Boundary Friction of ZDDP Tribofilms

The lubricant additive zinc dialkyldithiophosphate (ZDDP) has been used as an antiwear agent in engine oils for more than six decades. It limits wear by forming a protective, phosphate-based tribofilm on rubbing surfaces. In recent years its role as an antiwear additive in engine oils has become increasingly important because of the use of very low viscosity oils to improve fuel economy. Such low viscosities mean that engine components operate for longer periods in thin film boundary and mixed lubrication conditions where wear may occur. Unfortunately, as well as controlling wear, ZDDP is also believed to increase friction in both boundary and mixed lubrication conditions and consequently can be detrimental to fuel economy. It is therefore important to determine ways to reduce the friction of ZDDP tribofilms. This paper explores the boundary friction of ZDDP tribofilms and the influence of ZDDP molecular structure on this friction. The aim is to identify ZDDPs that are less harmful to fuel economy than those currently in use.