Effect of surface contamination on the UHV tribological behavior of the Cu(111)/Cu(111) interface

Friction and adhesion measurements have been made using two Cu(111) samples modified by the presence of atomic adsorbates. Copper surfaces with 10-15 angstrom thick contaminant films resulting from exposure to the atmosphere exhibited a static friction coefficient of mu(s) = 0.75 +/- 0.18. A sharp increase in the friction coefficient was observed when the contaminant layer was reduced to submonolayer thickness by sputtering and for the cleanest Cu(111) surfaces studied the static friction coefficient was mu(s) = 4.4 +/- 1.3. To further investigate the tribological effect of submonolayer coverages of surface contamination adhesion experiments were performed using surfaces modified with sulfur adsorbed at coverages in the range theta(s) = 0.0-0.39 ML (saturation). The adhesion coefficient of the clean surfaces (mu(aa) = 0.69 +/- 0.20) was markedly reduced by the presence of 0.05 ML of sulfur and decreased monotonically with increasing sulfur coverage to a limiting value of mu (ad) =0.26 +/- 0.07 at theta(s) = 0.39.