The effect of MoDTC-type friction modifier on the wear performance of a hydrogenated DLC coating

Abstract

Abstract: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 the engine components in contact.The mechanisms by which a non-ferrous material interacts with a variety of lubricant additives is becoming better understood as the research effort in this area increases 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.A pin-on-plate tribotester was used to run the experiments using HSS steel plates coated with 15 at.% hydrogenated DLC (a-C:15H) sliding against cast iron pins. One type of fully formulated oil with and without ZDDP and two levels of a MoDTC type friction modifier (Mo-FM) was used in this study. The friction and wear response of the fully formulated oils is discussed in detail.Furthermore, optical and scanning electron microscopes (SEM) were used to observe the wear scar and obtain wear mechanisms. Energy-Dispersive X-ray analysis (EDX) and X-ray Photoelectron Spectroscopy (XPS) analysis were performed on the tribofilms to understand the tribochemical interactions between oil additives and the DLC coating. A nano-indentation study was 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 the light of the physical observations and tribochemical analysis of the wear scar, the wear behaviour of a hydrogenated DLC (a-C:15H) coating was found to depend on the concentration of the MoDTC friction modifier and the wear performance is much better when ZDDP is present in the oil. The tribochemical mechanisms, which contribute to this behaviour, are discussed in this paper.

Wear Mechanisms of Hydrogenated DLC in Oils Containing MoDTC

“…Recently, the effect of MoDTC in increasing wear of the DLC coating in a DLC/steel contact has been reported by the current authors and others [4][5][6][7][8][9][10][11][12][13]. Different mechanisms by which lubrication by MoDTC leads to high wear to tribological systems including DLC have been proposed.…”

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

“…MoO 3 makes the DLC coating brittle. Finally, high wear is caused by the brittle surface [42]. Table 3 shows tribofilm formation on DLC coatings with GMO.…”

“…Tribofilm seems to be effective in reducing both friction and wear in [20][21][22]29,36,37,48]. 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.…”

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.…”