Investigation of Superlubricity Achieved by Polyalkylene Glycol Aqueous Solutions

Abstract

and investigated the fl uidity of hydration layers [26][27][28] to better understand the frictional behaviors in biological systems. Previously, our group reported different kinds of liquid superlubricity, e.g., boric acid with glycerol, [ 29 ] phosphoric acid, [ 30 ] as well as glycerol, [ 31 ] polyhydroxy alcohol solutions, [ 32 ] ethylene glycol, [ 33 ] and silicone oil [ 34 ] by running-in with an acid solution. However, the widespread application of these lubricating fl uids in industry is limited by the production of hydrogen ions and the high dependence on water content in these systems.Polyalkylene glycols (PAGs), a type of synthetic lubricants, are the copolymers from any combination of different alkylene oxides, typically of ethylene oxide (EO) and propylene oxide (PO). [ 35 ] Their high-temperature stability, low pour point, and high viscosity index are highly suited for the normal operation of mechanical equipment, for example, ore crushers, cement kilns, and steel roller bearings. In addition, most PAGs are biodegradable and some of them are even food-grade accredited, which means they satisfy the demands necessary for application in the agriculture, forestry, mining, and food-manufacturing industries. [ 35,36 ] By controlling the mixing ratio of EO/PO, the chemical structure of PAG allows for high-strength hydrogen bonding; thus, the solubility of PAG will be adjusted accordingly. [35][36][37] The presence of oxygen atoms in the polymer backbone results in the highly polar PAG being able to absorb onto the contact surface. As such, they have been widely applied in the textile industry, for household cleaners and polishes, and metal cutting. [ 36,38 ] In present work, considering that PAGs have a good hydration and adsorption capacity, we investigated the tribological properties of these aqueous solutions. Using various weight fractions of PAG, the friction coeffi cient was recorded in both droplet and full immersion states. As a result, ultralow friction coeffi cient can be achieved using a wide range of concentration. The contact region after test and the hydration state of PAG chains were analyzed to determine the mechanism of the superlubricity phenomenon in this system. Because of its superior lubrication property and the stable state of the system under ambient conditions, the application of this environmentally friendly solution, such as bearing and joint lubricating systems, was proposed to save energy via the reduction of friction, and prevent pollution from lubricant leakage.Ultralow friction coeffi cient ( µ ) of a polyalkylene glycol (PAG) aqueous solution obtained in both droplet state (40 µL) and full immersion state after a running-in period. Here, two key factors in achieving the superlubricity state ( µ < 0.01) demonstrated: the low shearing strength of the hydrated layer and the presence of a suitable amount of free water molecules. In the initial running-in period, a decrease in contact pressure contributes to the formation of elastohydrodynamic behavior of fl uid. The hydrogen-bon...

Ultralow Friction and Wear of Polymer Composites under Extreme Unlubricated Sliding Conditions

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

“…Thus, increasing the molecular weight of water-soluble molecules is a good idea to improve the performance of superlubricity systems. In 2016, Wang et al [40] achieved an ultralow COF (a minimum value of 0.0023) with a PAG aqueous solution in both the droplet state (40 μL) and full immersion state in a reciprocating mode, as shown in Fig. 10(a).…”

“…It is understood that the ULDH-NS additives were easy to enter the contact interface during the friction process. However, a relatively large dimension of LDHNPs might disturb the ultralow shear between the hydration layers, while the ULDH-NS additives (approximately 1 nm thick) were too thin to influence the continuity of the lubricant [40]. Next, 0.5 wt% ULDH-NS was added to water, and the COF was recorded under the same condition to investigate the lubrication mechanism of ULDH-NS as additives.…”

Novel phenylboronic acid derivatives containing phosphorous and nitrogen as lubricant additives: Synthesis, characterization, and tribological behaviors

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