Structure and Mechanical Properties of Unheated Chromium Aluminum Nitride Coatings Sputter-Deposited with Various Aluminum Content

Abstract

Chromium aluminum nitride (CrAlN) has been extensively studied because of high hardness, high oxidation and corrosion resistance, and good wear resistance. However, utilizing substrate treatments such as heating and voltage biasing during film deposition usually leads to relatively high surface roughness that affects wear rates. It has been found that sputter deposition at low substrate temperatures can produce nano-grain coatings with enhanced structure and mechanical properties. For this reason, the CrAlN in this study was prepared by a reactive co-sputtering technique without the substrate treatments. Effects of Al content on structure and mechanical properties were investigated by X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectrometry, atomic force microscopy, X-ray photoelectron spectroscopy, and nanoindentation. The results suggest that these CrAlN films formed as solid solutions by substitution of Al for Cr in the CrN crystalline structure. The deposition with increasing Al but fixed N leads to N deficiency, therefore at high Al content these films form under 1:1 stoichiometric nitride. This lowers film crystallinity and hence refined film morphology. Surface roughness and hardness of the films decreased from 5.737 to 1.135 nm and from 31.69 to 26.56 GPa, respectively. However, the solid solution strengthening arising from the further increase of the Al content causes these values to rebound to 2.466 nm and to 30.16 GPa.