Microstructural Changes of Vacuum Interrupter Contact Materials Caused by Switching Operations

Contact materials based on copper-chromium are widely used in vacuum interrupters for medium-voltage applications. The microstructures of contacts are rather coarse, with typical chromium particle sizes in the range of 100 microns. During switching operations, such as short-circuit current interruption, a vacuum arc is generated which in turn produces melted and resolidified layers at the contact surface. This paper describes the substantial microstructural changes caused by the vacuum arc. High-resolution scanning electron microscopy reveals the precipitation of chromium nanoparticles within the resolidified surface layers. This pronounced particle refinement within the melted and resolidified layer is a result of the high cooling rates involved in the solidification process. In separate metallurgical experiments copper-chromium materials were melted and rapidly solidified using splat-quenching and melt-spinning. The cooling rates in these experiments were adjusted to values that fit to calculated temperature-time profiles which numerically simulate the switching operations. It was found that the microstructures of both the experimentally solidified samples and the resolidified layers of vacuum interrupter contacts match very well.