Origin of low friction in hydrogenated diamond-like carbon films due to graphene nanoscroll formation depending on sliding mode: Unidirection and reciprocation

• Published in: Carbon (New York) 2021-03, Vol.173, p.696-704
• Main Authors: Liu, Yunhai, Chen, Lei, Jiang, Bangzheng, Liu, Yangqin, Zhang, Bin, Xiao, Chen, Zhang, Junyan, Qian, Linmao
• Format: Article
• Language: English
• Subjects: Atomic beam spectroscopy; Atomic force microscopy; Carbon; Coefficient of friction; Diamond-like carbon films; Friction; Graphene; Graphitization; Heat transfer; Hydrogenated diamond-like carbon; Image enhancement; Nanoscrolls; Raman spectra; Raman spectroscopy; Reciprocation; Sliding contact; Sliding motion; Thin films; Transfer layer; Transmission electron microscopy; Worn surface graphitization
• ISSN: 0008-6223; 1873-3891
• DOI: 10.1016/j.carbon.2020.11.039

The origins of low friction of hydrogenated diamond-like carbon (H-DLC) films depending on unidirectional and reciprocating motions were detected by the analyses of transmission electron microscopy (TEM), Raman spectroscopy and atomic force microscopy (AFM). Under the comparable experimental conditions, the reciprocating sliding of H-DLC films against a steel ball in humid air enables the stable friction coefficient to lower more than 70% but the surface wear to increase 50% compared to the unidirectional sliding. The uniformity and structure of transfer layer and worn H-DLC surface strongly depend on sliding motions. Two-way movements in the reciprocating sliding help to keep the transfer layer within contact region, enhancing its graphitization. TEM image of transfer layer shows the formation of abundant graphene nanoscrolls in reciprocating sliding but only few graphite layers in unidirectional sliding. AFM results and Raman spectra suggest that not only the transfer layer but also the graphitization of the rough worn H-DLC surface induced by high alternating stress plays a significant role in the low friction under reciprocating motion. Combining the atomic-scale, nanoscale and macroscale results, a comprehensive model is proposed for the statement of low friction sliding contact of H-DLC films depending on the sliding motions. [Display omitted] •Reciprocating motion enables lower COF but larger wear than unidirectional motion.•Abundant nanoscrolls and graphene in transfer layer formed in reciprocating motion.•Graphitization of worn H-DLC surface contributes to low COF in reciprocating motion.•Two-way movements in reciprocating motion confine transfer layer in contact region.•Alternating stress in reciprocating motion leads to a rougher worn H-DLC surface.