Molecular Tribology
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Molecular Tribology Steve Granick
On the scientific side, appréciation is growing that scientific understanding of thèse Systems, so complex and so far from equilibrium, is possible. Tribology is becoming recognized as an area with many exciting and useful surface science opportunities. The engineering significance is that while tribology design and tribologybased applications are rooted in our économie life, too often the technologies and formulations are empirically derived. One tends to take friction, wear, and tear for granted. A scientific understanding is needed so that better design can émerge by rational extension. This review seeks to bring out the excitement of new developments. The reader is referred to the original literature for full accounts. Surfaces in Dry Contact Arguments hâve waxed and waned for many years regarding the extent to which friction, adhésion, and wear stem primarily from bulk déformation of the underlying solids, or from events strictly at the interface. There is much to be said on both sides for thèse long-standing arguments. Bulk déformation is most pronounced as solids adhère or slide in dry contact. To appreciate developments in understanding dry contact, consider the récent large-
MRS BULLETIN/0CT0BER1991
scale molecular dynamics simulations of Landman and co-workers of the fate of a hard nickel tip as it approached a soft gold substrate.20 First, with quasi-static lowering of the tip towards the substrate, the outer planes of the gold crystal were calculated to bulge upward toward the tip. This graduai process ended in an abrupt jump into adhesive contact—a monolayer of gold wetting the nickel tip. When the tip was raised from the gold, ductile extension and eventual tear of the gold resulted. When the tip was pushed into the gold, gold came to wet the tip over a larger area than before. Also, slip planes formed in the gold lattice, accompanied by the advent of point defects. AU this was calculated in atomic détail. Favorable comparison was made with the hystérésis observed in laboratory experiments using atomic force microscopy (AFM). The importance of such studies is to link traditional formulations based on contact mechanics 30 with the inévitable granularity of matter. They show the limit down to which a continuum analysis can apply. They also serve a purpose in build-
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Introduction Molecular tribology—the study of tribology at atomic and molecular scales— constitutes a new frontier of tribology research. In a major surge of activity, expérimental methods hâve recently been developed to measure dynamic interfacial forces in shear. Building partly on earlier, somewhat neglected 1 " 3 friction studies, striking new findings hâve been obtained. The new methods include the surface forces apparatus for measuring adhésion and static interfacial forces as a function of surface séparation,45 new molecular tribometers for measuring friction in shear,6* atomic force microscopy,9"13 use of UHV tribometers,344 and the
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