EFFECT OF SURFACE STRUCTURE ON CONTACT RESISTANCE AS A FUNCTION OF OPERATING LIFE IN Ag-W CONTACTS

摘要

The change in contact resistance of Ag-W contacts as a function of operating life has been measured in a 20 A 110 V rms circuit. There was always a large statistical distribution of the contact resistances, but there was a definite trend which showed that the average of these distributions Increased directly with the number of operations. A scanning electron microscope (S.E.M.) study of the surface morphology of the Ag-W contacts as a function of the switching life showed bow the Ag content of the contact surface had changed. It was shown that a very thin Ag film was sufficient to produce a low contact resistance. It was also shown that the high contact resistances that occurred could not be accounted for by W to W contact alone. Debye-Scherer X-ray analysis of powder from the contact surfaces after 6000 operations revealed the presence of WO_3, W_3O and Ag_2WO_4 and also some contaminant non-conducting chemical compounds. It was the appearance of these compounds together with the gradual predominance of W on the surface of Ag-W contacts, that caused the trend towards the high contact resistances during the contact's operating life.