Microstructure and tribological performance of adaptive MoN–Ag nanocomposite coatings with various Ag contents

MoN–Ag coatings with different silver contents were synthesized using DC and RF magnetron sputtering. The microstructure and mechanical performance of the as-prepared coatings were investigated in detail. The friction and wear behaviors of the MoN–Ag coatings sliding against Si3N4 ceramic ball at RT and elevated temperature were also evaluated by a ball-on-disc tester with rotation method. The results showed that the MoN–Ag coatings were polycrystalline structures composed of γ-Mo2N and c-Ag. And the MoN–Ag coating with a 2.2% Ag content had the highest hardness and elastic modulus. The friction interface between the MoN–Ag coating and Si3N4 ball presented distinct oxidation after sliding at RT. In addition, the temperature greatly impacted the friction and wear behaviors of the MoN and MoN–Ag coatings. The friction coefficients of the investigated coatings decreased gradually when the temperature rose from 100 °C to 700 °C. However, the wear rates displayed opposite trends. The decrease in the friction coefficient for the coatings could be ascribed to the formation of lubricating phases (MoO3, Ag2MoO4, Ag2Mo4O13), while the increases in the wear rate were proven to be due to decreases in the residual compressive stresses of the coatings with increasing temperature. The MoN–Ag coating with 2.2% Ag had excellent tribological properties and an extremely low friction coefficient of 0.27 at 700 °C, which was superior to that of the pure MoN coating.