Effect of electrode material on electrical discharge machining of alumina

Technologies for machining advanced insulating ceramics are demanded in many industrial fields. Recently, several insulating ceramics, such as Si3N4, SiC and ZrO2, have been successfully machined by electrical discharge machining (EDM). As unstable discharges occur during the machining of Al2O3 ceramics, inferior machining properties have been obtained. The formation mechanism of the electrical conductive layer on the EDMed surface is much different as compared to other ceramics. In addition to this, the electrically conductive layers are not formed sufficiently to adhere to the EDMed workpiece surface and keep a stable and continuous discharge generation on the ceramics. Graphite is widely used as electrode material in EDM. It is expected that carbon from graphite electrode implant and generate a conductive layer. Copper, graphite (Poco EDM-3) and copper-infiltrated-graphite (Poco EDM-C3) electrodes were used to compare the effects of generation of a conductive layer on alumina corresponding to EDM properties. The electrical discharge machining of 95% pure alumina shows that the EDM-C3 performs very well, giving significantly higher material removal rate (MRR) and lower electrode wear ratio than the EDM-3 and copper electrodes. The value of MRR was found to increase by 60% for EDM-3 with positive electrode polarity. As for EDM-C3, MRR was increased by 80% under the same condition. When the results were investigated with energy dispersive spectroscopy (EDS), no element of copper was observed on the conductive layer with both EDM-3 and EDM-C3. However, surface resistivity of a conductive layer created with EDM-C3 is less than with EDM-3. Surface roughness was improved to 25 μm with positive polarity of EDM-C3.