Behaviour of boundary lubricating additives on DLC coatings

The friction properties of a range of boundary lubricating additives in diamond-like carbon (DLC)–DLC rolling–sliding contact have been investigated. Two types of commercial DLC have been studied, one hydrogenated diamond-like and the other Cr-doped, non-hydrogenated and graphitic. The graphitic-type DLC coating initially gave very low boundary friction with additive-free base oil but after a few minutes of rubbing in thin film conditions friction rose to a higher value, similar to that produced immediately by the diamond-like carbon. Some organic friction modifiers were able to preserve this low boundary friction behaviour of the graphitic-type DLC for a full 2-h rubbing test. One organic friction modifier, glycerol monooleate produced a considerable reduction in friction with both types of DLC coating in intermediate speed, though noting very slow speed conditions. The additive molybdenum dithiocarbamate formed a low friction film on both DLC surfaces, with lower friction on the graphitic than on diamond-like carbon one. Atomic force microscopy (AFM) measurements showed that this was due to the formation of regions of low lateral force on asperities within the rubbed track. ZDDP provided some reduction in friction on both DLC surfaces after rubbing but did not form thick ZDDP tribofilms like those generated on steel surfaces.