Surface and friction characterization of MoS2 and WS2 third body thin films under simulated wheel/rail rolling-sliding contact

The tribological behavior of MoS2 and WS2 third body thin films has been studied using thermogravimetric analysis (TGA), high-temperature rheometer (HTR) and an Amsler twin-roller tester that simulates wheel/rail contact in railroad use. These two metal disulfides are found to exhibit lower decomposition temperatures in tribochemical reactions compared to their thermal reactions. The chemical compositions probed by X-ray photoelectron spectroscopy (XPS) show that, under the Amster rolling-sliding test, both MoS2 and WS2 oxidize to the metal trioxide, SO42- and elemental S as solid phase products. These two S-containing products are found exclusively in tribochemical reactions, whereas gaseous SO2 is the sole product in thermal reactions. This highlights the effect of high pressure as well as micro-slip motion in the contact zone on the decomposition pathways of MoS2 and WS2. During the Amster wear test, the metal disulfide transfer films cover the counter surface as islands of thick patches. Rolling-sliding contact causes a decrease in the population and size of these patches, whose states largely determine the friction coefficient. This gives rise to the typical shape for the Amsler 'friction curve', with low/stable friction maintained while the thick patches of friction modifier are present. The conversion of a disulfide to trioxide, which has poorer lubricating properties, modifies the stable friction region and contributes to the slow increase in friction. The wear mechanism and chemical change of the MoS2 and WS2 during rolling-sliding are important factors governing the tribological performance in such aspects as friction level, retention time, lubrication regime and failure mode. (C) 2007 Elsevier B.V. All rights reserved.