Cross-Sectional Imaging of Boundary Lubrication Layer Formed by Fatty Acid by Means of Frequency-Modulation Atomic Force Microscopy

Abstract

To observe in situ the adsorption of fatty acid onto metal surfaces, cross-sectional images of the adsorption layer were acquired by frequency-modulation atomic force microscopy (FM-AFM). Hexadecane and palmitic acid were used as the base oil and typical fatty acid, respectively. A Cu-coated silicon wafer was prepared as the target substrate. The solvation structure formed by hexadecane molecules at the interface between the Cu substrate and the hexadecane was observed, and the layer pitch was found to be about 0.6 nm, which corresponds to the height of hexadecane molecules. This demonstrates that hexadecane molecules physically adsorbed onto the surface due to van der Waals forces with lying orientation because hexadecane is a nonpolar hydrocarbon. When hexadecane with palmitic acid was put on the Cu substrate instead of pure hexadecane, an adsorption layer of palmitic acid was observed at the interface. The layer pitch was about 2.5-2.8 nm, which matches the chain length of palmitic acid molecules well. This indicates that the original adsorption layer was monolayer or single bilayer in the local area. In addition, a cross-sectional image captured 1 h after observation started to reveal that the adsorbed additive layer gradually grew up to be thicker than about 20 nm due to an external stimulus, such as cantilever oscillation. This is the first report of in situ observation of an adsorbed layer by FM-AFM in the tribology field and demonstrates that FM-AFM is useful for clarifying the actual boundary lubrication mechanism.

Shear property of oil film containing oiliness additive in narrow gap measured with newly‐developed parallel‐disk viscometer supported by aerostatic bearing

“…We finally discuss on “where the interfacial slip face is.” One possibility is to slip in the boundary layer formed by the acid molecules. Some of recent observations showed that the fatty acid molecules take multilayered structure on the metal surfaces in lubricant . The adsorption of palmitic acid into a metal surface is known to be higher than that of oleic acid because the former has a straight chain while the latter has a flexed one because of the existence of a double bond, and a difference in adsorption properties may be appeared in our test.…”

“…The base oil was hexadecane and the acid concentration was 0.01 mass%. The detailed information on the capturing method is described in Hirayama et al The substrate was Cu because the bright area was almost not observed when we used Fe substrate instead of Cu one [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Some of recent observations showed that the fatty acid molecules take multilayered structure on the metal surfaces in lubricant. 13 The adsorption of palmitic acid into a metal surface is known to be higher than that of oleic acid because the former has a straight chain while the latter has a flexed one because…”

“…The base oil was hexadecane and the acid concentration was 0.01 mass%. The detailed information on the capturing method is described in Hirayama et al 13 The substrate was Cu because the bright area was almost not observed when we used Fe substrate instead of Cu one [Colour figure can be viewed at wileyonlinelibrary.com] of the existence of a double bond, and a difference in adsorption properties may be appeared in our test. Figure 15 shows the cross-sectional images of the interface between the Cu substrate and lubricant captured by FM-AFM (SPM-8000FM, Shimadzu Corp), as reference.…”

Three-dimensional solvation structure of ethanol on carbonate minerals

“…With the advent of high-resolution 3D atomic force microscopy (AFM) [1], the hydration structure of many interfaces has been studied, including the aqueous interface of mica [2] calcite [3,4] dolomite [5,6] and organic crystals [7]. However, while the majority of 3D AFM works have concentrated on water, comparatively fewer experimental studies exist addressing the interfacial arrangement of other solvent molecules [8][9][10]. This is unfortunate given the relevance of the interaction between organic molecules and carbonate surfaces, for example, in the field of biomineralization [11].…”