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.
Highlights
Since the hydrodynamic lubrication (HDL, 1886) theory was proposed by Reynolds [1], boundary lubrication (BL, 1921) [2] and elasto-hydrodynamic lubrication (EHL, 1949) [3] have been proposed to distinguish different lubrication regimes
PAG100 and PAO6 were injected to the same wear track and their coefficient of friction (CoF) were measured again between the steel/steel friction pairs that were treated by PEG(aq)
When the viscosity of both PAGs and PAOs is higher than 30 mPa·s, CoFs less than 0.01 can be obtained between the steel/steel friction pairs that are treated by PEG(aq)
Summary
Since the hydrodynamic lubrication (HDL, 1886) theory was proposed by Reynolds [1], boundary lubrication (BL, 1921) [2] and elasto-hydrodynamic lubrication (EHL, 1949) [3] have been proposed to distinguish different lubrication regimes. A type of weak acid (boric acid) was found to provide superlubricity when mixed with a polyethylene glycol aqueous solution (PEG(aq)) [27] The superlubricity of this lubricant can be attributed to the equilibrium between the production and consumption of H+ ions, making the superlubricity achievable under neutral conditions. Even though liquid superlubricity has been attained and investigated in both the BL [6, 21, 28] and EHL regime [29], it has been barely investigated in the TFL regime To solve this issue related to the TFL regime, in this work, a robust superlubricity state with a superlow CoF of 0.004 was attained in the TFL regime by the formation of tribochemical layers on the contact surfaces via a pre-wearing-in treatment. The robust superlubricity was investigated and analyzed in detail, and a possible model of superlubricity in the TFL regime was developed
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