Asperity Contact Evolution: Capillarity and Electromigration Effects

When two surfaces are brought together, contact occurs initially between asperities on the surface. If the mechanical loads are small, complete contact is never achieved and the behavior is dominated by asperity contact. The contact area and asperity morphology may evolve in time as a result of mechanical and capillarity (surface tension) effects, mediated by plastic deformation and/or diffusion. If a current passes through the contact, as in the case of micro-electro-mechanical switches, the evolution may be controlled by electromigration. This effect may be especially important if the voltage drop across the contact is fixed and the fractional contact area is small, such that the current is concentrated in a small number of contacts (see Fig. 1). Electromigration occurs as a result of the voltage driven electrons scattering off and imparting momentum to the atoms in the solid (see Fig. 2). Typically, the electromigration atom flux is opposite the direction of the electrical current (i.e., in the same direction as the electron flux). At small homologous temperatures (i.e., the temperature normalized by the melting temperature) and in small structures (such as asperities), atomic transport will be dominated by surface, rather than bulk, diffusion. In this presentation, we consider the evolution of an idealized asperity under the action of both capillarity and electromigration.