Film formation by colloidal overbased detergents in lubricated contacts

It has been reported in the literature that overbased deter-gents can posse ss good antifriction and antiwear properties although the origins of these properties are not fully clear In practice, overbased detergents are colloi dal dispersions and this may be important in determining their properties a nd mechanism of action. In the current study the lubricating properties of commercial, overbased ma gnesium and calcium sulfonates were measured in thin film, lubricated condi tions and compared to a neutral sulfonate additive. A range of techniques w as employed to evaluate the tribological performance of solutions of these additives. Film thickness measurements were carried out using optical inter ferometry and in-contact visualization, while friction and wear measurement s complemented the study. It has been found that, when operating in thin film conditions, overbased d etergents 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 t hickness corresponding to one colloid particle diameter between 10 and 20 n m. After prolonged rubbing, however the film thickness reaches the equivale nt of three particle diameters. No such thick boundary films are observed w ith the neutral sulfonate. The boundary films formed by overbased detergents produce a significant red uction in wear: However for the very smooth surfaces used in this study, th ey also result in an effective roughening of the very smooth surfaces studi ed. This leads to an increase in friction in the intermediate speed region by promoting solid-solid contact in thin fluid film conditions.