Squeezing molecular thin alkane lubrication films between curved solid surfaces with long-range elasticity: Layering transitions and wear

Abstract

The properties of alkane lubricants confined between two approaching solids are investigated by a model that accounts for the curvature and the elastic properties of the solid surfaces. We consider linear alkane molecules of different chain lengths, C 3 H 8 , C 4 H 10 , C 8 H 18 , C 9 H 20 , C 10 H 22 , C 12 H 26 , and C 14 H 30 confined between smooth gold surfaces. In most cases we observe well defined molecular layers develop in the lubricant film when the width of the film is of the order of a few atomic diameters. An external squeezing-pressure induces discontinuous, thermally activated changes in the number n of lubricant layers. We find that with increasing alkane chain length, the transition from n to nϪ1 layers occurs at higher pressure, as expected based on the increasing wettability ͑or spreading pressure͒ with increasing chain length. Thus, the longer alkanes are better boundary lubricants than the shorter ones, and this should result in less wear. We obtain good correlation between our theoretical results and wear experiments.

Shearing Nanometer-Thick Confined Hydrocarbon Films: Friction and Adhesion

“…It has been shown that simple fluids such as linear alcanes, or fluids consisting of (nearly) spherical molecules such as squalane, when confined at short separation between flat solid walls (wetted by the fluid) form layers parallel to the solid walls. Usually, as the squeezing pressure increases, one layer after another of the confined fluid is squeezed out from the interface [3,4], and the frictional shear properties of the interface changes in a discontinuous (quantized) manner as the number of confined monolayers changes.…”

The effect of surface nano-corrugation on the squeeze-out of molecular thin hydrocarbon films between curved surfaces with long range elasticity

“…Thus in Ref. [2] we have shown that adsorbed C 14 H 30 films between smooth surfaces may for short time withstand contact pressures approaching 1 GPa, which is similar to the plastic yieldstress of steel. In the present paper we will extend the study of [2] and present results for solid walls with periodic short-wavelength nano-roughness.…”

“…In the present paper we will extend the study of [2] and present results for solid walls with periodic short-wavelength nano-roughness. We note that this study, and that in [2], involve fluid squeeze-out between a curved surface and a nominally flat surface. This differs from our earlier (frictional) studies involving flat surfaces [3][4][5][6].…”

“…Fig. 1 shows the wear scar diameter measured on the ball as a function of the hydrocarbon molecular length [2]. Note that the wear volume depends non-linear on the wear scar diameter D as V ≈ πD 4 /(64R), where R is the radius of the ball.…”

“…If the base oil is squeezed out from the contact regions, the adsorbed film, at least if in a (twodimensional (2D)) solid-like state, can reduce the friction and the wear. However, even long-chain hydrocarbons can adsorb on solid walls and form protective coatings which can survive relative high contact pressures without getting removed [1,2]. Thus in Ref.…”