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

Abstract

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.

New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study

“…11 In Figure 2a, Raman peaks belonging to Fe2O3 were not observed; only the peak at 670 cm -1 belonging to Fe3O4 was observed. Hence, the spectra observed in Figure 2a cannot be attributed to tests being conducted in mineral oil or from the mineral oil itself ( Figure S10).…”

“…Several accumulations were obtained in each spectrum so as to increase the signal-to-noise ratio. At these Raman acquisition parameters, determined in a previous study 11 ,…”

Proposal of Biomimetic Tribological System to Control Friction Behavior under Boundary Lubrication by Applying 3D Metal Printing Process

“…Figure 10 shows a schematic diagram of the friction-reduction mechanism of the LSP and the conventional system. For the conventional system, a MoS 2 tribofilm derived from MoDTC formed on the sliding surface and led to low friction; this friction-reduction mechanism has been reported by many researchers [22][23][24][25][26][27][28][29]. On the other hand, for the LSP, a MoS 2 tribofilm also formed on the sliding surface, just like the conventional system.…”

“…In general, Raman spectroscopy is useful for analyzing MoDTC-derived compounds owing to its sensitivity to Mo-O and Mo-S bonding [29,30]. Figure 9 shows the Raman spectra of MoDTC alone and the worn surfaces of the LSP and the conventional system.…”

“…Tribofilms originating from ZDDP are known to contribute to the improvement in lubricity, and many studies have been carried out to reveal this mechanism [17][18][19][20][21]; ZnS, ZnO, FePO4, and compounds with P-S bonds were found to be produced during sliding, and a layer consisting of polyphosphate was formed on the outermost surface of the tribofilm. On the other hand, MoDTC, which is also used in automotive engine oils, is a friction modifier; MoS 2 derived from MoDTC shows low friction under boundary lubrication [22][23][24][25][26][27][28][29][30]. Friction tests were conducted at steel/steel contacts using the two methods and a cylinder-on-disk friction tester.…”