Transitional microfriction behavior of silicon induced by spontaneous water adsorption

The reciprocating sliding of a smooth silicon ball versus a flat silicon sample revealed significant differences in the friction behavior between pairs of hydrophilic and hydrophobic samples. Experiments were performed at sliding velocities of 40 and 100 μm/s and a constant normal load of 475 μN in ultrahigh vacuum and under a well-controlled residual atmosphere of water vapor at pressures ranging between 0.6×10−8 and 1.3×10−8 mbar. While hydrophobic sample pairs exhibited friction forces that only slightly increased with time, hydrophilic samples showed an abrupt transition from constant high-level friction values to a constant lower level value. This transition occurred at a certain point in time after beginning of sliding and it depended upon the duration of a preceding resting time, during which the contacting surfaces were exposed to the residual atmosphere in the vacuum chamber. These results are interpreted in terms of the spontaneous adsorption of water molecules into the slit between ball and flat due to superposition of the adsorption potentials of both surfaces.