Tribochemical approach toward mechanism for synergism of lubricant additives on antiwear and friction reducing properties

Abstract

The synergism and antagonism of additives on tribological properties is an important issue for lubricant formulations. In this study, combination effects of anti-wear additives, detergents, and dispersants were evaluated by a block-on-ring type tribometer under the boundary lubrication conditions. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) were employed to study the chemical structure of the boundary film formed on the rubbing surface. The correlation between tribological properties and chemical composition on the worn surface was investigated. Zinc dialkyldithiophosphates (ZnDTP) slightly reduced wear under these conditions. An over-based calcium sulfonate (Ca-Sul) reduced wear and friction remarkably, whereas alkylsuccinimdes (Suc-I) promoted wear. Metal oxides derived from the corresponding additives were found on the wear track lubricated with ZnDTP or Ca-Sul. On the other hand, only iron oxides were detected on the surface lubricated with Suc-I. Combination of Suc-I with ZnDTP or Suc-I with Ca-Sul changes tribological properties and surface contents. Taking the results into account, a classification of additives as “boundary film precursor” or “film formation controller” was proposed.