Performance of Friction Modifiers on ZDDP-Generated Surfaces

Most modern engine oils contain a friction modifier additive to reduce boundary friction and thus contribute to improved fuel economy. Considerable research work has been carried out to measure the effectiveness of friction modifiers, both in base oil and in fully-formulated engine oils. Most of this work has studied the behavior of friction modifiers on rubbing ferrous surfaces. However engine oils generally also contain the antiwear additive zinc dialkyldithiophosphate (ZDDP) and it is well-known that this additive reacts rapidly with rubbed ferrous surfaces to form a quite thick, iron and zinc phosphate-based coating. Thus, to be effective, a friction modifier additive has to form a friction-reducing film not on a ferrous surface but on a zinc phosphate one. In the current study, the friction-reducing properties of friction modifiers on steel surfaces have been compared with their performance on surfaces having a ZDDP reaction film coating. It has been found that additives which are effective in reducing friction on steel surfaces are not necessarily effective in reducing friction on a phosphate coating and vice versa. The reasons for these observed differences are discussed.