Molybdenum DialkyldiThioCarbamate (MoDTC) is a friction modifier that has been used in automotive engines for many years. However, its exact decomposition mechanism within tribocontacts is not fully understood. In this study, an attempt has been made towards
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 longer test duration is investigated. Secondly, the influence of temperature on the surface chemistry of the resulting wear scars is investigated. Results show that at the beginning of tribotests with MoDTC lubricant, iron oxides are formed in the tribocontact which result in high friction. At longer test durations, adsorbed MoDTC on the ferrous surface decomposes to form MoS 2 and low friction is observed. Surface chemistry at the tribocontact has been found to vary depending on the test temperature. At high temperatures, MoS 2 is formed which provides friction reduction while at low temperatures, molybdenum oxide and amorphous sulphur-rich molybdenum (MoS x ) compounds are formed which do not provide friction reduction. Furthermore, it has been shown that MoS 2 formed within the tribocontact at high temperatures has a slightly disordered crystal structure as a result of tribological processes.
In this study, the role of surface roughness and slide-roll ratio in the decomposition and friction performance of molybdenum dialkyldithiocarbamate (MoDTC) has been investigated. Tribotests were carried out in a minitraction machine (MTM) using steel discs of varying roughness rubbing against smooth steel balls in a sliding/rolling contact. Tests were conducted at slide-roll ratio (SRR) values of SRR = 100% and 200%. Raman spectroscopy was used to perform chemical characterisation on the resulting wear scars. The friction performance of rough discs was not affected by the SRR. On the other hand, increasing the SRR from 100% to 200% in tests with smooth discs resulted in higher friction with large instabilities. Raman analysis showed significant differences in chemical composition of the wear scars generated after tests with smooth and rough discs. Wear scars generated using rough discs were mainly composed of MoS 2 indicating complete MoDTC decomposition while those generated using smooth discs were composed of a mixture of MoS 2 , MoS x (x > 2) and FeMoO 4 indicating partial MoDTC decomposition. Numerical simulation of the contact revealed that under similar loading conditions rough surfaces have higher local pressures than smoother surfaces. It is proposed that higher local pressures in rough surfaces promoted complete MoDTC decomposition. The novel finding from results presented in this study is that at similar temperature and MoDTC concentration, the degradation of MoDTC within tribocontacts is highly dependent on the roughness of the tribopair. This is because surface roughness determines the local pressure at the asperity-asperity contact.
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