Effects of Abrasive Particles on Liquid Superlubricity and Mechanisms for Their Removal

Abstract

Liquid superlubricity results in a near-frictionless lubrication state, which can greatly reduce friction and wear under aqueous conditions. However, during the running-in process, a large number of abrasive particles are generated, and because these may lead to a breakdown in superlubricity performance, they should be effectively removed. In this paper, the morphology, size, and composition of abrasive particles were verified using scanning electron microscopy with energy-dispersive X-ray spectroscopy, and their influence on liquid superlubricity was explored through friction tests. Subsequently, different solvents were used to remove the abrasive particles, and the optimal cleaning process was determined by macroscopic tribo-tests and microscopic analysis. Finally, droplet-spreading experiments and a force-curve analysis were carried out to understand the abrasive-particle removal mechanism by different solvents. We found that SiO 2 was the main component in the abrasive particles, and micron-sized SiO 2 particles resulted in random "wave peaks" in the coefficient of friction and, thus, the superlubricity. Absolute ethanol + ultrapure water was determined to be the optimal solvent for effectively removing abrasive particles from friction-pair surfaces and helped the lubricant in exhibiting an ultralow friction coefficient for long periods of time. We proposed a "wedge" and "wrap" model to explain the abrasive-particle removal mechanism of different solvents. The SiO 2 removal mechanism outlined in this study can be applied under aqueous conditions to improve the stability and durability of liquid superlubricity in practical engineering applications.

High-Temperature Superlubricity Realized with Chlorinated-Phenyl and Methyl-Terminated Silicone Oil and Hydrogen-Ion Running-in

“…After running-in, the surface of the friction pair was scrubbed with petroleum ether, anhydrous ethanol, and ultrapure water to remove the surface solvent and the abrasive particles generated by running-in. 29 Thereafter, 100 μL of CPSO (lubricating medium) was dropped between the ball and disc, and the lubricating properties of CPSO after different running-in processes were measured. The normal load was 6 N, the rotation radius was 5 mm, the rotational speed was 300 rpm, and the linear sliding speed was 157 mm/s.…”

“…Before the friction test, a CPSO (190 °C) or H 3 PO 4 (pH = 1.5; 25 °C) solution was used for running-in under certain conditions (5 mm, 10 N, 300 rpm, 10 min) to form a small-sized contact platform on the surface of Si 3 N 4 . After running-in, the surface of the friction pair was scrubbed with petroleum ether, anhydrous ethanol, and ultrapure water to remove the surface solvent and the abrasive particles generated by running-in . Thereafter, 100 μL of CPSO (lubricating medium) was dropped between the ball and disc, and the lubricating properties of CPSO after different running-in processes were measured.…”

“…Transmission electron microscopy and atomic force microscopy further verified that the SiO 2 nanolayer generated by running-in considerably reduced the frictional resistance between ceramic surfaces under boundary lubrication at the macroscopic and microscopic scales. In addition, the SiO 2 nanolayer exhibited improved negative potential and hydrophilicity as well as adsorbed more hydrated cations, thereby promoting the superlubricity of hydrated ions. − In recent years, numerous scholars have found that the superlubricity of aqueous solutions, such as hydrated polyvalent ions, including Mg 2+ and Al 3+ ; polymers, including poly­(vinyl alcohol) and poly­(ethylene glycol); and polymer + salt solutions, − can be achieved by H + -ion running-in on a ceramic surface.…”

Significantly Affected Lubrication Behavior of Silicone Oil Lubricated Si3N4/Glass Contact after Cleaning with Different Solvents

“…The solvent water in this study was ultrapure water (UP) with a conductivity of 18.2 MΩ•cm and a total organic carbon content of less than 2 ppb. 36 The friction pair consisted of Si 3 N 4 spheres (S a = 3.0 nm) with a diameter of 3.969 mm and glass disks (S a = 5.0 nm) with a thickness of 3 mm. For the AFM SiO 2 spherical probe, SiO 2 particles (R = 20 μm; S a = 5.4 nm) were attached to the pointed end of the rectangular cantilever using epoxy glue (SINWE AB adhesive: a twocomponent fast curing transparent adhesive).…”

“…In contrast, the cleaning effect of the dropwise addition of petroleum ether and acetone was relatively weak, and several impurities remained on the surface (Figure 9b,c); these results are consistent with our previous research. 36 Subsequently, the force curves of the AFM probe and the surfaces with the blank/control samples were also measured in the silicone oil 200 solvent environment. The average and statistical analyses of the adhesion forces are shown in Figure 9e,f, respectively.…”