Adhesion and deformation properties of clean and characterized metal micro-contacts

Abstract

The adhesion and deformation properties of metal contacts are studied in a uhv scanning electron microscope, incorporating an Auger facility for surface characterization. Contact is made between a sharp tungsten point of about 1 μm radius of curvature and a nickel (111) single crystal. It is possible to measure applied loads and forces of adhesion down to 0.5 μN. The electrical resistance between the tungsten point and the crystal gives a measure of the area of contact.Experiments on clean metal surfaces show that for applied loads above 20 μN the contact is fully plastic, and surface forces have very little effect on the deformation. The loading-unloading cycle is highly irreversible, the junction finally separating in a ductile manner. For an applied load less than 5 μN the force of adhesion becomes independent of the load, and the deformation is determined by the surface forces. This suggests that the contact may be elastic. However, the loading-unloading cycle is still irreversible, and it is suggested that surface forces are strong enough to initiate plastic flow.When about two monolayers of oxygen (probably in the form of oxide) are present on the metal surface, the adhesion is reduced by up to a factor of 3 over the whole load-range examined. The loading-unloading behaviour is largely irreversible, and resembles that of clean surfaces. By contrast with the relatively small change in adhesion, the contact resistance is very considerably increased.