Film formation by colloidal overbased detergents in lubricated contacts

It has been reported in the literature that overbased deter-gents can possess good antifriction and antiwear properties although the origins of these properties are not fully clear In practice, overbased detergents are colloidal dispersions and this may be important in determining their properties and mechanism of action. In the current study the lubricating properties of commercial, overbased magnesium and calcium sulfonates were measured in thin film, lubricated conditions and compared to a neutral sulfonate additive. A range of techniques was employed to evaluate the tribological performance of solutions of these additives. Film thickness measurements were carried out using optical interferometry and in-contact visualization, while friction and wear measurements complemented the study. It has been found that, when operating in thin film conditions, overbased detergents deposit solid-like boundary films on the rubbing surfaces. These films form in both rolling and mixed rolling/sliding conditions and, unlike many other colloidally-formed boundary films, are able to survive in high speed, thick plm conditions. During formation, the film rapidly reaches a thickness corresponding to one colloid particle diameter between 10 and 20 nm. After prolonged rubbing, however the film thickness reaches the equivalent of three particle diameters. No such thick boundary films are observed with the neutral sulfonate. The boundary films formed by overbased detergents produce a significant reduction in wear: However for the very smooth surfaces used in this study, they also result in an effective roughening of the very smooth surfaces studied. This leads to an increase in friction in the intermediate speed region by promoting solid-solid contact in thin fluid film conditions.