Fundamentals of Friction
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volving a single contacting interface. This is a vastly simpler situation than that of contact between macroscopic objects, in which the friction necessarily reflects the collective behavior of a multitude of contacts. The development of faster parallel computers has meanwhile made it possible to address a number of longstanding microscopic and mesoscopic questions concerning the physical state of a sliding interface. Faster computers have allowed large-scale molecular-dynamics simulations of condensed systems involving as many as 108 atoms to be performed for significantly long times. The results of such simulations, which involve solution of Newton's equations of motion for a system of Np particles interacting via specified internal (possibly many-body) forces and externally applied forces,9 are increasingly comparable to experiment in a very direct fashion. Physicists and chemists thus have joined their materials-science-and-engineering colleagues in efforts to speed progress in this area. It is more important today than ever to be able to design tribomaterials a priori, be they for large-scale applications such as subterranean drilling or for small-scale applications such as those involving microelectromechanical systems (MEMS). Current issues of importance covered in this issue of MRS Bulletin include the following: (1) understanding the chemical and tribochemical reactions that occur in a sliding contact and the energydissipation mechanisms associated with such a contact, (2) characterization of the microstructural and mechanical properties of the contact regions between the sliding materials, (3) merging and coordinating information gained on the atomic scale with that observed at the macroscopic scale, (4) development of realistic interaction potentials for computer simulations of materials of interest to tribological applications, and (5) development of realistic laboratory setups that
are both well-controlled and relevant to operating machinery. The articles appear in order of increasing complexity of the system under discussion. The first, entitled "Interracial Friction: Where Does the Energy Go?" by M.O. Robbins and I. Krim, treats current theoretical and experimental efforts to establish the frictional energy dissipation mechanisms associated with "interfacial" friction—that is, those attributable to atoms and molecules immediately adjacent to a localized plane where sliding occurs. The next article, "Friction in the Presence of Molecular Lubricants and Solid/Hard Coatings," by J.A. Harrison and S.S. Perry, is a joint theoretical/experimental discussion of a more complicated scenario, whereby a molecularly thin lubricant and/or hard coating has been introduced at the sliding interface to modify its tribological performance. The third article, "Unlubricated Sliding Behavior of Metals," by D.A. Rigney and J.E. Hammerberg is an experimental and theoretical treatment of a scenario in which shearing occurs within the bulk of the sliding materials rather than being confined to a planar interface. The followin
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