Sci Rep 2019 Apr 18;9(1):6286. Epub 2019 Apr 18.
Université de Lyon, Ecole Centrale de Lyon, LTDS CNRS 5513, 69134, Ecully, France.
An impressive superlow coefficient of friction (CoF) as low as 0.004 (nearly equivalent to the rolling coefficient) was obtained by sliding a steel ball against a tetrahedral amorphous diamond-like carbon (ta-C) coating in glycerol under a boundary lubrication regime. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) revealed substantial changes in the surface chemistry and topography in the friction track. As shown by XPS analysis, a transfer of iron atoms from the steel ball to the ta-C layer occurred, forming iron oxy-hydroxide (FeOOH) termination on both surfaces. Between them, theoretical calculations show that a nanometre-thick fluid film consisting of glycerol and its degradation products prevents direct contact between the solid surfaces by nm-thick film EHL lubrication and results in the superlow friction, in agreement with the experiment. Furthermore, molecular dynamics (MD) simulations reveal that hydrogen atoms act as "low-friction brushes" between sliding layers of crystalline FeOOH, resulting also in low friction. A new model of sustainable green superlubricity is proposed. The tribo-formation of FeOOH with glycerol leads to a unique polishing process, which in turn leads to a self-sustained Elasto-Hydrodynamic Lubrication (EHL) regime until the very thin fluid film is no more than a few nanometres thick. At lower thicknesses, the hydroxide layer takes over. Wear of the ta-C coating is negligible, while wear on the steel ball is very moderate and acceptable for many practical applications, such as bio-tribology and the food industry, in which green lubrication is especially needed.