Tribological investigation of adaptive Mo2N/MoS2/Ag coatings with high sulfur content

Adaptive nanocomposite Mo2N/MoS2/Ag coatings were deposited on Inconel and silicon substrates by magnetron sputtering with individual targets of Mo, MoS2 and Ag. The tetragonal β-Mo2N structure in addition to Ag and MoS2 phases were detected using X-ray diffraction. The elemental composition of the coatings was investigated using Auger electron spectroscopy. The tribological properties of the coatings were studied at room temperature (RT), 350, and 600 °C against Si3N4 balls. The lowest friction coefficients that were obtained were 0.4, 0.3, and 0.1 at RT, 350 °C, and 600 °C, respectively. The average friction coefficient was maintained at 0.1 for more than 300,000 cycles at 600 °C due to the formation of lubricious silver molybdate phases at the contact surfaces. Three types of silver molybdate phases were detected by both X-ray diffraction and micro-Raman spectroscopy in the wear tracks, namely, Ag2Mo4O13, Ag2Mo2O7 and Ag2MoO4 depending on the Mo and Ag contents in the coatings. The superior performance of all three compounds is due to their layered structure with weaker Ag–O bridging bonds. These relatively weak bonds may shear or even break easily at high temperatures to account for the observed friction reduction.