Tribological performance and tribochemical processes in a DLC/steel system when lubricated in a fully formulated oil and base oil

Abstract

Abstract:Diamond-like carbon (DLC) coatings show extremely good promise for a number of applications in automotive components as they exhibit excellent tribological properties such as low friction and good wear resistance. This can impact on improved fuel economy and durability of the engine components. Much work has been reported on the dry sliding of DLC coatings with less so in lubricated contacts and, as such, there is a need to further understand the tribochemistry of lubricated DLC contacts. Commercially-available oils are normally optimized to work on ferrous surfaces. Previous studies on DLC lubricated contacts have tended to use model oil systems rather than fully formulated lubricants and from this an interesting picture of lubrication mechanisms is emerging. Optimising compatibility between a surface and a set of lubricant additives may lead to excellent durability (wear) as well as increased fuel economy (low friction). In this work, the friction and wear properties of a DLC coating under boundary lubrication conditions have been investigated and the tribological performance compared with that of an uncoated steel system. A pin-on-plate tribotester was used to run the experiments using High speed steel (HSS) M2 grade plates coated with 15 at.% hydrogenated DLC (a-C:15H) sliding against cast iron pins. A Group III mineral base oil, fully synthetic Group IV PAO and four different fully formulated oils were used in this study. Furthermore optical and scanning electron microscopes (SEM) were used to observe the wear scar and to assess the durability of the coatings. Energy-Dispersive X-ray analysis (EDX), X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy analyses were performed on the tribofilms to understand the tribochemical interactions between oil additives and the a-C:15H coating.This study show that the durability of the a-C:15H coating strongly depends on the selected additive package in the oils. In addition the effect of detergent, dispersant and antioxidants on the performance of the Molybdenum-based friction modifier (Mo-FM) and ZDDP anti-wear additive were investigated and results are reported in this paper.

Interaction of DLC and B4C coatings with fully formulated oils in boundary lubrication conditions

“…Equey et al [19] reported that the formation of ZDDP derived tribofilm was observed on the DLC coatings using SEM and the thickness of it was around 100 nm according to AFM analysis. Recently XPS analysis showed the formation of ZnS, ZnO and Znphosphates on the coating surface lubricated with fully formulated oils containing ZDDP [20]. This study also reported the presence of Ca and N elements, which are derived from detergent and dispersant in the oil respectively, on the coated contact surface.…”

Reducing Friction and Wear of Tribological Systems through Hybrid Tribofilm Consisting of Coating and Lubricants

“…When compared with [42], tribofilm seems to be effective in reducing the wear rate. However, a tribofilm in [23,30] did not reduce the wear rate. This tribofilm was made from ZnDTP on a DLC surface.…”

A Semi-deterministic Wear Model Considering the Effect of Zinc Dialkyl Dithiophosphate Tribofilm

“…Antiwear additives are generally known to form protective tribofilms on the surface of contacting bodies. The most common such additive is zinc dialkyl dithiophosphate (ZDDP) [16], which can form tribofilms on various materials including steel, diamond-like carbon (DLC) and Si/Al alloys [17][18][19][20][21][22][23][24][25][26][27]. Though extensively studied, the specific processes involved in formation and removal of tribofilms in these different systems are not yet fully understood.…”