Tribochemical mechanism of superlubricity in graphene quantum dots modified DLC films under high contact pressure

In this work, we designed three series of tribo-couples based on amorphous carbon films including GLC, DLC and PLC that were modified by graphene quantum dots (GQDs). The tribo-testing environment was controlled at harsh conditions (like heavy load and high speed) in dry nitrogen atmosphere using bare and film-coated bearing steel balls as counterbodies, respectively. Through the tribochemical interactions, the self-mated DLC system obtained a surperlubricity state (μ = 0.01). During the whole sliding, the contact surface of the upper counterfacing ball was covered by 2D-layered carbon and graphitic lubricants induced via structural transformation of GQDs. Meanwhile, the tribofilm of the disc wear track was composed of a silica-like SiOx boundary layer and a multicomponent mixed-layer induced by tribochemistry. Compared to the self-mated DLC system, the structural boundary enriched with SiOx compounds was not formed at the bottom region of the tribofilm for the bare steel system; meanwhile, the disc wear track was covered by a thicker tribofilm containing plenty of degraded GQDs. This inferred the fact that the formation of a nanostructured sliding interface was the key to realize superlubricity. These discoveries successfully afforded a lubrication mechanism of GQDs for solid lubricant in applications of engineering and industry. [Display omitted] •Graphene-quantum-dots lubricants with effective friction-reducing effects are achieved.•The lubricity depends strongly on the tribo-couple materials and surface strucutre.•The nanostructured tribofilms formed on the contacts govern the superlubricity mechanisms.