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Sun Q, Liu W, Huang D, Huang X, Xu S, Wang J, Ye Z, Wang X, Wu S, Yue Y. Molecular dynamics study on thermal conductance between a nanotip and a substrate under vertical forces and horizontal sliding. Phys Chem Chem Phys 2023; 25:5510-5519. [PMID: 36723186 DOI: 10.1039/d2cp04655e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
The heat transfer between a nanotip and its substrate is extremely complex but is a key factor in determining the measurement accuracy in tip-assisted nanomanufacturing and thermometry. In this work, the heat transfer from the nanotip to the substrate during sliding is investigated using molecular dynamics simulations. Interfacial interaction and bond formation are analyzed during the sliding process. The results show that the increase of vertical force would greatly improve the interface thermal conductance between the nanotip and the substrate. It is found that more bonds are formed and there are larger contact areas at the interface. In addition, we found that the thermal conductivity of the nanotip is another obstacle for heat transfer between the tip and substrate and it is greatly limited by the nanotip diameter near contact which is close to or even smaller than the phonon mean free path. Meanwhile, the dynamic formation and breakage of the covalent bonds during the sliding could gradually smoothen the tip apex and enhance the thermal transport at the interface. This work provides guidance for the thermal design of a nanotip-substrate system for nanoscale thermal transport measurements.
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Affiliation(s)
- Qiangsheng Sun
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Wenxiang Liu
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Dezhao Huang
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Xiaona Huang
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Shen Xu
- School of Mechanical and Automotive Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
| | - Jianmei Wang
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Zhijiang Ye
- Department of Mechanical and Manufacturing Engineering, Miami University, Ohio 45056, USA
| | - Xiaosun Wang
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Shijing Wu
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China.
| | - Yanan Yue
- School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China. .,Department of Mechanical and Manufacturing Engineering, Miami University, Ohio 45056, USA
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2
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Vishnubhotla SB, Chen R, Khanal SR, Li J, Stach EA, Martini A, Jacobs TDB. Quantitative measurement of contact area and electron transport across platinum nanocontacts for scanning probe microscopy and electrical nanodevices. NANOTECHNOLOGY 2019; 30:045705. [PMID: 30479311 DOI: 10.1088/1361-6528/aaebd6] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Conductive modes of atomic force microscopy are widely used to characterize the electronic properties of materials, and in such measurements, contact size is typically determined from current flow. Conversely, in nanodevice applications, the current flow is predicted from the estimated contact size. In both cases, it is very common to relate the contact size and current flow using well-established ballistic electron transport theory. Here we performed 19 electromechanical tests of platinum nanocontacts with in situ transmission electron microscopy to measure contact size and conductance. We also used molecular dynamics simulations of matched nanocontacts to investigate the nature of contact on the atomic scale. Together, these tests show that the ballistic transport equations under-predict the contact size by more than an order of magnitude. The measurements suggest that the low conductance of the contact cannot be explained by the scattering of electrons at defects nor by patchy contact due to surface roughness; instead, the lower-than-expected contact conductance is attributed to approximately a monolayer of insulating surface species on the platinum. Surprisingly, the low conductance persists throughout loading and even after significant sliding of the contact in vacuum. We apply tunneling theory and extract best-fit barrier parameters that describe the properties of this surface layer. The implications of this investigation are that electron transport in device-relevant platinum nanocontacts can be significantly limited by the presence and persistence of surface species, resulting in current flow that is better described by tunneling theory than ballistic electron transport, even for cleaned pure-platinum surfaces and even after loading and sliding in vacuum.
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Affiliation(s)
- Sai Bharadwaj Vishnubhotla
- Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, United States of America
| | - Rimei Chen
- Department of Mechanical Engineering, University of California-Merced, Merced, CA, United States of America
| | - Subarna R Khanal
- Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, United States of America
| | - Jing Li
- Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States of America
- Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, NY, United States of America
| | - Eric A Stach
- Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States of America
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, United States of America
| | - Ashlie Martini
- Department of Mechanical Engineering, University of California-Merced, Merced, CA, United States of America
| | - Tevis D B Jacobs
- Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, United States of America
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Yang F, Yang J, Qi Y, de Boer MP, Carpick RW, Rappe AM, Srolovitz DJ. Mechanochemical Effects of Adsorbates at Nanoelectromechanical Switch Contacts. ACS APPLIED MATERIALS & INTERFACES 2019; 11:39238-39247. [PMID: 31547645 DOI: 10.1021/acsami.9b09707] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Herein, classical molecular dynamics simulations are used to examine nanoscale adsorbate reactions during the cyclic opening and closing of nanoelectromechanical system (NEMS) switches. We focus upon how reactions change metal/metal conductive contact area, asperity morphology, and plastic deformation. We specifically consider Pt, which is often used as an electrode material for NEMS switches. The structural evolution of asperity contacts in gaseous environments with molecules which can potentially form tribopolymers is determined by various factors, for example, contact forces, partial pressure and molecular weight of gas, and the fundamental reaction rates of surface adsorption and adsorbate linkages. The modeled systems exhibit significant changes during the first few cycles, but as the number of contact cycles increases, the system finds a steady-state where the morphologies, Pt/Pt contact area, oligomer chain lengths, amount of Pt transfer between opposing surfaces, and deformation rate stabilize. The stress generated during asperity contact increases the rate of reactions among the adsorbates in the contact region. This makes the size of the adsorbate molecules increase and thus more exposed metal, which implies higher electrical conductance in the closed contact, but more plastic deformation, metal-metal transfer, and mechanical work expended in each contact cycle.
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Affiliation(s)
| | | | | | - Maarten P de Boer
- Department of Mechanical Engineering , Carnegie Mellon University , Pittsburgh , Pennsylvania 15213 , United States
| | | | | | - David J Srolovitz
- Department of Materials Science and Engineering , City University of Hong Kong , Kowloon , Hong Kong SAR
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Wu Z, Yang X, Wang Z. Size effect on the spontaneous coalescence of nanowires. NANOTECHNOLOGY 2019; 30:245601. [PMID: 30822770 DOI: 10.1088/1361-6528/ab0be6] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
This paper investigates the size effect on the coalescence process of contacting nanoparticles. It is revealed by molecular dynamics that the nanometer-sized surface curvature coupled with the effective melting temperature exhibits a strong influence on the atom diffusion at the interface, and is therefore critical to the coalescence time. This effect is particularly pronouncing for surface curvatures below 20 nm. A phenomenological model is derived from the melting point reduction approach to describe the kinetic process of nanowire coalescence and is validated against a variety of simulation datasets. The quantitative correlation between the sample size, the sintering temperature and the contact morphology evolution is demonstrated.
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Affiliation(s)
- Zhenyan Wu
- Department of Physics, Guangxi University, Nanning 530004, People's Republic of China
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Ouyang W, de Wijn AS, Urbakh M. Atomic-scale sliding friction on a contaminated surface. NANOSCALE 2018; 10:6375-6381. [PMID: 29560981 DOI: 10.1039/c7nr09530a] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Using non-equilibrium molecular dynamic simulations, we investigate the effect of adsorbates on nanoscopic friction. We find that the interplay between different channels of energy dissipation at the frictional interface may lead to non-monotonic dependence of the friction force on the adsorbate surface coverage and to strongly nonlinear variation of friction with normal load (non-Amontons' behavior). Our simulations suggest that the key parameter controlling the variation of friction force with the normal load, surface coverage and temperature is the time-averaged number of adsorbates confined between the tip and the substrate. Three different regimes of temperature dependence of friction in the presence of adsorbates are predicted. Our findings point on new ways to control friction on contaminated surfaces.
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Affiliation(s)
- Wengen Ouyang
- School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel. and The Sackler Center for Computational Molecular and Materials Science, Tel Aviv University, Tel Aviv 6997801, Israel
| | - Astrid S de Wijn
- Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway and Department of Physics, Stockholm University, 10691 Stockholm, Sweden
| | - Michael Urbakh
- School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel. and The Sackler Center for Computational Molecular and Materials Science, Tel Aviv University, Tel Aviv 6997801, Israel
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Zeng X, Peng Y, Lang H, Cao X. Nanotribological behavior of a single silver nanowire on graphite. NANOTECHNOLOGY 2018; 29:085706. [PMID: 29256869 DOI: 10.1088/1361-6528/aaa2e5] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The nanotribological characteristics of silver nanowires (Ag NWs) are of great importance for the reliability of their applications in flexible nanodevices involving mechanical interactions. The frictional behaviors of Ag NWs on graphite substrate were directly investigated by atomic force microscopy (AFM) nanomanipulation. The relatively short NWs demonstrate three forms of motion-rotation, translation and a combination of the two-whose frictional behaviors behave like rigid rods. The relatively long Ag NW shows characteristics of a flexible beam, whose friction increases with an increase in the bending angle of the NW. The friction between the NW and substrate increases linearly with an increase in the length of the NW. The long Ag NW displays extraordinary flexibility that can be folded to different shapes, and the friction of the folded NW becomes smaller due to the decreased bending deformation. The critical aspect ratio of the Ag NW on graphite substrate for two different frictional behaviors between the relatively long and short NWs is found to be 12-15. These findings can deepen the understanding of the frictional characteristics of Ag NWs and contribute to their quantitative interface design.
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Affiliation(s)
- Xingzhong Zeng
- College of Mechanical Engineering, Donghua University, Shanghai 201620, People's Republic of China
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8
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Summers AZ, Iacovella CR, Cummings PT, McCabe C. Investigating Alkylsilane Monolayer Tribology at a Single-Asperity Contact with Molecular Dynamics Simulation. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2017; 33:11270-11280. [PMID: 28915731 DOI: 10.1021/acs.langmuir.7b02479] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Chemisorbed monolayer films are known to possess favorable characteristics for nanoscale lubrication of micro- and nanoelectromechanical systems (MEMS/NEMS). Prior studies have shown that the friction observed for monolayer-coated surfaces features a strong dependence on the geometry of contact. Specifically, tip-like geometries have been shown to penetrate into monolayer films, inducing defects in the monolayer chains and leading to plowing mechanisms during shear, which result in higher coefficients of friction (COF) than those observed for planar geometries. In this work, we use molecular dynamics simulations to examine the tribology of model silica single-asperity contacts under shear with monolayer-coated substrates featuring various film densities. It is observed that lower monolayer densities lead to reduced COFs, in contrast to results for planar systems where COF is found to be nearly independent of monolayer density. This is attributed to a liquid-like response to shear, whereby fewer defects are imparted in monolayer chains from the asperity, and chains are easily displaced by the tip as a result of the higher free volume. This transition in the mechanism of molecular plowing suggests that liquid-like films should provide favorable lubrication at single-asperity contacts.
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Affiliation(s)
- Andrew Z Summers
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Christopher R Iacovella
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Peter T Cummings
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Clare McCabe
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
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Yang F, Carpick RW, Srolovitz DJ. Mechanisms of Contact, Adhesion, and Failure of Metallic Nanoasperities in the Presence of Adsorbates: Toward Conductive Contact Design. ACS NANO 2017; 11:490-500. [PMID: 27983792 DOI: 10.1021/acsnano.6b06473] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
The properties of contacting interfaces are strongly affected not only by the bulk and surface properties of contacting materials but also by the ubiquitous presence of adsorbed contaminants. Here, we focus on the properties of single asperity contacts in the presence of adsorbates within a molecular dynamics description of metallic asperity normal contact and a parametric description of adsorbate properties. A platinum-platinum asperity contact is modeled with adsorbed oligomers with variable properties. This system is particularly tailored to the context of nanoelectromechanical system (NEMS) contact switches, but the results are generally relevant to metal-metal asperity contacts in nonpristine conditions. Even though mechanical forces can displace adsorbate out of the contact region, increasing the adsorbate layer thickness and/or adsorbate/metal adhesion makes it more difficult for metal asperity/metal surface contact to occur, thereby lowering the electrical contact conductance. Contact separation is a competition between plastic necking in the asperity or decohesion at the asperity/substrate interface. The mechanism which operates at a lower tensile stress dominates. Necking dominates when the adsorbate/metal adhesion is strong and/or the adsorbate layer thickness is small. In broad terms, necking implies larger asperity deformation and mechanical work, as compared with decohesion. Optimal NEMS switch performance requires substantial contact conductance and minimal asperity deformation; these results indicate that these goals can be achieved by balancing the quantity of adsorbates and their adhesion to the metal surface.
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Affiliation(s)
- Fan Yang
- Department of Materials Science and Engineering and ‡Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania , Philadelphia, Pennsylvania 19104 United States
| | - Robert W Carpick
- Department of Materials Science and Engineering and ‡Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania , Philadelphia, Pennsylvania 19104 United States
| | - David J Srolovitz
- Department of Materials Science and Engineering and ‡Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania , Philadelphia, Pennsylvania 19104 United States
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10
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Cheng S, Robbins MO. Nanocapillary Adhesion between Parallel Plates. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2016; 32:7788-95. [PMID: 27413872 DOI: 10.1021/acs.langmuir.6b02024] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Molecular dynamics simulations are used to study capillary adhesion from a nanometer scale liquid bridge between two parallel flat solid surfaces. The capillary force, Fcap, and the meniscus shape of the bridge are computed as the separation between the solid surfaces, h, is varied. Macroscopic theory predicts the meniscus shape and the contribution of liquid/vapor interfacial tension to Fcap quite accurately for separations as small as two or three molecular diameters (1-2 nm). However, the total capillary force differs in sign and magnitude from macroscopic theory for h ≲ 5 nm (8-10 diameters) because of molecular layering that is not included in macroscopic theory. For these small separations, the pressure tensor in the fluid becomes anisotropic. The components in the plane of the surface vary smoothly and are consistent with theory based on the macroscopic surface tension. Capillary adhesion is affected by only the perpendicular component, which has strong oscillations as the molecular layering changes.
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Affiliation(s)
- Shengfeng Cheng
- Department of Physics, Center for Soft Matter and Biological Physics, and Macromolecules Innovation Institute, Virginia Polytechnic Institute and State University , Blacksburg, Virginia 24061, United States
| | - Mark O Robbins
- Department of Physics and Astronomy, Johns Hopkins University , 3400 North Charles Street, Baltimore, Maryland 21218, United States
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Abstract
Despite its fundamental importance, physical mechanisms that govern friction are poorly understood. While a state of ultra-low friction, termed structural lubricity, is expected for any clean, atomically flat interface consisting of two different materials with incommensurate structures, some associated predictions could only be quantitatively confirmed under ultra-high vacuum (UHV) conditions so far. Here, we report structurally lubric sliding under ambient conditions at mesoscopic (∼4,000–130,000 nm2) interfaces formed by gold islands on graphite. Ab initio calculations reveal that the gold–graphite interface is expected to remain largely free from contaminant molecules, leading to structurally lubric sliding. The experiments reported here demonstrate the potential for practical lubrication schemes for micro- and nano-electromechanical systems, which would mainly rely on an atomic-scale structural mismatch between the slider and substrate components, via the utilization of material systems featuring clean, atomically flat interfaces under ambient conditions. Structural lubricity—referring to ultralow levels of friction between atomically flat, incommensurate surfaces—has previously been observed under ultrahigh vacuum. Here, the authors report structural lubricity at gold-graphite interfaces under ambient conditions and on mesoscopic scales.
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12
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Ye Z, Egberts P, Han GH, Johnson ATC, Carpick RW, Martini A. Load-Dependent Friction Hysteresis on Graphene. ACS NANO 2016; 10:5161-5168. [PMID: 27110836 DOI: 10.1021/acsnano.6b00639] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Nanoscale friction often exhibits hysteresis when load is increased (loading) and then decreased (unloading) and is manifested as larger friction measured during unloading compared to loading for a given load. In this work, the origins of load-dependent friction hysteresis were explored through atomic force microscopy (AFM) experiments of a silicon tip sliding on chemical vapor deposited graphene in air, and molecular dynamics simulations of a model AFM tip on graphene, mimicking both vacuum and humid air environmental conditions. It was found that only simulations with water at the tip-graphene contact reproduced the experimentally observed hysteresis. The mechanisms underlying this friction hysteresis were then investigated in the simulations by varying the graphene-water interaction strength. The size of the water-graphene interface exhibited hysteresis trends consistent with the friction, while measures of other previously proposed mechanisms, such as out-of-plane deformation of the graphene film and irreversible reorganization of the water molecules at the shearing interface, were less correlated to the friction hysteresis. The relationship between the size of the sliding interface and friction observed in the simulations was explained in terms of the varying contact angles in front of and behind the sliding tip, which were larger during loading than unloading.
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Affiliation(s)
- Zhijiang Ye
- Department of Mechanical Engineering, University of California Merced , 5200 North Lake Road, Merced, California 95343, United States
| | - Philip Egberts
- Department of Mechanical and Manufacturing Engineering, University of Calgary , 40 Research Place NW, Calgary, Alberta T2L 1Y6, Canada
| | - Gang Hee Han
- Physics Division, Center for Integrated Nanostructure Physics, Sungkyunkwan University , Suwon 440-746, South Korea
| | | | | | - Ashlie Martini
- Department of Mechanical Engineering, University of California Merced , 5200 North Lake Road, Merced, California 95343, United States
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Summers AZ, Iacovella CR, Billingsley MR, Arnold ST, Cummings PT, McCabe C. Influence of Surface Morphology on the Shear-Induced Wear of Alkylsilane Monolayers: Molecular Dynamics Study. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2016; 32:2348-2359. [PMID: 26885941 DOI: 10.1021/acs.langmuir.5b03862] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Chemisorbed alkylsilane monolayer coatings have been shown to possess favorable lubrication properties; however, film degradation prevents the widespread use of these materials as lubricants in micro- and nanoelectromechanical systems (MEMS/NEMS). In this work, molecular dynamics (MD) simulations are used to provide insight into the conditions that promote the degradation and wear of these materials. This is achieved through removal of interfacial chain-substrate bonds during shear and the examination of the mobility of the resulting free, unbound chains. Specific focus is given to the effects of surface morphology, which has been shown previously to strongly influence frictional forces in monolayer systems. In-plane order of chain attachments is shown to lead to pressure-induced orientational ordering of monolayers, promoting film stability. This behavior is lost as nonideality is introduced into the substrate and chain patterning on the surface becomes disordered. The presence of surface roughness is found to reduce film stability, with localization of wear observed for chain attachment sites nearest the interface of contact. The influence of substrate nonideality on monolayer degradation is shown to diminish as chain length is increased.
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Affiliation(s)
- Andrew Z Summers
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Christopher R Iacovella
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Matthew R Billingsley
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Steven T Arnold
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Peter T Cummings
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
| | - Clare McCabe
- Department of Chemical and Biomolecular Engineering, ‡Multiscale Modeling and Simulation (MuMS) Center, and §Department of Chemistry, Vanderbilt University , Nashville, Tennessee 37235, United States
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Alonso-Marroquin F, Huang P, Hanaor DAH, Flores-Johnson EA, Proust G, Gan Y, Shen L. Static friction between rigid fractal surfaces. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2015; 92:032405. [PMID: 26465480 DOI: 10.1103/physreve.92.032405] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/01/2015] [Indexed: 06/05/2023]
Abstract
Using spheropolygon-based simulations and contact slope analysis, we investigate the effects of surface topography and atomic scale friction on the macroscopically observed friction between rigid blocks with fractal surface structures. From our mathematical derivation, the angle of macroscopic friction is the result of the sum of the angle of atomic friction and the slope angle between the contact surfaces. The latter is obtained from the determination of all possible contact slopes between the two surface profiles through an alternative signature function. Our theory is validated through numerical simulations of spheropolygons with fractal Koch surfaces and is applied to the description of frictional properties of Weierstrass-Mandelbrot surfaces. The agreement between simulations and theory suggests that for interpreting macroscopic frictional behavior, the descriptors of surface morphology should be defined from the signature function rather than from the slopes of the contacting surfaces.
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Affiliation(s)
| | - Pengyu Huang
- School of Civil Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia
| | - Dorian A H Hanaor
- School of Civil Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia
| | - E A Flores-Johnson
- School of Civil Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia
| | - Gwénaëlle Proust
- School of Civil Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia
| | - Yixiang Gan
- School of Civil Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia
| | - Luming Shen
- School of Civil Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia
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Ewers BW, Batteas JD. Utilizing atomistic simulations to map pressure distributions and contact areas in molecular adlayers within nanoscale surface-asperity junctions: a demonstration with octadecylsilane-functionalized silica interfaces. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2014; 30:11897-11905. [PMID: 24645696 DOI: 10.1021/la500032f] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
To achieve a better understanding of the mechanical effects of adsorbed films at surface contacts, methods were developed to map and examine the pressure distribution of nanoasperity contacts, modeled by molecular dynamics simulation. The methods employ smoothing functions to project the atomic forces obtained in contact simulation onto the contact plane for fitting to standard continuum contact models and subsequent analysis. Importantly, these methods allow for contact evolution between nanoscopic asperity-asperity contacts to be examined because these are the central load-bearing junctions at interfaces. To demonstrate the application and features of this approach, it was employed to examine the evolution of contact between silica nanoasperities, with an increasing density of octadecyltrichlorosilane (OTS) films employed as a model adsorbate film. Linearly increasing contact radius and linearly decreasing maximal pressure were observed as a function of the film packing density. Because contact between the underlying, high-energy silica surfaces is undesirable, the evolution of silica contact was also examined using these same methods. As more molecules were introduced into the contact, a sharp transition was observed from the narrow, high-pressure interaction between the underlying substrates, to a broad, substantially lower pressure interaction, indicating a sharp transition from the dry to lubricated condition. To study the dependence of these behaviors on contact morphology, silica nanoasperities in contact with a flat silica surface were also examined. Similar behavior, including the broadening of the contact area and the minimization of direct surface contact, were observed. The method developed herein is applicable to a variety of systems and can be employed to optimize surface protection and pressure redistribution by boundary lubricants. This method can also be extended to AFM adhesion measurements where a detailed understanding of the true contact area is critical for the quantitative determinations of molecular forces and local surface mechanics.
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Affiliation(s)
- Bradley W Ewers
- Department of Chemistry, Texas A&M University , P.O. Box 30012, College Station, Texas 77842-3012, United States
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Bai S, Murabayashi H, Kobayashi Y, Higuchi Y, Ozawa N, Adachi K, Martin JM, Kubo M. Tight-binding quantum chemical molecular dynamics simulations of the low friction mechanism of fluorine-terminated diamond-like carbon films. RSC Adv 2014. [DOI: 10.1039/c4ra04065a] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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Barthel AJ, Kim SH. Lubrication by physisorbed molecules in equilibrium with vapor at ambient condition: effects of molecular structure and substrate chemistry. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2014; 30:6469-6478. [PMID: 24827583 DOI: 10.1021/la501049z] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The effects of physisorbed organic vapor molecules on friction and wear were studied for various materials with different surface chemistries (metals, ceramics, glasses, carbons, polymers) and adsorbed species with distinct functional groups (short linear-chain, branched, and fluorinated alcohols with alkyl chain lengths up to five carbons as well as acetone and n-decane). Friction test results of stainless steel under equilibrium vapor adsorption conditions indicated that the longer chain length of the adsorbed alcohols results in lower friction and that n-pentanol gives the lowest friction and wear among the molecules investigated. The adsorption isotherm measurements revealed that the functional groups of the adsorbed molecules appear to play important roles in lubrication. Friction coefficients that ranged from 0.02 to 0.9 for the various materials in dry and humid environments converged to ∼0.15 for the inorganic solid materials tested in n-pentanol. These findings indicate that the molecular lubrication by the physisorbed species dominates the tribological behaviors of the inorganic solid materials, regardless of bulk mechanical properties. Tribotests using polymeric materials did not show the same lubricating effects for n-pentanol vapor. The failure of n-pentanol to lubricate polymeric materials may be due to vapor ingress into the polymer and the absence of an adsorbed surface layer.
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Affiliation(s)
- Anthony J Barthel
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University , University Park, Pennsylvania 16802, United States
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Cheng S, Robbins MO. Capillary adhesion at the nanometer scale. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 89:062402. [PMID: 25019789 DOI: 10.1103/physreve.89.062402] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/10/2014] [Indexed: 06/03/2023]
Abstract
Molecular dynamics simulations are used to study the capillary adhesion from a nonvolatile liquid meniscus between a spherical tip and a flat substrate. The atomic structure of the tip, the tip radius, the contact angles of the liquid on the two surfaces, and the volume of the liquid bridge are varied. The capillary force between the tip and substrate is calculated as a function of their separation h. The force agrees with continuum predictions based on macroscopic theory for h down to ∼5 to 10 nm. At smaller h, the force tends to be less attractive than predicted and has strong oscillations. This oscillatory component of the capillary force is completely missed in the macroscopic theory, which only includes contributions from the surface tension around the circumference of the meniscus and the pressure difference over the cross section of the meniscus. The oscillation is found to be due to molecular layering of the liquid confined in the narrow gap between the tip and substrate. This effect is most pronounced for large tip radii and/or smooth surfaces. The other two components considered by the macroscopic theory are also identified. The surface tension term, as well as the meniscus shape, is accurately described by the macroscopic theory for h down to ∼1 nm, but the capillary pressure term is always more positive than the corresponding continuum result. This shift in the capillary pressure reduces the average adhesion by a factor as large as 2 from its continuum value and is found to be due to an anisotropy in the pressure tensor. The component in the plane of the substrate is consistent with the capillary pressure predicted by the macroscopic theory (i.e., the Young-Laplace equation), but the normal pressure that determines the capillary force is always more positive than the continuum counterpart.
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Affiliation(s)
- Shengfeng Cheng
- Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, USA and Department of Physics, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA
| | - Mark O Robbins
- Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, USA
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Ewers BW, Batteas JD. The role of substrate interactions in the modification of surface forces by self-assembled monolayers. RSC Adv 2014. [DOI: 10.1039/c4ra01427h] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Detailed pressure and strain mapping of atomistic contact simulations elucidate the mechanical and tribochemical mechanisms of surface force modification with SAMs.
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Affiliation(s)
- B. W. Ewers
- Department of Chemistry
- Texas A&M University
- College Station, USA
| | - J. D. Batteas
- Department of Chemistry
- Texas A&M University
- College Station, USA
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Yang Y, Singh J, Ruths M. Friction of aromatic thiol monolayers on silver: SFA and AFM studies of adhesive and non-adhesive contacts. RSC Adv 2014. [DOI: 10.1039/c4ra01803f] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
(a) Friction coefficients and (b) critical shear stresses of thiol monolayers on silver, measured with SFA (○) and AFM (red circles).
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Affiliation(s)
- Y. Yang
- Department of Chemistry
- University of Massachusetts Lowell
- Lowell, USA
| | - J. Singh
- Department of Chemistry
- University of Massachusetts Lowell
- Lowell, USA
| | - M. Ruths
- Department of Chemistry
- University of Massachusetts Lowell
- Lowell, USA
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Affiliation(s)
- Jeong Young Park
- Center for Nanomaterials and Chemical Reactions, Institute for Basic Science , Daejeon 305-701, Republic of Korea
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Raos G, Sluckin TJ. Pulling Polymers on Energetically Disordered Surfaces: Molecular Dynamics Tests of Linear and Non-linear Response. MACROMOL THEOR SIMUL 2013. [DOI: 10.1002/mats.201200075] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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Labuda A, Lysy M, Paul W, Miyahara Y, Grütter P, Bennewitz R, Sutton M. Stochastic noise in atomic force microscopy. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2012; 86:031104. [PMID: 23030863 DOI: 10.1103/physreve.86.031104] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/02/2012] [Indexed: 06/01/2023]
Abstract
Having reached the quantum and thermodynamic limits of detection, atomic force microscopy (AFM) experiments are routinely being performed at the fundamental limit of signal to noise. A critical understanding of the statistical properties of noise leads to more accurate interpretation of data, optimization of experimental protocols, advancements in instrumentation, and new measurement techniques. Furthermore, accurate simulation of cantilever dynamics requires knowledge of stochastic behavior of the system, as stochastic noise may exceed the deterministic signals of interest, and even dominate the outcome of an experiment. In this article, the power spectral density (PSD), used to quantify stationary stochastic processes, is introduced in the context of a thorough noise analysis of the light source used to detect cantilever deflections. The statistical properties of PSDs are then outlined for various stationary, nonstationary, and deterministic noise sources in the context of AFM experiments. Following these developments, a method for integrating PSDs to provide an accurate standard deviation of linear measurements is described. Lastly, a method for simulating stochastic Gaussian noise from any arbitrary power spectral density is presented. The result demonstrates that mechanical vibrations of the AFM can cause a logarithmic velocity dependence of friction and induce multiple slip events in the atomic stick-slip process, as well as predicts an artifactual temperature dependence of friction measured by AFM.
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Affiliation(s)
- Aleksander Labuda
- Department of Physics, McGill University, Montreal, Quebec, Canada H3A 2T8
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