The role of substrate interactions in the modification of surface forces by self-assembled monolayers

Ewers, Bradley W.; Batteas, James D.

doi:10.1039/c4ra01427h

“…as well as amorphous silica nanoasperities protected by low coverage OTS monolayers. 44,45 Strong localisation of σ zz is indicative of larger local pressures than would be predicted using continuum contact mechanics models. 54 Since this only occurs for the sharpest tip, lowest coverage, and highest load considered, Hertz theory provides a reasonable approximation of the pressure distribution in these systems.…”

Section: Nanoscale Afm Experiments Have Generally Shown That µ Initiamentioning

confidence: 95%

“…They observed a spatial anisotropy of pressure-induced defects within ODPA domains; molecules whose initial tilt was in the direction of the pressure gradient increased in tilt angle, while those tilted away formed gauche-defects. 43 Ewers and Batteas 44,45 investigated the compression of OTS films adsorbed on amorphous silica (a-SiO 2 ) surfaces at a range of coverages (Γ = 1.5-3.5 nm −2 ) with a hemispherical a-SiO 2 tip (r tip = 4.5 nm).…”

Section: Introductionmentioning

confidence: 99%

“…As the surface coverage was increased, a sharp transition was observed from the narrow, high-pressure interaction between the underlying substrates, to a broad, substantially lower pressure interaction. 44,45 Some NEMD simulations have also been performed to study the frictional response of SAMs penetrated by single nanoscale asperities. For example, Knippenberg et al 46 studied friction in a close-packed (Γ = 4.6 nm −2 ) alkyl monolayer (C 14 ) indented with a spherical fullerene tip (r tip = 1.3 nm) at high sliding velocity (v s = 87 m s −1 ).…”

Section: Introductionmentioning

confidence: 99%

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Scale-Dependent Friction-Coverage Relations and Non-Local Dissipation in Surfactant Monolayers

Gao¹,

Ewen

²

,

Hartkamp³

et al. 2021

Preprint

0

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<div>Surfactant molecules, known as organic friction modifiers (OFMs), are added to lubricants to reduce friction and wear between sliding surfaces. In macroscale experiments, friction generally decreases as the coverage of OFM molecules on the sliding surfaces increases. However, recent nanoscale experiments with sharp atomic force microscopy (AFM) tips have shown increasing friction. To elucidate the origin of these opposite trends, we use nonequilibrium molecular dynamics (NEMD) simulations and study kinetic friction between OFM monolayers and an indenting nanoscale asperity. For this purpose, we study various coverages of stearamide OFMs on iron oxide surfaces and silica AFM tips with different radii of curvature. For our small tip radii, the friction coefficient and indentation depth both have a non-monotonic dependence on OFM surface coverage, with maxima occurring at intermediate coverage. This suggests that friction is dominated by plowing. We rationalise the non-monotonic relations through a competition of two effects (confinement and packing density) that varying the surface coverage has on the effective stiffness of the OFM monolayers. We also show that kinetic friction is not very sensitive to the sliding velocity in the range studied, indicating that it originates from instabilities. Indeed, while friction predominately originates from the plowing action of the monolayers by the leading edge of the tip, thermal dissipation is mostly localised in molecules towards the trailing edge of the tip.</div>

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“…Similar behaviour has previously been observed for compression simulations of contacting solid nanoasperities 54 as well as amorphous silica nanoasperities protected by low coverage OTS monolayers. 44,45 Strong localisation of σ zz is indicative of larger local pressures than would be predicted using continuum contact mechanics models. 54 Since this only occurs for the sharpest tip, lowest coverage, and highest load considered, Hertz theory provides a reasonable approximation of the pressure distribution in these systems.…”

Section: Frictionmentioning

confidence: 95%

Scale-Dependent Friction-Coverage Relations and Non-Local Dissipation in Surfactant Monolayers

Gao¹,

Ewen

²

,

Hartkamp³

et al. 2020

Preprint

0

View full text Add to dashboard Cite

<div>Surfactant molecules, known as organic friction modifiers (OFMs), are added to lubricants to reduce friction and wear between sliding surfaces. In macroscale experiments, friction generally decreases as the coverage of OFM molecules on the sliding surfaces increases. However, recent nanoscale experiments with sharp atomic force microscopy (AFM) tips have shown increasing friction. To elucidate the origin of these opposite trends, we use nonequilibrium molecular dynamics (NEMD) simulations and study kinetic friction between OFM monolayers and an indenting nanoscale asperity. For this purpose, we study various coverages of stearamide OFMs on iron oxide surfaces and silica AFM tips with different radii of curvature. For our small tip radii, the friction coefficient and indentation depth both have a non-monotonic dependence on OFM surface coverage, with maxima occurring at intermediate coverage. This suggests that friction is dominated by plowing. We rationalise the non-monotonic relations through a competition of two effects (confinement and packing density) that varying the surface coverage has on the effective stiffness of the OFM monolayers. We also show that kinetic friction is not very sensitive to the sliding velocity in the range studied, indicating that it originates from instabilities. Indeed, while friction predominately originates from the plowing action of the monolayers by the leading edge of the tip, thermal dissipation is mostly localised in molecules towards the trailing edge of the tip.</div>

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“…It measures single asperity contacts with an ultra-sharp cantilever without being inu-enced by the effect of surface roughness. 23,67,105 AFM is able to view both hard and so surfaces in liquid as well as air, hence images of polymer brushes have been taken without Fig. 6A shows a 3D AFM image where dodecane residues are clearly seen within a fully covered OTS SAM on a silicon substrate.…”

mentioning

confidence: 99%

Challenges and developments of self-assembled monolayers and polymer brushes as a green lubrication solution for tribological applications

Watson

¹

,

Nie

²

,

Wang

³

et al. 2015

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ab Self-assembled monolayers (SAMs), after originally being investigated due to their functions in changing surface wettability, have been significantly developed over the years. Many types of SAMs have been developed on a variety of substrates. However their formation mechanism, rate and quality are found to be influenced by many factors. A range of SAMs including single-and multi-component are included in this review with focus on the nano and macro tribological properties. More recently, surface initiated polymer brushes, i.e. macromolecular assemblies attached to a substrate, have emerged to be an alternative and promising method for surface modification. The ability to tether these macromolecules to tribological contacts is key to their resistance to shear under loaded contacts. This review also covers atom transfer radical polymerisation (ATRP) and the role of this technique in developing new lubrication solutions. Particular care has been taken to include the development of lubrication solutions for silicon nitride due to the importance of this material as an engineering ceramic. This paper reviews the stateof-the-art development of SAMs and polymer brushes especially the potential opportunities and challenges in applying them in tribological contacts as a lubrication solution.

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The role of substrate interactions in the modification of surface forces by self-assembled monolayers

Abstract: Detailed pressure and strain mapping of atomistic contact simulations elucidate the mechanical and tribochemical mechanisms of surface force modification with SAMs.

Cited by 11 publications

References 59 publications

Scale-Dependent Friction-Coverage Relations and Non-Local Dissipation in Surfactant Monolayers

Scale-Dependent Friction-Coverage Relations and Non-Local Dissipation in Surfactant Monolayers

Scale-Dependent Friction-Coverage Relations and Non-Local Dissipation in Surfactant Monolayers

Challenges and developments of self-assembled monolayers and polymer brushes as a green lubrication solution for tribological applications

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