Variation of the interfacial shear strength and adhesion of a nanometer-sized contact

We observe that the frictional force between a platinum-coated atomic force microscope (AFM) tip and the surface of mica in ultrahigh vacuum (UHV) varies with load in proportion to the contact area predicted by the Johnson-Kendall-Roberts (JKR) theory (Proc. R. Sec. London, Ser. A 1971, 324, 301) of adhesive elastic contacts. Using the JKR theory, the interfacial adhesion energy and shear strength can be determined. During the experiment, the tip-sample adhesion unexpectedly decreased by more than one order of magnitude, as did the measured frictional forces. These changes were induced by scanning the tip in contact with the mica sample. We attribute the substantial friction and adhesion decreases to changes of the interface, either structural or chemical, as opposed to changes in bulk structure or properties. The interfacial adhesion energy, gamma, dropped by more than one order of magnitude while the shear strength, tau, decreased to a lesser extent. Our observations indicate that, for a platinum-coated tip on mica, tau proportional to gamma(0.44). This is a new observation of a relation between adhesion and friction and is not explained by existing theories.