Low Friction Tribofilm Kinetics on Engine Tribological Components

Depletion of petroleum resources encourages researchers to find a way to improve fuel efficiency and reduce CO2 emissions. Decades of research is dedicated to overcoming the frictional losses in car engines. Engine lubricants are the subject of debate since oils are blended from additives, which influence lubricants' performance. Severe wear and energy losses are experienced when metal to metal contact occurs, predominately in the boundary lubrication conditions. Therefore, adding the performance-enhancing additives to the lubricants can support low friction tribofilm formation, protecting engine components. For such cause, Molybdenum DialkyldiThioCarbamate (MoDTC) is industry applied friction modifier, which, when blended in the lubricant, decomposes on the tribopair surfaces and forms low-friction MoS2 sheets. This study investigates the effect of MoDTC decomposition on engine components. The real-life engine and benchtop tribometer tests investigated MoDTC performance in the fully formulated low viscosity lubricants. In addition, Raman Spectroscopy and STEM-EDX were used to characterise MoS2 tribofilm 2D spatial distribution and tribofilm thickness. Engine test results show a friction force reduction when MoDTC is added to the lubricant. The observed friction reduction is dedicated to the MoS2 tribofilm formation in the piston/cylinder tribocontact, where a boundary regime is expected. Tribological tests revealed that in the fully formulated low viscosity lubricants, MoDTC concentration influence weakens as the Lambda ratio increases. The Raman Spectroscopy and TEM study on the MoS2 tribofilm evolution over time revealed that MoS2 formation occurs in several stages. Moreover, observed tribofilm area coverage and layer thickness correlate with the obtained friction. Firstly, MoS2 forms a high 2D coverage tribofilm which consists of the thin amorphous MoS2 layer, which enables initial friction to drop. After running in phase, the elongated tribofilm sheets form in the tribofilm matrix, which supports low friction in the tribopair, despite the 2D tribofilm coverage reduction.