Processes of molecular adsorption and ordering enhanced by mechanical stimuli under high contact pressure

Abstract

Adsorbed molecular films, referred to as boundary films in tribology, are widely used in various industrial products as a keyway for surface functionalisation, such as lubricity, wettability, and adhesion. Because boundary films are thin nanometre-scale molecular layers and can easily be removed, their formation process cannot be elucidated in detail. In this study, to analyse the growth dynamics of boundary films, the film thickness and molecular orientation of the boundary film of a fatty acid used as an additive in rolling contact as mechanical stimuli were measured in situ. The measurements were performed on simple test lubricants, which were composed of n-hexadecane and stearic acid, at rolling tribological condition between steel and glass (or sapphire) surfaces by ultrathin film interferometry combined with sum-frequency generation spectroscopy according to a unique protocol. The results quantitatively demonstrate shear-induced boundary film formation. The insight gained from these results is anticipated to enable the formulation of high-performance lubricant additives to further reduce friction loss and high-performance glues that can be freely designed for removability.

Advances in Tribology Driven by Surface Science

“…To date, research on the boundary lubrication mechanism has focused only on the action of an adsorption film consisting of polar molecules, but in-situ observation showed that non-polar solvent molecules, which are not directly involved in the formation of a strong chemisorbed film, are involved in the formation of a lubricating film at the interface. Furthermore, Watanabe et al reported that the orientation of the adsorbed film is largely influenced by shear stress 5) [69]. Frequency-modulated atomic force microscopy (FM-AFM) has attracted attention as a method for observing the structure of adsorbed molecules at the solid-liquid interface [70].…”

Layered Structure and Wear Mechanism of Concentrated Polymer Brushes

“…Prior studies have developed an apparatus featuring an in situ simultaneous observation system that incorporates “ultra-thin-film optical interferometry” and “friction measuring.” This apparatus measures the interfacial gap and “static” mechanical response, including the contact and friction forces, simultaneously. Using this apparatus, the lubrication mechanisms of various tribological systems have been elucidated by correlating the acquired information. ,− Meanwhile, for polymer materials such as CPBs, measuring “dynamic” mechanical response (i.e., viscoelasticity) is crucial because it strongly reflects the structure and conformation of polymers. , Using surface force and resonance shear measurements, Mizukami et al revealed that the elastic and damping parameters of PMMA-CPBs at the interfaces strongly depend on the contact and friction forces . Therefore, conducting simultaneous in situ measurements of the interfacial gap and dynamic mechanical response of CPBs at the contact interface potentially aids in elucidating the relationship between their layered structure and lubrication mechanisms.…”

“…Using this apparatus, the lubrication mechanisms of various tribological systems have been elucidated by correlating the acquired information. 18 , 23 − 25 Meanwhile, for polymer materials such as CPBs, measuring “dynamic” mechanical response (i.e., viscoelasticity) is crucial because it strongly reflects the structure and conformation of polymers. 26 , 27 Using surface force and resonance shear measurements, Mizukami et al revealed that the elastic and damping parameters of PMMA-CPBs at the interfaces strongly depend on the contact and friction forces.…”