Superlow friction and wear enabled by nanodiamond and hexagonal boron nitride on a-C:H films surfaces in dry nitrogen

Hydrogen-deficient amorphous carbon (a-C:H) is subjected to abnormally high friction in an inert environment, and the possibility of maintaining ultra-low friction is still a great challenge. Here, nanodiamond and hexagonal boron nitride are used to improve the tribological properties of a-C:H films. Particularly, an excellent synergistic lubrication effect was obtained after surface treatment using a solution droplet of nanodiamond + h-BN mixture with a mass ratio of 1:1 and a concentration of 0.1 mg/mL, and concurrently vacuum heated at 80 °C. The COF also decreased sharply to be below 0.001, about two orders of magnitude lower than the case without surface treatment. Correspondingly, the wear rate of steel ball was also decreased by ∼97%. The dominant factor to obtain ultra-low friction is the incommensurate contact between the tribolayer formed by nanodiamond + hexagonal boron nitride and the hydrogen passivation layer. Meanwhile, the incommensurate contact of mismatched h-BN nanoflakes promotes the formation of ultra-low friction. Besides, low shear slip at the interface formed by the in-situ growth of h-BN layered shear band and ND + h-BN nano-composited shear band, respectively, rubbing against the upper ND + h-BN mixed tribolayer also makes a contribution. The ultra-low friction is achieved under the joint action of these factors. These findings can enrich the understanding of the surface modification of a-C:H films by low dimensional nano-lubricants. It is helpful to develop solid carbon films with more adaptability and robustness under extreme working conditions and provide important scientific and technical guidance.