Comparison of Alcohol and Fatty Acid Adsorption on Hydrogenated DLC Coatings Studied by AFM and Tribological Tests

Abstract

Adsorption on hydrogenated diamond-like carbon (DLC) coatings with respect to boundary lubrication has been studied only very rarely, even though such adsorption has proved to be one of the main boundary-lubricating mechanisms for metal contacts. For this reason, we used atomic force microscopy (AFM) and tribotests to study the ability of hexadecanol and hexadecanoic acid to adsorb onto DLC and affect the tribological properties of DLC coatings, where steel was used as a reference. We have shown that alcohols and fatty acids can adsorb onto the DLC under static conditions. However, under dynamic tribocontact conditions, although alcohol and fatty acid molecules help to decrease the wear of the coatings, they are inefficient in reducing the friction. In all the experiments the fatty acid proved to have superior adsorption abilities compared to the corresponding alcohol. Based on our results and the existing literature, tentative adsorption mechanisms that include an environmental species effect, a temperature effect and a tribochemical effect are discussed for DLC surfaces.

Green Tribology for the Sustainable Engineering of the Future

“…This is in agreement with molecular-dynamics and quantum-chemistry investigations that showed very similar behavior [60]. A model for the interaction of simple alcohols and fatty acids was proposed, with a clear conclusion that fatty acids are more effective than alcohols [56], but friction can be clearly reduced with such mild, organic additives on DLC coatings [55] and [57].…”

Adsorption of alcohols and fatty acids onto hydrogenated (a-C:H) DLC coatings

“…Although both molecules enabled a similar decrease in the wear at high concentrations, the fatty acid proved to be more efficient at lower concentrations, which was assumed to be the effect of a better adsorption ability. In addition, we showed using AFM that the surface coverage with polar molecules increases with the concentration and is consistently larger in the case of the acid compared to the alcohol [36]. To further confirm these findings we performed neutron reflectometry analyses to determine the thickness and the density of the adsorbed layers on the a-C:H directly in the PAO oil.…”

“…We showed in our previous studies with tribotests that increasing the concentration of hexadecanol or hexadecanoic acid in PAO decreases the wear of the a-C:H coatings in boundary-lubricating conditions, Fig. 14 [36].…”

Tribological analyses of a new optimized gearbox biodegradable lubricant blended with reduced graphene oxide nanoparticles

“…The alkyl hydrophilic polar groups of r-GO assisted the molecule to keep it dispersed in oil, whereas the ester and triazole hydrophobic polar groups attached to the metal surfaces tribo-chemically (physisorption) reacting with them and thus forming a protective film on the metal surface. 79 Due to the unique geometry of the molecular structure of r-GO, they conduced to a high-surface film forming efficiency. 80 As it was interpreted from the results that a lower concentration of r-GO (0.1%) dispersed easily in oil and lubricated the surface although in this concentration, it was difficult to form a stable layer all over the contact area.…”