The unusual electrical erosion of high tungsten content, tungsten copper contacts switching load current in vacuum

The electrical erosion of high tungsten content, tungsten-copper (7-10 wt. % Cu) was investigated. The contacts were placed in a vacuum interrupter en velope with a background pressure of about 10(-6) torr. The contacts switch ed one half cycle of 60 Hz current per operation. The polarity of the curre nt was changed on each operation. Six contact pairs were investigated. Each pair was subjected to an ever-increasing number of operations: 1K, 5K, 10K , 20K, 40K, 50K, and 60K. The contact erosion was inferred by measuring the linear position of the moving contact terminal. On completing the electric al testing, the vacuum interrupters were dismantled and the contact surface s were observed. Unlike the previous work on Ag-WC (50 wt. % Ag) and Cu-Cr (75 wt. % Cu), the W-Cu contacts showed a localized build up of erosion pro ducts on the contact surfaces even beginning at 1K operations. The experime nts were repeated switching a unidirectional current i.e., the contacts rem ained at the same polarity throughout the experiments. Here an anode pip an d a cathode crater were formed immediately. The difference in the topograph ies of these contacts is discussed in terms of the metallographic analysis of the deposits on the contacts, the erosion deposits on the shields surrou nding the contacts and the expansion of the vacuum arc.