Behaviour of lubricant additives on DLC coatings

Abstract

Non-ferrous surfaces such as diamond-like carbon (DLC) coatings are becoming potential candidates for automotive engine parts because of fuel economy gains that these surfaces offer by operating with very low friction. In recent years, a wide range of DLC coatings have been developed and it is important to understand their film-forming, friction reduction and wear resistance mechanisms under lubricated conditions. This aim of the work described in this thesis is to improve our understanding of the tribological behaviour of DLC coatings with different engine oil additives. The main focus of the thesis is to study a wide range of available DLC coating types with currently available and widely-used additives such as ZDDP, friction modifiers, MoDTC etc., in order to establish general rules of their tribological behaviour that will help lubricant manufacturers produce new oil formulations. The research shows that tribofilms are formed on all DLCs by most of the currently used additives and that the film thickness depends on various factors such as type of DLC coating, doping elements present in the coatings, concentration of hydrogen and tungsten present in the coatings and the counterpart. Hydrogen-free coatings (a-C and ta-C) give lower boundary friction compared to the other coatings whereas hydrogenated amorphous carbon (a-C:H) coatings give better wear resistance properties. Study of a-C:H:W coatings shows that the concentration of tungsten present in the coatings has a significant influence on wear resistance properties but negligible influence on the friction properties when additives are present. The steel/steel couple is known to form a thick ZDDP tribofilm. If one of the contact surfaces is coated with DLC, the tribofilm forming properties on the steel vary and, for some cases, the low boundary friction properties of DLCs are degraded.