The effect of asperity array geometry on friction and pull-off force

The friction and pull-off forces between regular asperity arrays with various heights on a silicon wafer and a scanning probe of an atomic force microscope (AFM) were measured. We used two-dimensional periodic asperity arrays. The arrays were created by using a focused ion beam (FIB) to mill patterns on a silicon plate and on a platinum layer deposited on a silicon plate. For both materials the distance between adjacent peaks was about 240 nm and the groove depth ranged from about 3 to 49 nm. The probe of the AFM was a square flat 0.7 X 0.7 mu m(2). For the silicon array the pull-off force decreased with increasing groove depth and was proportional to the radius of curvature of the asperity. The friction force also decreased with asperity height and was proportional to both the asperity curvature and the pull-off force. For the platinum asperity array although both the pull-off and friction forces also decreased with groove depth, the friction coefficient (calculated by dividing the friction force by the pull-off force) was about half that of the silicon asperity array.