An experimental study on the rolling resistance of bearings coated by pure silver

An experimental study was performed to discover the effect of a soft metallic coating on the behavior of rolling resistance. Pure silver was coated on AISI 52100 bearing steel by a thermal evaporation method, where the coating thickness ranged from 100 nm to 4400 nm. A thrust ball bearing-type test rig was used and the rolling resistance was measured in various environmental conditions such as ambient air, vacuum, dry air and controlled humidity. The surfaces were analyzed by SEM and EPMA. Results generally showed that minimum rolling resistance was obtained after the initial coating film was apparently worn. The decrease of rolling resistance resulted mainly from shakedown phenomena which occurred at the contact patches of agglomerated wear particles. It was also found that rolling resistance was affected strongly by environmental conditions. Results in various environmental conditions suggested that humidity related closely to the agglomeration of particles that affected significantly the rolling resistance. The influence of humidity on particle agglomeration was further investigated by using a plasma surface modification that changed the wetting angle of coated films.