Friction Measured With a Surface Forces Apparatus

Abstract

A method is described with which friction can be measured between atomically smooth surfaces slid over distances from 1 nm to 1 μm, The force resolution is 0.5 μN and the pressure can range from zero (adhesive contact only) to 10⁹ N/m². The related method of surface forces measurement is also described, with which forces can be measured as a function of distance at separations from hundreds of nanometers down to molecular contact, with a distance resolution of 0.1 to 1 nm. Preliminary findings are reported for sliding of mica on mica in ambient atmosphere. The friction force varied little with load. The ratio of friction force to normal load therefore dropped with increasing load; it was 2 × 10⁻⁴ at a load of 1N. It is concluded that the friction coefficient is indeterminate for the present asperity-free surfaces. The shear strength was approximately 1 × 10⁶ N/m².