A methodology for Raman characterisation of MoDTC tribofilms and its application in investigating the influence of surface chemistry on friction performance of MoDTC lubricants

In this study, Raman spectroscopy has been employed to understand the influence of surface chemistry on friction in a tribocontact. Tribotests were conducted using molybdenum dialkyldithiocarbamate (MoDTC) lubricant in a steel/steel sliding contact. Firstly, surface chemistry in the high-friction regime, at the beginning of the test, and in the low-friction regime, after long test durations, is investigated. Secondly, the influence of temperature on the surface chemistry of the resulting wear scars was investigated. Results show that at the beginning of tribotests with MoDTC lubricant, iron oxides are formed at the tribocontact which result in high friction. At longer test durations, adsorbed MoDTC on the ferrous surface decomposes to form MoS2 and low friction is observed. Surface chemistry at the tribocontact has been found to vary depending on the test temperature. At high temperatures, MoS2 is formed which provides friction reduction, while at low temperatures, molybdenum oxide and amorphous sulphur-rich molybdenum (MoSx) compounds are formed which do not provide friction reduction. Furthermore, it has been shown that MoS2 formed within the tribocontact at high temperatures has a slightly disordered crystal structure as a result of tribological processes.