Diamond-like carbon coating under oleic acid lubrication: Evidence for graphene oxide formation in superlow friction

Abstract

The achievement of the superlubricity regime, with a friction coefficient below 0.01, is the Holy Grail of many tribological applications, with the potential to have a remarkable impact on economic and environmental issues. Based on a combined high-resolution photoemission and soft X-ray absorption study, we report that superlubricity can be realized for engineering applications in bearing steel coated with ultra-smooth tetrahedral amorphous carbon (ta-C) under oleic acid lubrication. The results show that tribochemical reactions promoted by the oil lubrication generate strong structural changes in the carbon hybridization of the ta-C hydrogen-free carbon, with initially high sp3 content. Interestingly, the macroscopic superlow friction regime of moving mechanical assemblies coated with ta-C can be attributed to a few partially oxidized graphene-like sheets, with a thickness of not more than 1 nm, formed at the surface inside the wear scar. The sp2 planar carbon and oxygen-derived species are the hallmark of these mesoscopic surface structures created on top of colliding asperities as a result of the tribochemical reactions induced by the oleic acid lubrication. Atomistic simulations elucidate the tribo-formation of such graphene-like structures, providing the link between the overall atomistic mechanism and the macroscopic experimental observations of green superlubricity in the investigated ta-C/oleic acid tribological systems.

Mechano-chemical decomposition of organic friction modifiers with multiple reactive centres induces superlubricity of ta-C

“…Accompanying atomistic simulations suggested that H/OH groups originating from glycerol can passivate the ta-C surfaces. These observations indicate that a complete decomposition of the lubricant allows passivation layers to form on top of ta-C, which is a necessary precondition for its superlubricity and nearly wearless sliding 6,7 . Despite these important investigations into the structural details of the lubrication mechanisms, the tribochemical processes underlying the lubricant’s fragmentation and the formation of passivating functional groups on ta-C remain elusive.…”

“…In particular, hydrogen-free DLC (tetrahedral amorphous carbon, ta-C) has attracted significant attention in the automotive industry 4 . In boundary lubrication experiments, ta-C coatings exhibited superior tribological performance compared to hydrogenated amorphous carbon (a-C:H) 5–7 . A seminal study by De Barros Bouchet and coworkers 5 reported pin-on-disc reciprocating sliding tests of steel/steel, a:C-H/a:C-H, and ta-C/ta-C pairs boundary-lubricated by a mixture of polyalpha-olefin base oil and glycerol-monooleate.…”

“…This study sparked interest into the lubrication of ta-C coatings 6–8 with organic friction modifiers 9 . While boundary lubrication by fatty acids tends to form self-assembled monolayers (SAMs) on a-C:H surfaces 10 and cause ultralow friction ( μ ≈ 0.06), experiments with an unsaturated fatty acid (oleic acid) between ta-C surfaces yielded ultralow friction ( μ ≈ 0.03–0.07) under boundary lubrication and even superlow friction ( μ ≈ 0.005) under mixed lubrication conditions 6,7 . Spectroscopic experiments provided evidence for fragmentation of the oleic acid, passivation of ta-C with hydrogen and hydroxyl groups 6 , and additional formation of monolayer-thin aromatic carbon structures 7 .…”

Ultra-Low Friction on Tetrahedral Amorphous Diamond-Like Carbon (ta-C) Lubricated with Ethylene Glycol

“…Surface chemical analysis using (time-of-flight) secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) measurements helps to elucidate the tribochemistry of DLC coatings. For example, the creation of graphite-like patches on the coatings could be established for taC/taC tribocontacts by means of XPS [10]. SIMS allowed for the detection of lubricant-fragments [11][12][13][14][15][16][17][18], although the detected fragments have not yet been correlated with the different friction levels.…”

Superlubricity of glycerol by self-sustained chemical polishing

“…They reported observing superlubricity when tetrahedral amorphous carbon (ta-C) was lubricated with an oleic acid. The formation of graphene oxide in the wear track was found to be responsible for superlubricity 12 . Additionally, they attributed abnormally low friction of the steel/steel tribo-pairs lubricated by pure glycerol to the formation of a “H-bond network” and to a water-like lubrication mechanism 13 .…”