The role of surface forces in metal–metal contacts

A real metal–metal contact involves the interaction between many small asperities on each surface. It is the way that these asperities deform and adhere that determines the more macroscopic properties of friction and wear. It is possible to investigate the interaction of a single asperity with another surface by contact experiments between a sharp metal point (1 μm radius) and a flat surface. This type of experiment also enables the interfacial adhesive energy to be directly measured. This parameter is important in thin film adhesion. In order to achieve true metal–metal contact, the experiments are performed on cleaned specimens in ultrahigh vacuum. The force required to separate the contact after a known load has been applied can be measured together wtih the area of contact (determined from electrical resistance). Results show that at very low applied loads the behavior is dominated by the action of surface forces which may themselves be strong enough to initiate plastic flow. There is also direct as well as independent evidence that these small deformed volumes are considerably harder than the bulk. The presence of oxide layers on the metal surface initially reduces the surface energy, but if sufficiently thick will also change the mechanical behavior of the contact. The latest results will be presented together with a discussion of simple models which include the role of surface forces in the contact between two surfaces. The work clearly shows the surface energy can be an important parameter in the behavior of metal–metal single asperity contacts.