Electron work function, adhesion, and friction between 3d transition metals under light loads

Adhesion and friction between metallic materials are of importance to many dynamic mechanical systems, particularly to those at nano- or micro-scales. Under light loads, friction is mainly dependent on the adhesion between two surfaces in contact, to which valence electrons have the predominant contribution. However, the correlation between the electronic behavior and adhesion as well as friction has not been well established. This paper reports the authors’ recent studies on the correlation among electron work function (EWF), adhesion and friction of transition metals. The EWF is the minimum energy required to remove an electron from the Fermi level. This parameter characterizes the electronic behavior of metals and can be determined experimentally. In this study, the metal–metal friction under light loads was evaluated employing a micro-tribometer. It was demonstrated that adhesion and friction of the transition metals were closely related to their electron work functions. The adhesion between two different metals in contact could be expressed as a function of their EWFs and electron densities. Consequently, the friction between the two metals under light loads could be estimated based on these two parameters.