Effect of Nano Surface Roughness on Friction Coefficient of Stainless Steel

Abstract

Surface modification of materials for sliding motion is very important to reduce friction as well as out-gassing for clean and smooth operation in a vacuum. We, therefore, tried to develop the advanced modifications for smooth motion in a vacuum by controlling surface roughness on a sub-micron scale. Friction measurement was carried out on type 304 austenitic stainless steel sheets known as typical vacuum material. They were surface-treated by chemical polishing or electrochemical buffing after mechanical polishing. A probe for friction measurement was a mechanically polished stainless steel ball of 3.18 mm in diameter. The probe was slided under the load of 0.49 N on surface of stainless steel sheets with surface roughness from 40 nm to 1550 nm. Friction measurement in an atmospheric pressure showed little change in friction coefficients of sheets treated either by chemical polishing or by electrochemical buffing. This shows that friction coefficients were almost independent of surface roughness in an atmospheric pressure. Friction measurement at the pressure of 10^-6 Pa showed that friction coefficients of sheets increased largely except sheets with surface roughness about 100 nm treated by chemical polishing or by electrochemical buffing. Stainless steel sheets with surface roughness about 100 nm can only keep low friction coefficient of 0.1 both in an atmospheric pressure and in a high vacuum. It is therefore concluded that the surface roughness about 100 nm will help smooth sliding motion under any pressure from atmospheric pressure down to high vacuum.