Morphology and tribological properties of metal (oxide) MoS2 nanostructured multilayer coatings

Multilayer coatings of metallic (Au, Ni, Pb or PbO) and MoS2 nanometer-scale layers were deposited by r.f. magnetron sputtering on 440C steel with a Cr adhesion layer. The structure, composition and thickness of the multilayers were studied by Rutherford back-scattering (RBS) and X-ray diffraction (XRD) spectroscopies. Pin-on-disk tribometer tests were performed in humid air of 50% RH. In order to investigate chemical and mechanical aspects of wear in humid air, the worn surfaces of the multilayers and of the counter parts were studied by scanning Auger microscopy (SAM) and scanning electron microscopy (SEM) after different sliding distances. The chemistry of wear was found to be associated with elimination of Mo from the contact surfaces. Relative to pure sputter-deposited MoS2 coatings, lower values and more stable mean friction coefficients were observed for the metal-MoS2 multilayers studied at higher loads. A correlation between the morphology and sliding properties of multilayers was found to be associated with an increased preferred basal orientation of MoS2 in nanoscaled layers. The role of metal is most likely associated with a structural modification of MoS2 rather than "gettering" of oxygen during sliding. It is concluded that the improvement in the tribological performance of sputter-deposited metal-MoS2 multilayers in humid air can be generally attributed to the optimization of MoS2 morphology in nanostructured multilayer coatings. (C) 1998 Elsevier Science S.A.