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.

We reveal the generation of the "Graphene Arch-Bridge" on a diamond (111) surface by Si doping via first-principles calculations. The "Graphene Arch-Bridge" is different from a simple graphene structure because both its ends are pinned to the diamond surface, and it has an interesting arched-type curved structure. The large stress around the doped Si atom leads to the transition of the six-membered C ring to a five-membered C ring. The C atom excluded from the ring by this transition changes from an sp 3 carbon to an sp 2 carbon and generates the "Graphene Arch-Bridge" on the diamond (111) surface. These results suggest that the generation of the fivemembered C ring by stress due to the Si doping is the reason why the "Graphene Arch-Bridge" is generated. Finally, we propose that the "Graphene Arch-Bridge" is the origin of the experimentally observed super-low friction of Si-doped diamond-like carbon (DLC). Furthermore, we suggest that the "Graphene Arch-Bridge" leads to the lower wear properties of Si-doped DLC compared with nondoped DLC because its ends of the bridge are pinned to the DLC surface.

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