Wear Mechanisms of Hydrogenated DLC in Oils Containing MoDTC

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.

Effect of ZnDTP triboflim's morphology on friction behaviors of DLC coatings: Tribofilm characterization by 3D scanning electron microscope observation

“…Diamond-like carbon (DLC) coatings have been extensively researched in recent years to further our understanding of their properties and increase the efficiency in their use to reduce energy toward a sustainable society. Their properties (such as high hardness, low friction coefficient, wear resistance and chemical inertness (Clausing et al, 2012;Erdemir, 2006;Robertson, 1992;Tasdemir, 2013)) or their tribological behaviors with different oil additives (Costa et al, 2011;Kosarieh et al, 2016;Renman, 2012) have been investigated over the years. Among additives for lubricating oils being used since 1920s, some are chemical compounds known to be able to absorb and/or react with the sliding surfaces to produce low-shear-strength tribochemical reaction layers that prevent direct contact between the sliding surfaces and are often used to reduce wear in the boundary-lubricated regime.…”

MoDTC lubrication of DLC-involving contacts. Impact of MoDTC degradation

“…In the past decade, Diamond-Like Carbon (DLC) coatings have been applied for automotive applications as surface protective layers due to the outstanding mechanical and tribological properties, such as high hardness, low friction and superior wear resistance [3,4]. Recent studies have shown that, despite the friction reduction obtained, the interaction between MoDTC and DLC coating often leads to a high wear rate [5,6,10]. Kosarieh et al [10], for example, correlate the high wear of DLC to the molybdenum-containing products decomposed from MoDTC.…”

Tribological properties of Ti‐doped diamond‐like carbon coatings under dry friction and PAO oil lubrication

“…The wear rates were calculated using Archard wear equation where k is the wear rate (m 3 N −1 m −1 ), V is the wear volume (m 3 ), F is the normal load (N), and s is the total sliding distance.…”

“…The wear rates were calculated using Archard wear equation. 22,23 k ¼ V= Fs (5) where k is the wear rate (m 3…”