Graphene nano scrolls responding to superlow friction of amorphous carbon

Amorphous carbon films are widely used as solid lubricant coating. However, the mechanism response for its superlow friction has not been well explored. The previous results indicated that graphitic tribofilms are formed at rubbing interfaces, which leading to decreasing of friction coefficient. But in some cases, the friction coefficients of graphite (0.1-0.6) are much high than the amorphous carbon films (0.05 0.01), where the graphitic theory is quite limited and the interpretation is poor. Using high resolution transmission electron microscopy and Raman spectra, we monitor the structure evolution of tribofilms and friction coefficient drop during running-in. We demonstrate that a kind of graphene nano scroll particle was developed in the tribofilms consisting of outer graphene shell and inner amorphous core. And the relationship between friction drop and graphene nano scroll evolution suggests that, incommensurate contact of such nano scrolls, may be the dominant dissipation modes for amorphous carbon. (C) 2017 Elsevier Ltd. All rights reserved.