Applications of sum-frequency generation vibrational spectroscopy in friction interface

Abstract

Sum-frequency generation (SFG) vibrational spectroscopy is a second-order nonlinear optical spectroscopy technique. Owing to its interfacial selectivity, SFG vibrational spectroscopy can provide interfacial molecular information, such as molecular orientations and order, which can be obtained directly, or molecular density, which can be acquired indirectly. Interfacial molecular behaviors are considered the basic factors for determining the tribological properties of surfaces. Therefore, owing to its ability to detect the molecular behavior in buried interfaces in situ and in real time, SFG vibrational spectroscopy has become one of the most appealing technologies for characterizing mechanisms at friction interfaces. This paper briefly introduces the development of SFG vibrational spectroscopy and the essential theoretical background, focusing on its application in friction and lubrication interfaces, including film-based, complex oil-based, and water-based lubricating systems. Real-time detection using SFG promotes the nondestructive investigation of molecular structures of friction interfaces in situ with submonolayer interface sensitivity, enabling the investigation of friction mechanisms. This review provides guidance on using SFG to conduct friction analysis, thereby widening the applicability of SFG vibrational spectroscopy.

A review of advances in tribology in 2020–2021

“…Zhang and Glovnea [4] firstly measured lubricant film thickness at the inner and outer raceways in grease lubricated rolling bearings by an original test rig using electrical techniques. Liu et al [5] reviewed the application of SFG vibrational spectroscopy on experimental configurations, contact and transfer of organic film analysis, molecular orientation alterations analysis, and molecular ordering alterations analysis on the friction interfaces. Bair [6] explored the rheological properties of the liquid film through the quantitative approach in which viscometers and rheometers were used at high pressure and classical approach where the same averaged properties were extracted from the contact itself, and they are essential for understanding and predicting the friction and the film thickness during elastohydrodynamic lubrication.…”

“…They found that the variation of morphology, instead Fig. 5 Schematic of lubrication states over different speed ranges for grease A and B [49]. Reproduced with permission from Ref.…”

Hydration lubrication modulated by water structure at TiO2-aqueous interfaces

“…However, there were still limited reports [36][37][38], in which the effects of interfacial water and particularly the hydrated layer structures on hydration lubrication have been investigated. In Ref.…”