Craters formation in a graphite cathode produced by pulsed arc at low pressure

DC pulsed arcs, between face to face opposite electrodes of graphite and stainless steel at pressures varying from 50 to 500 Pa into nitrogen environment, with a current and voltage of 300 A and 167 V, respectively, and a pulsed arc time of 30 ms were produced. Several consecutive craters, created by explosive way and by inclusions over the graphite cathode were found. These are located principally at the outskirts of the center and at the half of the graphite cathode radius; they are originated because of the cathode spots chaotic movement. The most representative craters have sizes over than 300 mm and some melted sectors were observed by means of the SEM technique, corroborating that there are temperatures higher or equal than the melting point of the graphite. The film surface is composed by small spheres with structures similar to the ones in the cauliflower diamond. Craters in some melted sectors on the anode surface were observed.