Abstract: A plane grinding tester capable of all-the-journey gradually-changing-direction at constant velocity was performed to investigate the tribological characteristics of Si_3N_4 ball ground by cast iron HT250 or AISI-1045 steel in a ball-on-disc configuration, in the presence of an aqueous medium and external electric potential. The ground surface of the Si_3N_4 ball was analyzed by means of SEM and AFM to explore the effect of the external voltage polarities in the auxiliary electrode mode on the wear scar size and ground surface quality of the Si_3N_4 ball. It was found that the polarity of the externally applied voltage had a significant effect on the tribological characteristics of the ceramic ball. The positive voltage applied on the metallic disc led to a significant increase in the wear of the Si_3N_4, while the friction coefficient remained unchanged or increased slightly. Contrary to the above, the negative voltage applied on the metallic disc caused a significant decrease in both the friction coefficient and the wear rate. The effect of external potential on the wear mechanism of the Si_3N_4 ball was analyzed. It was suggested to select the AISI-1045 steel disc as the counterpart and to apply a positive external voltage, so as to obtain a higher polishing efficiency and better polishing quality of Si_3N_4 ceramic.