Friction reduction mechanisms in boundary lubricated W-doped DLC coatings

Abstract

The applications of commercially available Diamond Like Carbon (DLC) coatings have seen a rapid increase whilst a lot of tribochemistry problems are not fully understood. This study looks into a tungsten doped hydrogenated DLC coating reciprocating against cast iron in model oils. Friction and wear of such system are then compared with steel. Raman spectroscopy is used extensively to understand the coating interactions with the lubricant/additives. Our results have demonstrated the way to characterize carbon coating structure in lubrication. It clarified the chemical decomposition of MoDTC to MoS 2 is the dominant process rather than the possible formation of WS 2 in such system. It also enbles application of in-situ tribochemical aspects using Raman spectroscopy in ambient conditions.

Effect of Lubricant Additives on the WDLC Coating Structure When Tested in Boundary Lubrication Regime

“…In other words, the carbon content at a particular spot size has more sp 2 content. This is in an agreement with other reported literature about the change in carbon structure [9,12,14], i.e., coating re-hybridization. However, the frictional performance cannot be simply linked to the change between post-test I D /I G ratio and the ratio without lubrication (original coating).…”

“…This is a phenomena referred to as superlubricity. However, a study by the authors previously using one GMO solution (0.2 wt%) in WDLC coating has shown removal of such DLC coating [14]. In the current study, the lowest friction observed at the WDLC/CI interface is 0.06 (GMO, 0.5 wt%).…”

“…However, tests in the single additive MoDTC system carried out in another study, resulted in the coating being worn out completely after very short periods [14]. The lubricant additive interactions on WDLC-lubricated systems have shown great importance.…”

“…Raman spectroscopy analyzes across a micrometre scale domain in depth from the top surface. Spectra are collected using 10% of the total laser power as is discussed in a previous study [14].…”

“…[1,2,[6][7][8][9][10][12][13][14]. Most of them suggest that the mechanisms are either by interactions with the carbon coating surfaces (GMO for example) [9,10,15], or chemical decomposition (MoDTC and ZDDP for example) [2,14], or formation of tribological films that claimed to have some superior structure or properties [7,13].…”