METALLIC FRICTION - THE EFFECT OF MOLECULAR ADSORBATES

A novel ultrahigh vacuum (UHV) tribometer was used to study the tribol ogical behavior of two Cu(111) surfaces modified by the adsorption of various molecular species (ethanol, butanol and heptafluorobutanol). F riction measurements were made at coverages ranging from submonolayer to several multilayers. These alcohols showed similar friction behavio r as a function of coverage. At coverages below one monolayer, the sur faces were not well lubricated; the sliding behavior was stick-slip, a nd the friction coefficients were high. At coverages greater than one monolayer, the friction coefficient was significantly reduced and decr eased monotonically with increasing coverage. The transition of slidin g behavior from ''stick-slip'' to ''slip'' occurred in the coverage ra nge of 2-7 ML. The friction coefficient continued to decrease with inc reasing coverage, reaching a limiting value only at coverages of sever al multilayers, (C) 1998 Elsevier Science B,V. All rights reserved.