Molecular Dynamics Simulations of Dry Sliding Asperities to Study Friction and Frictional Energy Dissipation

Abstract

Fundamental friction characteristics such as the friction coefficient, wear, adhesion, and dissipation of energy require an accurate understanding of how the surfaces in contact interact on nano or atomic scale. At nano scales since the ratio of surface area to volume is very high, quantum mechanics rather than classical laws for the bulk govern properties and process. Most of the known properties of materials are obtained from the bulk and hence cannot be used for analysis of nano components. Therefore, one of the biggest challenges in the miniaturization of systems is overcoming the tribological problems. With the rapid development in surface examining technologies like AFM, SFA and others as well as development of MEMS and NEMS devices, a better understanding of the atomistic mechanisms of sliding friction is essential in order to realize the true potential of such devices. Friction between sliding surfaces is almost always associated with the release of thermal energy and the dissipation of this thermal energy is critical for friction since most of the surface phenomena depend on the temperature. The energy dissipation in MEMS/NEMS devices is complicated as it involves different mechanisms such as non-equilibrium phonon and electron-hole creation, bond breaking and electron emission, defect formation and structural transformation, and wear. Quantitative data for the energy dissipation channels like elastic and plastic deformation are crucial for understanding the mechanics of deformation. Molecular modeling and simulation is a cost effective alternative to physical experiments for developing better insight on properties and phenomena at nano-scales. In this work, molecular dynamics simulations are used to study friction and energy dissipation in dry sliding friction of two hemispherical copper asperities. LAMMPS, which is a classical molecular dynamics code developed by Sandia National Labs, is used to perform all the simulations. The effect of interference, relative sliding velocity, asperity size, lattice orientation, and temperature control on friction and energy dissipation is investigated. The atomistic mechanisms of friction are studied and a novel method of energy dissipation analysis is presented. The results are analyzed and compared to similar work in nano tribology whenever possible and the key developments are presented.