Comparison of unidirectional and reciprocating tribometers in tests with MoDTC-containing oils under boundary lubrication. (September 2020) - Balarini, R.
Abstract: Friction reduction is a relevant topic in internal combustion engines (ICE), due to the usual concern of increasing efficiency and reducing pollutant gas emissions. Many lubricant additives intended to reduce friction, commonly classified as friction modifiers (FM), are oil soluble organic-molybdenum compounds, such as molybdenum dithiocarbamate (MoDTC). The action of this additive is associated with the formation of low-friction tribolayers of MoS2, which is strongly dependent on the mechanochemical properties of the tribosystem. One important variable affecting the MoDTC activation process is the type of the relative movement between the rubbing surfaces. In this context, the present work is focused on the investigation of the influence of both reciprocating and unidirectional rotating tribotests on the tribological performance of MoDTC-containing oils. The objective is to explore if both testing configurations, under equivalent contact conditions, may generate comparable results in terms of friction, wear and Mo-based tribofilm formation. The tribological tests were conducted in SRV-4 (reciprocating motion) and MTM-2 (unidirectional rotating motion) tribometers under boundary lubrication regime. A fully-formulated engine oil (SAE 0W-20) with MoDTC FM additive and a PAO 8 + 0.6 wt.% of MoDTC oil were used as lubricants. Friction and wear were compared over a range of speed/load ratios, as a function of sliding time, and the resulting tribofilms were analyzed by Scanning Electron Microscopy with Energy-Dispersive X-ray spectroscopy (EDX), Raman spectroscopy and Scanning Probe Microscopy (SPM). The obtained coefficient of friction (CoF) values were as low as 0.06 in MTM and SRV tribometers under some particular normalized sliding conditions, but the overall CoF behavior as a function of sliding time was distinct for both test rigs. MoS2 regions were observed in SRV discs after the tribological tests, confirming the MoDTC activation for reciprocating tests, whereas Mo-based tribofilms were not detected in MTM samples, despite a noticeable friction reduction during part of the tests. The fully-formulated oil has resulted in the lowest friction and wear levels in both tribometers. Highlights: Friction coefficient values around 0.06 were reached in both unidirectional and reciprocating motions. The lubricant used was more important for obtaining the low-friction regime in comparison with the type of relative motion. Reciprocating tests resulted in higher friction levels than unidirectional tests. The presence of MoS2 tribofilm was detected only in tests with reciprocating motion. MoS2 tribofilm areas were more easily detected at the reversal points and inside the roughness valleys. Fully-formulated oil performed better than PAO 8 + 0.6 wt.% of MoDTC in terms of low friction and wear resistance.