Effect of cathode microstructure on arc velocity and erosion rate of cold cathodes in magnetically rotated atmospheric pressure arcs

Abstract

Atmospheric pressure arc velocity and arc erosion measurements were performed on thermal sprayed copper cathodes having grain size distributions from submicrometre to a few micrometres in a continuous running arc system. Ultrahigh purity (UHP), 99.99% pure, argon and a mixture of UHP argon with 10% (volumetric) extra dry air were used as plasma forming gases. An external magnetic field of 0.10 T was used to rotate the arc which was operated at a constant power setting of 6 kW (150 A and 40 V). Cathodes having different microstructures were formed using high velocity oxygen fuel (HVOF) spraying and vacuum plasma spraying (VPS) methods with different annealing times. HVOF sprayed cathodes were tested for arc erosion and arc velocity in their as-sprayed state and also after annealing them at 300 and 600 °C for 1 and 8 h durations in an inert argon atmosphere. VPS sprayed coatings were tested as-sprayed. The effect of the initial coating thickness on erosion rates was investigated. Annealing HVOF coatings at 600 °C for 8 h produced near equi-axed grains with a 2–3 µm average size. These coatings showed 60% higher steady state arc velocities and up to 68% lower erosion rates compared with massive copper cathodes having a 20–23 µm average grain size. VPS coatings having a 0.9–1.5 µm average grain size gave up to 70% lower erosion rates when compared with massive copper cathodes. The results show a clear effect of the cathode microstructure on the arc velocity and the erosion rate.