The Mechanism of Action between Glyceryl Oleate Friction Modifier and Metal Friction Interface

Friction modifying additives (FM), which are widely used in lubricants, have attracted a lot of attention from researchers for their excellent friction reduction performance under boundary lubrication conditions. Friction modifiers are divided into four main categories, the first of which is organic friction modifiers (OFM). The second category is oil-soluble organic molybdenum compounds. The third category is friction polymers adsorbed on the abrasive surface. The fourth category is dispersed nanoparticles. Friction modifiers are now widely used in CVT lubricants to reduce friction and resist chattering. Among them, glyceryl oleate (GMO) as an organic friction modifier has attracted the attention of researchers due to its excellent friction improvement effect. There have been numerous studies on the tribological properties of glyceryl oleate in base oil systems. However, few studies have been conducted in recent years, especially on the friction reduction mechanism of GMO as PAO6 additive in ball discs made of GCr15, and few molecular simulations and lubrication mechanism analyses have been performed. In order to investigate the friction reducing mechanism of glycerol oleate (GMO) as a friction modifier in lubricating oil, a ball disk reciprocating friction and wear tester was used to investigate the effect of GMO containing poly α friction and wear behavior of olefin (PAO6) lubricating oil. The surface morphology and composition of wear marks were analyzed by scanning electron microscope, laser scanning microscope and Raman spectroscopy. The results showed that under certain conditions, carbon based films with GMO participating in the reaction would be formed on the friction surface, which would greatly reduce the friction coefficient and wear rate, and gradually stabilize the friction state. Also, molecular simulations of the adsorption process of GMO molecules on the substrate surface were performed. The results showed that there were several active sites in the GMO molecule, and the atomic chemical activity at C=C and C=O was high. During the friction process, the ester groups form chemisorption with the friction surface to provide a steady-state environment for the dehydrogenation reaction, and the dehydrogenated carbon chains were enriched on the surface of the substrate, thus forming a carbon-based film. Therefore, the lubrication mechanism of GMO as a friction modifier under high load was not limited to mere physical adsorption, but rather the generation of friction heat during the friction process, which leaded to the conversion of GMO from physical adsorption, where hydroxyl groups played a major role at the beginning, to chemisorption, where ester groups were involved. And Raman spectroscopy confirmed that the friction film formed by GMO consists of relatively soft graphitized carbon. However, this carbon film needed to be generated after a period of grinding under high load. Because, the high loading conditions can provide sufficient shear strength and a certain grinding time can provide enough frictional heat to make the film structure transformation leading to the change of interfacial chemical and physical interactions. From a microscopic point of view, the presence of ester groups in GMO molecules provided the possibility for chemisorption to occur and provided a stable state for dehydrogenation reactions to promote the formation of carbon based films, thus realizing the effect of GMO molecules as friction modifiers to improve tribological performance, and such carbon based films can be replenished by the lubricant itself without the use of special surface modification methods. © 2023 Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.