Advances in Friction Modifiers and Liquid Confinement

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

Deadline for manuscript submissions: closed (31 July 2023) | Viewed by 9504

Special Issue Editor


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Guest Editor
Department of Materials Science and Engineering, Saarland University, 66123 Saarbrücken, Germany
Interests: nanotribology; atomistic simulations; solid and liquid lubricants; force fields

Special Issue Information

Dear Colleagues,

Lubrication at small-scale contact is a complex matter given the prominent size effect and atomistic nature. For example, liquids under high confinement often exhibit a layering-like structure, with in-plane molecules arranged in a disordered manner. Because of their material nature, confining geometry, etc., such a confinement-induced molecular ordering can significantly extend the structural relaxation time and thereby result in a much-enhanced shear viscosity. When first-order or second-order phase transformation takes place, collective and solid-like interfacial behavior can be observed, which in turn can affect the tribological performance of the system, as well as its stability. Despite that, anisotropic stress tensor, essentially a natural consequence of liquid’s wavenumber dependent compressibility, is able to allow the confined film to be load-bearing. Do other roles come into play?

In some cases, friction modifiers are used as lubricant additives to lower friction and wear in the boundary lubrication regime. The combining polar head groups and long hydrocarbon chains enable them to adsorb onto a metal surface and self-assemble into a protecting monolayer, to prevent direct solid–solid contact. The viscoelastic nature of such an intermediate substance, however, complicates the interfacial behavior, given that its short-ranged deformation differs from typical elastic materials, and the so-called plowing mechanism renders classical models, e.g., JKR and DMT, inadequate. In this regard, further exploring size and molecular structural effects, alongside how energy dissipates during tip sliding, may be of interest, both scientifically and practically.

This Special Issue promotes the improved understanding of interfacial (tribological) behavior of liquid lubricants and additives in the boundary lubrication regime, and origins of the potential effects. Insights can be gained from investigations at different length scales and by means of experimental, modeling, and theoretical approaches.

Dr. Hongyu (Larry) Gao
Guest Editor

Manuscript Submission Information

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Keywords

  • boundary lubrication
  • friction modifiers
  • lubricants and additives
  • experiments and simulations at crossed scales
  • rheology
  • friction and wear
  • liquid confinement

Published Papers (4 papers)

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Research

24 pages, 12026 KiB  
Article
Effect of Friction Reducers with Unreinforced PEEK and Steel Counterparts in Oil Lubrication
by Davide Massocchi, Steven Chatterton, Marco Lattuada, Thomas Reddyhoff, Daniele Dini and Paolo Pennacchi
Lubricants 2023, 11(11), 487; https://doi.org/10.3390/lubricants11110487 - 10 Nov 2023
Viewed by 1543
Abstract
The increasing adoption of PEEK (polyetheretherketone) in many industrial applications has promoted intense research to optimize its lubrication along with the development of friction reducers (FRs), additives that help in reducing fuel consumption and, consequently, CO2 emissions. In this study, the effect [...] Read more.
The increasing adoption of PEEK (polyetheretherketone) in many industrial applications has promoted intense research to optimize its lubrication along with the development of friction reducers (FRs), additives that help in reducing fuel consumption and, consequently, CO2 emissions. In this study, the effect of FRs in improving the lubrication of PEEK–steel couplings was evaluated and their mechanism studied using the Mini Traction Machine (MTM) tribometer. Different types of FRs (such as Molybdenum dithiocarbamate, glycerol monooleate, amine and polymeric derivatives) and coupling combinations (steel/steel, steel/PEEK and PEEK/steel) were considered. The oil samples were evaluated as fresh and after a rubbing time considering different operative conditions (from high to low T, fixed load and type of contact motion), and a measurement of the tribofilm was acquired. The experimental campaign showed a ranking among FRs friction-reducing behavior and, in some cases, a synergistic effect was noted between the tribofilm containing the friction modifier and the PEEK surface. Comparing the top performing FRs with reference oil showed a reduction in friction of 22%, 21% and 37%, respectively, in steel–steel, PEEK–steel and steel–PEEK couplings, while in the standard steel–steel coupling, two out of four FRs did not reduce the friction. After conditioning in the presence of PEEK, all friction-modifier additives reduced the friction effectively. This demonstrates the promising performance of PEEK, its compatibility with friction-reducing additives, and its applicability to sliding machine parts in order to improve efficiency and thus reduce CO2 emissions. Full article
(This article belongs to the Special Issue Advances in Friction Modifiers and Liquid Confinement)
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16 pages, 5885 KiB  
Article
Effect of Molecular Weight on Tribological Properties of Polyether Amine Derivatives under Different Contact Modes
by Wenjing Hu and Jiusheng Li
Lubricants 2022, 10(6), 105; https://doi.org/10.3390/lubricants10060105 - 27 May 2022
Cited by 3 | Viewed by 2931
Abstract
The requirements for the fuel economy of modern industry continue to drive the progress of low-viscosity lubricants. The present work reports the application of polyether amine derivatives as friction modifiers to improve the tribological properties of low viscosity poly-alpha-olefin. Three polyether amine derivatives [...] Read more.
The requirements for the fuel economy of modern industry continue to drive the progress of low-viscosity lubricants. The present work reports the application of polyether amine derivatives as friction modifiers to improve the tribological properties of low viscosity poly-alpha-olefin. Three polyether amine derivatives with different molecular weights were synthesized, the tribological properties of which were systematically investigated under three different contact modes. These functionalized polymers exhibited significant friction reduction and wear resistance properties in the point-on-flat and line-on-flat friction tests, but just showed anti-wear performance in the severe point-to-point contact mode. The results exhibited that molecular weights of the polymers had a direct effect on their tribological properties. The increase of molecular weight in a certain range was beneficial to the improvement of tribological properties, but further undue increase will rather reduce the friction reduction and wear resistance performances. It can be indicated that the number of oxygen atoms increased with the molecular weight of the polymer, which will be conductive to the adsorption of the polymer on the metal surface. However, when the molecular weight of the polymer exceeds a certain value, the steric hindrance of the molecules adsorbing to the metal surface increases, which in turn has a negative impact on the tribological properties. Full article
(This article belongs to the Special Issue Advances in Friction Modifiers and Liquid Confinement)
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9 pages, 28252 KiB  
Article
Influence of Additive Chemistry on the Tribological Behavior of Steel/Copper Friction Pairs
by Huaigang Su, Yunlong Chen, Rui Ma, Weimin Li, Gaiqing Zhao, Yanxing Qi and Wenjing Lou
Lubricants 2022, 10(5), 91; https://doi.org/10.3390/lubricants10050091 - 10 May 2022
Cited by 2 | Viewed by 2090
Abstract
Tribological properties of five anti-wear additives for a steel-copper contact were investigated. It was found that the tribological performances are closely related to the molecular structure of additives. The protic ionic liquid anti-wear additive AW316 exhibits the best tribological performance with the lowest [...] Read more.
Tribological properties of five anti-wear additives for a steel-copper contact were investigated. It was found that the tribological performances are closely related to the molecular structure of additives. The protic ionic liquid anti-wear additive AW316 exhibits the best tribological performance with the lowest mean friction coefficient of 0.082, and the smallest wear volume, which is more than one order of magnitude smaller than base oil. Transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) tests reveal that a 10–15 nm thickness uniform boundary lubrication film composed of oxides, phosphates, and cuprous oxide was formed on the copper disc, which was responsible for its outstanding tribological performances. Full article
(This article belongs to the Special Issue Advances in Friction Modifiers and Liquid Confinement)
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10 pages, 943 KiB  
Article
History-Dependent Stress Relaxation of Liquids under High-Confinement: A Molecular Dynamics Study
by Hongyu Gao
Lubricants 2022, 10(2), 15; https://doi.org/10.3390/lubricants10020015 - 19 Jan 2022
Cited by 1 | Viewed by 2047
Abstract
When liquids are confined into a nanometer-scale slit, the induced layering-like film structure allows the liquid to sustain non-isotropic stresses and thus be load-bearing. Such anisotropic characteristics of liquid under confinement arise naturally from the liquids’ wavenumber dependent compressibility, which does not need [...] Read more.
When liquids are confined into a nanometer-scale slit, the induced layering-like film structure allows the liquid to sustain non-isotropic stresses and thus be load-bearing. Such anisotropic characteristics of liquid under confinement arise naturally from the liquids’ wavenumber dependent compressibility, which does not need solidification to take place as a prerequisite. In other words, liquids under confinement can still retain fluidity with molecules being (sub-)diffusive. However, the extensively prolonged structural relaxation times can cause hysteresis of stress relaxation of confined molecules in response to the motions of confining walls and thereby rendering the quasi-static stress tensors history-dependent. In this work, by means of molecular dynamics, stress tensors of a highly confined key base-oil component, i.e., 1-decene trimer, are calculated after its relaxation from being compressed and decompressed. A maximum of 77.1 MPa normal stress discrepancy has been detected within a triple-layer boundary film. Analyses with respect to molecular morphology indicate that among the effects (e.g., confinement, molecular structure, and film density) that can potentially affect confined stresses, the ordering status of the confined molecules plays a predominant role. Full article
(This article belongs to the Special Issue Advances in Friction Modifiers and Liquid Confinement)
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