Extraordinarily low friction of the diamond/H- and Ag-terminated Si(111) sliding system

Hydrogen-terminated Si(111) surfaces have been studied for tribological performance in the sliding of a diamond slider to obtain fundamental information on contact problems between a Si single crystal and a diamond slider. We focused on the tribological role of the atoms on the contact surfaces, and observed an extra-ordinarily low coefficient of friction 0.003, of the relevant sliding surfaces, a coefficient as low as that of the sliding system of a diamond/Ag monolayer/Si(111). The sliding tests were carried out under an ultra-high vacuum, lower than 1.5× 10 −7 Pa to avoid contamination, with a load of 250 mN and 0.1 mm/sec of speed. The Si(111) surface was H-terminated by a hydrofluoric acid solution, and by hydrogen gas adsorption before the sliding tests. By sliding both of the H-terminated Si surfaces, a very low and stable friction coefficient was observed, while the desorbed surface, which had been heated to 600°C, showed a very high friction of 0.4 or higher. The observed friction reduction of 2 order was attributed, to a monolayer of hydrogen atoms terminating the Si(111) dangling bonds. The results are discussed in terms of the skiding mechanism, surface source of friction, and design of the sliding system in a vacuum.