Molecular behaviors in thin film lubrication—Part one: Film formation for different polarities of molecules

Abstract

There are three unsolved problems in thin film lubrication (TFL) since it was proposed 20 years ago, i.e., the determination of the type of molecules that can enter the contact region efficiently during sliding, the orientation of molecules in the contact region, and the effect of solid surfaces on the liquid molecular orientation in TFL. In order to answer the first two questions, an in situ measurement system comprising a self-designed Raman microscopy and relative optical interference intensity (ROII) system was set up to study the molecular behaviors. A variety of binary mixtures were used as lubricants in the test, and the concentration distribution profile and orientation of the additive molecules in TFL were characterized. The molecular behavior was determined via a combination of shearing, confinement, and surface adsorption. Furthermore, the difference in molecular polarity resulted in different competing effect of surface adsorption and intermolecular interaction, the influence of which on molecular behavior was discussed. Polar additive molecules interacted with the steel surface and exhibited an enrichment effect in the Hertz contact region when added into a nonpolar base oil. No enrichment effect was observed for nonpolar molecules that were added into the nonpolar base oil and polar molecules added into polar base oil. The enrichment of additive molecules enhanced the film-forming ability of the lubricant and resulted in a reduction in the friction coefficient of up to 61%. The orderly arrangement of the additive molecules was another reason for the friction-reducing. A binary multilayer model was proposed to illuminate the molecular behavior in the TFL, and the model was supported by contrary experiment results in elastohydrodynamic lubrication. This research may aid in understanding the nanoscale lubrication mechanism in TFL and the development of novel liquid lubricants.

A review of recent advances in tribology

“…11. Their results indicate that the orientation of liquid molecules in central region of the gap is guided by the shear direction [109] and that of polar molecules near the solid surface (Ag) is guided by solid material [108]. The TFL model with a nanosandwich structure consisting of an adsorbed layer, an ordered-molecule layer, and a fluid layer has been verified.…”

“…The TFL molecular model proposed in 1996 had not been validated experimentally so far. However, there has been a major breakthrough in 2019 [108,109]. Gao et al [108] developed a method based on surfaceenhanced Raman spectroscopy which can show both the packing and orientation of liquid molecules in the TFL regime as shown in Fig.…”

Nanolubricant additives: A review

“…For instance, an enrichment effect in the Hertz contact region was observed when dispersing polar additive molecules into a nonpolar base oil, which can be explained by the interaction between polar additive molecules and the steel surface. However, there was no enrichment effect for nonpolar molecules dispersed into the nonpolar base oil and polar molecules dispersed into polar base oil [148]. Nanolubricant additives with low chemical activity such as copper, silver, and graphene always form a physical adsorption tribofilm.…”

Evaluation of 1,3-diketone as a novel friction modifier for lubricating oils

“…Owing to their high polarity and molecular orientation, liquid crystals exhibit attractive tribological properties both as base oil and as lubricant additives [17] . Luo et al [18][19][20] found that when a nematic liquid crystal 5CB (4-n-pentyl-4'-cyanobiphenyl) was added to hexadecane, the lubricant film became thicker but the coefficient of friction (COF) was even significantly reduced. This actually contradictory behavior was explained by the thin film lubrication theory where molecules attach themselves to the surface and the shear induces a specific molecular orientation.…”