Atomic frictional energy dissipation

It has been found from MD simulations that friction of SAMs on diamond decreases with the increasing chain length of hydrocarbon molecules, but it remains relatively constant when the number of carbon atoms in the molecule chain exceeds a certain threshold [44], which confirmed the experimental observations. In simulations of sliding friction of L-B films, Glosli and McClelland [45] identified two different mechanisms of energy dissipation, namely, the viscous mechanism, similar to that in viscous liquid under shear, and the plucking mechanism related to the system instability that transfers the mechanical energy into heat, similar to that proposed in the Tomlinson model (see Chapter 9). On the basis of a series work of simulations performed in the similar... 

The studies on friction originated from sliding-induced instability of interfacial atoms and consequent energy dissipation are reviewed in this section, including the author s work to extend the models of wearless friction. This type of friction has been addressed in the literature by different... 

The system could be energy conservative if the atoms moved smoothly over the potential field. In that case, an atom, when traveling over one period of the potential, would experience a symmetrically distributed lateral force so that its time average and the net work done by the force would be zero. In reality, however, this is not going to happen that way. The author will demonstrate in the following how the system becomes unstable which inevitably leads to energy dissipation and friction. 

Based on the discussion in earlier sections of this chapter, one may expect atomically flat incommensurate surfaces to be superlubric. Indeed the first suggestion that ultra-low friction may be possible was based on simulations of copper surfaces.6,7 Furthermore, the simulations of Ni(100)/(100) interfaces discussed in the previous section showed very low friction when the surfaces were atomically flat and misoriented (see the data for the atomically flat system between 30° and 60° in Figure 21). In general, however, it is reasonable to assume that bare metals are not good candidates for superlubric materials because they are vulnerable to a variety of energy dissipation mechanisms such as dislocation formation, plastic deformation, and wear. 

Investigation of the Atomic-Scale Friction and Energy Dissipation in Diamond using Molecular Dynamics. 

The availability of new experimental methods at the end of the 1980 s allowed us to study friction on the atomic scale and created the new field of nanotribology. The observed wearless friction on this scale can be understood using the model of Tomlinson where the plucking action of one atom on to the other leads to energy dissipation via the generation of phonons. 

Harrison JA, White CT, Colton RJ, Brenner DW (1995) Investigation of the atomic-scale friction and energy dissipation in diamond using molecular dynamics. Thin Solid Films 260 205-211... 

In the dielectric there is ionic or electronic conduction. In a metallic conductor the free, migrating electrons collide with the lattice of the bound ionized metal atoms, and the electrons transfer their excess energy to the lattice. With electrolytes the charge carriers are ions, and ordinary migration or local displacement is hindered by viscosity-based friction. In both cases the dielectric is heated up and energy dissipated, that is the Joule effect. 

Langevin dynamics simulates the effect of molecular collisions and the resulting dissipation of energy that occur in real solvents, without explicitly including solvent molecules. This is accomplished by adding a random force (to model the effect of collisions) and a frictional force (to model dissipative losses) to each atom at each time step. Mathematically, this is expressed by the Langevin equation of motion (compare to Equation (22) in the previous chapter) ... 

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