Fate of methanol molecule sandwiched between hydrogen-terminated diamond-like carbon films by tribochemical reactions: tight-binding quantum chemical molecular dynamics study

Abstract

Recently, much attention has been given to diamond-like carbon (DLC) as a solid-state lubricant, because it exhibits high resistance to wear, low friction and low abrasion. Experimentally it is reported that gas environments are very important for improving the tribological characteristics of DLC films. Recently one of the authors in the present paper, J.-M. Martin, experimentally observed that the low friction of DLC films is realized under alcohol environments. In the present paper, we aim to clarify the low-friction mechanism of the DLC films under methanol environments by using our tight-binding quantum chemical molecular dynamics method. We constructed the simulation model in which one methanol molecule is sandwiched between two hydrogen-terminated DLC films. Then, we performed sliding simulations of the DLC films. We observed the chemical reaction of the methanol molecule under sliding conditions. The methanol molecule decomposed and then OH-termination of the DLC was realized and the CH3 species was incorporated into the DLC film. We already reported that the OH-terminated DLC film is very effective to achieve good low-friction properties under high pressure conditions, compared to H-terminated DLC films. Here, we suggest that methanol environments are very effective to realize the OH-termination of DLC films which leads to the good low-friction properties.

Role of Water and Oxygen Molecules in the Lubricity of Carbon Nitride Coatings under a Nitrogen Atmosphere

“…Kubo et al used a tight-binding quantum chemical molecular dynamics simulation and found that hydrogen termination of DLC or CN X coatings and hydroxyl termination of DLC contributed to low friction [20][21][22][23]. They also reported that non-hydrogen-terminated DLC coatings display very high friction because of the formation of C-C bonds [22]. In this study, hydrogen and hydroxyl derived from [22,23].…”

“…When the water content of the nitrogen gas was high (> 1.0%RH, Mode II), friction increased because the structure of transferred carbon is not maintained and oxidation products formed on the surface of the Si 3 N 4 ball [14]. Kubo et al used a tight-binding quantum chemical molecular dynamics simulation and found that hydrogen termination of DLC or CN X coatings and hydroxyl termination of DLC contributed to low friction [20][21][22][23]. They also reported that non-hydrogen-terminated DLC coatings display very high friction because of the formation of C-C bonds [22].…”

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

“…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]. Some of the mechanisms suggested by experimental results have also found evidence in molecular dynamic simulations [10,13,16]. However, the WDLC coating has not been clarified yet with those mechanisms.…”

“…Some of the recent work on GMO with non-hydrogenated DLC coating has evidenced this hydrogen reduction observation via molecular dynamic modeling. [9,10,15,16]. Also the residual reduction in Oil 5 (GMO) does not only happen at the interface, but also on upper layers of the coating major structure (bulk), indicating a large scale upper layer structure change by GMO modification.…”

“…Reviewing literature of the DLC coating mechanism on GMO, the bond interaction of C-OH terminates and dehydrogenation process (formation of water) would be a possible reason for this coating with low friction [10,15,16]. However, the interaction and super lubricity of GMO on non-doped hydrogen free DLC coatings has only the very top interface dominantly terminated by the suggested mechanism [9,10].…”