Ag thin film on Si as the lubricant under ultra-high vacuum and the effect of adsorbed gas molecules on friction

A tribological analysis of the extraordinarily low coefficient of friction (μ) of diamond/Ag/Si (μ: 0.003) was carried out in an ultra-high vacuum (UHV), in terms of the effect of gas molecules adsorbed on the surfaces. To discuss the sliding mechanism of the thin Ag film, adsorption experiments of 0.1–100 L of N 2 and O 2 and sliding tests were conducted under UHV conditions. In preparing the Ag film on Si (111) substrates, the surface was cleaned by flashing at 1200°C prior to Ag deposition to desorb the adsorbed hydrocarbons and the oxidized layer of Si. After cleaning the Si surface by flashing, the Ag monolayer on the Si functioned as a lubricant, reducing the μ as low as the detection limit. On the other hand, when an Ag monolayer was deposited on an oxidized Si wafer without flashing, sliding tests proved that the coefficient of friction did not decrease below 0.3. The surface cleaning process is a necessary requirement for sliding with very low friction. N 2 was adsorbed on the Ag layer deposited on a cleaned Si surface, and a sliding test was conducted. The coefficient of friction did not increase over a minor level with exposure of 0.1 to 100 L. However, with O 2 adsorption to Ag on cleaned Si (111), a very low μ was still observed. These results suggest that these adsorbed gas molecules do not have a fatal effect on a low friction force sliding, within the limit of the cycles tested. A plausible model for the extraordinarily low friction in the diamond/Ag/Si system is presented to explain the observed results.