Special Issue "Friction Mechanisms"

A special issue of Lubricants (ISSN 2075-4442).

Deadline for manuscript submissions: closed (31 October 2019).

Special Issue Editor

Prof. Dr. Martin H. Müser
E-Mail Website
Guest Editor
Universität des Saarlandes, Department of Materials Science and Engineering, Saarbrücken, Germany
Interests: materials physics; condensed-matter theory; computer simulation; tribology; friction mechanisms; contact mechanics; phase transformations; force fields

Special Issue Information

Dear Colleagues,

There can be many different processes leading to friction. To name a few: Near-surface plastic deformation, viscous dissipation, sliding-induced elastic or adhesive instabilities, geometric interlocking of matching surfaces, and pinning of non-matching surfaces through third bodies, which can range from boundary lubricants to hard abrasive particles.

Although many friction mechanisms are well understood qualitatively, any quantitative prediction for specific tribological systems remains challenging. Part of the difficulty lies in the non-trivial scale-dependence of many processes leading to energy dissipation and the complex coupling between these processes. In the last one or two decades, important steps were made that improved our ability to predict friction in specific systems or to unravel the scale-dependence of processes leading to friction. However, the journey has just begun.

This Special Issue is aimed at further improving our understanding of the scale-dependence and the interplay of dissipation mechanisms. While the focus should lie on sliding or rolling contacts, systems may range from soft-matter systems, such as rubber moving past a rough surface to single-asperity metal on metal contacts. Theoretical, computational, and experimental submissions are welcome.

Prof. Martin H. Müser
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Lubricants is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Kinetic friction
  • Plastic deformation
  • Rubber friction
  • Boundary lubricants
  • Adhesive wear
  • Multiscale modeling
  • Multiphysics modeling

Published Papers (6 papers)

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Research

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Open AccessArticle
Contact Mechanics for Solids with Randomly Rough Surfaces and Plasticity
Lubricants 2019, 7(10), 90; https://doi.org/10.3390/lubricants7100090 - 16 Oct 2019
Abstract
We present experimental results for the elastic and plastic deformation of sandblasted polymer balls resulting from contacts with flat smooth steel and silica glass surfaces. Nearly symmetric, Gaussian-like height probability distributions were observed experimentally before and remarkably, also after the polymer balls were [...] Read more.
We present experimental results for the elastic and plastic deformation of sandblasted polymer balls resulting from contacts with flat smooth steel and silica glass surfaces. Nearly symmetric, Gaussian-like height probability distributions were observed experimentally before and remarkably, also after the polymer balls were deformed plastically. For all the polymers studied we find that the surface roughness power spectra for large wavenumbers (short length scales) are nearly unchanged after squeezing the polymer balls against flat surfaces. We attribute this to non-uniform plastic flow processes at the micrometer length scale. The experimental data are analyzed using the Persson contact mechanics theory with plasticity and with finite-element method (FEM) calculations. Full article
(This article belongs to the Special Issue Friction Mechanisms)
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Open AccessArticle
Polymer Brush Friction in Cylindrical Geometries
Lubricants 2019, 7(10), 84; https://doi.org/10.3390/lubricants7100084 - 25 Sep 2019
Abstract
Polymer brushes are outstanding lubricants that can strongly reduce wear and friction between surfaces in sliding motion. In recent decades, many researchers have put great effort in obtaining a clear understanding of the origin of the lubricating performance of these brushes. In particular, [...] Read more.
Polymer brushes are outstanding lubricants that can strongly reduce wear and friction between surfaces in sliding motion. In recent decades, many researchers have put great effort in obtaining a clear understanding of the origin of the lubricating performance of these brushes. In particular, molecular dynamics simulations have been a key technique in this scientific journey. They have given us a microscopic interpretation of the tribo-mechanical response of brushes and have led to the prediction of their shear-thinning behavior, which has been shown to agree with experimental observations. However, most studies so far have focused on parallel plate geometries, while the brush-covered surfaces might be highly curved in many applications. Here, we present molecular dynamics simulations that are set up to study the friction for brushes grafted on the exterior of cylinders that are moving inside larger cylinders that bear brushes on their interior. Our simulations show that the density distributions for brushes on the interior or exterior of these cylinders are qualitatively different from the density profiles of brushes on flat surfaces. In agreement with theoretical predictions, we find that brushes on the exterior of cylinders display a more gradual decay, while brushes on the interior of cylinders becomes denser compared to flat substrates. When motion is imposed, the density profiles for cylinder-grafted brushes adapt qualitatively differently to the shear motion than observed for the parallel plate geometry: the zone where brushes overlap moves away from its equilibrium position. Surprisingly, and despite all these differences, we observe that the effective viscosity is independent of the radius of the brush-grafted cylinders. The reason for this is that the viscosity is determined by the overlap between the brushes, which turns out to be insensitive to the exact density profiles. Our results provide a microscopic interpretation of the friction mechanism for polymer brushes in cylindrical geometries and will aid the design of effective lubricants for these systems. Full article
(This article belongs to the Special Issue Friction Mechanisms)
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Open AccessArticle
Friction vs. Area Scaling of Superlubric NaCl-Particles on Graphite
Lubricants 2019, 7(8), 66; https://doi.org/10.3390/lubricants7080066 - 06 Aug 2019
Abstract
Structural lubricity is an intriguing tribological concept, where extremely low friction is anticipated, if two surfaces in relative motion do not share the same lattice structure and consequently instabilities originating from interlocking surface potentials are strongly reduced. Currently, the challenges related to the [...] Read more.
Structural lubricity is an intriguing tribological concept, where extremely low friction is anticipated, if two surfaces in relative motion do not share the same lattice structure and consequently instabilities originating from interlocking surface potentials are strongly reduced. Currently, the challenges related to the phenomenon of structural lubricity are considered to be twofold. On one hand, experimental systems suitable for showing structural lubricity must be identified, while at the same time, it is also crucial to understand the intricate details of interface interaction. Here, we introduce a new material combination, namely NaCl-particles on highly oriented pyrolithic graphite (HOPG), where the nanoparticles coalesce under the influence of ambient humidity. Our experiments reveal that the interfacial friction can be described by the concept of structural lubricity despite the seemingly unavoidable contamination of the interface. By systematically analyzing the friction versus area scaling, this unlikely candidate for structural lubricity then shows two separate friction branches, with distinct differences of the friction versus area scaling. The exact tribological behavior of the nanoparticles can ultimately be understood by a model that considers the influence of nanoparticle preparation on the interface conditions. By taking into account an inevitable water layer at the interface between particle and substrate that can exist in different crystalline configurations all friction phenomena observed in the experiments can be understood. Full article
(This article belongs to the Special Issue Friction Mechanisms)
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Open AccessArticle
Effect of Plastic Deformation and Damage Development during Friction of fcc Metals in the Conditions of Boundary Lubrication
Lubricants 2019, 7(5), 45; https://doi.org/10.3390/lubricants7050045 - 23 May 2019
Abstract
The main goal of the presented work was the analysis of the interaction between deformed microstructures and friction and the wear properties of four face centered cubic (fcc) metals. Pure fcc metals such as Ag, Cu, Ni, and Al with different values of [...] Read more.
The main goal of the presented work was the analysis of the interaction between deformed microstructures and friction and the wear properties of four face centered cubic (fcc) metals. Pure fcc metals such as Ag, Cu, Ni, and Al with different values of stacking fault energy (SFE) were chosen for pin-on-disk tests in lubricated conditions. Friction properties of the four fcc metals are presented herein as their Stribeck curves. The transition from elasto-hydrodynamic lubrication (EHL) to boundary lubrication (BL) regions depends mainly on the values of SFE, hardness, and contact temperature. The acoustic emission (AE) parameters were analyzed in the transition from the EHL to the BL region. The models of friction in different lubricant conditions related to the AE waveforms were proposed. The nanocrystalline top surface layers characterized the deformed structure during friction of Ag in the BL region. The lamellar cross-sectional microstructure was formed in the subsurface layers of Ag, Cu, and Ni in the friction direction. Steady state friction and wear in the BL conditions were explained by a balance between the hardening and the dynamic recovery, which was strongly dependent on the SFE and the temperature. The interaction between the deformed structure, the friction, and the wear properties of the studied metals rubbed in the BL region is discussed herein. Full article
(This article belongs to the Special Issue Friction Mechanisms)
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Open AccessArticle
Scratching Cu|Au Nanolaminates
Lubricants 2019, 7(5), 44; https://doi.org/10.3390/lubricants7050044 - 21 May 2019
Abstract
We used molecular dynamics simulations to study the scratching of Cu|Au nanolaminates of 5 nm layer thickness with a nanoscale indenter of 15 nm radius at normal forces between 0.5 μ N and 2 μ N. Our simulations show that Au layers wear [...] Read more.
We used molecular dynamics simulations to study the scratching of Cu|Au nanolaminates of 5 nm layer thickness with a nanoscale indenter of 15 nm radius at normal forces between 0.5 μ N and 2 μ N. Our simulations show that Au layers wear quickly while Cu layers are more resistant to wear. Plowing was accompanied by the roughening of the Cu|Au heterointerface that lead to the folding of the nanolaminate structure at the edge of the wear track. Our explorative simulations hint at the complex deformation processes occurring in nanolaminates under tribological load. Full article
(This article belongs to the Special Issue Friction Mechanisms)
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Review

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Open AccessReview
Solid Lubrication with MoS2: A Review
Lubricants 2019, 7(7), 57; https://doi.org/10.3390/lubricants7070057 - 02 Jul 2019
Cited by 2
Abstract
Molybdenum disulfide (MoS2) is one of the most broadly utilized solid lubricants with a wide range of applications, including but not limited to those in the aerospace/space industry. Here we present a focused review of solid lubrication with MoS2 by [...] Read more.
Molybdenum disulfide (MoS2) is one of the most broadly utilized solid lubricants with a wide range of applications, including but not limited to those in the aerospace/space industry. Here we present a focused review of solid lubrication with MoS2 by highlighting its structure, synthesis, applications and the fundamental mechanisms underlying its lubricative properties, together with a discussion of their environmental and temperature dependence. The review also includes an extensive overview of the structure and tribological properties of doped MoS2, followed by a discussion of potential future research directions. Full article
(This article belongs to the Special Issue Friction Mechanisms)
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