Arc characteristics and microstructure evolution of W–Cu contacts during the vacuum breakdown

•The experimental results revealed that cathode spots probably occurred on the Cu phase during the first breakdown.•The segregation of the Cu-rich rims has an important effect on the performance of W–Cu alloys.•The theoretical calculations were in exact accordance with the experimental results.•Liquid phase separation was involved in the microstructure evolution of the W–Cu alloys during vacuum breakdowns.

The microstructures of surface melt layers for W–Cu contact materials after vacuum breakdown were analyzed in detail and the arc motion characteristic was investigated with a digital high-speed video camera. The experimental results revealed that cathode spots probably occurred on the Cu phase during the first breakdown. The Cu-rich melts were sprayed out of the cathode spots due to the high plasma pressure and great volume expansion resulting from the partial boiling of Cu. The spraying of the droplets was clearly observed by a digital high-speed video camera. The calculated work function of the close-packed plane of Cu (111) was smaller than that of W (110) and the arc would firstly appear on the Cu phase of the W–Cu alloys during the breakdown. The theoretical calculations were in exact accordance with the experimental results. The experiment also indicated that liquid phase separation was involved in the microstructure evolution of W–Cu alloys.