Comparative study of the microspot splitting characteristics of direct-current and pulsed cathodic vacuum arc

Abstract

Ejection of macroparticles is an inherent and unavoidable characteristic of cold cathode arc discharges. The size of the cathode arc spot's melt pool is closely related to the emitted particle size, and existing studies mostly indicate that motion velocity of arc spots can be heightened primarily by magnetic fields, thereby reducing the ejection of macroparticles. However, scant consideration has been given to avoid the fundamental nature of arc spot current concentration discharge by exploring the phenomenon of arc spot splitting. In this paper, microspot splitting characteristics of direct current and pulsed cathodic vacuum arc were compared and analyzed to investigate the effect of pulsed discharges on the arc spot's internal structure, splitting, and kinematic characteristics. The results showed that pulsed arc discharges emit a dense, highly ionized plasma flow which impacts the relatively thin positive charge layer instantaneously during the peak pulsed period. This then produces a highly ionized plasma region, while also promoting an effect by which the arc spot is split into multiple microspots. Additionally, the degree of dispersion resulting from microspot splitting is positively correlated with the ratio of the plasma flow density to the concentration of the positive charge layer at the moment of microexplosion. Therefore, the microspot splitting effect of a second-order pulsed arc is weaker than that of a first-order pulsed arc. Moreover, the splitting characteristics indicated a trend toward the progressive splitting state of the direct current cathode arc. When the peak current of the single-order pulsed arc was increased from 100 to 400 A, the particle refinement effect on the surface of the CrN coating became much more evident compared to the effect of a 200 A arc in the direct current mode. And upon reaching a peak current of 500 A (and up to 600 A), the number of splits did not increase linearly due to the continuous increase in the discrete degree of microspot splitting and concurrent decrease in microspot stability. This resulted in relatively high individual microspot currents, while some relative macroparticles still appeared on the coating surface.