Effect of capillary formation on friction and pull-off forces measured on submicron-size asperities

Asperities with hemispherical peaks were fabricated on a silicon substrate using a focused ion beam. Pull-off and friction forces were measured on each asperity using atomic force microscopy (AFM) in high vacuum (HV) of 2x10(-5) Pa. The probe of the AFM cantilever had a flat square tip, approximately 1x1 mu m(2) in area. The radius of curvature of the asperity peaks ranged from 70 to 610 nm. The results showed that the pull-off force was roughly proportional to this radius. The friction force was proportional to the pull-off force. Effects of the substrate temperature on pull-off force on a plane (the flat substrate) and friction force on an asperity were also examined. The pull-off force on the flat substrate increased with increasing contact time at a substrate temperature of 100 degrees C or lower, but was independent of contact time at 190 degrees C or higher. This suggests that the capillary cannot form at a substrate temperature of 190 degrees C or higher. The friction force increased with lower sliding velocities at 100 degrees C or lower, suggesting the capillary has a lubricating effect that prevents direct solid contact.