Superlubrication obtained with mixtures of hydrated ions and polyethylene glycol solutions in the mixed and hydrodynamic lubrication regimes
Graphical abstract
Introduction
Superlubricity, which refers to the lubrication state where friction vanishes, has potential applications on energy and materials saving, thus extending the service life of mechanical equipment in industry and of human joints [1], [2], [3]. In the field of tribology, the term superlubricity is used to describe any sliding state of ultra-low friction, where the coefficient of friction (COF) is below 0.01 or even lower [4], [5], [6]. In the past thirty years, great progress has been made in liquid superlubricity, and different approaches have been explored to study the possibilities for achieving ultra-low friction coefficient [4], [7], [8]. Until now, various materials have been reported to obtain liquid superlubricity mainly by using ball-on-disk or pin-on-disk tribometer, surface force apparatus (SFA), and atomic force microscope (AFM). These materials include water between ceramics [9], [10], aqueous solutions of acids and alcohols [11], [12], [13], ionic liquids [14], [15], [16], hydrated materials (including hydrated ions, amphiphilic surfactants, polymer brushes, and liposomes) [2], [17], [18], additives or coatings of two dimensional materials (including graphene, graphene oxide, a-C:H films, and diamond-like carbon) [19], [20], biological fluids [21], oil-based materials [22], [23]. It is well known that there are three tribological regimes as defined from the Stribeck diagram: boundary lubrication, mixed lubrication, and hydrodynamic lubrication. Superlubricity can be observed in three different situations based on the lubrication regimes. In the hydrodynamic lubrication regime, the liquid lubricant film is thick enough to separate the surfaces completely, and the frictional losses come from internal shear of lubricant molecules. Generally, hydrodynamic lubrication is the main contributor of macroscale superlubricity and originates from a thick lubrication film between solid surfaces during friction [24], [25]. The formation and maintenance of the lubricating film is governed by three elastohydrodynamic factors, namely, the sliding speed, the contact pressure or normal load, and the lubricant viscosity, as well as the mechanical properties of the surface materials (including elastic modulus, Poisson's ratio, surface roughness) [25], [26]. In the boundary lubrication regime, the sliding surfaces are separated by a very thin molecular film of lubricant, so the chemical and physical natures of the interfaces and the interfacial characteristics of the lubricant are of major significance in producing effective lubrication. In this regime, surface hydration layer formed by hydrated materials is a key contributor to superlubricity. The surface hydration layers composed of water molecules surrounding surface-bound charges can sustain high normal loads and counter balance van der Waals attraction through hydration repulsive forces [27], [28], [29], [30]. Meanwhile, the hydration layers retain a fluid response under shear conditions which is attributed to the rapid exchange of water molecules between hydration shells and the bulk solution during friction process [31], which allows to maintain high frictional dissipation and therefore superlubrication. Also present in the boundary lubrication regime is the formation of a tribo-induced film especially during the running-in period, which can also contribute to the occurrence of superlubricity. Such film originates from tribochemical transformations between the lubricant and surface asperities in contact as they slide past each other, which is composed of easy-shear materials [32], [33], [34]. For example, the tribo-induced silica layer generated on ceramic surfaces is known to contribute to macroscale superlubricity by water and salt solutions [10], [32].
Besides our thorough understanding of superlubricity, it is still a challenge today to engineer tribo-systems exhibiting robust superlubricating properties. For example, during tribo-induced film formation, a long running-in period often leads to serious wear of the tribopair [14], [35]. The lubricants known to have superlubricating properties are usually not environmentally friendly and may corrode friction materials or contaminate the environment, such as acidic lubricants [11], [35]. Therefore, there is a need to design more efficient and more environmentally friendly lubricants, which should be capable of sustaining large compressive stress and maintaining very low shear resistance, but these two properties are usually mutually exclusive [4]. We previously reported macroscale superlubricity of hydrated monovalent and multivalent ions between ceramic surfaces under high contact pressures above 0.25 GPa [36], [37]. The results indicated for the first time that hydration superlubrication under macroscopic conditions and at high pressures was possible. The significant role of surface forces, especially hydration repulsion, on superlubricity was also proposed through numerical simulation on the basis of unified mixed lubrication numerical model, which was consistent with the experimental findings [38]. However, a running-in period of 300 s was found to be necessary to reach the ultra-low friction regime, resulting in a significant wear of the surfaces. Polyethylene glycol (PEG), known as a common green lubricant and solvent in many applications ranging from industrial manufacturing to medicine, has been found to exhibit good lubrication performances and significantly reduce the running-in time, but the COF of PEG lubricants is as high as 0.1 between steel surfaces and 0.03 between ceramic surfaces and hence never reaches the superlubrication state [39], [40].
The present work overcomes these limitations and solve these problems by combining the hydration and hydrodynamic contributions to reach robust superlubrication. A systematic study on friction and wear characteristics of mixtures of hydrated ions and PEG solutions was conducted with a Si3N4 ball and sapphire disk as tribo-pair. The different PEG solutions covered a range of molecular weight (MW) from 200 to 1000 g/mol while the hydrated ions were chosen based on their valency from monovalent ions (Li+, Na+, and K+) to multivalent ions (Mg2+ and Al3+). The influence of PEG on tribochemical reaction between lubricants and surfaces as well as ion adsorption on negatively charged ceramic surfaces were both studied using X-ray photoelectron spectroscopy (XPS) and -potential measurements. The effect of salts on interfacial adsorption and conformational properties of PEG was further studied by QCM-D and DLS analyses. The results proved that stable liquid superlubricity can be achieved under high contact pressures (>500 MPa) even if the running-in period is very short (<1 min).
Section snippets
Materials
All the monovalent and multivalent salts (LiCl, NaCl, KCl, MgCl2·6H2O, and AlCl3·6H2O) were 99.99% purity from Shanghai Aladdin Bio-Chem Technology Co., Ltd. PEG with six different average molecular weights (200, 300, 400, 600, 800, and 1000 g/mol) was 99.5% purity from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Ultrapure water with conductivity and nominal TOC of 18.2 MΩ·cm and <2 ppb was purified by the water purification system (Thermo Fisher Scientific). The silicon nitride ball with a
Excellent lubrication and anti-wear properties of PEG300/KCl mixtures
Tribological properties of mixed PEG300/KCl solutions were measured between the Si3N4 ball and sapphire disk, as shown in Fig. 1. The COF of mixed solutions with KCl (100 mM) and PEG300 (25 wt%) dropped quickly from 0.1 to 0.3 to 0.01 in a short running-in period lower than 1 min, and then remained stable at the low value of 0.003–0.005. As a comparison, the average COF of pure water and KCl solution (100 mM) was as high as 0.073 and 0.027 after a friction test of 30 min, demonstrating that
Conclusions
Robust and rapid superlubricity (COF = 0.003–0.004) can be achieved under lubrication of aqueous mixtures of hydrated ions with different valence (Li+, Na+, K+, Mg2+, and Al3+) and PEG with various molecular weights (200–1000 g/mol) between Si3N4 and sapphire surfaces under a wide range of normal loads (1–9 N) and velocities (0.05–0.5 m/s). Outstandingly, the Si3N4 ball had extremely low wear and the sapphire disk had almost no wear because of the short running-in period (Trun < 1 min), which
Credit authorship contribution statement
Tianyi Han: Methodology, Visualization, Data curation, Writing - original draft, Investigation, Formal analysis. Shuang Yi: Methodology, Visualization, Data curation. Chenhui Zhang: Supervision, Conceptualization, Funding acquisition, Writing - review & editing. Jinjin Li: Investigation, Methodology. Xinchun Chen: Investigation, Methodology. Jianbin Luo: Supervision, Conceptualization, Funding acquisition. Xavier Banquy: Writing - review & editing, Visualization, Funding acquisition.
Acknowledgements
This work is financially supported by the National Key Research and Development Program, China (No. 2018YFB2002204), and the National Natural Science Foundation of China, China (Grant Nos. 51925506, 51527901). T. H. gratefully acknowledges the financial support from the non-profit China Scholarship Council, China. X.B. acknowledges financial support from the Canada Research Chair program, Canada. We thank Prof. Yonggang Meng for his technical assistance in QCM-D measurements and analyses.
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