The effect of contact area on nano/micro-scale friction

Abstract

The effect of contact area on nano/micro- scale friction was experimentally studied. Glass balls with various radii were used in order to change the contact area. Borosilicate glass balls (radii-0.32 mu m, 0.5 mu m, 1.25 mu m and 2.5 mu m) attached on the top of AFM tip (NPS, DI) were applied for nano-scale contact and Soda Lime balls with radii 0.25 mm, 0.5 mm and I mm were applied for micro-scale contact. At the nano-scale, the friction between ball and surface was measured with the applied normal load using an atomic force microscope (AFM), and at the micro-scale it was measured using a ball-on-flat type micro-tribotester. All experiments were conducted on silicon wafer and diamond-like carbon (DLC) coated silicon samples, at controlled conditions of temperature of 24 +/- 1 degrees C and relative humidity of 45 +/- 5%. Friction was measured with the applied load in the range of 0-160 nN at the nano-scale and at 1000 mu N, 1500 mu N, 3000 mu N and 4800 mu N at the micro-scale. Results at the nano-scale showed that the friction increased with the applied normal load and tip size, for both kinds of samples. Similar behavior of friction with the applied normal load and ball size was observed for silicon at the micro-scale. However, for DLC friction decreased with the ball size. This distinct difference in the behavior of friction in DLC at the nano- and micro-scale was attributed to the difference in the operating mechanisms. At nano-scale, friction in DLC was affected by adhesion, whereas at the micro-scale it was affected mainly by plowing. Evidences of the operating mechanisms at micro-scale were obtained using scanning electron microscope (SEM). At micro-scale, solid-solid adhesion was dominant in silicon, while DLC showed plowing. Contrary to the nano-scale that is almost a 'wear-less' situation, wear was prominent at the micro-scale. At both the nano- and micro-scales, the effect of applied normal load and the tip/ball size on friction was discussed as the influence of contact area on these parameters. (c) 2005 Elsevier B.V. All rights reserved.