Anode Erosion During Pulsed Arcing

An experimental study of the anodic erosion rates of Cu, Zr, Ti, Mo, Ta and W is presented under conditions similar to those used for electrodischarge coating. The arcs were conducted between a small anode and a larger cathode in air with pressures ranging from 10^-4 to 10³ torr. Unipolar arc pulses of 200–400 A peak current and 0.1 ms duration were produced at a 100 Hz pulse repetition rate by an RC circuit. For most materials, the electrode mass loss is primarily from the anode, and the mass loss is independent of pressure for pressures less than 0.1 torr. The mass loss decreases steeply with increasing pressures in the range 0.1 to 10 torr. Above 10 torr the mass loss decreases more gradually with increasing pressure. The experimental results are explained by using a limiting case of the integral conservation laws in which the momentum input to the anode spot is expended on the acceleration of the anodic plasma jet, and the energy input is expended on the acquisition of kinetic energy for the mass eroded from the anode and on accelerating and heating entrained background gases. A particular solution of the system of equations is found using the hydrodynamic approach. A numerical solution for the Cu anode shows good quantitative agreement with the experimental measurements. In the low pressure region the input energy is expended mainly in the acceleration of the metal vapor, and thus the erosion rate is independent of pressure. In the intermediate pressure region the metal vapor jet is braked by its interaction with the surrounding gas. In the high pressure region the vapor jet is completely halted, and vapor transport takes place only by diffusion through the surrounding gas.