Superlubricity of glycerol by self-sustained chemical polishing

Yun Long,Tasuku Onodera,Maria-Isabel De Barros Bouchet,Ton Lubrecht,Jean Michel Martin

doi:10.1038/s41598-019-42730-9

Abstract

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.

Highlights

Friction is inevitable when two solid bodies are in contact under relative movement
The steel flat has a roughness of 2.2 nm, and the tetrahedral amorphous carbon (ta-C)-coated ball has a roughness of 32 nm
The ta-C flat surface is much smoother, with an Ra equal to 2.7 nm, and the black points correspond to holes typically found in this type of coating after carbon droplets from the deposition process have been removed by polishing

Summary

Introduction

Friction is inevitable when two solid bodies are in contact under relative movement. in 1990, through calculation, Hirano and Shinjo demonstrated that static friction could entirely vanish[1]. Superlubricity was achieved for the first time under real conditions when a molybdenum disulfide (MoS2) coating slid against a steel pin under ultrahigh vacuum. The formation of graphene oxide in the wear track was found to be responsible for superlubricity[12] They attributed abnormally low friction of the steel/steel tribo-pairs lubricated by pure glycerol to the formation of a “H-bond network” and to a water-like lubrication mechanism[13]. Ge et al reported that, under neutral conditions, mixing a weak acid (boric acid) with polyethylene glycerol (PEG) as lubricant effectively achieves superlubricity[19]. By changing the steel flat to a ta-C-coated one and for the first time, we achieved superlubricity under boundary lubrication conditions (the lambda ratio defined as the ratio of oil film thickness to the composite roughness of the two surfaces is less than unity).

Methods

Results

Conclusion