Friction and Adhesion of Clean and Contaminated Mica Surfaces

Abstract

In order to determine the shear strength of boundary lubricants it is essential to use a substrate which is smooth on a molecular scale. An apparatus is described in which it is possible to apply normal and tangential loads to sheets of mica covered with monomolecular layers of boundary lubricant. The shear strength obtained from these experiments explains in part why frictional force and pickup are not reduced proportionately, in the presence of a boundary lubricant. The remaining difference is probably due to elastic hysteresis losses in the sliding solids. The surface energy of mica in air has also been measured by determining the force necessary to propagate a crack in the material. Cycles of opening and closing the crack have been performed and the difference in energy which is observed is attributed to the adsorption and migration of an interfacial film of air or water vapor. An electronmicrographic study of the structure of monolayers deposited by retraction from nonpolar solution is also described. This indicates that the area covered by the monolayer is only about ⅓ the total surface area. This poor coverage arises from incorporation of solvent molecules in the monolayer, which later evaporate.