Super lubricity of diamond sliding on the Ag monolayer/Si(1 1 1) and the effect of gas adsorption

Sliding tests of diamond/Ag monolayers/Si(1 1 1) were carried out under ultra-high vacuum conditions, and an extraordinarily low coefficient of friction 0.003 was observed. After cleaning the Si substrate surface by flashing, about 1–2 monolayers of Ag was deposited on the Si and functioned as a lubricant. Where an Ag monolayer was deposited on an oxidized Si substrate without flash cleaning, sliding tests proved that the coefficient of friction did not decrease lower than 0.3. These results show that the effect of the topmost monolayer on the friction force during sliding is very strong in this system. cuss the mechanism of the sliding on thin Ag films with a very low friction force, gas adsorption and sliding experiments were conducted as a function of the dose (0.1–100 L) of gases such as N2, O2, and water vapor, to compare with the friction measurements in a constant gas pressure. The coefficient of friction of sliding in vacuum did not increase by exposure to gases between 0.1 and 100 L. On the other hand, sliding in a constant gas pressure shows that the low friction is completely disturbed by O2 gas higher than 10−6 Pa. The results suggest that the chemical state of the nanostructure on the Si(1 1 1) surface strongly affects the macroscopic friction force in this system. A plausible model of sliding for the extraordinarily low friction of the diamond/Ag/Si system is discussed.