Tribochemistry of boundary lubricated DLC/steel interfaces and their influence in tribological performance

The application of Diamond-Like Carbon (DLC) coatings for automotive components is becoming a promising strategy to cope with the new challenges faced by automotive industries. DLC coatings simultaneously provide low friction and excellent wear resistance which could potentially improve fuel economy and durability of engine components in contact. The mechanisms by which a non-ferrous material interacts with a variety of lubricant additives is becoming better understood as research efforts in this area increase. However there are still significant gaps in the understanding. A better understanding of DLC wear may lead to lubricant additive solutions being tailored for DLC surfaces to provide excellent durability (wear) as well as similar or increased fuel economy (low friction). In this work, the wear and friction properties of DLC coating under boundary lubrication conditions have been investigated.
In this study, tribological performance of DLC coatings was evaluated using a pin-on-plate tribometer. The experiments were conducted using (High Speed Steel) HSS plates coated with 15 at.% hydrogenated DLC (a-C:15H) sliding against cast iron pins. Oils with different formulations were used in this study and the friction and wear response of the fully formulated oils is discussed in detail.
Using different surface analysis techniques such as optical and scanning electron microscopes (SEM), Energy-Dispersive X-ray analysis (EDX), X-ray Photoelectron Spectroscopy (XPS) analysis, Focused Ion Beam (FIB) and Transmission Electron Microscopy (TEM) were performed on the surfaces to understand physical characterization and the tribochemical interactions between oil additives and the DLC coating. A nano-indentation study was also conducted to observe the changes in the structure of the coating, which can provide a better insight into the wear mode and failure mechanism of such hard coatings.
In light of the physical observations and tribochemical analysis of the wear scar, the tribological performance of a hydrogenated DLC (a-C:15H) coating was found to depend on the oil formulation. The level of Molybdenum Dialkyl Dithiocarbamate (MoDTC) friction modifier (Mo-FM) blended in the oils greatly influenced the friction and the wear performance of the DLC coatings. High concentration of Mo-FM resulted in lower friction but higher wear of a-C:15H DLC coating. However, the addition of Zinc dialkyldithiophosphate (ZDDP) to the oils showed a positive effect in mitigating such high wear. The tribochemical mechanisms, which contribute to this behaviour, are discussed in detail.