Friction and pull-off forces on submicron-size asperities measured in high-vacuum and in both dry and humid nitrogen at atmospheric pressure

Asperities having spherical 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 a high vacuum (HV) of 2 x 10(-5) Pa and in both dry and humid nitrogen at atmospheric pressure. The radius of curvature of the asperity peaks ranged from 70 to 610 nm. The probe of the AFM cantilever had a flat square tip, approximately 1 x 1 mum(2) in area. The results showed that the pull-off force was roughly proportional to the radius of curvature of the asperity peaks in each atmosphere. The friction force was proportional to the pull-off force. The gradient of the friction force against the pull-off force was slightly higher in the humid nitrogen than in the HV, which suggests the viscous resistance of the capillary is part of the friction force. The friction force in HV increased with lower sliding velocities without heating the substrate, which suggests the capillary also has a lubricating effect that prevents direct solid contact.