Effect of a Fatty Acid Additive on the Kinetic Friction and Stiction of Confined Liquid Lubricants

Abstract

The kinetic friction and stiction properties of the dilute solution of palmitic acid (PA, friction modifier additive) in poly(a-olefin) (PAO) confined between molecularly smooth mica surfaces were investigated using the surface forces apparatus (SFA). The results were contrasted with those of confined PAO system; the effects of PA additive on the confined film structure and friction/ stiction properties were discussed. The hard-wall thickness of the PA/PAO system was 4.2 ± 0.1 nm, and that of the PAO system was below 1.4 nm. Considering the molecular size, the PA/PAO system has a monomolecular layer of PAO confined between PA adsorbed monolayers on mica surfaces. The kinetic friction of the PA/PAO system decreased with the increase of sliding velocity V, whereas the kinetic friction of the PAO system increased with the increase of V. The stiction spike height of the PA/PAO system increased with the increase of applied load L (pressure P), whereas the spike height of the PAO system decreased with the increase of L (P). These results imply totally different confined structures/sliding mechanisms for the two systems: solid-like slippage of PAO monomolecular layer between PA adsorbed monolayers for the PA/ PAO system; and the extreme viscosity increase of PAO and resulting glass-like transition for the PAO system. The kinetic friction and stiction were larger for the PA/PAO system, which comes from the extreme confinement of PAO (only monomolecular layer) and strong van der Waals adhesion between the solid-like interfaces. The SFA results for smooth surfaces obtained here seem inconsistent with the observations in macroscopic tribology; palmitic acid dissolved in PAO reduces kinetic friction and stiction of engineering (rough) surfaces. The discrepancy of the friction behaviors between smooth and rough surfaces is discussed, which gives insights into designing low-friction surfaces in the oil-based lubrication of macroscopic tribology.

Friction Laws for Saturated/Unsaturated Fatty Acid Layers

“…Nevertheless, Homola et al [14] showed that the value of the constant shear stress depended on the number of layers of OMCTS trapped between mica surfaces. When loose-packed monolayers made of alkane molecules coadsorbed with carboxylic acids and separated by squalane, hexadecane, or more viscous polyalphaolefin, were sheared, a linear increase in friction force with the normal force was measured, regardless of the contact pressure [15][16][17]. The solvent separating opposite monolayers also play a role: a low pull-off force was measured in larger dielectric constant solvent and was associated to an Amontons-like linear friction evolution with normal force [13,18].…”

Analysis of the Interfacial Molecular Behavior of a Lubrication Film of n-Dodecane Containing Stearic Acid under Lubricating Conditions by Sum Frequency Generation Spectroscopy

“…Their results indicate that adsorption films with oiliness additives behave as solids and prevent the direct contact of two sliding materials. 45 Therefore, under a shear field, the stearic acid adsorption film behaves as solid-like, and the alignment of the n-dodecane molecular structure on the lubricant bulk side parries the fluid resistance. As a lubrication mechanism, the results obtained in this study for n-dodecaned 26 with stearic acid (Figure 2c) are consistent with the conventional model of preventing direct contact of sliding materials.…”

“…Yamada et al reported the kinetic friction and stiction properties of poly­(α-olefin) containing palmitic acid confined between molecularly smooth mica surfaces based on measurements using surface forces apparatus (SFA). Their results indicate that adsorption films with oiliness additives behave as solids and prevent the direct contact of two sliding materials . Therefore, under a shear field, the stearic acid adsorption film behaves as solid-like, and the alignment of the n-dodecane molecular structure on the lubricant bulk side parries the fluid resistance.…”

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

“…The measurements afforded by the SFA provide details of the friction mechanism, where confined and sliding structures are related to the presence or absence of organic friction modifiers. For example, it was demonstrated that the base oil on a surface coated with a single layer of organic friction modifier forms a molecular layer and behaves as a non-adsorbed layer, exhibiting solid-like slippage at the outermost surface of the friction modifiers layer 17 . SPR has also been used for the in situ measurement of the surface coverage of friction modifier molecules adsorbed onto gold surfaces in the presence of shear forces 18 .…”