The Friction Reducing Properties of Molybdenum Dialkyldithiocarbamate Additives: Part I – Factors Influencing Friction Reduction

Engine efficiency has become an increasingly important issue in automobile development. Since engine fuel efficiency can be improved by the use of appropriately-designed oils, engine lubricants are now required to demonstrate fuel efficiency in standardized engine tests. Introduction of these tests has led to a strong interest in understanding the influence of the composition of the lubricant on friction. There are two main and complementary approaches to reducing friction via engine oil design. One is to employ low viscosity oils, which reduce losses in pumping and under hydrodynamic lubrication conditions. The second is to reduce friction in the boundary lubrication regime by the incorporation of appropriate friction reducing additives. A very important class of friction reducing additives is the oil soluble molybdenum-containing compounds such as molybdenum dialkyldithiocarbamates (MoDTCs). Previous work has shown that this type of additive is able to reduce friction coefticients in thin film, boundary lubrication conditions to very low values, typically 0.06 to 0.075. The aim of the work described in this paper was to explore the conditions under which MoDTCs produce low friction. Attention was focussed on simple MoDTC solutions in base oil in the absence of other additives. The friction response of selected MoDTCs was examined over a range of conditions of temperature, concentration and sliding configuration. Friction measurements were correlated with the formation of the reaction species MoS2 using surface Raman spectroscopy and atomic force microscopy. This paper examines the behavior of fresh, undegraded molybdenum additive solutions. The degradation of MoDTC and the impact of this on durability of performance are investigated in a companion paper (Graham, et al. (2001)).