Behaviour of Boundary Lubricating Additives on DLC Coatings

The friction properties of a range of boundary lubricating additives in DLC-DLC sliding-rolling, lubricated contact have been investigated. Two types of DLC have been studied, one hydrogenated diamond-like and the other a non-hydrogenated graphitic type. The graphitic-type DLC coating gave initially very low boundary friction with additive-free base oil but after a few minutes of rubbing in thin film conditions this rose to the higher value produced immediately by the diamond-like DLC. With the graphitic-type DLC, ZDDP formed a thin patchy film while MoDTC produced a marked reduction of friction. Lateral force microscopy showed that the latter resulted from the formation of low friction material on the asperity peaks. Some organic friction modifiers preserved the low friction of g-DLC during prolonged rubbing. MoDTC and ZDDP were also effective in reducing friction on diamond-like DLC.