Superlubricity achieved with two-dimensional nano-additives to liquid lubricants

Abstract

The topic of superlubricity is attracting considerable interest around the world while humanity is facing an energy crisis. Since various liquid superlubricity systems can be commonly achieved on the macroscale in ambient conditions, it is considered an effective solution to reduce unnecessary energy and material losses. However, certain practical problems such as low load-bearing pressure, dependence on hydrogen ions, and relatively long running-in processes still limit its widespread application. Twodimensional (2D) nano-additives with ultrathin longitudinal dimensions can lower the shear resistance between sliding solid surfaces, and thus further optimize the applied conditions. In this review, the latest studies on 2D nano-additives with a combination of various water-based lubricants in the state of superlubricity are reported, typically including black phosphorus (BP), graphene oxide (GO), and layered double hydroxide. During the sliding process, composite lubricants effectively improved the load capacity (up to 600 MPa), reduced wear, and accelerated the running-in period (within 1,000 s) of the liquid superlubricity system. Both macromechanical experiments and microscopic tests are conducted to precisely analyze various interactions at the interfaces of the nano-additives and solid surfaces. These interactions can be described as tribochemical reactions, physical protection, and adsorption enhancement, and improved wear resistance. This review provides better guidance for applying 2D nanomaterials in liquid superlubricity systems.

Macroscale superlubricity achieved via hydroxylated hexagonal boron nitride nanosheets with ionic liquid at steel/steel interface

“…Given recent innovative progress in reducing friction and wear, superlubricity, defined as a friction coefficient (COF) of less than 0.01, is considered a viable strategy [4]. Nowadays, superlubricity, including solid superlubricity [5] and liquid superlubricity [6], has attracted continuously attention in many fields including machinery, energy, aerospace, and biology [7][8][9][10]. As for solid superlubricity, the achievement of extremely low COF (< 0.01) usually relied on a specific structure (e.g., graphite, MoS 2 , a-C:H, and diamond like carbon (DLC) film) [11][12][13][14][15] or a certain atmosphere (e.g., inert gas and vacuum) [16][17][18].…”

“…Ge et al [30] utilized the induced tribochemical reaction to situ-form ionic liquids (ILs) on ceramic interfaces and achieved ultralow COF (0.003) and excellent antiwear properties. Other studies on liquid superlubricity have been reported, including the aqueous lubrication of charged polymer brushes [31], in situ formation of tribofilm with MXene nanoflakes in glycerol, the synergistic effect of graphene-oxide nanoflakes and ethanediol [32], IL-modified carbon quantum dots (CQDs) [33], two-dimensional nanoadditives [6], and macromolecular structures [34].…”

Inorganic nanomaterial lubricant additives for base fluids, to improve tribological performance: Recent developments

“…Black phosphorus (BP) is emerging as a unique material and has attracted much scientific attention [121,122]. In particular, BP exhibits weak interlayer interactions, an anisotropic lamellar structure, high thermal stability, a tunable bandgap, and high carrier mobility; this makes it suitable as a liquid lubricant additive in numerous applications, including tribology [123][124][125].…”

Superlubricity Achieved with Zwitterionic Brushes in Diverse Conditions Induced by Shear Actions

“…Spencer et al used the chemical grafting method to graft a poly­(methacrylic acid) brush (anionic polymers) onto Si/SiO 2 substrates, and the friction coefficient was lower than 0.005 under a low-contact pressure . In addition to anionic polymers, poly­(2-(dimethylamino)­ethyl methacrylate) is widely studied as a cationic polymer because it is protonated with the positive charge under acidic conditions, leading to the achievement of superlubricity (friction coefficient < 0.01). − Although the lubrication performance of charged polymer brushes was good, the charged polymer brushes usually have poor biocompatibility resulting from the biofouling problem, which, in turn, increases the surface friction, greatly limiting the applications …”