The effect of organic friction modifiers on ZDDP tribofilm

Abstract

In recent years a major concern in lubricant formulation has become the design of lubricants that can deliver very low friction and thus reduce energy consumption. This can be achieved in two main ways, by lowering the effective viscosity of lubricants and thus reducing hydrodynamic friction, and by reducing boundary friction through the use of friction modifier additives (FMs). Both approaches carry risk. Reducing lubricant viscosity can result in thinner lubricant films and this can lead to excessive wear, while some FM additives may inhibit the action of other lubricant additives present in a formulation. The aim of this thesis was to investigate the behaviour of one type of organic friction modifier, the Ethomeens, in rubbing tribological contacts, with respect to their effect on the widely-used antiwear additive zinc dialkyldithiophosphate (ZDDP). Of particular interest was to determine the extent to which Ethomeens reduce friction with ZDDP and to see whether they damage the protective antiwear films formed by ZDDPs. The tribofilm-forming ability and friction properties of a range of different types of ZDDP was studied. As in previous work, it was found that most ZDDPs rapidly form thick tribofilms with a pad-like structure on rubbed surfaces and these significantly increase friction in mixed/boundary lubrication. A new class of ZDDP was identified based on a predominantly tertiary alkyl structure. This forms tribofilms more rapidly that other ZDDPs but its films appear to be unstable and are lost during prolonged rubbing. Parallel study of the topography of ZDDP tribofilms using three different methods showed that ZDDP tribofilm topography is most reliably characterised using atomic force microscopy (AFM). 2 Transmission electron microscopy (TEM) and X-ray near edge spectroscopy (XANES) were used to characterise ZDDP tribofilms. It was found that care must be taken in preparation of samples for TEM analysis since some commonly-used methods cause considerable damage to the films under study. Using TEM, the ZDDP film studied was found to be crystalline and, using XANES, to be composed primarily of zinc orthophosphate. The immersion of pre-formed ZDDP tribofilms in Ethomeen solutions produced an immediate reduction in boundary friction and also some loss of the ZDDP film. The extent of both of these effects was found to be strongly dependent on the molecular structure of the Ethomeen and in particular its number of ethoxy groups. Ethomeens with just two ethoxy groups gave a lower reduction in friction but more removal of the ZDDP tribofilm that those with 15 ethoxy groups. This is believed to result from the easier access of the Ethomeen with fewer ethoxy groups to the ZDDP tribofilm surface. Overall the study illustrates the need to balance carefully the competing roles of different lubricant additives in the design of low friction lubricants