Wear Tests and Pull-Off Force Measurements of Single Asperities by Using Parallel Leaf Springs Installed on an Atomic Force Microscope

At the micro-scale level, the adhesion force dominates the friction force when the normal load approaches zero. For determining the effects of micro wear on the adhesion (pull-off) force, the wear-induced changes in surface topography of asperities and the pull-off force between the asperities and leaf springs were determined. First, single asperities were formed on a single-crystal gold plate and the asperities were rubbed with a silicon leaf spring attached to an AFM (atomic force microscope). A focused ion beam (FIB) system was used to form gold pyramid-shaped asperities on the surface of a single crystal gold plate. The FIB was also used to create the two types of single crystal silicon leaf springs tested here; single and parallel. The single leaf spring was created by flattening the probe-head of a commercially available AFM cantilever for AC mode. The parallel leaf spring was created by removing the central portion of a single-crystal silicon beam (25 μm×50 μm×300 μm). For the single leaf spring, the pull-off force no longer increased when the sliding distance exceeded 5 mm at a load of more than 200 nN. On the other hand, for the parallel leaf spring, the pull-off force increased monotonically with sliding distance, showing a more rapid increase at the higher normal load. The worn area of the asperity peak (measured by using an ordinary AFM probe) was proportional to the pull-off force. The wear volume per unit distance (i.e., wear rate) was estimated from the change in pull-off force, and was found to increase monotonically with the external load. There was no effect of adhesion force on the wear volume. [S0742-4787(00)01102-4]