Nano-scale adhesion and friction on Si wafer with the tip size using AFM

Nano-scale studies on adhesion and friction were conducted in Si-wafer (100) using Atomic Force Microscopy (AFM). Glass (Borosilicate) balls of radii 0.32<TEX>$mu extrm{m}$</TEX>, 1.25<TEX>$mu extrm{m}$</TEX>, and 2.5<TEX>$mu extrm{m}$</TEX>, mounted on cantilever (Contact Mode type NPS) were used as tips. Adhesion and friction between Si-wafer and glass tips were measured at ambient temperature (24<TEX>${pm}$</TEX>1<TEX>$^{circ}C$</TEX>) and humidity (45<TEX>${pm}$</TEX>5%). Friction was measured as a function of applied normal load in the range of 0-160 nN. Results showed that, both adhesion and friction increased with the tip radii. Also, friction increased linearly as a function of applied normal load. The effect of tip size on adhesion and friction was explained as the influence of the capillary force exerted by meniscus and that of the contact area on these parameters respectively. The coefficient of friction was estimated in two different ways, as the slope from the plot of friction force against the applied normal load and as the ratio between the friction force and the applied normal load. Both these estimates showed that the coefficient of friction increased with the tip size. Further, the influence of the adhesion force on the coefficient of friction was also discussed.