Molecular behaviors in thin film lubrication—Part three: Superlubricity attained by polar and nonpolar molecules

Abstract

In thin-film lubrication (TFL), generally, the viscosity of the lubricant and its coefficient of friction (CoF) increase. Finding a method to reduce the CoF in TFL is a significant challenge for tribologists. In the present work, we report a robust superlubricity attained by using polyalkylene glycols (PAGs, polar molecules) and poly-α-olefins (PAOs, nonpolar molecules) as lubricants on steel/steel friction pairs that have been pre-treated by wearing-in with polyethylene glycol aqueous solution (PEG(aq)). A steady superlubricity state with a CoF of 0.0045 for PAG100 and 0.006 for PAO6 could be maintained for at least 1 h. Various affecting factors, including the sliding velocity, normal load, and viscosity of the lubricants, were investigated. Element analysis proved that composite tribochemical layers were deposited on the worn region after the treatment with PEG(aq). These layers were formed by the tribochemical reactions between PEG and steel and composed of various substances including oxides, iron oxides, FeOOH, and Fe(OH) 3 , which contributed to the superlubricity. In addition to the tribochemical layers, ordered layers and a fluid layer were formed by the PAGs and PAOs during the superlubricity periods. All the three types of layers contributed to the superlubricity, indicating that it was attained in the TFL regime. Accordingly, a mechanism was proposed for the superlubricity of the PAGs and PAOs in the TFL regime in this work. This study will increase the scientific understanding of the superlubricity in the TFL regime and reveal, in the future, the potential for designing superlubricity systems on steel surfaces for industrial applications.

A review of recent advances in tribology

“…Therefore, finding ways to realize superlubricity in thin film lubrication become a great challenge for tribologists. Luo's group successfully reduced the friction coefficient and achieved the superlubricity state under thin film lubrication conditions in 2018 [78].…”

“…Due to its high thermal stability and promising tribological properties, the application of ILs as boundary lubricants is becoming more and more interesting. Dašić et al [81] have studied the influence [78]. © The author(s), 2018.…”

Oscillating friction of nanoscale capillary bridge

“…(2), it is possible to observe the oscillation by controlling the radius of the capillary bridge, the distance between the surfaces, the elastic modulus of the surface, and by using a certain liquid. As a rule of thumb for experimentalists, in Table 1, we list the critical radius, r c , for some typical friction pairs with confined liquid at the interface [35]. A potential experimental system could be graphene layers deposited by exfoliation onto a smooth surface (e.g., a Si/SiO 2 surface), which represent a large category of experimental systems [26,36,37].…”

Molecular behaviors in thin film lubrication—Part two: Direct observation of the molecular orientation near the solid surface

“…In the work reported in Ref. [50], a robust liquid superlubricity has been achieved in TFL regime, which facilitates the industrial applications of TFL theory.…”