Fundamental Aspects of Friction and Wear Contacts in <100> Surfaces
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FUNDAMENTAL ASPECTS OF FRICTION AND WEAR CONTACTS IN 〈100〉 SURFACES William W. Gerberich, Natalia I. Tymiak and Donald E. Kramer1 Department of Chemical Engineering and Materials Science University of Minnesota, Minneapolis, MN 55455 1 Now at National Institute for Standards and Technology, Gaithersburg, MD 20899 ABSTRACT Unexpected friction and wear transitions occur in transition metals associated with dislocation emission, dislocation storage, and oxide break-through phenomena. Both normal nanoindentation and nanoscratch evaluations of conical diamond tips driven into tungsten {100} single crystal surfaces have been conducted. In terms of initiating plasticity undert the contact, this represents a high Peierl’s barrier for dislocation motion in transition metals. Both quasiequilibrium and kinetic aspects are reported along with current but speculative ideas on multiple friction and wear transitions. Preliminary results show that yielding under contacts can produce a 250 nm displacement excursion. Ramifications are seen in terms of friction coefficients which can double during the near-instantaneous yield excursion but then continue to triple from about 0.05 to 0.15 in the pile-up phase in front of the sliding contact. Implications of how nanotribological issues such as adhesion connect through this mesoscale activity to macroscopic friction and wear are discussed. INTRODUCTION Mechanism maps, first for creep and then fracture and wear became popular in the 1980 s. Lim and Ashby s [1] wear mechanism map was highlighted in Blau s [2] monograph on friction and wear transitions. To that time most of the studies on transitions were fairly qualitative in nature and only major changes like transcending from severe wear to seizure were noted. Detailed mechanisms as might be noted in creep mechanism maps were absent. Availability of such information has increased exponentially in the 90 s with widespread usage of surface forces, scanning probe, and nanoindentation devices. For example, we now know that for only one point on such a wear mechanism map that originally had five regimes, this one point may represent four or five stages of friction and wear development with at least four different nano/micro-scale mechanisms. Still, if one examines the explosion of information, there is a distinct void represented at the mesoscale regime considered here to be at the nm to µm scale with forces from the µN to mN levels. For example, from the OPML Nanomechanics Links [3] website, one can identify 13 scanning probe books, at least three of which deal with scanning force microscopy. The closest to the friction and wear subject is one on probing friction and rheology by Meyer, et al. [4]. The web site lists 29 investigative groups mostly in Chemistry and Physics. Contrast that to nanoindentation where few books apparently exist and only three research groups are mentioned. Of course there are many more than that in both camps arbitrarily defined here as atomistic and mesoscale. For three levels of scale we took the liberty of codifyi
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