Humidity Effects on Friction and Wear Between Dissimilar Metals

The effect of water vapor on the friction and wear between copper and 440C stainless steel was studied using a ball-on-flat tribometer, polarization-modulation reflection-absorption infrared spectroscopy, and Auger electron spectroscopy. The wear behavior changed drastically as the relative humidity (RH) varied in inert gas (nitrogen or argon). In a dry environment, a small degree of abrasive wear of soft copper was observed. In the RH range of 10-70 %, catastrophic adhesive wear of the soft copper surface was dominant. A high RH (> 80 %) environment exhibited wear of the hard 440C stainless steel surface and the steel wear debris was deposited onto the copper. The adsorption isotherm measurements for copper and stainless steel revealed that water adsorption increases quickly between zero and 10 % RH and then the adsorption proceeds more slowly as RH increases further. The adsorbed water layer thickness increases rapidly again as saturation is approached. It seems that the thin layer of adsorbed water under 70 % RH facilitates the adhesive wear through passivation of grain boundaries or acceleration of crack propagations, but the thick water layer formed over 80 % RH acts as an electrolyte medium allowing galvanic corrosion to commence.