Calcium Sulfonate Complex Grease as a Next-Generation Lubricant: Tribofilm Formation and Effect of Additives

Calcium sulfonate complex grease (CSCG) is a promising lithium-free alternative owing to its excellent corrosion resistance, loadcarrying capacity, and structural stability.However, tribofilm formation in CSCGs and the interfacial reactions between CSCG thickeners and additives remain insufficiently understood, particularly regarding their microstructural and crystallographic characteristics.In this study, the effects of zinc dialkyldithiophosphate and molybdenum dialkyldithiocarbamate on the tribological behavior and tribofilm formation of CSCG were investigated.Friction and wear tests were conducted using a reciprocating ball-on-disk tribometer, and nanoscale analyses were performed using transmission electron microscopy, selected area electron diffraction analysis, and spectroscopic techniques.The additive-free CSCG exhibited superior wear resistance compared to the CSCGs containing additives under the tested conditions.The tribofilm primarily consisted of nanocrystalline calcium carbonate with a calcite structure and formed a dense and stable protective layer.When additives were introduced, Mo-derived tribochemical species contributed to friction reduction, whereas wear resistance decreased, indicating that the additive reactions modified the crystallinity of the Ca-based tribofilm.These findings demonstrate the strong influence of tribofilm crystallinity on wear resistance.In particular, the intrinsic ability of the CSCG thickener to form a stable nanocrystalline Ca-based tribofilm was identified as the primary origin of its excellent wear resistance.