Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z27: Focus Session: Friction and Wear at the Nano- and Micro-Scales |
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Sponsoring Units: DCOMP Chair: Michael Chandross, Sandia National Laboratories Room: 501 |
Friday, March 7, 2014 11:15AM - 11:51AM |
Z27.00001: Friction and Adhesion Behavior of Graphene and other Two-Dimensional Materials Invited Speaker: Robert Carpick Two-dimensional materials provide a rich playground for exploring new and unexpected physical phenomena, including tribological behavior such as friction and wear. This talk will focus on friction and adhesion behavior of nanoscale contacts with such materials. For contacts to graphene, MoS$_{2}$, NbS$_{2}$, and BN, we find that the friction force exhibits a significant dependence on the number of 2-D layers [1] which we attribute to an out-of-plane ``puckering'' deformation that occurs when the 2-D material is weakly bound to its substrate. However, adhesive behavior does not follow this dependence. Instead, we find that sliding can induce an increased adhesive force due to local delamination of the topmost layer of graphene [2]. Finally, we observe a large, order-of-magnitude increase in friction that occurs when graphene is fluorinated. This result is interpreted in the context of the Prandtl-Tomlinson model of stick-slip friction. \\[4pt] [1] \textit{Frictional characteristics of atomically thin sheets}. C. Lee, Q. Li, W. Kalb, X.-Z. Liu, H. Berger, R. W. Carpick, J. Hone. \textbf{Science}, 328, 76-80 (2010). \\[0pt] [2] \textit{Nanoscale adhesive properties of graphene: The effect of sliding history}. X.-Z. Liu, Q. Li, P. Egberts, and R.W. Carpick, \textbf{Adv. Mat. Interf.}, accepted (2013). [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z27.00002: Ab-initio modelling of energy dissipation in nanotribological systems. A DFT study of fcc Cu(111) Michael Wolloch, Gregor Feldbauer, Peter Mohn, Josef Redinger, Andras Vernes Accurate modelling of the energy dissipation in sliding friction with \textit{ab-initio} methods in nanotribological systems poses a fundamental challenge in modern tribology. Here we present a quasi-static model to obtain the nanofrictional response of dry, wearless systems based on quantum mechanical all electron calculations. We propose a mechanism for energy dissipation, which relies on the atomic relaxations during sliding. We define two different ways of calculating the mean nanofriction force, both leading to an exponential friction versus load behaviour for all sliding directions. Since our approach does not impose any limits on lengths and directions of the sliding paths, we investigate arbitrary sliding directions for an fcc Cu(111) interface and detect two periodic paths which form the upper and lower bound of nanofriction. For long aperiodic paths the friction force convergences to a value in between these limits. For low loads we retrieve the Derjaguin generalization of Amontons-Coulomb kinetic friction law which appears to be valid all the way down to the nanoscale. We observe a non-vanishing Derjaguin-offset even for atomically flat surfaces in dry contact. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z27.00003: \textit{Ab-initio} simulations on adhesion and material transfer between contacting Al and TiN surfaces Gregor Feldbauer, Michael Wolloch, Peter Mohn, Josef Redinger, Andras Vernes Contacts of surfaces at the atomic scale are crucial in many modern applications from analytical techniques like indentation or AFM experiments to technologies such as nano- and micro-electro-mechanical-systems (N-/M-EMS). Furthermore, detailed insights into such contacts are fundamental for a better understanding of tribological processes like wear. A series of simulations is performed within the framework of Density Functional Theory (DFT) to investigate the approaching, contact and subsequent separation of two atomically flat surfaces consisting of different materials. Aluminum (Al) and titanium-nitride (TiN) slabs have been chosen as a model system representing the interaction between a soft and a hard material. The approaching and separation is simulated by moving one slab in discrete steps and allowing for electronic and ionic relaxations after each one. The simulations reveal the influences of different surface orientations ((001), (011), (111)) and alignments of the surfaces with respect to each other on the adhesion, equilibrium distance, charge distribution and material transfer between the surfaces. Material transfer is observed for configurations where the interface is stronger than the softer material. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:51PM |
Z27.00004: Friction at Interfaces of Metals and Alloys Invited Speaker: Shengfeng Cheng Pure metals such as gold that are frequently used in electrical contacts usually exhibit high adhesion and friction. However, nanocrystalline gold alloyed with minute amounts of Ni or Co can have low friction while still possessing low contact resistance. We used large-scale molecular dynamics simulations with validated EAM potentials to study the atomistic origin of friction reduction in metallic alloys. Three systems will be focused on in this talk: pure Ag, Ag-Cu alloy, and Ag-Au alloy. Our results reveal that different friction coefficients of metals and alloys are due to different sliding mechanisms. Dislocation-mediated plasticity dominates in pure metals or lattice-matched alloys and leads to high friction, while grain-boundary sliding mainly occurs in lattice-mismatched alloys that leads to low friction. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z27.00005: Failure of brittle heterogeneous materials: Intermittency or continuum regime Jonathan Bar\'es, Daniel Bonamy, Luc Barbier The problem of the solid fracture has occupied scientists and engineers for centuries. This phenomenon is classically addressed within the framework of continuum mechanics. Still, stress enhancement at crack tips makes the failure behavior observed at the continuum-level scale extremely dependent on the presence of microstructure inhomogeneities down to very small scales. This yields statistical aspects which, by essence, cannot be addressed using the conventional engineering continuum approaches. We addressed the problem numerically. The simulations invoke a recent statistical model mapping heterogeneous fracture with the depinning transition of an elastic manifold in a random potential. The numerical exploration of the parameter space allowed us to unravel when (i.e. which loading conditions, microstructure material parameters, material constants...) regular dynamics compatible with continuum approaches are expected to be observed, and when crackling dynamics calling for statistical approaches are observed. In this latter case, we have characterized quantitatively the dynamics statistic and its variations as a function of the input parameters. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z27.00006: Homogeneous Dislocation Nucleation - role of geometrical parameters and interatomic potentials Akanksha Garg, Asad Hasan, Craig Maloney We perform atomistic simulations of dislocation nucleation in defect free crystals in 2D and 3D during indentation with circular (2D) or spherical (3D) indenters of radius R. We study realistic interatomic potentials such as embedded atom method (EAM) potentials for Al in addition to simple pair-wise interactions such as linear springs. The dislocation embryo is localized along a line (or plane in 3D) of atoms with a lateral extent, $\xi$, at some depth, D, below the surface. For all potentials, in 2D, the scaled critical - load, $F_c/R$, and contact length, $C_c/R$, decrease to R independent values in the limit of large R. However, despite the R independence of $F_c/R$ and $C_c/R$, $\xi/R$ and $D/R$ display non-trivial scaling with R. Although both the interaction potential and the orientation of lattice affect the \emph{prefactors} in the scaling relations (e.g. crystal with springs is much harder than EAM Aluminum), all the \emph{scaling laws} are robust. Furthermore, we show that, despite the excellent prediction for the relation between F and C, Hertzian contact theory fails to correctly predict the strain underneath the indenter. This observation gives us hope that local nucleation criteria based on appropriate local strain may capture the nontrivial scaling laws. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z27.00007: Molecular Dynamics Simulation of the Phonon Conductivity in Cu-Ni Binary Alloy Yusuke Konishi, Tetsuya Fukushima, Kazunori Sato, Yoshihiro Asai, Hiroshi Katayama-Yoshida In 2010, a giant Peltier effect was observed in a Cu-Ni/Au junction [1]. It is considered that this giant Peltier effect is caused by nano-scale phase separation formed in the sputtering process. The giant Peltier coefficient in the Cu-Ni/Au junction indicates the great Seebeck coefficient in Cu-Ni alloy. Although this alloy is a prospective thermoelectric material because of its great Seebeck coefficient, the low phonon thermal conductivity is also necessary for a large thermoelectric coefficient ZT. In order to find conditions for the low phonon conductivity, we calculate the thermal conductivity in Cu-Ni Alloy in various shapes with or without nanostructures by using nonequilibrium molecular dynamics simulation. In this simulation, we use a semi-empirical potential [2] and the reverse nonequilibrium molecular dynamics [3] method. [1] A. Sugihara et al., Appl. Phys. Exp. 3, 065204 (2010). [2] J. Cai and Y. Y. Ye Phys. Rev. B 54, 8398 (1996). [3] F. M\"{u}ller-Plathe J. Chem. Phys. 106, 6082 (1997). [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z27.00008: Micrometer-scale molecular dynamics simulations on the lattice thermal conductivities of graphene and silicon Minkyu Park, Sun-Chul Lee, Yong-Sung Kim We calculated lattice thermal conductivity of graphene and silicon by using large-scale molecular dynamics simulations. In the molecular dynamics simulations, whether the non-equilibrium systems reach the steady states is rigorously investigated, and the times to reach the steady states are found to drastically increase with the lengths of the system. From the ballistic to the diffusive regime, the lattice thermal conductivities are explicitly calculated and found to keep increasing in a wide range of lengths with finally showing a converging behavior at 16 micrometer for graphene and 8 micrometer for silicon. That obtained macroscopic values of the lattice thermal conductivity of graphene and silicon are 3200 and 210 W/mK respectively. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z27.00009: Using Markov State Models to Study Self-Assembly Matthew Perkett, Michael Hagan In recent years, a number of algorithms have been developed to study rare events, which has resulted in paradigm shift from running a few long trajectories to gathering statistics from many shorter trajectories. Running many simulations in parallel to build a Markov State Model (MSM) is one such technique, which has been applied to protein folding with great success. We present an adaptation to the MSM framework that enables its application to a wide range of systems undergoing self-assembly. The feasibility of this approach is demonstrated on two different coarse-grained models for virus self-assembly. We find good agreement between the MSM calculations and brute force long simulations, with up to several orders of magnitude reduction in simulation time. [Preview Abstract] |
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