Silver Based Refractory Metal/metal Carbide Electrical Contacts for Circuit Breaker Application

The purpose of a switch, relay or circuit breaker is to open and close an electrical circuit as desired by the user and sometimes in order to safeguard the application from failures like overload and short circuit. The circuit breaker is an essential part of the protection system of every electrical network, from common household to automotive and industrial networks. The electrical contact material is an indispensable component of a circuit breaker which unlike an expendable fuse can be re-used for several cycles of opening and closure. In order to comply with the safety protocols framed by the Underwriters Laboratories (UL), these contact materials have to satisfy a set of stringent ‘properties’ depending on their switching application, i.e. operating voltage and current.

Historically, contact materials are metal-metal or metal-ceramic composites which tend to incorporate multiple properties of distinct materials and provide a compromise between several irreconcilable ‘switching properties’. In this work, Ag-WC and Ag-W contact materials have been investigated, where a conductive soft metal like silver is integrated with a refractory material without losing its intrinsic properties, due to the negligible solid and liquid solubility of the two compounds. The framework of the thesis is based on the processing-microstructure-property relationship of these materials and their dependence on the composite composition.

These materials are processed via powder metallurgical routes and are densified into a solid compact by more traditional methods, which are commonly known in industry as ‘press-sinter-infiltration’ and ‘press-sinter’. These densified compacts are further brazed on a copper substrate and incorporated inside a circuit breaker. In this work, this traditional method has been scrutinized and compared with a novel approach of simultaneous spark plasma sintering of such a composite contact material and in-situ bonding it to a copper substrate. The microstructure of these materials has been altered by varying the particle size of the starting powders and the milling method. Moreover, the microstructure also has a strong dependence on the composite composition. The composition was varied between the prescribed ranges of Ag and WC by changing the starting powder compositions and by altering the sintering shrinkage prior to infiltration. Transition metals like Ni, Co, Fe and Cu were alloyed in small amounts and the role of Ni was studied extensively.

Amongst the properties important for contact materials, the two most relevant switching properties which have been studied in this context are the arc-erosion resistance and the electrical contact resistance. Ag-WC contact materials have an excellent arc-erosion behavior compared to other silver based contact materials, thus the primary objective was to improve the contact resistance without compromising the arc-erosion resistance of these materials. These properties were investigated in a break-only model switch at the Technical Materials division of Umicore AG & Co. KG in Hanau, Germany.

Since the processing-composition-microstructure-property corners of the materials tetrahedron are strongly correlated, specific combinations of microstructures and compositions have been highlighted in this work. The results allow the reader to define and select the composition and processing method of arc-resistant Ag-W/WC contact materials demonstrating a low electrical contact resistance in order to avoid unnecessary heating of electrical appliances.