Contact resistance, adhesion and wear of tarnished and untarnished silver subjected to an oscillating friction load

Abstract

As a follow-up to earlier investigations, silver chips with and without Ag 2S tarnish layers were subjected to an osculating friction load from a hard gold stud. Contact resistance, adhesion and wear were investigated with contact forces of 10 cN at a friction frequency of 150 Hz using frictional movements smaller than the diameter of the stud-plate contact area ( i.e smaller than 100 μm). The increased contact resistance caused by the tarnish layer dropped appreciably as a function of movement after only a short frictional contact. For larger ( e.g50 μm) amplitudes the contact resistances were only slightly higher than on the untarnished chip. In the presence of tarnish layers, the breakaway forces which increase with increasing frictional movement (10–70 μm range) may be attributed partly to adhesion due to tarnishing and partly to cold welding. However, none of the tests with the tarnished chips produced any significant silver surface wear. Without a tarnish layer typical forms of transfer wear between chip and stud occurred.