The effect of humidity on the rolling resistance of silver coatings modified by plasma surface treatments

Rolling behavior of plasma-modified silver films was experimentally evaluated under various environments using a thrust ball bearing-typed test rig. The silver films were coated on AISI 52100 bearing races by a thermal evaporation method where the thickness was about 1.4 μm. Plasma surface modification was performed on the silver-coated specimens in order to change the water-wetting characteristics of the surfaces. Tests were performed in ambient air, dry air and vacuum under the normal load of 147 N and at the rotating speed of 31 mm/s, and the rolling resistances were measured with the contact cycles. The wetting angles were measured by Contact Anglemeter and the surfaces were analyzed by Fourier transformation infrared spectroscope and scanning electron microscope. Results showed that rolling resistance of plasma-modified silver-coated films was greatly affected by humidity. Rolling resistance of plasma-modified hydrophobic silver film showed its minimum value at a relative humidity of 80% while that of the modified hydrophilic silver film showed its minimum value at a relative humidity of 10%. It was found that the changes in wetting angle influenced remarkably the silver debris agglomeration and the tendency of contact patch formation. Analyses suggested that the dependence of the rolling resistance on humidity could be explained by the fact that adsorbed water vapors affected the wear debris agglomeration and resulted in the different behavior of rolling resistance with humidity.