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Bhuiyan FH, Li YS, Kim SH, Martini A. Shear-activation of mechanochemical reactions through molecular deformation. Sci Rep 2024; 14:2992. [PMID: 38316829 PMCID: PMC10844542 DOI: 10.1038/s41598-024-53254-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2023] [Accepted: 01/30/2024] [Indexed: 02/07/2024] Open
Abstract
Mechanical stress can directly activate chemical reactions by reducing the reaction energy barrier. A possible mechanism of such mechanochemical activation is structural deformation of the reactant species. However, the effect of deformation on the reaction energetics is unclear, especially, for shear stress-driven reactions. Here, we investigated shear stress-driven oligomerization reactions of cyclohexene on silica using a combination of reactive molecular dynamics simulations and ball-on-flat tribometer experiments. Both simulations and experiments captured an exponential increase in reaction yield with shear stress. Elemental analysis of ball-on-flat reaction products revealed the presence of oxygen in the polymers, a trend corroborated by the simulations, highlighting the critical role of surface oxygen atoms in oligomerization reactions. Structural analysis of the reacting molecules in simulations indicated the reactants were deformed just before a reaction occurred. Quantitative evidence of shear-induced deformation was established by comparing bond lengths in cyclohexene molecules in equilibrium and prior to reactions. Nudged elastic band calculations showed that the deformation had a small effect on the transition state energy but notably increased the reactant state energy, ultimately leading to a reduction in the energy barrier. Finally, a quantitative relationship was developed between molecular deformation and energy barrier reduction by mechanical stress.
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Affiliation(s)
- Fakhrul H Bhuiyan
- Department of Mechanical Engineering, University of California Merced, 5200 N. Lake Road, Merced, CA, 95343, USA
| | - Yu-Sheng Li
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, PA, 16802, USA
| | - Seong H Kim
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, PA, 16802, USA
| | - Ashlie Martini
- Department of Mechanical Engineering, University of California Merced, 5200 N. Lake Road, Merced, CA, 95343, USA.
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2
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Ogbomo E, Bhuiyan FH, Latorre CA, Martini A, Ewen JP. Effects of surface chemistry on the mechanochemical decomposition of tricresyl phosphate. Phys Chem Chem Phys 2023; 26:278-292. [PMID: 38059507 DOI: 10.1039/d3cp05320b] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/08/2023]
Abstract
The growth of protective tribofilms from lubricant antiwear additives on rubbing surfaces is initiated by mechanochemically promoted dissociation reactions. These processes are not well understood at the molecular scale for many important additives, such as tricresyl phosphate (TCP). One aspect that needs further clarification is the extent to which the surface properties affect the mechanochemical decomposition. Here, we use nonequilibrium molecular dynamics (NEMD) simulations with a reactive force field (ReaxFF) to study the decomposition of TCP molecules confined and pressurised between sliding ferrous surfaces at a range of temperatures. We compare the decomposition of TCP on native iron, iron carbide, and iron oxide surfaces. We show that the decomposition rate of TCP molecules on all the surfaces increases exponentially with temperature and shear stress, implying that this is a stress-augmented thermally activated (SATA) process. The presence of base oil molecules in the NEMD simulations decreases the shear stress, which in turn reduces the rate constant for TCP decomposition. The decomposition is much faster on iron surfaces than iron carbide, and particularly iron oxide. The activation energy, activation volume, and pre-exponential factor from the Bell model are similar on iron and iron carbide surfaces, but significantly differ for iron oxide surfaces. These findings provide new insights into the mechanochemical decomposition of TCP and have important implications for the design of novel lubricant additives for use in high-temperature and high-pressure environments.
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Affiliation(s)
- Egheosa Ogbomo
- Department of Mechanical Engineering, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK.
- Institute of Molecular Science and Engineering, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK
- The Thomas Young Centre, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK
| | - Fakhrul H Bhuiyan
- Department of Mechanical Engineering, University of California-Merced, 5200 N. Lake Road, Merced 95343, CA, USA
| | - Carlos Ayestarán Latorre
- Department of Mechanical Engineering, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK.
- Institute of Molecular Science and Engineering, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK
- The Thomas Young Centre, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK
| | - Ashlie Martini
- Department of Mechanical Engineering, University of California-Merced, 5200 N. Lake Road, Merced 95343, CA, USA
| | - James P Ewen
- Department of Mechanical Engineering, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK.
- Institute of Molecular Science and Engineering, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK
- The Thomas Young Centre, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK
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3
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Hu C, van Bonn P, Demco DE, Bolm C, Pich A. Mechanochemical Synthesis of Stimuli Responsive Microgels. Angew Chem Int Ed Engl 2023; 62:e202305783. [PMID: 37177824 DOI: 10.1002/anie.202305783] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2023] [Revised: 05/12/2023] [Accepted: 05/12/2023] [Indexed: 05/15/2023]
Abstract
Mechanochemical approaches are widely used for the efficient, solvent-free synthesis of organic molecules, however their applicability to the synthesis of functional polymers has remained underexplored. Herein, we demonstrate for the first time that mechanochemically triggered free-radical polymerization allows solvent- and initiator-free syntheses of structurally and morphologically well-defined complex functional macromolecular architectures, namely stimuliresponsive microgels. The developed mechanochemical polymerization approach is applicable to a variety of monomers and allows synthesizing microgels with tunable chemical structure, variable size, controlled number of crosslinks and reactive functional end-groups.
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Affiliation(s)
- Chaolei Hu
- DWI-Leibniz Institute for Interactive Materials e.V., Forckenbeckstraße 50, 52074, Aachen, Germany
- Institute of Technical and Macromolecular Chemistry, RWTH Aachen University, Forckenbeckstraße 50, 52074, Aachen, Germany
| | - Pit van Bonn
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074, Aachen, Germany
| | - Dan E Demco
- DWI-Leibniz Institute for Interactive Materials e.V., Forckenbeckstraße 50, 52074, Aachen, Germany
| | - Carsten Bolm
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074, Aachen, Germany
| | - Andrij Pich
- DWI-Leibniz Institute for Interactive Materials e.V., Forckenbeckstraße 50, 52074, Aachen, Germany
- Institute of Technical and Macromolecular Chemistry, RWTH Aachen University, Forckenbeckstraße 50, 52074, Aachen, Germany
- Aachen Maastricht Institute for Biobased Materials (AMIBM), Maastricht University, Urmonderbaan 22, 6167 RD, Geleen, The Netherlands
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4
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Kim JI, Lee WY, Tokoroyama T, Umehara N. Superlubricity with Graphitization in Ti-Doped DLC/Steel Tribopair: Response on Humidity and Temperature. ACS APPLIED MATERIALS & INTERFACES 2023; 15:19715-19729. [PMID: 37029740 DOI: 10.1021/acsami.3c01704] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/19/2023]
Abstract
The anti-friction of diamond-like carbon (DLC) is achieved by a well-developed carbonaceous transfer layer, and Ti-doped DLC is developed into a robustly built-up carbonaceous transfer layer. The friction performance of DLC depends on the operating environment, e.g., ambient gas, humidity, temperature, lubricants, and mating material. In this study, we aimed to reveal the environmental sensitivities of Ti-DLC on friction characteristics. To this end, a Ti-DLC was rubbed against a steel ball, and friction behaviors were evaluated with different gas compositions, humidity, and temperature. Finally, we identified that fractional coverage of water on surfaces affected the anti-graphitization on Ti-DLC, leading to avoiding friction reduction.
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Affiliation(s)
- Jae-Il Kim
- Department of Micro-Nano Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Aichi, Japan
| | - Woo-Young Lee
- Department of Micro-Nano Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Aichi, Japan
- Intelligent Optical Module Research Center, Korea Photonics Technology Institute (KOPTI), Cheomdan venture-ro 108-gil 9, Buk-gu, Gwangju 61007, Republic of Korea
| | - Takayuki Tokoroyama
- Department of Micro-Nano Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Aichi, Japan
| | - Noritsugu Umehara
- Department of Micro-Nano Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Aichi, Japan
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5
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Ducker RE, Brügge OS, Meijer AJHM, Leggett GJ. Tribochemical nanolithography: selective mechanochemical removal of photocleavable nitrophenyl protecting groups with 23 nm resolution at speeds of up to 1 mm s -1. Chem Sci 2023; 14:1752-1761. [PMID: 36819865 PMCID: PMC9931061 DOI: 10.1039/d2sc06305k] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2022] [Accepted: 01/14/2023] [Indexed: 01/18/2023] Open
Abstract
We describe the mechanochemical regulation of a reaction that would otherwise be considered to be photochemical, via a simple process that yields nm spatial resolution. An atomic force microscope (AFM) probe is used to remove photocleavable nitrophenyl protecting groups from alkylsilane films at loads too small for mechanical wear, thus enabling nanoscale differentiation of chemical reactivity. Feature sizes of 20-50 nm are achieved repeatably and controllably at writing rates up to 1 mm s-1. Line widths vary monotonically with the load up to 2000 nN. To demonstrate the capacity for sophisticated surface functionalisation provided by this strategy, we show that functionalization of nanolines with nitrilo triacetic acid enables site-specific immobilization of histidine-tagged green fluorescent protein. Density functional theory (DFT) calculations reveal that the key energetic barrier in the photo-deprotection reaction of the nitrophenyl protecting group is excitation of a π-π* transition (3.1 eV) via an intramolecular charge-transfer mechanism. Under modest loading, compression of the adsorbate layer causes a decrease in the N-N separation, with the effect that this energy barrier can be reduced to as little as 1.2 eV. Thus, deprotection becomes possible via either absorption of visible photons or phononic excitation transfer, facilitating fast nanolithography with a very small feature size.
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Affiliation(s)
- Robert E. Ducker
- Department of Chemistry, University of SheffieldBrook HillSheffield S3 7HFUK
| | - Oscar Siles Brügge
- Department of Chemistry, University of Sheffield Brook Hill Sheffield S3 7HF UK
| | | | - Graham J. Leggett
- Department of Chemistry, University of SheffieldBrook HillSheffield S3 7HFUK
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6
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Bhuiyan FH, Li YS, Kim SH, Martini A. Shear-activated chemisorption and association of cyclic organic molecules. Faraday Discuss 2023; 241:194-205. [PMID: 36134558 DOI: 10.1039/d2fd00086e] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Mechanochemical activation has created new opportunities for applications such as solvent-free chemical synthesis, polymer processing, and lubrication. However, mechanistic understanding of these processes is still limited because the mechanochemical response of a system is a complex function of many variables, including the direction of applied stress and the chemical features of the reactants in non-equilibrium conditions. Here, we studied shear-activated reactions of simple cyclic organic molecules to isolate the effect of chemical structure on reaction yield and pathway. Reactive molecular dynamics simulations were used to model methylcyclopentane, cyclohexane, and cyclohexene subject to pressure and shear stress between silica surfaces. Cyclohexene was found to be more susceptible to mechanochemical activation of oxidative chemisorption and subsequent oligomerization reactions than either methylcyclopentane or cyclohexane. The oligomerization trend was consistent with shear-driven polymerization yield measured in ball-on-flat sliding experiments. Analysis of the simulations showed the distribution of carbon atom sites at which oxidative chemisorption occurred and identified the double bond in cyclohexene as being the origin of its shear susceptibility. Lastly, the most common reaction pathways for association were identified, providing insight into how the chemical structures of the precursor molecules determined their response to mechanochemical activation.
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Affiliation(s)
- Fakhrul H Bhuiyan
- Department of Mechanical Engineering, University of California Merced, 5200 N. Lake Road, Merced, California 95343, USA.
| | - Yu-Sheng Li
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA
| | - Seong H Kim
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA
| | - Ashlie Martini
- Department of Mechanical Engineering, University of California Merced, 5200 N. Lake Road, Merced, California 95343, USA.
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7
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The Effects of Physical-Chemical Evolution of High-Sulfur Petroleum Coke on Hg 0 Removal from Coal-Fired Flue Gas and Exploration of Its Micro-Scale Mechanism. INTERNATIONAL JOURNAL OF ENVIRONMENTAL RESEARCH AND PUBLIC HEALTH 2022; 19:ijerph19127082. [PMID: 35742330 PMCID: PMC9222546 DOI: 10.3390/ijerph19127082] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/29/2022] [Revised: 06/02/2022] [Accepted: 06/08/2022] [Indexed: 02/05/2023]
Abstract
As the solid waste by-product from the delayed coking process, high-sulfur petroleum coke (HSPC), which is hardly used for green utilization, becomes a promising raw material for Hg0 removal from coal-fired flue gas. The effects of the physical-chemical evolution of HSPC on Hg0 removal are discussed. The improved micropores created by pyrolysis and KOH activation could lead to over 50% of Hg0 removal efficiency with the loss of inherent sulfur. Additional S-containing and Br-containing additives are usually introduced to enhance active surface functional groups for Hg0 oxidation, where the main product are HgS, HgBr, and HgBr2. The chemical-mechanical activation method can make additives well loaded on the surface for Hg0 removal. The DFT method is used to sufficiently explain the micro-scale reaction mechanism of Hg0 oxidation on the surface of revised-HSPC. ReaxFF is usually employed for the simulation of the pyrolysis of HSPC. However, the developed mesoporous structure would be a better choice for Hg0 removal in that the coupled influence of pore structure and functional groups plays a comprehensive role in both adsorption and oxidation of Hg0. Thus, the optimal porous structure should be further explored. On the other hand, both internal and surface sulfur in HSPC should be enhanced to be exposed to saving sulfur additives or obtaining higher Hg0 removal capacity. For it, controllable pyrolysis with different pyrolysis parameters and the chemical-mechanical activation method is recommended to both improve pore structure and increase functional groups for Hg0 removal. For simulation methods, ReaxFF and DFT theory are expected to explain the micro-scale mechanisms of controllable pyrolysis, the chemical-mechanical activation of HSPC, and further Hg0 removal. This review work aims to provide both experimental and simulational guidance to promote the development of industrial application of Hg0 adsorbent based on HSPC.
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8
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Ayestarán Latorre C, Remias JE, Moore JD, Spikes HA, Dini D, Ewen JP. Mechanochemistry of phosphate esters confined between sliding iron surfaces. Commun Chem 2021; 4:178. [PMID: 36697879 PMCID: PMC9814736 DOI: 10.1038/s42004-021-00615-x] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2021] [Accepted: 11/26/2021] [Indexed: 01/28/2023] Open
Abstract
The molecular structure of lubricant additives controls not only their adsorption and dissociation behaviour at the nanoscale, but also their ability to reduce friction and wear at the macroscale. Here, we show using nonequilibrium molecular dynamics simulations with a reactive force field that tri(s-butyl)phosphate dissociates much faster than tri(n-butyl)phosphate when heated and compressed between sliding iron surfaces. For both molecules, dissociative chemisorption proceeds through cleavage of carbon-oxygen bonds. The dissociation rate increases exponentially with temperature and stress. When the rate-temperature-stress data are fitted with the Bell model, both molecules have similar activation energies and activation volumes and the higher reactivity of tri(s-butyl)phosphate is due to a larger pre-exponential factor. These observations are consistent with experiments using the antiwear additive zinc dialkyldithiophosphate. This study represents a crucial step towards the virtual screening of lubricant additives with different substituents to optimise tribological performance.
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Affiliation(s)
- Carlos Ayestarán Latorre
- Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK
- Department of Materials, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK
| | | | - Joshua D Moore
- Afton Chemical Corporation, Richmond, VA, 23219, USA
- Dassault Systèmes Americas Corporation, Waltham, MA, 02451, USA
| | - Hugh A Spikes
- Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK
| | - Daniele Dini
- Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK
- Institute of Molecular Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK
- Thomas Young Centre for the Theory and Simulation of Materials, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK
| | - James P Ewen
- Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK.
- Institute of Molecular Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK.
- Thomas Young Centre for the Theory and Simulation of Materials, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK.
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9
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Chen Z, Khajeh A, Martini A, Kim SH. Origin of High Friction at Graphene Step Edges on Graphite. ACS APPLIED MATERIALS & INTERFACES 2021; 13:1895-1902. [PMID: 33347272 DOI: 10.1021/acsami.0c18098] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
On graphite, friction is known to be more than an order of magnitude larger at step edge defects as compared to on the basal plane, especially when the counter surface slides from the lower terrace of the step to the upper terrace. Very different mechanisms have been proposed to explain this phenomenon, including atomic interactions between the counter surface and step edge (without physical deformation) and buckling or peeling deformation of the upper graphene terrace. Here, we use atomic force microscopy (AFM) and reactive molecular dynamic (MD) simulations to capture and differentiate the mechanisms proposed to cause high friction at step edges. AFM experiments reveal the difference between cases of no deformation and buckling deformation, and the latter case is attributed to the physical stress exerted by the sliding tip. Reactive MD simulations explore the process of peeling deformation due to tribochemical bond formation between the tip and the step edge. Combining the results of AFM experiments and MD simulations, it is found that each mechanism has identifiable and characteristic features in the lateral force and vertical height profiles recorded during the step-up process. The results demonstrate that buckling and peeling deformation of the graphene edge rarely occur under typical AFM experimental conditions and thus are unlikely to be the origin of high friction at step edges in most measurements. Instead, the high step-up friction is due to stick-slip behavior facilitated by the topographical change and atomic interactions between the tip and step edge without deformation of the graphene itself.
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Affiliation(s)
- Zhe Chen
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States
| | - Arash Khajeh
- Department of Mechanical Engineering, University of California Merced, Merced, California 95343, United States
| | - Ashlie Martini
- Department of Mechanical Engineering, University of California Merced, Merced, California 95343, United States
| | - Seong H Kim
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States
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Chen Z, Khajeh A, Martini A, Kim SH. Identifying Physical and Chemical Contributions to Friction: A Comparative Study of Chemically Inert and Active Graphene Step Edges. ACS APPLIED MATERIALS & INTERFACES 2020; 12:30007-30015. [PMID: 32496047 DOI: 10.1021/acsami.0c08121] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Friction has both physical and chemical origins. To differentiate these origins and understand their combined effects, we study friction at graphene step edges with the same height and different terminating chemical moieties using atomic force microscopy (AFM) and reactive molecular dynamics (MD) simulations. A step edge produced by physical exfoliation of graphite layers in ambient air is terminated with hydroxyl (OH) groups. Measurements with a silica countersurface at this exposed step edge in dry nitrogen provide a reference where both physical topography effects and chemical hydrogen-bonding (H-bonding) interactions are significant. H-bonding is then suppressed in AFM experiments performed in alcohol vapor environments, where the OH groups at the step edge are covered with physisorbed alcohol molecules. Finally, a step edge buried under another graphene layer provides a chemically inert topographic feature with the same height. These systems are modeled by reactive MD simulations of sliding on an OH-terminated step edge, a step edge with alkoxide group termination, or a buried step edge. Results from AFM experiments and MD simulations demonstrate hysteresis in friction measured during the step-up versus step-down processes in all cases except the buried step edge. The origin of this hysteresis is shown to be the anisotropic deflection of terminal groups at the exposed step edge, which varies depending on their chemical functionality. The findings explain why friction is high on atomically corrugated and chemically active surfaces, which provides the insight needed to achieve superlubricity more broadly.
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Affiliation(s)
- Zhe Chen
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States
| | - Arash Khajeh
- Department of Mechanical Engineering, University of California Merced, Merced, California 95343, United States
| | - Ashlie Martini
- Department of Mechanical Engineering, University of California Merced, Merced, California 95343, United States
| | - Seong H Kim
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States
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Abstract
Tribochemistry, the study of chemical reactions in tribological interfaces, plays a critical role in determining friction and wear behavior. One method researchers have used to explore tribochemistry is “reactive” molecular dynamics simulation based on empirical models that capture the formation and breaking of chemical bonds. This review summarizes studies that have been performed using reactive molecular dynamics simulations of chemical reactions in sliding contacts. Topics include shear-driven reactions between and within solid surfaces, between solid surfaces and lubricating fluids, and within lubricating fluids. The review concludes with a perspective on the contributions of reactive molecular dynamics simulations to the current understanding of tribochemistry, as well as opportunities for this approach going forward.
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12
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He X, Ngo D, Kim SH. Mechanochemical Reactions of Adsorbates at Tribological Interfaces: Tribopolymerizations of Allyl Alcohol Coadsorbed with Water on Silicon Oxide. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2019; 35:15451-15458. [PMID: 31390866 DOI: 10.1021/acs.langmuir.9b01663] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Mechanochemical reactions of adsorbed molecules at tribological interfaces can benefit or impede lubrication, depending on the type of reactions induced by the interfacial shear or friction. Shear-induced polymerization of oxidatively chemisorbed organic species can occur at tribological interfaces, and their products can mitigate the wear of the surface in the case of the intermittent cessation of the lubricant supply. In contrast, tribochemical reactions involving water molecules impinging from the ambient air could facilitate surface wear. In this study, we investigated how such processes are affected when a silicon oxide surface is exposed to the environment containing both water and polymerizable organic molecules. For the polymerizable organic moiety, allyl alcohol was chosen because it is known to have a good tribopolymerization activity and can compete with water for surface adsorption sites. The adsorbate composition can be divided into two regimes: water-rich and alcohol-rich. The tribopolymerization yield was found to be significantly enhanced, compared to the alcohol-only case, in both water-rich and alcohol-rich regimes. The coadsorbed water molecules appeared to be incorporated into the tribopolymerization product of allyl alcohol. The friction coefficient qualitatively correlated with the tribopolymerization yield. Surprisingly, a small degree of surface wear was observed in the alcohol-rich regime, although wear was completely suppressed in the water-rich regime and the alcohol-only condition. These results suggested that the wear prevention effect does not necessarily correlate with the tribopolymerization effects.
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Affiliation(s)
- Xin He
- Department of Chemical Engineering and Materials Research Institute , Pennsylvania State University , University Park , Pennsylvania 16802 , United States
| | - Dien Ngo
- Department of Chemical Engineering and Materials Research Institute , Pennsylvania State University , University Park , Pennsylvania 16802 , United States
| | - Seong H Kim
- Department of Chemical Engineering and Materials Research Institute , Pennsylvania State University , University Park , Pennsylvania 16802 , United States
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13
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Khajeh A, Chen Z, Kim SH, Martini A. Effect of Ambient Chemistry on Friction at the Basal Plane of Graphite. ACS APPLIED MATERIALS & INTERFACES 2019; 11:40800-40807. [PMID: 31578847 DOI: 10.1021/acsami.9b13261] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Graphite is widely used as a solid lubricant due to its layered structure, which enables ultralow friction. However, the lubricity of graphite is affected by ambient conditions and previous studies have shown a sharp contrast between frictional behavior in vacuum or dry environments compared to humid air. Here, we studied the effect of organic gaseous species in the environment, specifically comparing the adsorption of phenol and pentanol vapor. Atomic force microscopy experiments and reactive molecular dynamics simulations showed that friction was larger with phenol than with pentanol. The simulation results were analyzed to test multiple hypotheses to explain the friction difference, and it was found that mechanically driven chemical bonding between the tip and phenol molecules plays a critical role. Bonding increases the number of phenol molecules in the contact, which increases the adhesion as well as the number of atoms in registry with the topmost graphene layer acting as a pinning site to resist sliding. The findings of this research provide insight into how the chemistry of the operating environment can affect the frictional behavior of graphite and layered materials more generally.
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Affiliation(s)
- Arash Khajeh
- Department of Mechanical Engineering , University of California Merced , 5200 N. Lake Road , Merced , California 95343 , United States
| | - Zhe Chen
- Department of Chemical Engineering and Materials Research Institute , Pennsylvania State University , University Park , Pennsylvania 16802 , United States
| | - Seong H Kim
- Department of Chemical Engineering and Materials Research Institute , Pennsylvania State University , University Park , Pennsylvania 16802 , United States
| | - Ashlie Martini
- Department of Mechanical Engineering , University of California Merced , 5200 N. Lake Road , Merced , California 95343 , United States
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14
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Chen Z, Khajeh A, Martini A, Kim SH. Chemical and physical origins of friction on surfaces with atomic steps. SCIENCE ADVANCES 2019; 5:eaaw0513. [PMID: 31448329 PMCID: PMC6688867 DOI: 10.1126/sciadv.aaw0513] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/13/2018] [Accepted: 06/27/2019] [Indexed: 05/16/2023]
Abstract
Friction occurs through a complex set of processes that act together to resist relative motion. However, despite this complexity, friction is typically described using a simple phenomenological expression that relates normal and lateral forces via a coefficient, the friction coefficient. This one parameter encompasses multiple, sometimes competing, effects. To better understand the origins of friction, here, we study a chemically and topographically well-defined interface between silica and graphite with a single-layer graphene step edge. We identify the separate contributions of physical and chemical processes to friction and show that a single friction coefficient can be separated into two terms corresponding to these effects. The findings provide insight into the chemical and topographic origins of friction and suggest means of tuning surfaces by leveraging competing frictional processes.
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Affiliation(s)
- Zhe Chen
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA
| | - Arash Khajeh
- Department of Mechanical Engineering, University of California, Merced, Merced, CA 95343, USA
| | - Ashlie Martini
- Department of Mechanical Engineering, University of California, Merced, Merced, CA 95343, USA
- Corresponding author. (A.M.); (S.H.K.)
| | - Seong H. Kim
- Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA
- Corresponding author. (A.M.); (S.H.K.)
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Wu H, Khan AM, Johnson B, Sasikumar K, Chung YW, Wang QJ. Formation and Nature of Carbon-Containing Tribofilms. ACS APPLIED MATERIALS & INTERFACES 2019; 11:16139-16146. [PMID: 30951286 DOI: 10.1021/acsami.8b22496] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Minimizing friction and wear at a rubbing interface continues to be a challenge and has resulted in the recent surge toward the use of coatings such as diamond-like carbon (DLC) on machine components. The problem with the coating approach is the limitation of coating wear life. Here, we report a lubrication approach in which lubricious, wear-protective carbon-containing tribofilms can be self-generated and replenishable, without any surface pretreatment. Such carbon-containing films were formed under modest sliding conditions in a lubricant consisting of cyclopropanecarboxylic acid as an additive dissolved in polyalphaolefin base oil. These tribofilms show the same Raman D and G signatures that have been interpreted to be due to the presence of graphite- or DLC films. Our experimental measurements and reactive molecular dynamics simulations demonstrate that these tribofilms are in fact high-molecular weight hydrocarbons acting as a solid lubricant.
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Affiliation(s)
- Hongxing Wu
- Key Laboratory of Education Ministry for Modern Design and Rotor Bearing Systems , Xi'an Jiaotong University , Xi'an 710049 , P. R. China
| | | | | | - Kiran Sasikumar
- Avant-Garde Materials Simulation , Deutschland GmbH , Merzhausen 79249 , Germany
- Center for Nanoscale Materials , Argonne National Laboratory , Argonne , Illinois 60439 , United States
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16
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Bolm C, Hernández JG. Mechanochemistry of Gaseous Reactants. Angew Chem Int Ed Engl 2019; 58:3285-3299. [DOI: 10.1002/anie.201810902] [Citation(s) in RCA: 164] [Impact Index Per Article: 32.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2018] [Indexed: 12/17/2022]
Affiliation(s)
- Carsten Bolm
- Institute of Organic Chemistry; RWTH Aachen University; Landoltweg 1 52074 Aachen Germany
| | - José G. Hernández
- Institute of Organic Chemistry; RWTH Aachen University; Landoltweg 1 52074 Aachen Germany
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17
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Affiliation(s)
- Carsten Bolm
- Institut für Organische Chemie; RWTH Aachen University; Landoltweg 1 52074 Aachen Deutschland
| | - José G. Hernández
- Institut für Organische Chemie; RWTH Aachen University; Landoltweg 1 52074 Aachen Deutschland
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18
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Mohammadtabar K, Eder SJ, Bedolla PO, Dörr N, Martini A. Reactive Molecular Dynamics Simulations of Thermal Film Growth from Di- tert-butyl Disulfide on an Fe(100) surface. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:15681-15688. [PMID: 30475634 DOI: 10.1021/acs.langmuir.8b03170] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Iron sulfide films are present in many applications, including lubricated interfaces where protective films are formed through the reactions of lubricant additive molecules with steel surfaces during operation. Such films are critical to the efficiency and useful lifetime of moving components. However, the mechanisms by which films form are still poorly understood because the reactions occur between two surfaces and so cannot be directly probed experimentally. To address this, we explore the thermal contribution to film formation of di- tert-butyl disulfide-an important extreme pressure additive-on an Fe(100) surface using reactive molecular dynamics simulations, where the reactive potential parameters are validated by comparison to ab initio calculations. The reaction pathway leading to the formation of iron sulfide surfaces is characterized using the reactive simulations. Then, the film formation process is mimicked by simulations where di- tert-butyl disulfide molecules are cyclically added to the surface and subjected to temperatures comparable to those expected due to frictional heating. The use of a reactive empirical potential is a novel approach to modeling the iterative nature of thermal film growth with realistic lubricant additive molecules.
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Affiliation(s)
- Karen Mohammadtabar
- Department of Mechanical Engineering , University of California Merced , 5200 N. Lake Road , Merced , California 95343 , United States
| | - Stefan J Eder
- AC2T research GmbH , Viktor-Kaplan-Straße 2/C , 2700 Wiener Neustadt , Austria
- Institute for Engineering Design and Logistics Engineering , Vienna University of Technology , Getreidemarkt 9 , 1060 Vienna , Austria
| | - Pedro O Bedolla
- AC2T research GmbH , Viktor-Kaplan-Straße 2/C , 2700 Wiener Neustadt , Austria
| | - Nicole Dörr
- AC2T research GmbH , Viktor-Kaplan-Straße 2/C , 2700 Wiener Neustadt , Austria
| | - Ashlie Martini
- Department of Mechanical Engineering , University of California Merced , 5200 N. Lake Road , Merced , California 95343 , United States
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