Achieving Macroscale Liquid Superlubricity Using Lubricant Mixtures of Glycerol and Diols

Friction and wear are ubiquitous in moving mechanical systems, and achieving vanishing friction and wear could significantly improve energy efficiency and extend the service life of mechanical components. In this paper, various diols, viz. ethylene glycol (EG), 1,3-propanediol (13-PD), and 1,2-propdiol (12-PD), have been selected to be mixed with glycerol for superlubricity performance. The results show that the lubricant mixture of EG and glycerol (EG/glycerol) and the mixture of 13-PD and glycerol (13-PD/glycerol) are effective in providing superlow friction (COF < 0.01) for steel tribopairs under ambient atmosphere environment with little surface damage caused. However, 12-PD, which exhibits the same chemical formula as 13-PD except for the configuration of hydroxyl groups, is ineffective for superlubrication. Furthermore, compared with 13-PD, EG is more efficient in preparing superlubric lubricants. Experimental and molecular dynamics simulation results show that the superlow friction realized by the lubricant mixtures of glycerol and diols is related to their intermolecular hydrogen-bonding interaction and the adsorbed formation of adsorbed molecular layers. The intermolecular interaction could affect the rheological property of lubricant mixtures and the hydrodynamic lubricant film-forming capability at the interface, while the quality of the adsorbed molecular layers determines the passivating efficiency for asperity interactions between opposite surfaces. Due to the atomic structure difference, EG is the most desirable diol for this objective, followed by 13-PD, while 12-PD is ineffective. These findings could help enable the rational design of novel lubricants for superlubricating performance and push the development of liquid superlubricity for future engineering applications.