In tribo vs ex tribo MoS2: mechanochemical insights into additivation strategies for lubricating greases

Molybdenum disulfide (MoS₂) is widely regarded as an effective material for reducing friction. It is used in lubricated contacts, either directly as an additive or via an oil-soluble precursor (molybdenum dithiocarbamate, MoDTC) which forms a MoS₂ tribofilm during operation. However, specific strategies for implementing its use in lubricating greases, where challenges often occur due to complex systems involving grease thickeners that actively participate in lubrication processes, are rarely discussed. This study evaluates two distinct grease systems, a lithium complex (LiX) grease and a polypropylene (PP) grease, assessing the effectiveness of various MoS₂ additivation strategies under high-load rolling–sliding conditions. The tribological performance is assessed through traction tests and surface analyses, including Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The results show that additive performance in greases depends not only on the chemistry of the additive but also on interactions with the thickener matrix and co-additives. In particular, zinc dialkyldithiophosphate (ZDDP) plays a key role, as its tribofilm is essential for the formation of an effective MoS₂ tribofilm. For lithium grease, the only way to achieve friction reduction using MoS₂ is to incorporate it directly as an additive, i.e. , ex tribo origin. However, with polymer-thickened grease, much lower traction levels can be attained through in tribo MoS₂ generation. A further reduction in the traction coefficient can be achieved when both additivation strategies are used together, i.e. , synergy between ex tribo and in tribo . The extent of these gains depends on the thickener system, making a combined strategy almost unnecessary for the polymer-thickened system. Overall, the study emphasizes the need to optimize additivation strategies for each system.