Abstract
Superlubricity is generally classified as solid superlubricity and liquid superlubricity according to the lubricants involved at the interfaces, and is a popular research topic in tribology, which is closely linked to energy dissipation. Significant advancements in both experimental studies and theoretical analysis have been made regarding superlubricity in the past two decades. Compared with solid superlubricity, liquid superlubricity has many advantages; e.g., it is more easily achieved at the macroscale and less sensitive to the surface smoothness and atmospheric conditions. In the present study, the advancements in liquid superlubricity at the macroscale are reviewed, and the corresponding mechanisms for various types of liquid lubricants are discussed. This investigation is important for engineering traditional mechanical lubricating systems. Finally, the issues regarding the liquid superlubricity mechanism and the future development of liquid superlubricity are addressed.
Highlights
Because the energy consumption and material losses derived from friction and wear in mechanical systems place large economic and environmental burdens on all nations (Erdemir and Eryilmaz, 2014), studies on lubrication technology and mechanisms are very important for energy conservation and emission reduction
Studies on liquid superlubricity at the macroscale are focused on the mechanism, which differs for different lubricants and is usually attributed to multiple factors, such as the silica layers and hydrodynamic lubrication for water, the tribochemical layer, and hydrogen-bond network for viscous lubricants, and the stern layer and hydrogen-bond network for acid-based lubricants
Hydrophilic polymer brushes may be suitable for silicone rubbers in microfluidics, marine paints, biomedical devices, etc
Summary
Because the energy consumption and material losses derived from friction and wear in mechanical systems place large economic and environmental burdens on all nations (Erdemir and Eryilmaz, 2014), studies on lubrication technology and mechanisms are very important for energy conservation and emission reduction. The superlubricity mechanism of these acid-based mixtures is as follows: the hydrogen-bond network formed by polyhydroxy alcohols and water molecules is adsorbed onto the surface of the stern layer (induced by H+ ions) during the wearing-in period; ,. A new type of acid-based lubricant, which is a mixture of BA and a PEG aqueous solution (PEG(aq)), was synthesized and found to provide superlubricity (COF ≈ 0.004) in neutral conditions (pH ≈ 6.4) at Si3N4/SiO2 interfaces, as depicted in Figure 8 (Ge et al, 2018a). The mechanism of this superlubricity achieved in neutral conditions is similar to that of acidbased lubricants discussed previously. Such liquid superlubricity achieved in neutral conditions is of immense importance to both scientific understanding and industrial technology implementation
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