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Now showing items 1 - 16 of 26

  • A shear localization mechanism for lubricity of amorphous carbon materials

    Ma, Tian-Bao   Wang, Lin-Feng   Hu, Yuan-Zhong   Li, Xin   Wang, Hui  

    Amorphous carbon is one of the most lubricious materials known, but the mechanism is not well understood. It is counterintuitive that such a strong covalent solid could exhibit exceptional lubricity. A prevailing view is that lubricity of amorphous carbon results from chemical passivation of dangling bonds on surfaces. Here we show instead that lubricity arises from shear induced strain localization, which, instead of homogeneous deformation, dominates the shearing process. Shear localization is characterized by covalent bond reorientation, phase transformation and structural ordering preferentially in a localized region, namely tribolayer, resulting in shear weakening. We further demonstrate an anomalous pressure induced transition from stick-slip friction to continuous sliding with ultralow friction, due to gradual clustering and layering of graphitic sheets in the tribolayer. The proposed shear localization mechanism sheds light on the mechanism of superlubricity, and would enrich our understanding of lubrication mechanism of a wide variety of amorphous materials.
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  • Modeling Atomic-Scale Electrical Contact Quality Across Two-Dimensional Interfaces

    Song, Aisheng   Shi, Ruoyu   Lu, Hongliang   Gao, Lei   Li, Qunyang   Guo, Hui   Liu, Yanmin   Zhang, Jie   Ma, Yuan   Tan, Xin   Du, Shixuan   Li, Xin   Liu, Xiao   Hu, Yuan-Zhong   Gao, Hong-Jun   Luo, Jianbin   Ma, Tian-Bao  

    Contacting interfaces with physical isolation and weak interactions usually act as barriers for electrical conduction. The electrical contact conductance across interfaces has long been correlated with the true contact area or the "contact quantity". Much of the physical understanding of the interfacial electrical contact quality was primarily based on Landauer's theory or Richardson formulation. However, a quantitative model directly connecting contact conductance to interfacial atomistic structures still remains absent. Here, we measure the atomic-scale local electrical contact conductance instead of local electronic surface states in graphene/Ru(0001) superstructure, via atomically resolved conductive atomic force microscopy. By defining the "quality" of individual atom-atom contact as the carrier tunneling probability along the interatomic electron transport pathways, we establish a relationship between the atomic-scale contact quality and local interfacial atomistic structure. This real-space model unravels the atomic-level spatial modulation of contact conductance, and the twist angle-dependent interlayer conductance between misoriented graphene layers.
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  • Vanishing stick-slip friction in few-layer graphenes: the thickness effect

    Xu, Liang   Ma, Tian-Bao   Hu, Yuan-Zhong   Wang, Hui  

    We report the thickness dependence of intrinsic friction in few-layer graphenes, adopting molecular dynamics simulations. The friction force drops dramatically with decreasing number of layers and finally approaches zero with two or three layers. The results, which are robust over a wide range of temperature, shear velocity, and pressure are quantitatively explained by a theoretical model with regard to lateral stiffness, slip length, and maximum lateral force, which could provide a new conceptual framework for understanding stick-slip friction. The results reveal the crucial role of the dimensional effect in nanoscale friction, and could be helpful in the design of graphene-based nanodevices.
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  • Large-scale high performance computation on 3D explosion and shock problems

    Fei, Guang-lei   Ma, Tian-bao   Hao, Li  

    Explosion and shock often involve large deformation, interface treatment between multi-material, and strong discontinuity. The Eulerian method has advantages for solving these problems. In parallel computation of the Eulerian method, the physical quantities of the computaional cells do not change before the disturbance reaches to these cells. Computational efficiency is low when using fixed partition because of load imbalance. To solve this problem, a dynamic parallel method in which the computation domain expands with disturbance is used. The dynamic parallel program is designed based on the generally used message passing interface model. The numerical test of dynamic parallel program agrees well with that of the original parallel program, also agrees with the actual situation.
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  • Numerical Simulation and Experimental Investigation of Shaped Charge Jet

    Ma, Tian-Bao   Wang, Cheng  

    In this paper, the multi-material Eulerian method was used to study the fluid-solid coupling issues, such as the formation of shaped charge jet. An adaptive grid refinement algorithm, based on the Youngs' interface reconstruction algorithm, was proposed to treat the transportation of the mixed grids. In order to verify the efficiency of this algorithm in the actual physical field, the algorithm was added to the indigenously developed EXPLOSION-2D hydrocode. The copper liner shaped charge jet was numerically simulated. The simulation results were in good agreement with the experimental results. It demonstrated that the adaptive grid refinement algorithm was efficient.
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  • The damage and failure mechanism of the concrete subjected to shaped charge loading

    Xu, Xiang-zhao   Ma, Tian-bao   Ning, Jian-guo  

    In this paper, the experiments of the large-size shaped charge jet penetration in concrete target were carried out. Then, the concrete target was cut off to obtain the internal structure and measure the shape of the penetration hole. Moreover, the cube concrete samples with the sizes of 100 mm length in different location of the concrete target were incised, and the material compressive strength was test by the material testing machine. The test results show that the material strength of the concrete target is enhanced with the increase of the distance to the penetration hole. Therefore, the damage of concrete target can be rough evaluated according to the compressive strength of the concrete samples. Based on the test results, the damage factor was added in the dynamic constitutive, which can describe mechanical behavior of concrete subjected to intensive impact loading was proposed. In this model, the concrete is assumed to be homogenous and consecutive in macroscopically. The results of the model were compared with the experimental results and the results which not considering the damage factor. The comparing results show that the model can be used to describe the dynamic mechanical behavior of concrete.
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  • Tunable giant anisotropic diffusion of water sub-monolayers between graphene layers

    Xu, Liang   Hu, Yuan-zhong   Ma, Tian-bao   Wang, Hui  

    We investigate the in-plane confinement effect of two graphene layers on the diffusion behaviour of water sub-monolayers using molecular dynamics simulations. An unexpected fast diffusion state with giant anisotropy is observed when the two confining graphene walls have certain shifts applied to their relative positions. The phenomenon is mainly attributed to the smooth one-dimensional potential channels produced by the composition effect of the potential energy landscapes of the two graphene walls, and the concerted motion of water molecules due to hydrogen bonding. Unique duality in the diffusion mechanism is observed in the fast diffusion state, as is ballistic motion along the potential channels and Fickian diffusion across such channels. The smooth potential channels can be created in certain directions simply by shifting the confining walls, which provides a novel measure to manipulate the motion of confined molecules in real-time.
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  • Numerical Analysis of Blasting Effect on Concrete

    Ma, Tian-Bao   Ning, Jian-Guo   Wang, Cheng  

    In this study, the blasting effect on concrete is numerically simulated by the 2-D multi-material Eulerian method. Through operator splitting of the governing equations into Lagrangian and Eulerian steps, the Eulerian method is discussed in detail. For the material interface and transport of the mixed Eulerian cells, a modified Young's interface reconstruction algorithm is proposed. The simulation results agree with the experimental data, indicating that the model and algorithm presented in this paper are valid and the numerical method can be used for engineering design.
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  • Molecular dynamics simulation of the interlayer sliding behavior in few-layer graphene

    Xu, Liang   Ma, Tian-bao   Hu, Yuan-zhong   Wang, Hui  

    The interlayer sliding behavior of few-layer (3-8) graphene (FLG) is investigated using molecular dynamics simulations. A constant velocity is imposed on the top layer, inducing relative sliding between layers. With multiple interlayer interfaces, the sliding is found to present unique behavior, such as coherent sliding at multiple interfaces and a periodic layer-stacking transition, which is quite different from previously reported flake-on-substrate cases. It is observed that sliding is usually "stick-slip", and can be divided into three stages, i.e. an initial stage, a developing stage and a stable sliding stage. A novel mechanism is proposed which explains the origin of the unique sliding behavior of FLG. (C) 2011 Elsevier Ltd. All rights reserved.
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  • Superlubricity of two-dimensional fluorographene/MoS2 heterostructure: a first-principles study

    Wang, Lin-Feng   Ma, Tian-Bao   Hu, Yuan-Zhong   Zheng, Quanshui   Wang, Hui   Luo, Jianbin  

    The atomic-scale friction of the fluorographene (FG)/MoS2 heterostructure is investigated using first-principles calculations. Due to the intrinsic lattice mismatch and formation of periodic Moire patterns, the potential energy surface of the FG/MoS2 heterostructure is ultrasmooth and the interlayer shear strength is reduced by nearly two orders of magnitude, compared with both FG/FG and MoS2/MoS2 bilayers, entering the superlubricity regime. The size dependency of superlubricity is revealed as being based on the relationship between the emergence of Moire patterns and the lattice mismatch ratio for heterostructures.
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  • Eulerian Numerical Method for Analysis of the Explosion and Impact Effects of Blast in Concrete

    Ma, Tian-Bao   Hao, Li   Ning, Jian-Guo   Wang, Cheng  

    In this study, a 2D multimaterial Eulerian numerical method and an MMIC-2D hydrocode are presented to analyze the explosion and impact effects. The operator of the governing equations is spitted into the Lagrangian and Eulerian parts, so that the multimaterial Eulerian method can be applied. A modified Young's interface reconstruction algorithm is used to treat the material interface in mixed Eulerian cells. The effects of blast in concrete are studied numerically. The numerical results are in good agreement with the experimental data, indicating that the proposed model and algorithm are effective and can be used for engineering design.
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  • Robust microscale superlubricity under high contact pressure enabled by graphene-coated microsphere

    Liu, Shu-Wei   Wang, Hua-Ping   Xu, Qiang   Ma, Tian-Bao   Yu, Gui   Zhang, Chenhui   Geng, Dechao   Yu, Zhiwei   Zhang, Shengguang   Wang, Wenzhong   Hu, Yuan-Zhong   Wang, Hui   Luo, Jianbin  

    Superlubricity of graphite and graphene has aroused increasing interest in recent years. Yet how to obtain a long-lasting superlubricity between graphene layers, under high applied normal load in ambient atmosphere still remains a challenge but is highly desirable. Here, we report a direct measurement of sliding friction between graphene and graphene, and graphene and hexagonal boron nitride (h-BN) under high contact pressures by employing graphene-coated microsphere (GMS) probe prepared by metal-catalyst-free chemical vapour deposition. The exceptionally low and robust friction coefficient of 0.003 is accomplished under local asperity contact pressure up to 1 GPa, at arbitrary relative surface rotation angles, which is insensitive to relative humidity up to 51% RH. This ultralow friction is attributed to the sustainable overall incommensurability due to the multi-asperity contact covered with randomly oriented graphene nanograins. This realization of microscale superlubricity can be extended to the sliding between a variety of two-dimensional (2D) layers.
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  • Formation of linear carbon chains during the initial stage of nanostructured carbon film growth

    Ma, Tian-Bao   Hu, Yuan-Zhong   Wang, Hui  

    The initial stage of nanostructured carbon film growth is investigated by molecular dynamics simulations. The carbon film exhibits amorphous structures with linear chains and cyclic rings on the surface at low incident energies. The structural transformations from linear chains to cyclic rings and to atom networks are observed during the growth process, which is explained in terms of system stability. The atomic adsorption behavior is analyzed through the calculation of the surface potential field. The formation of linear chain structure is due to the predominance of inhomogeneous adsorption of incident atoms on the surface and preferential growth at the tip of the chain. The formation of nanostructures on the surface is argued to be the initial nucleation process of amorphous carbon films. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2978358]
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  • Molecular Dynamics Study on Friction Due to Ploughing and Adhesion in Nanometric Scratching Process

    Zhu, Peng-zhe   Hu, Yuan-zhong   Ma, Tian-bao   Wang, Hui  

    In this study, three-dimensional MD simulations are carried out to study the nanometric scratching process. The ploughing friction coefficient and the adhesion friction coefficient are distinguished for the first time using MD simulations. The contribution of chip to friction coefficient is also evaluated. The simulation results show that the macroscale theory can qualitatively evaluate the ploughing friction coefficient, but it slightly overestimates the ploughing friction coefficient on the nanoscale for the scratching depths studied. It is found that the adhesion friction coefficient is independent of the scratching depth as predicted by macroscale theory. It is also found that the contribution of chip to friction coefficient is independent of the scratching depth and cannot be neglected on the nanoscale.
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  • Tribochemistry of Phosphoric Acid Sheared between Quartz Surfaces:A Reactive Molecular Dynamics Study

    Yue, Da-Chuan   Ma, Tian-Bao   Hu, Yuan-Zhong   Yeon, Jejoon   van Duin, Adri C. T.   Wang, Hui   Luo, Jianbin  

    Tribochemical processes have profound consequences on tribological performance. In the present paper, the tribochemical mechanism of low friction state in the silica/phosphoric acid system is elucidated by reactive molecular dynamics (ReaxFF) simulations. The friction coefficient is found having strong positive correlation with the number of interfacial hydrogen bonds, which suggests that a weaker interfacial hydrogen bond network would favor a lower friction. The friction reduction mechanisms have been analyzed in two temperature regimes: For 300 <=3D T <=3D 600 K, no indication of tribochemical reaction is observed, and the friction coefficient decreases because of the accelerated molecular rotational and translational motion and the corresponding weakened hydrogen bond network. For 800 K <=3D T <=3D 1400 K, the occurrence of tribochemical reactions leads to a clustering and polymerization of the phosphoric acid molecules and generation of a considerable quantity of water molecules distributed mainly in the sliding interface which could act as lubricant, and a low friction state is reached with a friction coefficient of 0.02.
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  • Growth of ultrathin diamond-like carbon films by C60 cluster assembly: Molecular dynamics simulations

    Ma, Tian-Bao   Hu, Yuan-Zhong   Wang, Hui  

    The deposition of C(60) clusters to produce cluster-assembled ultrathin diamond-like carbon (DLC) films is investigated using the molecular dynamics (MD) simulations. The deposition dynamics, especially the fragmentation process of the C(60) cluster are studied. The trajectory and instantaneous velocity of each atom in the cluster reveal a close relationship between cluster deposition and atom-by-atom deposition of DLC film. The atomistic structures, sp(3) fractions. and radial distribution functions of DLC films are quantitatively studied. For relatively low energies (E<20 eV/atom), the C(60) structural features preserve partially. The cluster-assembled films show big cavities, non-uniform structures, and rough surfaces. For relatively high energies (E>20 eV/atom), the films turn out to be structurally amorphous, densely-packed and sp(3) dominated. A marked discrepancy is observed between the sp(3) fraction of cluster and atom-by-atom deposited DLC films. The kinetic energy dissipation and the oblique incidences of freed atoms are the main reason for this discrepancy. (C) 2008 Elsevier B.V. All rights reserved.
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