The strength of an interface between two metal surfaces can be drastically affected by the presence of chemical species in the interface. The two surfaces may form the boundary between two grains inside a solid block of metal, the interface between a substrate and a metal film or the interface between two solid bodies brought into physical contact. Interface impurities such as oxygen atoms generally reduce the cohesion between the two surfaces. Conventional fracture experiments have difficulty in determining whether this results from a reduction in the interfacial energy, or a hardening of the interface, either of these making brittle fracture easier. The experiments which we have carried out by contacting two clean and characterised surfaces in ultra high vacuum, enable a direct study to be made of the change in interfacial energy and mechanical behaviour as a result of adsorbed species.
A sharp metal point of about 1μm radius of curvature is contacted against a flat metal specimen and the force of adhesion and contact area (measured from electrical resistance) are determined. The extent of ductile or elastic behaviour of the interface can be determined from the reversibility of the contact area as a function of applied load during separation.
We have shown directly from a contact experiment, that monolayer quantities of oxygen reduce the interfacial energy by a factor of three in the metal contacts studied. Interfacial ductility is only reduced by the presence of oxide layers of the order of 5nm thick.