Surface and friction characterization of MoS{sub}2 and WS{sub}2 third body thin films under simulated wheel/rail rolling-sliding contact

摘要

The tribological behavior of MoS{sub}2 and WS{sub}2 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 Amsler rolling-sliding test, both MoS{sub}2 and WS{sub}2 oxidize to the metal trioxide, (SO{sub}4){sup}(2-) and elemental S as solid phase products. These two S-containing products are found exclusively in tribochemical reactions, whereas gaseous SO{sub}2 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 MoS{sub}2 and WS{sub}2. During the Amsler 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 MoS{sub}2 and WS{sub}2 during rolling-sliding are important factors governing the tribological performance in such aspects as friction level, retention time, lubrication regime and failure mode.