Film Forming and Friction Properties of Sulphur-Free Antiwear Additives Zinc Dialkylphosphates (ZDPs)

Abstract

ZDPs (zinc dialkylphosphates) are sulphur-free analogues of the conventional antiwear additives ZDDPs (zinc dialkyldithiophosphates). Recently, the application of ZDPs in practical use has been investigated as an alternative of ZDDPs. The key benefit of ZDPs is that they do not contain sulphur which may damage some catalysts in after-treatment exhaust systems. Utilizing ZDPs instead of ZDDPs contributes to decrease in sulphur content in engine oils. In addition, ZDP-formulated oils show better base number retention than ZDDPs, which could contribute to prolongation of engine oil life. This is because ZDPs do not generate sulphuric acid in their decomposition process, while ZDDPs generate sulphuric acid which consumes detergents. However there is still little known about the mechanism of action of ZDPs as antiwear additives. Therefore, the purpose of this study is to clarify how ZDP antiwear additives work by investigating the film forming and friction properties of ZDPs. The main technique employed was MTM-SLIM (mini traction machine - spacer layer interferometry). This machine can measure friction properties under various rubbing conditions and monitor film formation by additives during rubbing tests. This study has shown that ZDPs form stable reaction films by rubbing, which protect metal surfaces from wear. This is fundamentally similar to the mechanism for antiwear performance of ZDDPs. However, some differences between ZDPs and ZDDPs are seen in their filmforming properties, which may affect their antiwear performances. For example, the filmforming rates of ZDPs are slower than those of ZDDPs at the initial stage. Stabilized films formed by ZDPs are thinner than films formed by ZDDPs. By contrast, ZDPs show similar friction properties to ZDDPs. In the boundary and mixed lubrication regions, there is no noticeable difference in friction between stabilized films formed by ZDPs and ZDDPs. In addition, these properties are largely affected by various factors such as rubbing conditions, concentrations, and alkyl structures of the additives. For example, ZDPs having linear alkyl chains show slower film formation and lower boundary friction than ZDPs having branched alkyl chains. Moreover, other additives such as dispersants and detergents largely affect the properties of ZDPs. Generally, highly polar additives appear to retard the film formation of ZDPs. The mechanisms for the above features are also discussed in this thesis.