Comparison of unidirectional and reciprocating tribometers in tests with MoDTC-containing oils under boundary lubrication

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% 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.
Source: Tribology International - Category: Materials Science Source Type: research