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 (similar to 10(11) 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 (similar to 0.20) over a wide range of relative humidity values. However, copper presents lower friction (similar to 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. (C) 2007 Elsevier Ltd. All rights reserved.