Sliding Wear of Metallic Contacts

The wear of metallic contacts occurs by adhesion, abrasion, brittle fracture, fretting, and delamination processes. Adhesive wear can be in mild or severe regimes and is promoted by surface cleanliness and contact pressure. The severe adhesive wear mechanism in early sliding ofAu, Pd, Ag, Sn, SnPb alloys, and many other metals is prow formation, characterized by the transfer of metal from the member with the larger surface involved in sliding to the smaller. On repeat-pass movement a transition to rider wear occurs in which the direction of metal transfer reverses. Prow formation occurs readilywith ductile metals because it requires a large growth of mating asperities. Neither the hardness nor the presence of codeposited polymer in pure or alloy gold e!ectrodeposits is as significant as their ductility in determining performance. Codeposited polymer is not a lubricant, but it may inhibit plastic flow of the metal, reducing ductility and thereby indirectly minimizing the tendency to adhesive wear. Golds that contain a few tenths of a percent of cobalt or nickel from dc-plated acid cyanide baths have good adhesive wear resistance. Abrasive wear of a metal occurs when it is slid against a rough hard surface. Ductile metals resist fracture during abrasion; both high hardness and ductility are desirable. Wrought metals, such as inlays, are generally more durable than the cobalt and nickel dc gold electrodeposits in abrasive wear. Fretting, or oscillatory sliding through a small amplitude, leads quickly to high contact resistance with base metals, because oxide debris accumulates between the members. Fretting may also degrade the contact resistance of mated connectors if gold finishes on their contacts wear through, thereby exposing the base substrate. Delamination wear, which appears first as a substrate, then as surface cracks, followed by the loosening of surface material, contributes to contact failure in fretting. Hard substrates and underplates are desirable because they reduce adhesive, abrasive, and fretting wear. Smooth surfaces are superior to rough surfaces when adhesive and abrasive wear can occur.