Use of zirconium diboride-copper as an electrode in plasma applications

Abstract

Frequent replacement of electrodes, due to their high wear rate, is an undesired feature of most thermal plasma processes. Hence, the discovery of a high spark-resistive tool, ZrB2-Cu, is of interest. Performance evaluation of this metal matrix ceramic (MMC) employed electrical discharge machining (EDM), where steel is used as the cathode workpiece and the MMC is used as the anode tool. Compared with the performance of copper and graphite tools, ZrB2-Cu yields the highest workpiece removal rate,; and the lowest tool wear rate at high plasma heat flux conditions, resulting in an extremely low wear ratio. Energy dispersive spectroscopy shows deposition of workpiece materials (Fe, Cr, Ni and S) on the ZrB2-Cu surface after EDM. This is due to the difference between the surface temperature of the tool and the workpiece. Scanning electron microscopy and elemental mapping analysis reveal that the composite electrode erodes by a combination of dominant evaporation and melting of the metal phase, negligible melting and thermal spalling in the ceramic phase, quick refreezing of the metal phase back to the surface, and deposition of the workpiece (steel) on the tool surface. Most of the heat is conducted through the Cu phase, reducing thermal stress in the ceramic phase. This causes lower surface temperatures for the molten ZrB2 matrix; hence, the Cu tends to refreeze quickly near the surrounding ceramic matrix.