Effects of Oxygen and Water Vapor Adsorption on Friction of Pure Copper during Fretting

Abstract

Fretting friction experiments for pure copper in contact with itself were carried out in air and in argon. Variation in coefficient of friction μ was continuously measured before and after the rapid change in relative humidity RH from 0% to the desired RH levels, namely RH jump.
The μ in air at a steady state just after the RH jump increased with jump range ΔRH from the minimum value of ΔRH=0%. The μ in argon was quite higher than that in air but less changed with ΔRH. The physical conditions of rubbing surfaces can be kept constant even after the RH jump in air because of the short testing time. Therefore, the μ in air is affected only by the variation in surface functional group of the rubbing surfaces. The increase in μ with ΔRH can be explained by an adsorption model of oxygen and water molecules on the rubbing surfaces.
Variation in μ during fretting without RH jump was also measured at various RH levels. The steady μ in air changed complicatedly against RH and was smaller than the μ with RH jump. Roughness of the rubbing surfaces was varied with RH. Therefore, the variation in physical factors affecting μ should be taken into consideration in the fretting experiments at RH levels without RH jump.