INFLUENCE OF ADSORBED GASES ON THE SURFACE-ENERGY OF GRAPHITE - CONSEQUENCES ON THE FRICTION BEHAVIOR

The tribological behaviour of graphite depends on the orientation of the crystallites and on the nature of the environment. The basal planes of graphite are dense and the electrons between carbon atoms in these planes are bended with covalent character. The friction coefficient parallel to these planes is low. A sliding outside of the basal planes leads to the creation of dangling bonds and to an increase of the Fermi level. Then the wear is abrasive and the friction coefficient increases. Moreover, a correlation between the tribological behaviour of the graphite and the presence of an inert gas environment is observed. An inert gas surrounding the contact favours the reorientation of the crystallites parallel to the sliding direction. That leads to a decrease of the Fermi level and the observed lower friction coefficient is explained. In parallel, the adsorption of atomic oxygen on the basal planes leads to a lower Fermi level by decreasing the conduction electron density and explains the low friction coefficient. Analogous reasoning about the surface energy leads to the same conclusions. The aim of this paper is to correlate the friction behaviour of graphite with its energy level under various environments.