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 fo
rces. These changes were induced by scanning the tip in contact with t
he mica sample. We attribute the substantial friction and adhesion dec
reases to changes of the interface, either structural or chemical, as
opposed to changes in bulk structure or properties. The interfacial ad
hesion energy, gamma, dropped by more than one order of magnitude whil
e the shear strength, tau, decreased to a lesser extent. Our observati
ons indicate that, for a platinum-coated tip on mica, tau proportional
to gamma(0.44). This is a new observation of a relation between adhes
ion and friction and is not explained by existing theories.