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Abstract: Tribological performance of sub-nano to nanometer-thick Ag layers deposited on Si(111) have been examined to understand the effect of surface thin layers to the wear and friction characteristics. The slider was made of diamond sphere of 3mm in diameter. Sliding tests were carried out in an ultra-high vacuum environment (lower than 2×10−8 Pa) and analyzed in situ by Auger electron spectroscopy (AES) for the quantitative thickness-measurements, by reflection high-energy electron diffraction (RHEED) to clarify the substrate cleanliness and crystallography of the Ag films, and by scanning probe microscopy (SPM) for the morphology of the deposited/slid film surfaces. The Ag and substrate surfaces were atomically clean during the tribological tests so that the chemical effect of the Ag and Si surfaces appeared directly on the friction and wear processes. Experimental results of the sliding tests indicated that the minimum coefficient of friction (μ) was 0.007 for the 1–5nm thick as-deposited films of Ag on a Si(111) surface. Beyond the minimum thickness region, μ increased to 0.01 or higher. Observations of the slid surfaces showed that the Ag(111) sliding planes orientate parallel to the sliding direction, although the surface morphology of the deposited surface was totally deformed by the slider on the track. The Ag crystallites were not, therefore, destructed crystallographically by sliding but the fine Ag(111) sliding sheets were rotated, sheared, and displaced layer by layer, to orientate the Ag(111) plane parallel to the sliding direction by the reciprocal movement of the slider. After 100 cycles of the reciprocal movements, no worn particles were found around the sliding track and the low friction sliding lasted over 1000 cycles of the reciprocal sliding. The observed results indicated that the atomic force between surface atoms, especially diamond Cal bond, Ag(111) interlayer bonding force, and also the orientation of the sliding planes strongly reflect to the friction force, in addition to the asperity of the surface. A sliding mechanism of very thin layers of Ag film of very low wear and friction is discussed. [Copyright &y& Elsevier]