Microstructure and tribological behaviors of MoN-Cu nanocomposite coatings sliding against Si3N4 ball under dry and oil-lubricated conditions

MoN and MoN-Cu coatings were deposited on the substrates of 304 stainless steel using direct current (DC) magnetron sputtering. The chemical composition and microstructure of the as-prepared MoN and MoN-Cu coatings were analyzed by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The hardness and elastic modulus of the coatings were tested by nanoindentation. The tribological performances of both kinds of coatings sliding against Si3N4 ball under dry and PAO oil-lubricated conditions were evaluated using a ball-on-disc wear tester. The characterization results show that both MoN and MoN-Cu coatings exhibit polycrystalline face-centered cubic (FCC) structures. The MoN coating presents an equiaxed crystal growth manner, while the MoN-Cu coating grows in an obvious columnar crystal. The doping of Cu element exhibits the grain refining effect, endowing the MoN-Cu coating with grain size of 9 nm smaller than that of 12 nm for MoN coating. The hardness and elastic modulus of the MoN-Cu were slightly lower than those of the MoN, due to the doping of Cu element into the coating. The tribo-test results show that the MoN-Cu coating presents superior tribological performances than MoN coating under dry and oil-lubricated conditions, which is closely related to a large number of mixed oxides with good lubrication effect generated on the wear surface for the MoN-Cu coating. The produced oxides on the wear interfaces can also prevent the MoN-Cu coating and the counterpart ball from direct contact, which greatly reduces the friction and wear of the composite coating. Finally, the possible wear mechanisms of both kinds of coatings under oil-lubricated conditions were illustrated by schematic model.