Superlubricity mechanism of diamond-like carbon with glycerol. Coupling of experimental and simulation studies

Abstract

Abstract. We report a unique tribological system that produces superlubricity under boundary lubrication conditions with extremely little wear. This system is a thin coating of hydrogen-free amorphous Diamond-Like-Carbon (denoted as ta-C) at 353 K in a ta-C/ta-C friction pair lubricated with pure glycerol. To understand the mechanism of friction vanishing we performed ToF-SIMS experiments using deuterated glycerol and 13 C glycerol. This was complemented by first-principlesbased computer simulations using the ReaxFF reactive force field to create an atomistic model of ta-C. These simulations show that DLC with the experimental density of 3.24 g/cc leads to an atomistic structure consisting of a 3D percolating network of tetrahedral (sp 3 ) carbons accounting for 71.5% of the total, in excellent agreement with the 70% deduced from our Auger spectroscopy and XANES experiments. The simulations show that the remaining carbons (with sp 2 and sp 1 character) attach in short chains of length 1 to 7. In sliding simulations including glycerol molecules, the surface atoms react readily to form a very smooth carbon surface containing OH-terminated groups. This agrees with our SIMS experiments. The simulations find that the OH atoms are mostly bound to surface sp 1 atoms leading to very flexible elastic response to sliding. Both simulations and experiments suggest that the origin of the superlubricity arises from the formation of this OH-terminated surface.

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).…”

“…The mechanism needs to be clarified and compared. In the literature, the coatings used for the superlubricity studies were mostly hydrogen free, non-doped DLC coatings against steel/DLC counter parts [9,10]. The widely reported literature on WDLC coatings normally reports moderate friction with sometimes high wear depending on selected additive systems [1,7,11].…”

“…This coating is commercially-available and has been extensively investigated. However, reported literature showed no superior performance over other metal-doped or non-doped DLCs [1,6,7,[9][10][11]. There are a number of possible reasons, i.e., different coating composition, hydrogenation, tribological and processing conditions, and the various additive packages used [1,7,11].…”

“…[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].…”

“…However, the WDLC coating has not been clarified yet with those mechanisms. The change in the coating structure in different lubricants has been studied in the last 20 years with the aim being to clarify the potential for graphitization and the possible relationship with friction, wear and durability of DLC coatings [9,12,17]. The relationship between mechanical properties of the coating interface and different additives is still not yet clear although recent work has started to deal with aspects of this [8].…”