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 evaluat ed under various environments using a thrust ball bearing-typed test rig. T he silver films were coated on AISI 52100 bearing races by a thermal evapor ation method where the thickness was about 1.4 mum. Plasma surface modifica tion was performed on the silver-coated specimens in order to change the wa ter-wetting characteristics of the surfaces. Tests were performed in ambien t air, dry air and vacuum under the normal load of 147 N and at the rotatin g speed of 31 mm/s, and the rolling resistances were measured with the cont act 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 pla sma-modified silver-coated films was greatly affected by humidity. Rolling resistance of plasma-modified hydrophobic silver film showed its minimum va lue at a relative humidity of 80% while that of the modified hydrophilic si lver film showed its minimum value at a relative humidity of 10%. It was fo und that the changes in wetting angle influenced remarkably the silver debr is agglomeration and the tendency of contact patch formation. Analyses sugg ested that the dependence of the rolling resistance on humidity could be ex plained by the fact that adsorbed water vapors affected the wear debris agg lomeration and resulted in the different behavior of rolling resistance wit h humidity. (C) 2001 Elsevier Science B.V. All rights reserved.