sputtering processes in the reactive mode. In this study, high entropy nitride thin films of AlCoCrCu0.5FeNi were deposited using RF reactive magnetron sputtering with three different nitrogen gas flow fractions (RN) of 6.25, 12.5, and 25%. X-ray diffraction technique and transmission electron microscopy revealed that the crystallinity of the HEN films changed from a mixed cubic FCC and BCC to a partially amorphous phase with increasing RN due to the accumulation of more interstitial nitrogen atoms in the film. Modifications of film morphology, such as decreases in grain size and surface roughness, were observed from scanning electron microscopy and atomic force microscopy, respectively. Energy dispersive spectroscopy recorded the highest nitrogen concentration of 10.6% for the film deposited at an intermediate RN of 12.5%. X-ray photoelectron spectroscopy detected significant levels of surface protective oxides (such as Al2O3 and Cr2O3) and nitrides (such as AlN and CrN) for the HEN thin films deposited at higher gas fractions of 12.5 and 25%. According to nanoindentation results, the film deposited at the RN of 25% attained a hardness of 9.8 GPa. The highest hardness, significant oxidation resistance and the highest hydrophobicity with a WCA of 112° were achieved for the HEN film deposited using the highest RN of 25%. Such high entropy nitride films deposited by reactive magnetron sputtering have important implications as protective coatings for a wide range of applications including turbines, engine blades and aircraft bodies used in aerospace industries.