1
|
Zhang D, Huang M, Klausen LH, Li Q, Li S, Dong M. Liquid-Phase Friction of Two-Dimensional Molybdenum Disulfide at the Atomic Scale. ACS Appl Mater Interfaces 2023; 15:21595-21601. [PMID: 37070722 DOI: 10.1021/acsami.3c00221] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
Tribological properties depend strongly on environmental conditions such as temperature, humidity, and operation liquid. However, the origin of the liquid effect on friction remains largely unexplored. Herein, taking molybdenum disulfide (MoS2) as a model system, we explored the nanoscale friction of MoS2 in polar (water) and nonpolar (dodecane) liquids through friction force microscopy. The friction force exhibits a similar layer-dependent behavior in liquids as in air; i.e., thinner samples have a larger friction force. Interestingly, friction is significantly influenced by the polarity of the liquid, and it is larger in polar water than in nonpolar dodecane. Atomically resolved friction images together with atomistic simulations reveal that the polarity of the liquid has a substantial effect on friction behavior, where liquid molecule arrangement and hydrogen-bond formation lead to a higher resistance in polar water in comparison to that in nonpolar dodecane. This work provides insights into the friction on two-dimensional layered materials in liquids and holds great promise for future low-friction technologies.
Collapse
Affiliation(s)
- Deliang Zhang
- Key Laboratory of Colloid and Interface Chemistry of the Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
| | - Mingzheng Huang
- State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
| | | | - Qiang Li
- Key Laboratory of Colloid and Interface Chemistry of the Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
| | - Suzhi Li
- State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
| | - Mingdong Dong
- Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Aarhus C DK-8000, Denmark
| |
Collapse
|
2
|
Milne ZB, Hasz K, McClimon JB, Castro J, Carpick RW. A modified multibond model for nanoscale static friction. Philos Trans A Math Phys Eng Sci 2022; 380:20210342. [PMID: 35909363 DOI: 10.1098/rsta.2021.0342] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/30/2021] [Accepted: 02/28/2022] [Indexed: 06/15/2023]
Abstract
Several key features of nanoscale friction phenomena observed in experiments, including the stick-slip to smooth sliding transition and the velocity and temperature dependence of friction, are often described by reduced-order models. The most notable of these are the thermal Prandtl-Tomlinson model and the multibond model. Here we present a modified multibond (mMB) model whereby a physically-based criterion-a critical bond stretch length-is used to describe interfacial bond breaking. The model explicitly incorporates damping in both the cantilever and the contacting materials. Comparison to the Fokker-Planck formalism supports the results of this new model, confirming its ability to capture the relevant physics. Furthermore, the mMB model replicates the near-logarithmic trend of increasing friction with lateral scanning speed seen in many experiments. The model can also be used to probe both correlated and uncorrelated stick slip. Through greater understanding of the effects of damping and noise in the system and the ability to more accurately simulate a system with multiple interaction sites, this model extends the range of frictional systems and phenomena that can be investigated. This article is part of the theme issue 'Nanocracks in nature and industry'.
Collapse
Affiliation(s)
- Zachary B Milne
- Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA, USA
| | - Kathryn Hasz
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, USA
| | - J B McClimon
- Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA, USA
| | - Juan Castro
- Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA, USA
| | - Robert W Carpick
- Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA, USA
| |
Collapse
|
3
|
Vazirisereshk MR, Hasz K, Zhao MQ, Johnson ATC, Carpick RW, Martini A. Nanoscale Friction Behavior of Transition-Metal Dichalcogenides: Role of the Chalcogenide. ACS Nano 2020; 14:16013-16021. [PMID: 33090766 DOI: 10.1021/acsnano.0c07558] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Despite extensive research on the tribological properties of MoS2, the frictional characteristics of other members of the transition-metal dichalcogenide (TMD) family have remained relatively unexplored. To understand the effect of the chalcogen on the tribological behavior of these materials and gain broader general insights into the factors controlling friction at the nanoscale, we compared the friction force behavior for a nanoscale single asperity sliding on MoS2, MoSe2, and MoTe2 in both bulk and monolayer forms through a combination of atomic force microscopy experiments and molecular dynamics simulations. Experiments and simulations showed that, under otherwise identical conditions, MoS2 has the highest friction among these materials and MoTe2 has the lowest. Simulations complemented by theoretical analysis based on the Prandtl-Tomlinson model revealed that the observed friction contrast between the TMDs was attributable to their lattice constants, which differed depending on the chalcogen. While the corrugation amplitudes of the energy landscapes are similar for all three materials, larger lattice constants permit the tip to slide more easily across correspondingly wider saddle points in the potential energy landscape. These results emphasize the critical role of the lattice constant, which can be the determining factor for frictional behavior at the nanoscale.
Collapse
Affiliation(s)
- Mohammad R Vazirisereshk
- Department of Mechanical Engineering, University of California, Merced, California 95343, United States
| | - Kathryn Hasz
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - Meng-Qiang Zhao
- Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
- Department of Chemical and Materials Engineering, New Jersey Institute of Technology, Newark 07102, United States
| | - A T Charlie Johnson
- Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - Robert W Carpick
- Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - Ashlie Martini
- Department of Mechanical Engineering, University of California, Merced, California 95343, United States
| |
Collapse
|
4
|
Reichenbach T, Mayrhofer L, Kuwahara T, Moseler M, Moras G. Steric Effects Control Dry Friction of H- and F-Terminated Carbon Surfaces. ACS Appl Mater Interfaces 2020; 12:8805-8816. [PMID: 31971767 DOI: 10.1021/acsami.9b18019] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
A stable passivation of surface dangling bonds underlies the outstanding friction properties of diamond and diamond-like carbon (DLC) coatings in boundary lubrication. While hydrogen is the simplest termination of a carbon dangling bond, fluorine can also be used as a monoatomic termination, providing an even higher chemical stability. However, whether and under which conditions a substitution of hydrogen with fluorine can be beneficial to friction is still an open question. Moreover, which of the chemical differences between C-H and C-F bonds are responsible for the change in friction has not been unequivocally understood yet. In order to shed light on this problem, we develop a density functional theory-based, nonreactive force field that describes the relevant properties of hydrogen- and fluorine-terminated diamond and DLC tribological interfaces. Molecular dynamics and nudged elastic band simulations reveal that the frictional stress at such interfaces correlates with the corrugation of the contact potential energy, thus ruling out a significant role of the mass of the terminating species on friction. Furthermore, the corrugation of the contact potential energy is almost exclusively determined by steric factors, while electrostatic interactions only play a minor role. In particular, friction between atomically flat diamond surfaces is controlled by the density of terminations, by the C-H and C-F bond lengths, and by the H and F atomic radii. For sliding DLC/DLC interfaces, the intrinsic atomic-scale surface roughness plays an additional role. While surface fluorination decreases the friction of incommensurate diamond contacts, it can negatively affect the friction performance of carbon surfaces that are disordered and not atomically flat. This work provides a general framework to understand the impact of chemical structure of surfaces on friction and to generate design rules for optimally terminated low-friction systems.
Collapse
Affiliation(s)
- Thomas Reichenbach
- Fraunhofer IWM, MicroTribology Center μTC , Wöhlerstraße 11 , 79108 Freiburg , Germany
- Institute of Physics , University of Freiburg , Hermann-Herder-Straße 3 , 79104 Freiburg , Germany
| | - Leonhard Mayrhofer
- Fraunhofer IWM, MicroTribology Center μTC , Wöhlerstraße 11 , 79108 Freiburg , Germany
- Cluster of Excellence livMatS@FIT-Freiburg Center for Interactive Materials and Bioinspired Technologies , University of Freiburg , Georges-Köhler-Allee 105 , 79110 Freiburg , Germany
| | - Takuya Kuwahara
- Fraunhofer IWM, MicroTribology Center μTC , Wöhlerstraße 11 , 79108 Freiburg , Germany
| | - Michael Moseler
- Fraunhofer IWM, MicroTribology Center μTC , Wöhlerstraße 11 , 79108 Freiburg , Germany
- Institute of Physics , University of Freiburg , Hermann-Herder-Straße 3 , 79104 Freiburg , Germany
- Freiburg Materials Research Center , University of Freiburg , Stefan-Meier-Straße 21 , 79104 Freiburg , Germany
- Cluster of Excellence livMatS@FIT-Freiburg Center for Interactive Materials and Bioinspired Technologies , University of Freiburg , Georges-Köhler-Allee 105 , 79110 Freiburg , Germany
| | - Gianpietro Moras
- Fraunhofer IWM, MicroTribology Center μTC , Wöhlerstraße 11 , 79108 Freiburg , Germany
| |
Collapse
|
5
|
Pawlak R, Vilhena JG, D'Astolfo P, Liu X, Prampolini G, Meier T, Glatzel T, Lemkul JA, Häner R, Decurtins S, Baratoff A, Pérez R, Liu SX, Meyer E. Sequential Bending and Twisting around C-C Single Bonds by Mechanical Lifting of a Pre-Adsorbed Polymer. Nano Lett 2020; 20:652-657. [PMID: 31797665 DOI: 10.1021/acs.nanolett.9b04418] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Bending and twisting around carbon-carbon single bonds are ubiquitous in natural and synthetic polymers. Force-induced changes were so far not measured at the single-monomer level, owing to limited ways to apply local forces. We quantified down to the submolecular level the mechanical response within individual poly-pyrenylene chains upon their detachment from a gold surface with an atomic force microscope at 5 K. Computer simulations based on a dedicated force field reproduce the experimental traces and reveal symmetry-broken bent and rotated conformations of the sliding physisorbed segment besides steric hindrance of the just lifted monomer. Our study also shows that the tip-molecule bond remains intact but remarkably soft and links force variations to complex but well-defined conformational changes.
Collapse
Affiliation(s)
- Rémy Pawlak
- Department of Physics , University of Basel , Klingelbergstrasse 82 , 4056 Basel , Switzerland
| | - J G Vilhena
- Department of Physics , University of Basel , Klingelbergstrasse 82 , 4056 Basel , Switzerland
| | - Philipp D'Astolfo
- Department of Physics , University of Basel , Klingelbergstrasse 82 , 4056 Basel , Switzerland
| | - Xunshan Liu
- Department of Chemistry and Biochemistry , University of Bern , Freiestrasse 3 , Bern , CH 3012 , Switzerland
| | - Giacomo Prampolini
- CNR-Consiglio Nazionale delle Ricerche , Istituto di Chimica dei Composti Organo Metallici (ICCOM-CNR) , Pisa , Italy
| | - Tobias Meier
- Department of Physics , University of Basel , Klingelbergstrasse 82 , 4056 Basel , Switzerland
| | - Thilo Glatzel
- Department of Physics , University of Basel , Klingelbergstrasse 82 , 4056 Basel , Switzerland
| | - Justin A Lemkul
- Department of Biochemistry , Virginia Tech , 303 Engel Hall, 340 West Campus Drive , Blacksburg , Virginia 24061 , United States
| | - Robert Häner
- Department of Chemistry and Biochemistry , University of Bern , Freiestrasse 3 , Bern , CH 3012 , Switzerland
| | - Silvio Decurtins
- Department of Chemistry and Biochemistry , University of Bern , Freiestrasse 3 , Bern , CH 3012 , Switzerland
| | - Alexis Baratoff
- Department of Physics , University of Basel , Klingelbergstrasse 82 , 4056 Basel , Switzerland
| | - Rubén Pérez
- Departamento de Física Teórica de la Materia Condensada , Universidad Autónoma de Madrid , E-28049 Madrid , Spain
- Condensed Matter Physics Center (IFIMAC) , Universidad Autónoma de Madrid , E-28049 Madrid , Spain
| | - Shi-Xia Liu
- Department of Chemistry and Biochemistry , University of Bern , Freiestrasse 3 , Bern , CH 3012 , Switzerland
| | - Ernst Meyer
- Department of Physics , University of Basel , Klingelbergstrasse 82 , 4056 Basel , Switzerland
| |
Collapse
|
6
|
He F, Yang X, Bian Z, Xie G, Guo D, Luo J. In-Plane Potential Gradient Induces Low Frictional Energy Dissipation during the Stick-Slip Sliding on the Surfaces of 2D Materials. Small 2019; 15:e1904613. [PMID: 31639269 DOI: 10.1002/smll.201904613] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/16/2019] [Revised: 09/22/2019] [Indexed: 06/10/2023]
Abstract
Understanding the nanoscale friction properties of 2D materials and further manipulating their friction behaviors is of great significance for the development of various micro/nanodevices. Recent studies, taking advantage of the close relationship between friction and surface charges, use an external out-of-plane electric field to control the interfacial friction. Nevertheless, friction increases with the application of the out-of-plane electric field in most cases. Here, an in-plane potential gradient is applied for the investigation of the contribution of electric charges to friction on the surfaces of 2D materials. Experimental results show that the friction between an atomic force microscope tip and the flakes of 2D materials decreases with the application of the in-plane potential gradient, and the higher the potential gradient, the greater the friction decrease. By comparing the in situ atomic-level stick-slip maps before and after the application of the in-plane potential gradient, it is proposed that the promotion of low friction dissipative motion during the stick-slip process owing to the presence of the potential gradient gives rise to the friction reduction. These results not only help to reveal the origin of friction, but also provide a novel way to manipulate friction through an electrically-controlled sliding process.
Collapse
Affiliation(s)
- Feng He
- State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Haidian District, Beijing, 100084, China
| | - Xiao Yang
- State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Haidian District, Beijing, 100084, China
| | - Zhengliang Bian
- Department of Engineering Mechanics, Tsinghua University, Haidian District, Beijing, 100084, China
| | - Guoxin Xie
- State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Haidian District, Beijing, 100084, China
| | - Dan Guo
- State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Haidian District, Beijing, 100084, China
| | - Jianbin Luo
- State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Haidian District, Beijing, 100084, China
| |
Collapse
|
7
|
Milne ZB, Schall JD, Jacobs TDB, Harrison JA, Carpick RW. Covalent Bonding and Atomic-Level Plasticity Increase Adhesion in Silicon-Diamond Nanocontacts. ACS Appl Mater Interfaces 2019; 11:40734-40748. [PMID: 31498997 DOI: 10.1021/acsami.9b08695] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Nanoindentation and sliding experiments using single-crystal silicon atomic force microscope probes in contact with diamond substrates in vacuum were carried out in situ with a transmission electron microscope (TEM). After sliding, the experimentally measured works of adhesion were significantly larger than values estimated for pure van der Waals (vdW) interactions. Furthermore, the works of adhesion increased with both the normal stress and speed during the sliding, indicating that applied stress played a central role in the reactivity of the interface. Complementary molecular dynamics (MD) simulations were used to lend insight into the atomic-level processes that occur during these experiments. Simulations using crystalline silicon tips with varying degrees of roughness and diamond substrates with different amounts of hydrogen termination demonstrated two relevant phenomena. First, covalent bonds formed across the interface, where the number of bonds formed was affected by the hydrogen termination of the substrate, the tip roughness, the applied stress, and the stochastic nature of bond formation. Second, for initially rough tips, the sliding motion and the associated application of shear stress produced an increase in irreversible atomic-scale plasticity that tended to smoothen the tips' surfaces, which resulted in a concomitant increase in adhesion. In contrast, for initially smooth tips, sliding roughened some of these tips. In the limit of low applied stress, the experimentally determined works of adhesion match the intrinsic (van der Waals) work of adhesion for an atomically smooth silicon-diamond interface obtained from MD simulations. The results provide mechanistic interpretations of sliding-induced changes and interfacial adhesion and may help inform applications involving adhesive interfaces that are subject to applied shear forces and displacements.
Collapse
Affiliation(s)
- Zachary B Milne
- Department of Mechanical Engineering and Applied Mechanics , University of Pennsylvania , Philadelphia , Pennsylvania 19104 , United States
| | - J David Schall
- Mechanical Engineering Department , North Carolina Agricultural and Technical State University , Greensboro , North Carolina 27411 , United States
| | - Tevis D B Jacobs
- Department of Mechanical Engineering and Materials Science , University of Pittsburgh , Pittsburgh , Pennsylvania 15261 , United States
| | - Judith A Harrison
- Chemistry Department , United States Naval Academy Annapolis , Maryland 21402 , United States
| | - Robert W Carpick
- Department of Mechanical Engineering and Applied Mechanics , University of Pennsylvania , Philadelphia , Pennsylvania 19104 , United States
| |
Collapse
|
8
|
Abstract
We demonstrate snake-like motion of graphene nanoribbons atop graphene and hexagonal boron nitride ( h-BN) substrates using fully atomistic nonequilibrium molecular dynamics simulations. The sliding dynamics of the edge-pulled nanoribbons is found to be determined by the interplay between in-plane ribbon elasticity and interfacial lattice mismatch. This results in an unusual dependence of the friction-force on the ribbon's length, exhibiting an initial linear rise that levels-off above a junction-dependent threshold value dictated by the pre-slip stress distribution within the slider. As part of this letter, we present the LAMMPS implementation of the registry-dependent interlayer potentials for graphene, h-BN, and their heterojunctions that were used herein, which provides enhanced performance and accuracy.
Collapse
Affiliation(s)
- Wengen Ouyang
- School of Chemistry and The Sackler Center for Computational Molecular and Materials Science , Tel Aviv University , Tel Aviv 6997801 , Israel
| | - Davide Mandelli
- School of Chemistry and The Sackler Center for Computational Molecular and Materials Science , Tel Aviv University , Tel Aviv 6997801 , Israel
| | - Michael Urbakh
- School of Chemistry and The Sackler Center for Computational Molecular and Materials Science , Tel Aviv University , Tel Aviv 6997801 , Israel
| | - Oded Hod
- School of Chemistry and The Sackler Center for Computational Molecular and Materials Science , Tel Aviv University , Tel Aviv 6997801 , Israel
| |
Collapse
|
9
|
Tangpatjaroen C, Grierson D, Shannon S, Jakes JE, Szlufarska I. Size Dependence of Nanoscale Wear of Silicon Carbide. ACS Appl Mater Interfaces 2017; 9:1929-1940. [PMID: 27997110 DOI: 10.1021/acsami.6b13283] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Nanoscale, single-asperity wear of single-crystal silicon carbide (sc-SiC) and nanocrystalline silicon carbide (nc-SiC) is investigated using single-crystal diamond nanoindenter tips and nanocrystalline diamond atomic force microscopy (AFM) tips under dry conditions, and the wear behavior is compared to that of single-crystal silicon with both thin and thick native oxide layers. We discovered a transition in the relative wear resistance of the SiC samples compared to that of Si as a function of contact size. With larger nanoindenter tips (tip radius ≈ 370 nm), the wear resistances of both sc-SiC and nc-SiC are higher than that of Si. This result is expected from the Archard's equation because SiC is harder than Si. However, with the smaller AFM tips (tip radius ≈ 20 nm), the wear resistances of sc-SiC and nc-SiC are lower than that of Si, despite the fact that the contact pressures are comparable to those applied with the nanoindenter tips, and the plastic zones are well-developed in both sets of wear experiments. We attribute the decrease in the relative wear resistance of SiC compared to that of Si to a transition from a wear regime dominated by the materials' resistance to plastic deformation (i.e., hardness) to a regime dominated by the materials' resistance to interfacial shear. This conclusion is supported by our AFM studies of wearless friction, which reveal that the interfacial shear strength of SiC is higher than that of Si. The contributions of surface roughness and surface chemistry to differences in interfacial shear strength are also discussed.
Collapse
Affiliation(s)
- Chaiyapat Tangpatjaroen
- Department of Materials Science & Engineering, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States
| | - David Grierson
- Department of Materials Science & Engineering, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States
| | - Steve Shannon
- Department of Nuclear Engineering, North Carolina State University , Raleigh, North Carolina 27695, United States
| | - Joseph E Jakes
- Forest Biopolymers Science and Engineering, USDA Forest Service, Forest Products Laboratory , One Gifford Pinchot Drive, Madison, Wisconsin 53726, United States
| | - Izabela Szlufarska
- Department of Materials Science & Engineering, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States
| |
Collapse
|
10
|
Zekonyte J, Polcar T. Friction Force Microscopy Analysis of Self-Adaptive W-S-C Coatings: Nanoscale Friction and Wear. ACS Appl Mater Interfaces 2015; 7:21056-21064. [PMID: 26340161 DOI: 10.1021/acsami.5b05546] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Transition metal dichalcogenides (TMD) are increasingly popular due to unique structural and mechanical properties. They belong, together with graphene and similar 2D materials, to a small family of solid lubricants with potential to produce ultralow friction state. At the macroscale, low friction stems from the ability to form well-oriented films on the sliding surface (typically up to 10 nm thick), with the TMD basal planes aligned parallel to the surface. In this study, we quantitatively evaluate tribological properties of three sputtered tungsten-sulfur-carbon (W-S-C) coatings at a nanoscale using friction force microscopy. In particular, we investigate possible formation of well-ordered tungsten disulfide (WS2) layers on the coating surface. The coefficient of friction decreased with increasing load independently of coating composition or mechanical properties. In contrast, hard coatings with high tungsten carbide content were more resistant to wear. We successfully identified a WS2 tribolayer at the sliding interface, which peeled off as ultrathin flakes and attached to AFM tip. Nanoscale tribological behavior of WSC coatings replicates deviation of Amonton's law observed in macroscale testing and strongly suggests that the tribolayer is formed almost immediately after the start of sliding.
Collapse
Affiliation(s)
- Jurgita Zekonyte
- School of Engineering, University of Portsmouth , Anglesea Building, Anglesea Road, Portsmouth PO1 3DJ, United Kingdom
| | - Tomas Polcar
- National Centre for Advanced Tribology (nCATS), Faculty of Engineering and Environment, University of Southampton , Southampton SO17 1BJ, United Kingdom
- Department of Control Engineering, Faculty of Electrical Engineering, Czech Technical University in Prague , Technicka 2, Prague 166 27, Czech Republic
| |
Collapse
|