AVS 46th International Symposium
    Nanometer-scale Science and Technology Division Monday Sessions
       Session NS1-MoA

Invited Paper NS1-MoA7
Tribological Proprties of Single Crystalline Metal Surface

Monday, October 25, 1999, 4:00 pm, Room 612

Session: Nanoscale Tribology and Adhesion
Presenter: A.J. Gellman, Carnegie Mellon University
Authors: A.J. Gellman, Carnegie Mellon University
J.S. Ko, Carnegie Mellon University
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The tribological properties of single crystal metal surfaces have been measured under the ultra-high vacuum conditions of a surface analysis apparatus. This experiment allows us to measure both friction and adhesion between two single crystal surfaces brought into contact under a wide range of loads, and sheared with a wide range of sliding velocities. Most importantly it is possible to maintain strict control over the chemistry and properties of the surfaces. The experiments performed to date have systematically varied a number of surface characteristics in order to observed their effects on tribological properties. The clean surfaces of singles crystals can be brought together under varying orientation. Experiments with Ni(100) surfaces have shown that the crystallographic orientation can affect the frictional properties of the interface. These result in variations of the friction coefficients over a fivefold range. Furthermore the effects of crystallographic oreintation propagate through adsorbed layer of thickness up to four monolayers. The effects of orientation are thought to result from plastic deformation in the bulk solid of the metal crystals. The effects of adsorbed species can be measured with an extremely high level of control over the nature of the adsorbate, its coverage and its chemistry. On both Cu(111) and Ni(100) surfaces we have observed that at coverages less than one monolayer adsorbed species have little or no influence on interfacial friction. This has been observed using both atomic and molecular adorbates. It is thought that for adsorbed films at coverages less than one monolayer direct metal-metal junctions across the interface cause displacement of adsorbed species from the contact region. The result is a metal-metal interface with high shear strength similar to that formed between clean surfaces. Finally, the friction between single crystal surfaces can be influenced by fine characteristics of the structure of adsorbed layers. Measurements reveal discontinuous breaks in the friction versus adsorbate coverage for both trifluoroethanol on the Cu(111) surface and ethanol on the Ni(100) surface. These discontinuities are attributed to layering of the adsorbate on the surface. The observation of such effects has been made in several laboratories using the surface forces apparatus to measure friction between perfectly flat mica surfaces. It is remarkable to see similar effects occuring between crystalline metallic surfaces that are plastically deformable.