Optimizing the Mo concentration in low viscosity fully formulated oils

Abstract

Molybdenum dithiocarbamate (MoDTC) is usually added in low viscosity engine oils to improve its ability to reduce friction. However, increasing the amount of MoDTC added results in unwanted sulfated ash increases and other adverse effects. Thus, it is necessary to investigate the optimization of the amount of MoDTC added. The current work investigates this topic in terms of the Mo concentration in the lubricant. Seven different Mo concentrations with the same additive package are tested, whose tribological properties and chemistry of the tribofilm are investigated. For the selected operating conditions in this study, the critical concentration of Mo is determined to be ≈ 350 ppm due to its ability to reduce the friction to ≈ 0.04 under a constant and varying lambda ratio by forming the required threshold thickness of MoS2 within the tribofilm matrix, dominated by zinc dialkyl dithiophosphate (ZDDP) species. In general, increasing the Mo concentration increases the formation rate of MoS2 in the initial stages of the traction, directly forming a thicker MoS2 layer within the tribofilm up to a specific Mo concentration. Induction time also decreases with increasing Mo concentration, and increasing the Mo concentration from 350 to 1000 ppm does not negatively impact the wear generated.