Gross plastic fretting mechanical deterioration of silver coated electrical contacts

Abstract

This paper sets out to describe the particular fretting situation of power connectors, with emphasis on deformation mechanisms, contact resistance, friction forces and tendencies to cold welding. In power applications, silver coated copper contacts are subjected to very high contact forces (typically 10–100 N) and thus inevitably deform plastically when mated under stationary conditions.Under such conditions the fretting mechanisms deviate substantially from those described for fretting under near elastic conditions. If extensive contact surface flow occurs, owing to high normal and friction forces, large displacements can be accommodated without slip. Yielding of the surface is increased by application of soft coatings. The fretting under massive plastic deformation described here can be considered a new mechanism for wear and surface damage. The name gross plastic fretting is proposed.As also established in the field of fretting under near elastic conditions, three fretting regimes were distinguished in gross plastic fretting. All three regimes, denominated gross weld fretting, temporary weld fretting and gross slip fretting, involve plastic deformation and have distinctive effects on the surface damage, the friction, the adhesion to the counter face and the contact resistance. The regime prevailing is primarily determined by the combination of displacement amplitude and applied normal force.For silver coated copper contacts, welding between the contact surfaces strongly influences the fretting behaviour. It is concluded that only the fretting regime found at low displacement amplitudes, in combination with high normal forces, gives acceptable contact resistances. Fretting at lower normal force and/or higher displacement amplitude rapidly deteriorate the electrical contact due to fretting corrosion of copper or due to severe wear of the contact surfaces.