Synergistic effect of graphene and nanodiamonds to achieve ultra-low friction on rough DLC coatings

Diamond-like carbon (DLC) with its protective, antiwear and self-lubricant properties represents an outstanding coating material to improve the tribological performance of bare metal-to-metal contacts. The DLC friction response is strictly related to the generation of a carbon-rich transfer layer on the sliding countersurfaces. This key process may benefit from the DLC surface functionalization with carbon nanomaterials, as attested by the recent observation of wearless sliding and ultra-low friction for nanorough model interfaces. In this study, we show by ball-on-disc tribometer that the tribological response of microrough steel-DLC contacts is considerably improved in dry atmosphere through DLC functionalization with graphene sheets (GSs) and nanodiamonds (NDs). The functionalization effectiveness is demonstrated exclusively with both allotropic forms, that synergically lower friction after a unique run-in period. We attribute the enhanced lubricity, with steady coefficient of friction <0.05 at 1 N load, to the formation of a transfer layer which incorporates GSs, NDs and nanoscroll structures self-assembled during sliding. High-resolution electron microscopy and Raman spectroscopy indicate that NDs, besides enabling the erosion of the highest interfacial micro-asperities, do assist the transfer layer development via GSs milling. Our findings contribute to the current quest for superlubricity in realistic tribocontacts meeting industrial standards.