Apparatus for Environmental Friction-Adhesion Studies Using Ultrahigh Vacuum Techniques

Abstract

An apparatus is described for the real-time analysis of sliding friction and normal force adhesion phenomena using ultrahigh vacuum techniques. Developed for the purpose of investigating the effects of surface cleaning, temperature, and deliberate gaseous contamination on the adhesion behavior of selected candidate materials for use in the NERVA nuclear rocket engine program, the system was designed to measure contact resistance by a four terminal ac (constant current-voltage drop) instrument calibrated in resistance. Using a torsion balance technique the relationship between the normal compressive load applied across crossed rod samples and the resultant contact resistance is used to ascertain the extent of contact area growth under anticipated service conditions. The apparatus can accommodate sample temperatures to 3000 K and maintain gaseous environments from pressures of 1 atm to 10−11 Torr. Contact resistance in the range 103–10−4 Ω, normal compressive loads from 1 mg to 600 g, and/or tangential loads to 155 g are displayed continuously in an analog fashion. Contact resistance data taken for a variety of metallic, intermetallic, and graphitic materials combinations have indicated the rate and extent of contact area growth to be expected during service and the effectiveness of gaseous lubricants in preventing adhesion. Typical data are presented for the materials combination 440C stainless steel vs 440C stainless steel.