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Tribofilms and Solid Lubrication

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

Deadline for manuscript submissions: closed (29 February 2016) | Viewed by 87251

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Special Issue Editors

Dr. Werner Oesterle

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Guest Editor

Federal Institute for Materials Research and Testing, 12200 Berlin, Germany
Interests: nanocharacterization of materials; subsurface microstructural analysis; tribofilm functionality; dry friction applications

Prof. Dr. Ga Zhang

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Guest Editor

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, Gansu, China
Interests: tribology; tribofilm; polymer composites

Special Issue Information

Dear Colleagues,

Despite of the appreciated benefits of using lubricating oils or greases, many tribological applications require dry conditions, or, for another class of applications, it is necessary that the systems work reliably under both wet and dry conditions. Examples of technical devices depending on a predictable and reliable coefficient of friction (COF) without taking credit of hydrodynamic lubrication are given in the following (in the order of decreasing COF): brakes, clutches, tire-road contact, wheel-rail contact, bearings, space applications, hard disks for data storage.

The specific challenge of friction control during dry or mixed lubricating conditions is triggering the formation of a third body, which provides the desired friction behavior, and protects first body surfaces from being worn severely. In contrast to oil-lubricated systems, a certain amount of wear is desired and necessary for obtaining enough third body material for screening the contacting surfaces. Ideally, such systems are self-healing, i.e., if surface films get lost a new film will form from wear products of the underlying substrates. Solid lubricants, either incorporated in first body materials, or applied as coatings, play an important role in stabilizing the COF at a desired level.

This Special Issue will examine current advances and future trends of solid lubricant usage in dry friction applications. Contributions from both academic and industrial research are welcome. The papers should either aid obtaining a better understanding of tribofilm functionalities or give insight into new concepts for smart materials for dry friction applications.

Prof. Dr. Ga Zhang
Dr. Werner Oesterle
Guest Editors

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Keywords

solid lubricants
friction modifiers
friction materials
antifriction materials
tribofilms
transfer films
third body
wear reduction

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Published Papers (12 papers)

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6353 KiB

Open AccessArticle

Tribological Performance of MoS₂ Coatings in Various Environments

by Thomas Gradt and Thomas Schneider

Lubricants 2016, 4(3), 32; https://doi.org/10.3390/lubricants4030032 - 7 Sep 2016

Cited by 40 | Viewed by 7669

Abstract

Molybdenum disulfide (MoS₂) is a well-known solid lubricant for tribosystems running in vacuum or dry gases. Problems arise due to its sensitivity to humidity, which is a drawback for its application under ambient conditions. However, by using a physical vapor deposition (PVD) process, deposition parameters can be optimized not only to gain a coatings structure with favorable frictional properties but also to minimize the sensitivity to attack by water molecules. Therefore, an improved tribological behavior even under moist conditions can be achieved. MoS₂ coatings are also candidates for being applied at cryogenic temperatures. They already have proven their suitability, e.g., for sliding support elements between superconducting magnets of the nuclear fusion-experiment Wendelstein 7-X. However, these coatings were exclusively produced for this particular application and the utilization for more common tribosystems may be precluded due to cost considerations. In view of a wider range of applications, pure and Cr containing PVD-MoS₂ coatings with an optimized structure were tested under varying environments including hydrogen gas and cryogenic temperatures. Results of the most promising variant are presented in this paper. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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2643 KiB

Open AccessArticle

Tribofilms Forming in Oil-Lubricated Contacts

by Matthias Scherge, Angelika Brink and Dominic Linsler

Lubricants 2016, 4(3), 27; https://doi.org/10.3390/lubricants4030027 - 14 Jul 2016

Cited by 10 | Viewed by 4390

Abstract

The subject of the present paper is the characterization of third bodies of run-in systems. By means of continuous friction and wear measurement, lubricated steel-steel and steel-aluminum contacts were evaluated. Microstructure, chemical composition and response of the materials to shear were analyzed by XPS/AES and focused ion beam technique. After a proper running-in, both systems developed a third body. The third body differs significantly from the base materials. In addition to adapted microstructure and near-surface chemistry, the third body exhibited a substructure characterized by a near-surface zone that accommodates shear and a second, deeper region that ensures strengthening. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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13194 KiB

Open AccessFeature PaperArticle

MD Sliding Simulations of Amorphous Tribofilms Consisting of either SiO₂ or Carbon

by Andrey I. Dmitriev, Anton Yu. Nikonov and Werner Österle

Lubricants 2016, 4(3), 24; https://doi.org/10.3390/lubricants4030024 - 29 Jun 2016

Cited by 35 | Viewed by 5619

Abstract

The sliding behaviors of two simplified tribofilms with amorphous structure consisting either of SiO₂ molecules or C atoms were simulated by molecular dynamics modeling. The objective was to identify mechanisms explaining the experimentally observed lubricating properties of the two amorphous films. The impacts of layer thickness, normal pressure, temperature and different substrate materials were studied systematically, while the sliding velocity was kept constant at 30 m/s. While the layer thickness was not critical, all the other parameters showed special effects under certain conditions. Normal pressure impeded void formation and could even eliminate voids if applied at high temperature. Stick-slip sliding was changed to smooth sliding at high temperature due to void healing. Considering the carbon film, high friction forces and shearing of the entire film was observed with diamond substrates, whereas interface sliding at low friction forces and an amorphous layer of iron mixed with carbon was observed if the supporting substrates consisted of α-Fe. Both films show a decrease of friction forces and smooth sliding behavior at elevated temperature, corresponding well to the tribological behavior of an advanced nanocomposite sliding against a steel disc under severe stressing conditions when high flash temperatures can be expected. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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3898 KiB

Open AccessArticle

On the Growth Rate of Tribomaterial in Bovine Serum Lubricated Sliding Contacts

by Alfons Fischer, Daniel Stickel, Christian Schoss, Rob Bosman and Markus A. Wimmer

Lubricants 2016, 4(2), 21; https://doi.org/10.3390/lubricants4020021 - 21 Jun 2016

Cited by 5 | Viewed by 5274

Abstract

Considering total hip arthroplasty, so-called tribolayers (aka tribomaterial), consist of carbonaceous material from the periprosthetic joint fluid or bovine serum mixed with nanometer size metal and oxide wear particles. Currently, its growth sequence and rate are unknown. Thus, smooth surfaces of low-Carbon (LC-) vs. high-Carbon (HC-)CoCrMo (Cobalt-Chromium-Molybdenum) alloys have been worn in a conforming contact under bovine serum lubrication by means of a pin-on-ball wear tester. These tests were interrupted at certain numbers of cycles in order to weigh the specimens, characterize the topography, and investigate the wear appearances. In addition, after cleaning in ethanol and anionic detergent, before-and-after comparison rendered the weight of the tribomaterial. This revealed that, during run-in, the specimens gained weight by generating tribomaterial. Afterwards the loss of material surpassed the generation of new tribomaterial and a steady weight-loss was measured. Topography measurements were used as input data for contact mechanics calculations. Apparently the incipient, locally high contact stresses accelerated tribochemical reactions. After run-in, the contact situation changes and leads to a much smaller generation rate. This paper provides information about the growth sequence and rate of such tribomaterial formation. It further highlights the significance of highly localized contact stress as an important factor for tribomaterial generation. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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4169 KiB

Open AccessFeature PaperArticle

Tribology of Graphite-Filled Polystyrene

by Raffaele Gilardi

Lubricants 2016, 4(2), 20; https://doi.org/10.3390/lubricants4020020 - 9 Jun 2016

Cited by 14 | Viewed by 5836

Abstract

Self-lubricating polymer compounds are currently used for a wide range of applications such as bearings, gears, and water meters. Under severe conditions such as high pressure, high velocity, and/or high temperatures, the material fails (PV limit). In this study, we investigated the effect of graphite on the tribological properties of polystyrene (PS) with “ball-on-three-plates” tests. Graphite-filled PS plates were produced via an internal mixer and compression molding. Unhardened steel (1.4401) and nylon (PA66) balls were used for the tribological tests. Our results indicate that graphite loading, graphite type, and particle size have a big influence on the friction coefficient, the wear resistance, and the PV limit of PS both against steel and PA66. In particular, primary synthetic graphite performs better than secondary synthetic graphite due to the higher degree of crystallinity. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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3861 KiB

Open AccessArticle

Microstructural and Chemical Characterization of the Tribolayer Formation in Highly Loaded Cylindrical Roller Thrust Bearings

by Carsten Gachot, ChiaJui Hsu, Sebastián Suárez, Philipp Grützmacher, Andreas Rosenkranz, Andreas Stratmann and Georg Jacobs

Lubricants 2016, 4(2), 19; https://doi.org/10.3390/lubricants4020019 - 8 Jun 2016

Cited by 39 | Viewed by 8243

Abstract

Zinc dithiophosphates (ZDDP) have been widely applied in automobile industry for over 70 years as a lubricant additive for wear protection. Tribolayers have been described as blue- and brown-colored layers on surfaces observed by microscopical observation or even bare eye presumably as a consequence of layer thickness or chemical composition. However, the reaction pathways of ZDDP tribolayers are still not yet fully understood. In the present study, the difference between the blue- and brown-colored tribolayers has been revealed by high resolution methods in cylindrical roller thrust bearings at relatively high contact pressures of around 1.92 GPa. After running a FE8 standard bearing test with a normal load of 80 kN and a temperature of 60 °C, said tribolayers could be identified on the bearing surfaces. By using Raman spectroscopy, it could be shown that the blue-colored layers are enriched by FeS and ZnS whereas the brown-colored layers show a significant amount of Fe₃O₄. This is an interesting finding as it clearly shows a correlation between the color appearance of the films and the chemical composition besides potential film thickness variations. Finally, transmission electron microscopy verified the amorphous nature of the formed tribolayer which is in a good agreement with literature. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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4074 KiB

Open AccessArticle

The Role of Counter-Face Roughness on the Tribological Performance of a Clutch System Tested with a Pin-On-Disc Tribometer

by Graciliano Fernandes, Paulo Zanotto and Amilton Sinatora

Lubricants 2016, 4(2), 18; https://doi.org/10.3390/lubricants4020018 - 31 May 2016

Cited by 6 | Viewed by 4110

Abstract

This study was a continuous investigation of the roles played by the tribofilm on dry automotive clutch system performance. Tribological experiments were performed by the addition of wear debris at the beginning of tribometer tests and by reducing the surface roughness of a cast iron counter-face. The initial surface conditions of cast discs were 0.2 and 1.2 µm. The pin-on-disc tests were carried out at three different PV levels: 3.08, 7.88, and 10.09 MPa·ms⁻¹, and the current results were correlated to those previously obtained in the standard tribometer procedure. When the wear debris was added into the tribosystem, the friction coefficient level dropped drastically while the assembly wear rate rose. In contrast, the modified cast discs provided a reduced wear rate of assembly and a higher and more stable friction coefficient level. These improvements were obtained in a severe condition where higher temperature levels were reached. For the tests with added debris, SEM observations revealed a more intense tribofilm development over the worn surfaces of the clutch friction material. The smoothest cast disc did not damage the developed tribofilms and maintained them more stably due to a reduction in contact area stresses at the highest tribometer test. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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2530 KiB

Open AccessArticle

Tribo-Mechanisms of Carbon Nanotubes: Friction and Wear Behavior of CNT-Reinforced Nickel Matrix Composites and CNT-Coated Bulk Nickel

by Leander Reinert, Sebastian Suárez and Andreas Rosenkranz

Lubricants 2016, 4(2), 11; https://doi.org/10.3390/lubricants4020011 - 19 Apr 2016

Cited by 58 | Viewed by 6891

Abstract

In this study, nickel matrix composites reinforced by carbon nanotubes (CNTs) are compared to unreinforced CNT-coated (by drop-casting) bulk nickel samples in terms of their friction and wear behavior, thus gaining significant knowledge regarding the tribological influence of CNTs and the underlying tribo-mechanism. It has been shown that the frictional behavior is mainly influenced by the CNTs present in the contact zone, as just minor differences in the coefficient of friction between the examined samples can be observed during run-in. Consequently, the known effect of a refined microstructure, thus leading to an increased hardness of the CNT reinforced samples, seems to play a minor role in friction reduction compared to the solid lubrication effect induced by the CNTs. Additionally, a continuous supply of CNTs to the tribo-contact can be considered isolated for the reinforced composites, which provides a long-term friction reduction compared to the CNT-coated sample. Finally, it can be stated that CNTs can withstand the accumulated stress retaining to some extent their structural state for the given strain. A comprehensive study performed by complementary analytical methods is employed, including Raman spectroscopy and scanning electron microscopy to understand the involved friction and wear mechanisms. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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1489 KiB

Open AccessArticle

Influence of Workpiece Material on Tool Wear Performance and Tribofilm Formation in Machining Hardened Steel

by Junfeng Yuan, Jeremy M. Boyd, Danielle Covelli, Taib Arif, German S. Fox-Rabinovich and Stephen C. Veldhuis

Lubricants 2016, 4(2), 10; https://doi.org/10.3390/lubricants4020010 - 19 Apr 2016

Cited by 11 | Viewed by 5406

Abstract

In addition to the bulk properties of a workpiece material, characteristics of the tribofilms formed as a result of workpiece material mass transfer to the friction surface play a significant role in friction control. This is especially true in cutting of hardened materials, where it is very difficult to use liquid based lubricants. To better understand wear performance and the formation of beneficial tribofilms, this study presents an assessment of uncoated mixed alumina ceramic tools (Al2O3+TiC) in the turning of two grades of steel, AISI T1 and AISI D2. Both workpiece materials were hardened to 59 HRC then machined under identical cutting conditions. Comprehensive characterization of the resulting wear patterns and the tribofilms formed at the tool/workpiece interface were made using X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy. Metallographic studies on the workpiece material were performed before the machining process and the surface integrity of the machined part was investigated after machining. Tool life was 23% higher when turning D2 than T1. This improvement in cutting tool life and wear behaviour was attributed to a difference in: (1) tribofilm generation on the friction surface and (2) the amount and distribution of carbide phases in the workpiece materials. The results show that wear performance depends both on properties of the workpiece material and characteristics of the tribofilms formed on the friction surface. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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1197 KiB

Open AccessArticle

Mechanism of Friction and Wear in MoS₂ and ZDDP/F-PTFE Greases under Spectrum Loading Conditions

by Sujay D. Bagi and Pranesh B. Aswath

Lubricants 2015, 3(4), 687-711; https://doi.org/10.3390/lubricants3040687 - 18 Dec 2015

Cited by 21 | Viewed by 10827

Abstract

Two different greases formulated using MoS₂ and a combination of ZDDP and functionalized PTFE (F-PTFE) were examined under spectrum loading conditions where loads, frequency, and duration of the steps were treated as variables. Combination of ZDDP and F-PTFE were synergistic resulting in a significant reduction in the wear and friction under spectrum loading condition. Decreasing the time step during the ramp up and ramp down cycles resulted in larger wear for the grease containing MoS₂ particles in comparison to ZDDP/F-PTFE in grease. The tribofilm formed on the surface was analyzed using various characterization techniques like SEM, EDS, and Stereo Optical Microscopy. Tribofilms from MoS₂ additives had extensive amounts of abrasive and adhesive wear and showed the formation of MoS₂ on the surface on the other hand the tribofilms from ZDDP/F-PTFE had smaller amounts of severe wear and exhibited patchy tribofilms of Zn-phosphates as well as sulfides of Zn and Fe. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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Review

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748 KiB

Open AccessReview

The Role of Solid Lubricants for Brake Friction Materials

by Werner Österle and Andrey I. Dmitriev

Lubricants 2016, 4(1), 5; https://doi.org/10.3390/lubricants4010005 - 29 Feb 2016

Cited by 79 | Viewed by 12891

Abstract

This review article comprises of three parts. Firstly, reports of brake manufacturers on the beneficial impact of solid lubricants for pad formulations are surveyed. Secondly, since tribofilms were identified to play a crucial role in friction stabilization and wear reduction, the knowledge about tribofilm structures formed during automotive braking was reviewed comprehensively. Finally, a model for simulating the sliding behavior of tribofilms is suggested and a review on modelling efforts with different model structures related to real tribofilms will be presented. Although the variety of friction composites involved in commercial brake systems is very broad, striking similarities were observed in respect to tribofilm nanostructures. Thus, a generalization of the tribofilm nanostructure is suggested and prerequisites for smooth sliding performance and minimal wear rates have been identified. A minimum of 13 vol % of soft inclusions embedded in an iron oxide based tribofilm is crucial for obtaining the desired properties. As long as the solid lubricants or their reaction products are softer than magnetite, the main constituent of the tribofilm, the model predicts smooth sliding and minimum wear. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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2140 KiB

Open AccessReview

A Review of Transfer Films and Their Role in Ultra-Low-Wear Sliding of Polymers

by Jiaxin Ye, David L. Burris and Ting Xie

Lubricants 2016, 4(1), 4; https://doi.org/10.3390/lubricants4010004 - 26 Feb 2016

Cited by 103 | Viewed by 9007

Abstract

In dry sliding conditions, polytetrafluoroethylene (PTFE) composites can form thin, uniform, and protective transfer films on hard, metallic counterfaces that may play a significant role in friction and wear control. Qualitative characterizations of transfer film morphology, composition, and adhesion to the counterface suggest they are all good predictors of friction and, particularly, wear performance. However, a lack of quantitative transfer film characterization methods and uncertainty regarding specific mechanisms of friction and wear control make definitive conclusions about causal relationships between transfer film and tribological properties difficult. This paper reviews the state of the art in the solid lubricant transfer film literature and highlights recent advances in quantitative characterization thereof. Full article

(This article belongs to the Special Issue Tribofilms and Solid Lubrication)

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