MoS₂ tribofilm distribution from low viscosity lubricants and its effect on friction

The current study analyses the friction performance of low viscosity fully-formulated oils containing the Molybdenum Dialkyl Dithiocarbamate (MoDTC) friction modifier at different concentrations. The MoDTC friction modifier is known to produce MoS2 sheets in the tribocontact providing a low coefficient of friction under boundary lubrication conditions. However, there is a little knowledge around the quantitative relationship between the concentration of MoDTC in the oil and MoS2 amount and distribution in the contact. The study uses Raman spectroscopy mapping capability to characterise the tribofilm formed from different chemistry lubricants and under different tribological conditions as defined by the lambda ratio. After qualitative and quantitative chemical surface characterisation a discussion is presented to highlight some important aspects to relate the formed MoS2 sheets, their spatial distribution in tribofilms and the subsequent tribological performance

Low friction tribofilm formation and distribution on an engine cylinder tested with MoDTC-containing low viscosity engine lubricants

The piston-liner tribological contact has a key role in controlling friction caused energy losses in a passenger car engine. This study analyses the MoDTC-containing low viscosity lubricant performance and the resulting low-friction MoS2 tribofilm distribution on the liner surface. The engine liners were tested in a floating liner measurement system, where fully-formulated lubricants with different MoDTC concentrations were utilised to assess the impact of friction modifiers on the friction performance. Tested engine liners were dismantled from the measurement system, and MoS2 tribofilm formation on the liner was quantitatively characterised by Raman Microscopy mapping. The MoS2 tribofilm chemical composition and lamellar structure were further investigated by Transmission Electron Microscopy (TEM), High-Angle Annular Dark-Field Scanning (HAADF) and Energy Dispersive X-ray (STEM-EDX) Microscopy. The findings presented in this paper confirm that MoS2 tribofilm spatial distribution plays a pivotal role in reducing friction in piston-liner contact. 60% friction torque reduction was achieved at BDC area with 0.7% MoDTC concentration in the fully formulated lubricant. Moreover, MoS2 presence in the middle of the liner indicates MoDTC effectiveness throughout the whole engine cycle. Therefore, contributing to the overall energy savings in passenger cars