Macroscale superlubricity breakdown of lubricating materials caused by substrate surface roughening and mechanochemical modification poses great challenges for their practical tribological applications. Here, a facile way is reported to access robust macroscale superlubricity in a vacuum environment, via the operando formation of graphene/transition‐metal dichalcogenide (TMDC) heterostructures at wear‐induced rough surfaces. By trapping active amorphous carbon (a‐C) wear products between TMDC flakes, the sandwich structures readily transform into graphene/TMDC heterostructures during running‐in stage, based on shear‐induced confinement and load‐driven graphitization effects. Then they assemble into multipoint flake‐like tribofilms to achieve macroscale superlubricity at steady stage by reducing contact area, eliminating strong cross‐interface carbon–carbon interactions and polishing a‐C rough nascent surface. Atomistic simulations reveal the preferential formation of graphene/TMDC heterostructures during running‐in stage and demonstrate the superlubric sliding of TMDCs on the graphene. The findings are of importance to achieve robust superlubricity and provide a good strategy for the synthesis of other van der Waals heterostructures.
The solid‐state conversion of amorphous carbon into graphene is extremely difficult, but it can be achieved in the friction experiments that induce macroscale superlubricity. However, the underlying conversion mechanisms remain elusive. Here, the friction experiments with Cu nanoparticles and (non‐hydrogen (H) or H) a‐C in vacuum, show the H‐induced conversion of mechanical to chemical wear, resulting in the a‐C's tribosoftening and nanofragmentating that produce hydrocarbon nanoclusters or molecules. It is such exactly hydrocarbon species that yield graphene at hydrogen‐rich a‐C friction interface, through reaction of them with Cu nanoparticles. In comparison, graphene isn't formed at Cu/non‐H a‐C friction interface. Atomistic simulations reveal the hydrogen‐enhanced tribochemical decomposition of a‐C and demonstrate the energetically favorable graphitization transformation of hydrocarbons on Cu substrates. The findings are of importance to achieve solid‐state transformation between different carbon allotropes and provide a good strategy to synthesize other graphitic encapsulated catalysts with doped elements.
Achieving macroscale superlubricity of van der Waals (vdW) nanopowders is particularly challenging, due to the difficulty in forming ordered junctions before friction and the friction‐induced complex contact restructuration among multiple nanometer‐sized junctions. Here, a facile way is reported to achieve vdW nanopowder‐to‐heterojunction conversion by graphene edge‐oxygen (GEO) incorporation. The GEO effectively weakens the out‐of‐plane edge–edge and in‐plane plane–edge states of the vdW nanopowder, leading to a coexistent structure of nanoscale homojunctions and heterojunctions on the grinding balls. When sliding on diamond‐like carbon surfaces, the ball‐supported structure governs macroscale superlubricity by heterojunction‐to‐homojunction transformation among the countless nanoscale junctions. Furthermore, the transformation guides the tunable design of superlubricity, achieving superlubricity (µ ≈ 0.005) at wide ranges of load, velocity, and temperature (−200 to 300 °C). Atomistic simulations reveal the GEO‐enhanced conversion of vdW nanopowder to heterojunctions and demonstrate the heterojunction‐to‐homojunction transformation superlubricity mechanism. The findings are of significance for the macroscopic scale‐up and engineering application of structural superlubricity.
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