Elastic Lubricious Effect of Solidlike Boundary Films in Oil-Starvation Lubrication

• Published in: Journal of physical chemistry. C Vol. 123; no. 3; pp. 1677 - 1691
• Main Authors: Xu, Jiao, Nian, Jingyan, Wang, Peng, Guo, Zhiguang, Liu, Weimin
• Format: Journal Article
• Language: English
• Published: American Chemical Society 24.01.2019
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.8b07795

Herein, a boundary lubrication (BL) mechanism based on the carbon-based tribofilms formed via Ni­(Cu)-catalytic dehydrogenation of hydrocarbon oils was queried by performing standard ball-on-disk tribotests on magnetron-sputtered WN, Ni, and W–N–Ni coatings in the oil-starvation condition. The experimental results indicate that both the bare GCr15 steel substrate and the sputtered W–N–Ni/MoN–Cu coatings exhibited comparable fine antiwear performance with friction coefficients below 0.09; however, no clues regarding the formation of the effectiveness of the graphite-like tribofilms on the contact surfaces were found, no matter whether the tested coatings were composited in amorphous or nanocrystallized structures. An understanding of the antiwear mechanism was achieved by conducting a nonequilibrium molecular dynamics (NEMD) simulation of the confined shearing process, taking 1-decene chains as a model of the poly-α-olefin 10 base oil. It was found that as the normal load increases from 0.14 to 9.2 GPa, the “sandwiched” sheared oil molecules undergo a liquid-to-solidlike phase transition at a threshold pressure of approximately 0.54 GPa, above which the solidified oil film responds as an elastic protective layer between rigid walls and provides lower friction coefficients and energy dissipation than those of laminar shear motion. The main reasons are the elastic compliance of solidlike oil molecules and the occurrence of boundary slippage on the liquid–wall interface. The comparison of NEMD simulation among confined sheared boundary films of nonhydrogenated diamond-like carbon (DLC), hydrogen-terminated DLC, laminar fluid layers, and solidified oil films further reveals that it is the flexibility of C–C backbones as well as the elastic compliance of terminal C–H bonds that play a dominant role in the low-friction behavior of solidlike oil films in oil-starvation lubrication. Moreover, the simulation results indicate that the temperature increment of the confined sheared 1-decene under 0.14–9.2 GPa loads is generally below 20 K, which is far below the active temperature of 1-decene’s catalytic dehydrogenation and pyrolysis reaction. These results support that in a steady state BL with low friction coefficients at room temperature, the formation of lubricious tribofilms via catalytic dissociation of alkanes can hardly be triggered due to the limited temperature increment, even when the catalysts of Ni/Cu are composited in the sliding surfaces.