Research

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30 pages, 2517 KiB

Open AccessArticle

by Anle Wang, Yunong Zhou and Martin H. Müser

Lubricants 2021, 9(2), 17; https://doi.org/10.3390/lubricants9020017 - 08 Feb 2021

Cited by 15 | Viewed by 2782

When an elastomer approaches or retracts from an adhesive indenter, the elastomer’s surface can suddenly become unstable and reshape itself quasi-discontinuously, e.g., when small-scale asperities jump into or snap out of contact. Such dynamics lead to a hysteresis between approach and retraction. In [...] Read more.

When an elastomer approaches or retracts from an adhesive indenter, the elastomer’s surface can suddenly become unstable and reshape itself quasi-discontinuously, e.g., when small-scale asperities jump into or snap out of contact. Such dynamics lead to a hysteresis between approach and retraction. In this study, we quantify numerically and analytically the ensuing unavoidable energy loss for rigid indenters with flat, Hertzian and randomly rough profiles. The range of adhesion turns out to be central, in particular during the rarely modeled approach to contact. For example, negligible traction on approach but quite noticeable adhesion for nominal plane contacts hinges on the use of short-range adhesion. Central attention is paid to the design of cohesive-zone models for the efficient simulation of dynamical processes. Our study includes a Griffith’s type analysis for the energy lost during fracture and regeneration of a flat interface. It reveals that the leading-order corrections of the energy loss are due to the finite-range adhesion scale at best, with the third root of the linear mesh size, while leading-order errors in the pull-off force disappear linearly. Full article

(This article belongs to the Special Issue Friction Mechanisms)

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18 pages, 652 KiB

Open AccessArticle

Shear Thinning in the Prandtl Model and Its Relation to Generalized Newtonian Fluids

by Martin H. Müser

Lubricants 2020, 8(4), 38; https://doi.org/10.3390/lubricants8040038 - 25 Mar 2020

Cited by 10 | Viewed by 3426

Abstract

The Prandtl model is certainly the simplest and most generic microscopic model describing solid friction. It consists of a single, thermalized atom attached to a spring, which is dragged past a sinusoidal potential representing the surface energy corrugation of a counterface. While it [...] Read more.

The Prandtl model is certainly the simplest and most generic microscopic model describing solid friction. It consists of a single, thermalized atom attached to a spring, which is dragged past a sinusoidal potential representing the surface energy corrugation of a counterface. While it was primarily introduced to rationalize how Coulomb’s friction law can arise from small-scale instabilities, Prandtl argued that his model also describes the shear thinning of liquids. Given its success regarding the interpretation of atomic-force-microscopy experiments, surprisingly little attention has been paid to the question how the Prandtl model relates to fluid rheology. Analyzing its Langevin and Brownian dynamics, we show that the Prandtl model produces friction–velocity relationships, which, converted to a dependence of effective (excess) viscosity on shear rate

η (\dot{γ})

, is strikingly similar to the Carreau–Yasuda (CY) relation, which is obeyed by many non-Newtonian liquids. The two dimensionless parameters in the CY relation are found to span a broad range of values. When thermal energy is small compared to the corrugation of the sinusoidal potential, the leading-order

{\dot{γ}}^{2}

corrections to the equilibrium viscosity only matter in the initial part of the cross-over from Stokes friction to the regime, where

η

obeys approximately a sublinear power law of

1 / \dot{γ}

. Full article

(This article belongs to the Special Issue Friction Mechanisms)

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19 pages, 850 KiB

Open AccessArticle

Contact Mechanics for Solids with Randomly Rough Surfaces and Plasticity

by Avinash Tiwari, Anle Wang, Martin H. Müser and B. N. J. Persson

Lubricants 2019, 7(10), 90; https://doi.org/10.3390/lubricants7100090 - 16 Oct 2019

Cited by 9 | Viewed by 4037

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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18 pages, 4644 KiB

Open AccessArticle

Polymer Brush Friction in Cylindrical Geometries

by Karel J. van der Weg, Guido C. Ritsema van Eck and Sissi de Beer

Lubricants 2019, 7(10), 84; https://doi.org/10.3390/lubricants7100084 - 25 Sep 2019

Cited by 6 | Viewed by 3415

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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12 pages, 5573 KiB

Open AccessArticle

Friction vs. Area Scaling of Superlubric NaCl-Particles on Graphite

by Felix Hartmuth, Dirk Dietzel, Astrid S. de Wijn and André Schirmeisen

Lubricants 2019, 7(8), 66; https://doi.org/10.3390/lubricants7080066 - 06 Aug 2019

Cited by 8 | Viewed by 3117

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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18 pages, 12597 KiB

Open AccessArticle

Effect of Plastic Deformation and Damage Development during Friction of fcc Metals in the Conditions of Boundary Lubrication

by Alexey Moshkovich, Vladislav Perfilyev and Lev Rapoport

Lubricants 2019, 7(5), 45; https://doi.org/10.3390/lubricants7050045 - 23 May 2019

Cited by 12 | Viewed by 3704

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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12 pages, 4934 KiB

Open AccessArticle

Scratching Cu|Au Nanolaminates

by Adrien Gola and Lars Pastewka

Lubricants 2019, 7(5), 44; https://doi.org/10.3390/lubricants7050044 - 21 May 2019

Cited by 3 | Viewed by 3603

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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35 pages, 8727 KiB

Open AccessReview

Solid Lubrication with MoS₂: A Review

by Mohammad R. Vazirisereshk, Ashlie Martini, David A. Strubbe and Mehmet Z. Baykara

Lubricants 2019, 7(7), 57; https://doi.org/10.3390/lubricants7070057 - 02 Jul 2019

Cited by 322 | Viewed by 21190

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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