Nonequilibrium molecular dynamics simulations of metallic friction at Ta/Al and Cu/Ag interfaces
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8/10/04
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Nonequilibrium Molecular Dynamics Simulations of Metallic Friction at Ta/Al and Cu/Ag Interfaces J.E. HAMMERBERG, B.L. HOLIAN, T.C. GERMANN, and R. RAVELO With the advent of large-scale parallel computers in the past decade, it has become possible to study the microphysics of complex nonlinear many-body systems at scales approaching the mesoscale. One of the areas where nonequilibrium deformation arises is that of dry sliding friction between ductile metals. We discuss the results of atomistic simulation studies of sliding friction in the velocity range 10 to 103 m/s in the high-pressure regime of 5 to 15 GPa for Ta/Al and Cu/Ag tribopairs. We discuss the velocity dependence and the near interface deformation seen in these simulations.
I. INTRODUCTION
THE physics and materials science of sliding friction are more complex than those of bulk elastic-plastic flow. This is due to many factors, among them surface roughness, surface chemistry, and surface electronic structure.[1,2] Even dry sliding, where clean surfaces slide over each other under vacuum conditions, lacks a comprehensive and predictive theory. A macroscopic theory for the effective tangential force acting on two metals expressed as a constitutive law, such as those currently used in materials dynamics continuum codes for the material flow stress,[3–8] has not yet been formulated. From a theoretical perspective, dry sliding friction is an example of a nonequilibrium material flow phenomenon. The boundaries are characterized by steady forcing conditions, not by thermal isolation, and global thermodynamic equilibrium is not attained, although for sufficiently weak driving, local thermodynamic equilibrium may very well be a reasonable approximation. Rather, the system adjusts until a timeindependent steady state is achieved, consistent with the prescribed flow conditions at the boundary. Although the steady state is a nonequilibrium one, the conservation laws for mass, momentum, and energy are always obeyed. This leads, in situations where local thermodynamic equilibrium is valid and local constitutive relations can be defined, to the macroscopic hyperbolic partial differential equations of materials continuum mechanics.[9] On the microscale, the analogue is the direct integration of Newton’s equations of motion—the method known as nonequilibrium molecular dynamics (NEMD).[10,11] One of the characteristics of systems driven out of equilibrium is the emergence of a steady state characterized by multiple length and time scales. This is particularly the case for dry sliding friction. Rigney and co-workers[12–16] and others[17] have found a wide variety of substructure that develops J.E. HAMMERBERG and T.C. GERMANN, Technical Staff Members, Applied Physics Division, and B.L. HOLIAN, Technical Staff Member, Theoretical Division, are with the Los Alamos National Laboratory, Los Alamos, NM 87545. Contact e-mail: [email protected] R. RAVELO, Professor of Physics, is with the Department of Physics, University of Texas, El Paso, TX 79968-051
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