Fretting behaviour of tinned connectors under grease lubrication

Fretting is a well known phenomena due to unwanted micro displacements leading to drastic electrical contact resistance increase. Much work has been done to understand and evaluate the critical parameters in view of finding solutions to alleviate the problem. Several paths have been proposed involving for example connector design and surface modification. The simplest surface modification process is depositing an organic thin film with specific properties. The effect of liquid lubricants on fretting of electrical contacts has been well studied and the lubrication efficiency has been shown to depend on the degradation modes of the contacts which involve the nature and type of metallic surface (Au, Ni, Sn, Ag, plating thickness). Another type of lubrication involves semi liquid films such as greases. While much work has been done on the effect of greases on the fretting-wear of mechanical devices, less has been devoted to electrical contacts. In such field the grease films must also maintain a low electrical resistance for durations corresponding to field service. The work presented here was performed with a standard fretting device imposing controlled constant amplitude sine vibrations under constant normal load to pin on flat tinned contacts. This laboratory test allows comparing the behaviour of different types of greases for a given set of experimental conditions. Different commercial greases are compared involving different thickeners and different base oils. Tests at room temperature show little variations in the electrical behaviour of the grease coated contacts. Nevertheless wear particles dispersion seems to vary. A very high increase of contact resistance is observed in some cases after heat treatment 10 days at 150°C. Polymerization was observed to occur with one of the formulations and it is suggested that the base oil/thickener choice might be unfavourable to contact application.