Dispersion forces and equilibrium distance between deposited rough films in contact

Abstract

The average distance ${h}_{0}$ separating two contacting bodies is a critical parameter for many problems, for example, for control of unwanted stiction in micro/nanoelectromechanical systems. Here this problem is analyzed in relation to precise determination of the dispersion forces in a difficult for measurement range of $5\ensuremath{-}30\phantom{\rule{0.222222em}{0ex}}\mathrm{nm}$. The unloaded contact between two deposited rough films characterized by a relatively large number of high asperities is considered. The equilibrium distance ${h}_{0}$ can be found from the balance of attractive dispersion forces and repulsive forces acting in the spots of real contact. A simple columnar model associated with AFM images of rough surfaces is used to describe the balance. The numerical analysis, which treats the high asperities as elastoplastic semispheroids, demonstrates that the columnar model describes the contact adequately. It is shown that in contrast with the value of ${h}_{0}$ the adhesion energy between the surfaces is nearly entirely defined by the dispersion interaction, but the effects of contact interaction and plastic deformations can be neglected. This property is proposed to use for more precise determination of the equilibrium distance.