Nanometer-Scale Distribution of a Lubricant Modifier on Iron Films: A Frequency-Modulation Atomic Force Microscopy Study Combined with a Friction Test

Abstract

Liquid lubricants used in mechanical applications are low-vapor-pressure hydrocarbons modified with a small quantity of polar compounds. The polar modifiers adsorbed on the surface of sliding solids dominate the friction properties when the sliding surfaces are in close proximity. However, a few methods are available for the characterization of the adsorbed modifiers of a nanometer-scale thickness. In this study, we applied frequency-modulation atomic force microscopy to evaluate the vertical and lateral density distributions of the adsorbed modifier in a real lubricant, namely, poly-α-olefin (PAO) modified with an orthophosphoric acid oleyl ester. The liquid-induced force on the probing tip was mapped on a plane that was perpendicular to the lubricant–iron interface with a force sensitivity on the order of 10 pN. The PAO in the absence of the ester modifier was directly exposed to the film, which produced a few liquid layers parallel to the film surface with layer-to-layer distances of 0.6–0.7 nm. A monomolecular layer of the modifier was intermittently adsorbed with increasing ester concentration in the bulk lubricant, with complete coverage seen at 20 ppm. The C18H35 chains of the oleyl esters fluctuating in the lubricant produced a repulsive force on the tip, which monotonically decayed with the tip-to-surface distance. The dynamic friction coefficient of sliding steel–lubricant–steel interfaces, which was separately determined using a friction tester, was compared with the force map determined on the iron film immersed in the corresponding lubricant. The complete monomolecular layer of the ester modifier on the static lubricant–iron boundary is a requirement for achieving smooth and stable friction at the sliding interface.

Atomic-Scale Structural Analysis on the Interfaces between Molten Gallium and Solid Alloys by Atomic Force Microscopy

“…That is, atomic-resolution AFM imaging was achieved in molten Ga. Although many studies on atomic-resolution AFM analysis in liquid environments have been reported, − no AFM studies in molten metal have been performed. This is the first result showing that AFM has an atomic resolution in molten metal.…”

“…Atomic force microscopy (AFM) is known as a powerful technique for atomic-scale characterization of surface structures. In recent years, AFM operating in liquid environments has made significant progress, and atomic-resolution analysis in various liquids, such as aqueous solutions, − organic solvents, , and ionic liquids, − has been reported. Since AFM can also characterize the density distribution of liquid molecules on the solid/liquid interfaces, the so-called solvation structures, it has become an essential analytical technique in the physical chemistry of solid/liquid interfaces.…”

Combined use of neutron reflectometry and frequency-modulation atomic force microscopy for deeper understanding of tribology

“…Both NR and FM-AFM are quite useful for in situ analysis of an adsorbed additive layer, and their combined use is one of the most powerful tools for revealing the true I. 50) The number of papers discussing interfacial structures between the lubricant and substrate using NR [62][63][64] or FM-AFM 65,66) has markedly increased over recent years.…”

Intrinsic Friction Behaviour Originating from Molecular Structure of Lubricant in Nanoscale Gap Fabricated by Microelectromechanical Systems

“…These devices have been used to verify the existence of adsorption layers and to clarify their thickness and quantity. These devices have evolved into a variety of analysis methods: an SFA is used with resonance shear measurement (RSM) [30][31][32], an AFM is used with frequency-modulation AFM (FM-AFM) [33,34], and a QCM is used with dissipation monitoring (QCM-D) [35][36][37].…”