  Contact electrification and triboelectrification are well-known in the case
of dissimilar materials, however the case of charge exchange during friction
between nominally identical insulating materials is less documented. We
experimentally investigated the triboelectrification between two smooth
monocrystalline &#945;-Al 2O 3 (sapphire) antagonists by surface force
measurements with a Surface Force Apparatus (SFA). The force between a sphere
and a plane, both in sapphire, was measured as a function of the sphere-plane
distance D, before and after nano-friction tests, under dry argon atmosphere.
Respective contributions of van der Waals, water meniscus and electrostatic
forces were determined. The estimated Hamaker constant was in good agreement
with the Lifshitz theory, and the dominant meniscus attraction at low
separation could be overcome with small radius sphere. We demonstrated that
electrostatic forces were generated by the nano-friction test and we quantified
the adhesion that results from this contact-electrification. In the first stage
of the unloading process, the short range electrostatic force was found to vary
both with time and distance D. Experimental results were correlated with
surface densities of mobile charges on the two surfaces, and the
time-dependence was related to classical surface transport phenomena on alumina
surfaces.
