Microtribological analysis of gold and copper contacts

There are millions of electrical contacts subject to relative motion: connectors, relays, chips in cards, switches… Friction and wear in this kind of devices are still a source of concern. Even more, recent developments in MEMS RF switches make compulsory a further understanding of tribological processes in order to get higher operating life (∼1011 cycles). In this work two commonly used electrical conductors are characterized and compared: copper and gold. Both materials have been deposited by PVD sputtering on silicon wafers and plano-convex lenses. Surfaces were characterized by AFM and roughnesses around 1 nm are obtained. Tribological testing with normal loads in the range 1–20 mN have been carried out. Gold presents quite a constant friction (∼0.20) over a wide range of relative humidity values. However, copper presents lower friction (∼0.10) at 33% RH and higher friction when humidity is increased. Contact angle measurements have been performed on both surfaces (Au and Cu) using two different liquids: water (polar) and diiodomethane (non-polar). Surface energy and interfacial energy calculations show that energy in the gold–water interface (23 mN/m) doubles that of copper–water interface. Capillary forces play a key role generating friction in these contacts and water absorption capability of both materials determine their frictional properties in the analyzed range of relative humidity.