Abstract
Diamond-Like Carbon (DLC) coatings are well known for offering excellent tribological properties. They have been shown to offer low friction and outstanding wear performance in both dry and lubricated conditions. Application of these coatings for automotive components is considered as a promising strategy to cope with the emerging requirements regarding fuel economy and durability. Commercially available oils are generally optimised to work on conventional ferrous surfaces and are not necessarily effective in lubricating non-ferrous surfaces. Recently, the adverse effect of the Molybdenum DialkyldithioCarbamate (MoDTC) friction modifier additive on the wear performance of the hydrogenated DLC has been reported. However, the mechanisms by which MoDTC imposes this high wear to DLC are not yet well understood. A better understanding of DLC wear may potentially lead to better compatibility between DLC surfaces and current additive technology being achieved. In this work, the wear properties of DLC coatings in the DLC/cast iron (CI) system under boundary lubrication conditions have been investigated to try to understand what appears to be a tribocorrosion-type process. A pin-on-plate tribotester was used to run the experiments using High Speed Steel (HSS) plates coated with 15 at.% hydrogenated DLC (a-C:15H) sliding against CI pins or ceramic balls. The lubricants used in this study are typical examples of the same fully formulated oil with and without ZDDP. The friction and wear responses of the fully formulated oils are discussed in detail. Furthermore, Optical Microscopy (OM) and Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDX), Focused Ion Beam (FIB) and Transmission Electron Microscopy (TEM) were used to observe the wear scar and propose wear mechanisms. The X-ray Photoelectron Spectroscopy (XPS) analysis was performed on the tribofilms to understand the tribochemical interactions between oil additives and the DLC coating. Nano-indentation analysis was conducted to assess potential structural modifications of the DLC coating. Coating hardness data could provide a better insight into the wear mode and failure mechanism of such hard coatings. Given the obtained results, the wear behaviour of the hydrogenated DLC coating was found to depend not only on the presence of ZDDP in the oil formulation but also on the counterpart type. This study revealed that the steel counterpart is a critical component of the tribocouple leading to MoDTC-induced wear of the hydrogenated DLC.
Highlights
It has been recognized that coating the surface of automotive components with Diamond-Like Carbon (DLC) is a promising strategy to deal with the emerging challenges faced by automotive industries
Tests were conducted for different time intervals to quantify the evolution of tribochemical reactions occurring in the DLC/cast iron (CI) contacts lubricated in oils containing Molybdenum DialkyldithioCarbamate (MoDTC)
The X-ray Photoelectron Spectroscopy (XPS) elemental quantification of the tribofilms formed on DLC wear scars (Table 4) shows that additive-derived elements were present in the tribofilm irrespective of the counterpart type
Summary
It has been recognized that coating the surface of automotive components with Diamond-Like Carbon (DLC) is a promising strategy to deal with the emerging challenges faced by automotive industries. DLC coatings provide exceptional properties by offering low friction and by exhibiting excellent wear resistance. Current additive technology is generally optimised to work on ferrous surfaces, Tribol Lett (2016) 64:4 and in the current and near future market, it is unlikely that an optimised oil for DLC only will be brought to the market. A better understanding of the mechanism by which a non-ferrous materials interact with a variety of lubricant additives may potentially lead to lubricant additive solutions being tailored for DLC surfaces in the future. Molybdenum Dithiocarbamate (MoDTC) is a wellknown friction modifier, used for ferrous surfaces. Used lubricant additives are designed to form tribofilms on ferrous-based surfaces. It is, essential to optimize coating and lubricant compatibility to enable additive solutions to be tailored to DLC surfaces
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