INFLUENCE OF WATER-VAPOR ON NANOTRIBOLOGY STUDIED BY FRICTION FORCE MICROSCOPY

To simulate the nanotribological effects of adsorbed water molecules and the capillary condensation of water around contacting asperities, we have studied the friction and adhesive forces acting on the tip of a friction force microscope in contact with a flat surface as function of humidity. On the hydrophilic surface of silicon oxide strong capillary formation occurs around the tip. In contrast, on the hydrophobic surface of silicon oxide covered with a perfluoropolyether lubricant, the water capillary formation around the tip is greatly suppressed. Friction and adhesive forces decrease substantially with increasing humidity during sliding of the tip across both sample surfaces, implying that the surfaces are lubricated by adsorbed water.