The detection and effects of oxide layers on silver-refractory contact surfaces

It is known that high contact resistance after repetitive switching operations or the interruption of elevated currents can be a problem when silver-refractory contacts are used. One previously identified cause of high contact resistance is the formation of oxides on the contact surfaces. This paper shows how oxidation of contact surfaces can be effectively evaluated using a scanning electron microscope with a light element X-ray detector. Ways in which surface examination alone can be misleading are discussed. The general oxidation characteristics of three silver-tungsten or silver-tungsten carbide materials and the contact resistances of these materials after oxidation are presented and compared. There appeared to be a critical oxidation layer thickness below which the contact resistance was low and stable while thicker layers had resistances of over one milliohm. The results indicated that there can be appreciable oxidation on contact surfaces while still maintaining acceptable contact resistance. Possible causes of the discontinuity of contact resistance with respect to layer thickness are presented.