The use of Ag-W-CdO and AgSi3N4as Contact Materials

Three different contact combinations were investigated: (a) Ag-W operated against Ag-CdO, (b) Ag-W-CdO contact pair, (c) Ag-Si
3
N
4
contact pair. A hot pressing technique was used to make the contacts for (b) and (c). The contacts were tested by switching a 20 A, 110V circuit and then examined using a Scanning Electron Microscope. It was found that: (a) The Ag-W vs Ag-CdO produced a high resistance contact pair which was the result of migration of a W layer onto the Ag-CdO contact. (b) The high temperature arc roots caused the W to reduce the CdO in the Ag-W-CdO contact material, which led to severe contact erosion and degradation. (c) The Si
3
N
4
dissociated in the switching arc and the Si formed a SiO
2
layer on the contact surface, which would eventually lead to a high contact resistance. These results are discussed in terms of the thermodynamics of the material combinations and their interaction with the switching arc.