The Friction of Lead Films in Spherical Contact

Recent advances in applied mechanics are utilized to show that Rabinowicz's experi-mental results for the friction of lead films as a function of film thickness can be relate-! to the dependence of real area of contact on film thickness. L I HE friction of solid film lubricants as a function of film thickness, geometry of contact, and materials properties has recently been a topic of considerable interest (references [1-4]). ' I t is now generally recognized tha t the friction coefficient of coat-ings is a systems property rather than a materials property. I t critically depends on the applied mechanics of contact, with film thickness being a major parameter. An analytical understanding has emerged in recent years for two types of situations. First for bonded solid lubricant films (references [1 and 2]); their mechanics of contact can be modeled after an elastic layer on a rigid foundation pressed on by a rigid indentor, as they have Young's moduli about two orders of mag-nitude below that of steel. Second, an understanding now exists for very thin films such as those formed on surfaces by chemical reaction [4]. The key to understanding the film thick-ness dependence of these films is the realization that the stiffness of the film is thickness dependent. The case of soft metallic coatings is today far less well under-stood than those systems mentioned previously. These metals have Young's moduli which at most are below those of commonl