Research On Energy Dissipation In Atomic-scale Friction

Since the 1980 s, research on tribology has come to a new stage. In the aspect of experiment, due to the development of nanotechnology, especially the development of SPM instruments, the phenomenon of atomic-scale friction has been observed. In theory, due to the rapid progress of computer science, simulations of big data amounts become possible and thus promote the theoretical study of mechanism of atomic-scale friction. In this paper, molecular dynamics simulation and theoretical models are used to investigate the dynamic behavior, the energy dissipation mechanism, lattice vibrations and phonons dissipation, illustrating related disciplines and mechanisms of atomic-scale friction.Atomic-scale friction can be divided into two forms according to its dynamic behaviors, continuous sliding and stick-slip. The consistent analysis of mathematical model and molecular dynamics simulation show that it is the multi-stability of interfacial atoms that directly leads to the discontinuous movements. Consequently, the study of factors which could influence the multi-stability becomes an important theme of research on atomic-scale friction. Our calculation results based on Prandtl-Tomlinson model reveal that surface potential corrugation and lateral stiffness of system are two crucial factors determining the multi-stability. In addition, dynamic behavior can also be influenced by damping. High damping can introduce a longer slip period and decrease the oscillation after the sudden slip, so that the system can rapidly keep quasi-stable state.Energy dissipation is the essential cause of friction. We believe that the quantitative analysis of energy dissipation can provide us a deeper and more accurate comprehension of friction. A systematic analysis has been conducted based on the above model, to investigate the energy transitions and dissipation during stick-slip friction. In the simulation experiments, a reversible energy exchange is found between friction s ystem and outer system which indicates that the energy accumulated during the stick stage do not always dissipate completely in the stick-slip. We definite the Energy Dissipation Ratio(EDR) according to describe the relationship between the energy dissipated permanently in the system and the total energy accumulated in the stick period. A continuous change from 0 to 100% has been found, covering stick-slip, intermediate and smooth sliding regimes. We found that in the perspective of energy dissipation, two pathways of superlubricity, smooth sliding and thermolubricity are found to be perfectly unified, which is energy reversible superlubricity.At the atomic-scale, electronic dissipation and phononic dissipation are considered to be two fundam ental energy dissipation channels in friction. However, the latter is highly related to lattice vibrations and thus a more universal form of dissipation. A method named Velocity Auto-Correlation Function is adopted by us to describe the collective motion. We have studied the properties of vibrational spectra of single-crystal diamond structure and interface-hydrogenated diamond structure. The variations of spectra during a complete stick-slip process are particularly emphasized in our research. Different frequencies of vibrations have been observed in the slip moment, which indicate the release of phonons. And thus, we give an essential explanation of energy dissipation in atomic-scale friction.