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Abstract: Friction characteristics of self-assembled monolayers (SAMs) coated on Si-wafer (100) by chemical vapor deposition technique were studied experimentally at nano and micro-scales. Four self-assembled monolayers, such as dimethyldichlorosilane (DMDC), diphenyldichlorosilane (DPDC), perfluorooctyltrichlorosilane (PFOTS) and perfluorodecanoicacid (PFDA) coated on Si-wafer (100) were used as test materials. Nano-scale friction was measured using atomic force microscopy (AFM) in the range of 0–40nN normal loads, in LFM (lateral force microscopy) mode, using a contact mode type Si3N4 tip. Results showed that the friction of SAMs at this scale was influenced by their physical/chemical properties, while that of Si-wafer by its inherent adhesion. Further, micro-scale friction tests were also performed with a ball-on-flat type micro-tribotester under reciprocating motion. Friction was measured in the range of 1500–4800μN applied normal loads using glass balls of varying radii, viz., 0.25, 0.5 and 1mm. It was observed that the performance of SAMs was more superior to Si-wafer even at micro-scale, except for PFDA. Evidences obtained using scanning electron microscope showed that Si-wafer and PFDA exhibited wear at this scale. Wear in the case of Si-wafer was due to solid–solid adhesion and that in the case of PFDA due to the influence of humidity (moisture). The micro-scale friction in both these materials was severely influenced by their wear. [Copyright &y& Elsevier]

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—0.32μm, 0.5μm, 1.25μm and 2.5μm) attached on the top of AFM tip (NPS, DI) were applied for nano-scale contact and Soda Lime balls with radii 0.25mm, 0.5mm and 1mm 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°C and relative humidity of 45±5%. Friction was measured with the applied load in the range of 0–160nN at the nano-scale and at 1000μN, 1500μN, 3000μN and 4800μN 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. [Copyright &y& Elsevier]