Effect of surface topography on the frictional behavior at the micro/nano-scale

In this paper, the effect of surface topography on the frictional behavior is investigated at micro/nano-scale in order to better understand the influence of asperity contact angle on friction. Experiments were performed to observe the variation in the frictional force as a spherical ball slides across a grooved surface. Specimens with single and multiple grooves of tens of micrometers in width were fabricated on silicon wafers. The frictional behavior between these specimens and steel balls of different diameters were observed with a tribometer built inside a scanning electron microscope (SEM). Normal load in the range of 20 mN and sliding speed of about 1-6 μm/s were applied in the experiments. It was shown that for relative ball/groove dimension that resulted in low contact angle, the overall frictional force was less than that observed for surface without the groove. Also, in situations where there was a great change in the contact angle stick-slip behavior could be observed. This stick-slip behavior is attributed to mechanical interlocking action.In addition to the above experiments, tests were conducted using lateral force microscopy (LFM). Unlike the previous finding that LFM output is dependent on the slope alone, it was found that the signal was more sensitive to the change in slope, especially when the slope was relatively large. Overall, both micro and nano-scale topographic effect on friction was similar. These results will ultimately aid in design of surface topography for micro-systems for best tribological performance.