Special Issue "Tribofilms and Solid Lubrication"

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

Deadline for manuscript submissions: closed (29 February 2016)

Special Issue Editors

Guest Editor
Dr. Werner Oesterle

Federal Institute for Materials Research and Testing, 12200 Berlin, Germany
Website | E-Mail
Fax: +493081041517
Interests: nanocharacterization of materials; subsurface microstructural analysis; tribofilm functionality; dry friction applications
Guest Editor
Prof. Dr. Ga Zhang

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, Gansu, China
Website | E-Mail
Phone: +86 (0) 931 4968041
Fax: +86 (0) 931 8277088
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

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 550 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

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

Published Papers (12 papers)

View options order results:
result details:
Displaying articles 1-12
Export citation of selected articles as:

Research

Jump to: Review

Open AccessArticle
Tribological Performance of MoS2 Coatings in Various Environments
Received: 29 February 2016 / Revised: 23 August 2016 / Accepted: 29 August 2016 / Published: 7 September 2016
Cited by 3 | PDF Full-text (6353 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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)
Figures

Figure 1

Open AccessArticle
Tribofilms Forming in Oil-Lubricated Contacts
Received: 7 May 2016 / Revised: 28 June 2016 / Accepted: 7 July 2016 / Published: 14 July 2016
Cited by 2 | PDF Full-text (2643 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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)
Figures

Figure 1

Open AccessFeature PaperArticle
MD Sliding Simulations of Amorphous Tribofilms Consisting of either SiO2 or Carbon
Received: 21 April 2016 / Revised: 17 June 2016 / Accepted: 21 June 2016 / Published: 29 June 2016
Cited by 16 | PDF Full-text (13194 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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)
Figures

Figure 1

Open AccessArticle
On the Growth Rate of Tribomaterial in Bovine Serum Lubricated Sliding Contacts
Received: 16 February 2016 / Revised: 8 April 2016 / Accepted: 5 May 2016 / Published: 21 June 2016
Cited by 2 | PDF Full-text (3898 KB) | HTML Full-text | XML Full-text
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-) [...] Read more.
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)
Figures

Figure 1

Open AccessFeature PaperArticle
Tribology of Graphite-Filled Polystyrene
Received: 14 March 2016 / Revised: 6 April 2016 / Accepted: 18 May 2016 / Published: 9 June 2016
PDF Full-text (4169 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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)
Figures

Figure 1

Open AccessArticle
Microstructural and Chemical Characterization of the Tribolayer Formation in Highly Loaded Cylindrical Roller Thrust Bearings
Received: 24 March 2016 / Revised: 13 May 2016 / Accepted: 18 May 2016 / Published: 8 June 2016
Cited by 7 | PDF Full-text (3861 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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)
Figures

Figure 1

Open AccessArticle
The Role of Counter-Face Roughness on the Tribological Performance of a Clutch System Tested with a Pin-On-Disc Tribometer
Received: 18 February 2016 / Revised: 20 April 2016 / Accepted: 21 April 2016 / Published: 31 May 2016
Cited by 1 | PDF Full-text (4074 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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)
Figures

Figure 1

Open AccessArticle
Tribo-Mechanisms of Carbon Nanotubes: Friction and Wear Behavior of CNT-Reinforced Nickel Matrix Composites and CNT-Coated Bulk Nickel
Received: 1 March 2016 / Revised: 6 April 2016 / Accepted: 14 April 2016 / Published: 19 April 2016
Cited by 14 | PDF Full-text (2530 KB) | HTML Full-text | XML Full-text
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. [...] Read more.
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)
Figures

Figure 1

Open AccessArticle
Influence of Workpiece Material on Tool Wear Performance and Tribofilm Formation in Machining Hardened Steel
Received: 23 February 2016 / Revised: 5 April 2016 / Accepted: 6 April 2016 / Published: 19 April 2016
Cited by 6 | PDF Full-text (1489 KB) | HTML Full-text | XML Full-text
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, [...] Read more.
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)
Figures

Figure 1

Open AccessArticle
Mechanism of Friction and Wear in MoS2 and ZDDP/F-PTFE Greases under Spectrum Loading Conditions
Lubricants 2015, 3(4), 687-711; https://doi.org/10.3390/lubricants3040687
Received: 26 July 2015 / Revised: 8 December 2015 / Accepted: 9 December 2015 / Published: 18 December 2015
Cited by 4 | PDF Full-text (1197 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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)
Figures

Figure 1

Review

Jump to: Research

Open AccessReview
The Role of Solid Lubricants for Brake Friction Materials
Received: 11 December 2015 / Revised: 18 February 2016 / Accepted: 19 February 2016 / Published: 29 February 2016
Cited by 13 | PDF Full-text (748 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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)
Figures

Figure 1

Open AccessReview
A Review of Transfer Films and Their Role in Ultra-Low-Wear Sliding of Polymers
Received: 16 January 2016 / Revised: 3 February 2016 / Accepted: 16 February 2016 / Published: 26 February 2016
Cited by 20 | PDF Full-text (2140 KB) | HTML Full-text | XML Full-text
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 [...] Read more.
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)
Figures

Figure 1

Lubricants EISSN 2075-4442 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
Back to Top