Influence of electric potentials on the tribological behaviour of silicon carbide

Thanks to their superior friction and wear properties in aqueous environments, silicon carbide (SiC) ceramics are commonly used for slide bearings and face seals in pumps. Tribochemical reactions of SiC with water have a major influence on the frictional behaviour as well as on the wear rates. It has been presumed that low friction coefficients (in the order of 0.1) are reached by smoothening of the surfaces, which favours hydrodynamic conditions even in water, and by the formation of lubricious oxide scales through tribochemical reaction of SiC with water. Silicon carbide is electrically semiconducting. Due to additives like boron or aluminium, silicon carbide ceramics can exhibit a considerable specific electrical conductivity. Therefore, electrochemical reactions with water or aqueou s electrolytes and subsequent friction, wear and corrosion may be influenced electrochemically. A pin-on-disc tester was modified to study SiC under the influence of electric potentials between −1 and +1 V (vs. Ag/AgCl). It was observed that under cathodic polarisation of the SiC samples, both friction coefficient and wear rates were drastically decreased. Surprisingly, despite the formation of oxide scales under positive (anodic) polarisation the friction coefficients increased significantly, which contradicts the fundamental hypothesis of a lubricating effect of the oxide scales. After switching off the electric potentials, the friction coefficient instantly changed to adopt normal values of ca. 0.1. These results show that it is possible to influence or even control the tribologic al behaviour of electrically conductive ceramics in aqueous media by electric potentials. New models based on electrochemical surface effects are necessary to describe the observed effects.