A possible degeneration mechanism in stationary electrical contacts

A mechanism is proposed for the degeneration of stationary electrical contacts. The mechanism derives from W.W. Mullins' (1959) model of flattening of free surfaces through the action of capillarity forces. Material transport is assumed to occur by volume self-diffusion. Illustrative calculations of surface flattening and contact degeneration are presented for the case of Al. Calculations indicate that deformation rates increase with asperity curvature and are sensitive to activation energy for volume self-diffusion. These results suggest that the deformation of relatively small surface asperities (peak curvature of approximately 1 mu m/sup -1/ or larger) on defect-bearing Al is relatively rapid near room temperature and should be observable over a time interval of a few to several days. A simple electrical contact model suggests that the increase in contact resistance stemming from surface flattening in Al at room temperature can become noticeable in a time interval of a few days ( approximately 10/sup 6/ s). This result may explain the previously observed deterioration of contaminated Al/Al stationary electrical contacts operated at room temperature, after run times of 10/sup 5/-10/sup 6/. Since many metals are characterized both by rapid volume or surface diffusion and by a large surface energy, the results suggest that asperity flattening can lead to degeneration in all bulk electrical interfaces. Finally, since surface flattening leads to separation of contact surfaces, it makes it possible for fluid contaminant films to spread into the contact interface, suggesting that the mechanism of contaminant ingress proposed earlier as a primary cause of contact generation may instead be a consequence of surface operation in electrical junctions.<
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