Superlubrication obtained with mixtures of hydrated ions and polyethylene glycol solutions in the mixed and hydrodynamic lubrication regimes

Abstract

Hypothesis

Superlubricity is known to dramatically reduce frictional energy consumption and to improve service life of mechanical devices and biological systems. However, reduction of wear during the running-in period of friction pairs, especially under high contact pressures, still remains an unresolved issue affecting all machines.

Experiments

Here the lubrication, adsorption, and conformational properties of hydrated ions and polyethylene glycol (PEG) mixtures were evaluated at different mass fractions and concentrations of PEG and salts by ball-on-disc tribometer, $\zeta$-potential, quartz crystal microbalance with dissipation (QCM-D), and dynamic light scatting (DLS) analyses.

Findings

These mixtures exhibited superlubricity between Si₃N₄ and sapphire surfaces in a wide range of concentrations and ions valency. Interestingly, a running-in phase shorter than 1 min and low wear rate of 1.85 μm³/(N·m) were observed at contact pressures up to 555 MPa, significantly higher to earlier findings. PEG chains retain random coils filling the bulk of the interfacial film without strongly adsorbing on the interfaces but significantly increasing the viscosity of lubricating film, thereby favoring hydrodynamic lubrication. Hydrated ions are strongly adsorbed on the negatively charged ceramic surfaces, ensuring a sustained hydration effect maintaining superlubricity. The outstanding lubrication characteristics of the PEG/ions mixtures were attributed to the synergistic action of hydration and hydrodynamic lubrication, which appears as a promising avenue for developing new green lubricants and has implications for industrial and biological applications.

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 $\zeta$-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).