Nanotribology
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ibology
James F. Belak, Guest Editor
MRS BULLETIN/MAY 1993
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Nanotribology
junctions exhibit crystalline structure, and their elongation mechanism, on slow retraction of the tip from the surface after contact, involves a sequence of plastic deformations and yield processes, coupled with structural rearrangements. The molecular liquid is layered in the capillary junction for small separations between the two confining solid surfaces. Further separation results in a transition to a liquidlike region in the middle of the elongated column. This layering phenomena of fluid films confined between two solid walls is further investigated by M.O. Robbins and P.A. Thompson. They first describe the frictional coupling between adsorbed monolayers of spherical molecules and a substrate. The strength of the coupling depends strongly on whether the monolayer is a fluid, a commensurate solid, or an incommensurate solid. Confining the fluid between two solid walls separated by only a few molecular diameters changes the structure and the dynamics. As the film thickness decreases, the ordering influence of the two walls begins to overlap. This ultimately may lead to a phase transition in the film. Spherical molecules crystallize, while chain molecules enter a glassy state. The onset of this glass transition is heralded by a dramatic increase in viscosity and by universal non-Newtonian behavior. Crystalline or glassy films have a nonzero yield stress and exhibit stick-slip motion at slow velocities. In the next article, by J.A. Harrison and co-workers, the MD method is used to investigate the atomic-scale friction that arises from two diamond (111) surfaces placed in sliding contact. Diamond surfaces were chosen because of the increasing interest in using diamond as a low-friction and wear-resistance coating. The interatomic forces used in these simulations were derived from a many-body classical potential energy function that was originally developed to model the deposition of diamond films and can be used to study the chemical reactivity of carbon-based systems. These simulations predict and examine the behavior of the friction coefficient /A as a function of normal load, temperature, crystallographic sliding direction, sliding speed, and surface morphology. Unlike macroscopicscale studies, where changes in friction are observed and correlated with wear and subsequent chemical adhesion, no chemical reactions have been observed in these simulations. As expected, the frictional forces are small for small loads, but increase significantly with increasing load, approaching a constant value at high loads. This high value of /x is shown to be
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associated with mechanical excitation of hydrogen atoms on the opposing surfaces. Coating the surface with chemisorbed hydrocarbon chains causes a dramatic reduction in the friction coefficient, particularly at higher loads. At these higher loads the chains are forced to occupy valleys on the surface, reducing the mechanical excitation within the chains and resulting in a smoother surface.
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