Atomistic Computer Simulations of Nanotribology
Molecular dynamics (MD) and related simulation techniques are powerful tools for improving our understanding of nanotribology. In simulations, materials properties and boundary conditions can be varied at will, and the resulting changes in both macroscopi
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Atomistic Com
Molecular dynamics (MD) and related simulation techniques are powerful tools for improving our understanding of nanotribology. In simulations, materials properties and boundary conditions can be varied at will, and the resulting changes in both macroscopic variables and the dynamics of individual atoms can be observed. This allows one to study systematically the effects of many different factors on friction and wear at the nano-scale. Some examples that are considered in this chapter are (i) the symmetry of contacting surfaces (disordered vs. crystalline and with or without common periods), (ii) surface elasticity, (iii) surface curvature or topology, (iv) interfacial adhesion, and (v) lubricant and/or contaminant molecules present at the interface. Results from simulations and experiments on isolated nanoscale contacts often contradict our experience from macroscopic systems. Kinetic friction coefficients can be orders of magnitude smaller than those observed in macroscopic experiments. Detailed calculations even suggest that there should be no static friction between most pairs of clean, chemically passivated surfaces unless the load is large enough to produce wear. Simulations that test a series of possible mechanisms for static friction are described. Geometrical interlocking can produce static friction in contacts containing only a few atoms, such as an atomic force microscope
23.2 Friction Mechanisms at the Atomic Scale. 23.2.1 Geometric Interlocking ................. 23.2.2 Elastic Instabilities ....................... 23.2.3 Role of Dimensionality and Disorder 23.2.4 Elastic Instabilities vs. Wear in Atomistic Models ...................... 23.2.5 Hydrodynamic Lubrication and Its Confinement-Induced Breakdown . 23.2.6 Submonolayer Films .....................
718 719 720 721 721 722 723 723 724 727 727 729 731
23.3 Stick-Slip Dynamics .............................. 732 23.4 Conclusions .......................................... 734 References .................................................. 735
tip. Larger contacts only exhibit static friction when there is wear, or when the surfaces are separated by a glassy contaminant layer that locks them together. Most surfaces are coated by such films and they are shown to yield friction forces that agree with both nanoscale and macroscopic experiments.
standing of the molecular underpinnings of macroscopic tribology. There is a growing recognition that the behavior of nanometer-thick interfacial layers is crucial to tribology in systems of all scales, which has led to a fruitful convergence of researchers from many disciplines. Understanding friction is an interesting and challenging scientific task, owing to the many factors that come into play: elastic and plastic properties of the two solids in relative sliding motion, surface roughness in multi-asperity contacts and tip geometry in single asperity contacts, geometric correlation between the solids in a microcontact, the amount of lubricant between the two solids, the physical nature of the lub
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