Scaling up to macroscale superlubricity of sp2-dominated structural carbon films: Graphene and carbon onion

Sp2-dominated structural carbon films exhibit macroscale superlubricity. Nevertheless, the mechanisms governing the superlubricity are still not well comprehended, mainly due to the lack of direct link between graphene (or carbon onion) and superlubricity scale-up. Here, we establish the link by the combination of atomic force microscopy (AFM), transmission electron microscopy (TEM) and Raman analyses of the frictional surfaces of sp2dominated structural carbon films. The friction-induced graphene and carbon onion products locate strictly at the wear track center and take on flake-like and nanoparticle-like morphologies, respectively. Then they eliminate the cross-interface carbon-carbon interactions and reduce contact area, thereby promoting the achievement of macroscale superlubricity. Atomistic simulations reveal the key role of the curvature and cross-linking degree of graphene basal planes in the sp2-dominated carbon films on their frictional products. The results answer the question how the nanoscale tribology of graphene and carbon onion can be scaled up to macroscale superlubricity.