Structure characterization of switched Ag - metal oxide contact materials

The structural changes of several Ag ‐ metal oxide composites during switching operations were investigated. During the switching operation the contact material is melting and for a short moment (ca.10 ms) the metal oxide particles will be floating in the liquid silver resulting in structural changes. These processes affect the switching performance of contact materials and shall be described in this work. The low hardness of the Ag‐matrix requires careful sample preparation in order to avoid preparation artifacts and to reveal the actual structural features. This was achieved by a final step, in which the polished surface was ion beam etched. The samples will be analyzed by stereography, light microscopy, field emission scanning electron microscopy (FE‐SEM) and electron back scatter diffraction (EBSD). It was found that the structure of a switched material can be divided into four category regions: Firstly, the bulk material, secondly, a very small heat affected zone, thirdly, a broad fusion zone and finally, the surface layer. The fusion zone is characterized by large columnar grains, increasing porosity and oxide agglomeration towards the surface. These structural phenomena can only be revealed by ion beam etching, as will be illustrated by examples of selected contact material.