Open Access Research Article Issue
Extremely low friction on gold surface with surfactant molecules induced by surface potential
Friction 2023, 11 (4): 513-523
Published: 31 May 2022

An extremely low friction state was observed on the gold surface induced by applying a specific negative potential in cationic surfactant solution. The friction force showed a remarkable reduction from 8.3 to 3.5 × 10−2 nN (reduced by 99.6%) with increasing the period of negative applied potential, and the final friction coefficient could reduce down to 3 × 10−4. The extremely low friction state was robust, and it also exhibited an excellent load bearing capacity, which cannot be damaged by a high load. Moreover, the extremely low friction state achieved under negative applied potential could keep stable even after the removal of potential, but failed in a short time, once a specific positive potential was applied. It was demonstrated that there was a stable electro-adsorption of surfactant molecules on the gold surface induced by applying a negative potential, leading to the formation of a bilayer structure on the gold surface. The hydration layers of the bilayer on the gold surface and micelles on the silica probe provided a shear plane with an extremely low shear strength, leading to the extremely low friction state on the gold surface. This study provides a method to achieve extremely low friction state by applied potential.

Open Access Research Article Issue
Functionalized graphene-oxide nanosheets with amino groups facilitate macroscale superlubricity
Friction 2023, 11 (2): 187-200
Published: 25 April 2022

Graphene-oxide (GO) has been recognized as an excellent lubrication material owing to its two-dimensional structure and weak interlayer interactions. However, the functional groups of GO that can contribute to anti-friction, anti-wear, and superlubricity are yet to be elucidated. Hence, further improvement in GO-family materials in tribology and superlubricity fields is impeded. In this study, macroscale superlubricity with a coefficient of friction of less than 0.01 is achieved by exploiting the high adhesive force between amino groups within aminated GO (GO–NH2) nanosheets and SiO2. It was observed that GO–NH2 nanosheets form a robust adsorption layer on the worn surfaces owing to the high adsorption of amino groups. This robust GO–NH2 adsorption layer not only protects the contact surfaces and contributes to low wear, but also causes the shearing plane to transform constantly from solid asperities (high friction) into GO–NH2 interlayers (weak interlayer interactions), resulting in superlubricity. A SiO2-containing boundary layer formed by tribochemical reactions and a liquid film are conducive to low friction. Such macroscale liquid superlubricity provides further insights into the effect of functional groups within functionalized GO materials and a basis for designing functionalized GO materials with excellent tribological performances.

Open Access Research Article Issue
Two-dimensional molybdenum carbide (MXene) as an efficient nanoadditive for achieving superlubricity under ultrahigh pressure
Friction 2023, 11 (3): 369-382
Published: 12 April 2022

In this study, a robust macroscale liquid superlubricity with a coefficient of friction of 0.004 was achieved by introducing molybdenum carbide (Mo2CTx) MXene nanoparticles as lubricating additives in a lithium hexafluorophosphate-based ionic liquid at Si3N4–sapphire interfaces. The maximal contact pressure in the superlubricity state could reach 1.42 GPa, which far exceeds the limit of the superlubricity regime in previous studies. The results indicate that a composite tribofilm (mainly containing molybdenum oxide and phosphorus oxide) that formed at the interface by a tribochemical reaction contributed to the excellent antiwear performance. Furthermore, the extremely low shear strength of the tribofilm and the interlayers of Mo2CTx MXene contributed to the superlubricity. This work demonstrates the promising potential of Mo2CTx MXene in improving superlubricity properties, which could accelerate the application of superlubricity in mechanical systems.

Open Access Research Article Issue
Molecular behaviors in thin film lubrication—Part three: Superlubricity attained by polar and nonpolar molecules
Friction 2019, 7 (6): 625-636
Published: 12 November 2018

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.

Open Access Short Communication Issue
New achievements in superlubricity from International Workshop on Superlubricity: Fundamental and Applications
Friction 2015, 3 (4): 344-351
Published: 23 December 2015

Since the term “superlubricity” was put forward at the beginning of 1990s, it has become one of the hottest researches in tribology due to it being close linked to the energy problems. Recently, the International Workshop on “Superlubricity: Fundamental and Applications” was successfully held on 19–20 October 2015 in Beijing, which has attracted many researchers in this field. The recent scientific results in both solid superlubricity and liquid superlubricity have been presented according to these invited wonderful lectures and posters. In the communication, we gave an introduction to the Workshop on Superlubricity, and also summarized the new achievements of superlubricity during recent years according to these reports. Finally, the problems of superlubricity mechanism and the future development direction of superlubricity are discussed.

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