Wear and friction characteristics of PVD-coated roller bearings

On the basis of cylindrical roller thrust bearings it was systematically examined to what extent physical vapour deposition (PVD)-coatings are able to take over the function of EP/AW-additives. The bearings were tested under heavy-duty conditions in order to distinguish very fast the efficiency of different coating–substrate-systems. Four Me-C:H-coatings showed the best performance of the investigated coatings and fulfilled the required criterion for roller bearings in the boundary friction. Least wear was produced, if only the bearing washers were coated. Two different ZrCg coatings were varied in their design of gradient and carbon top layer. It could be shown that a homogenous transient from the Zr-bonding layer to the graded ZrC coating and to the carbon top layer reveals a better wear protection than sudden transients, which built a inadvertent breaking point in the coating system. The different hardness and thickness of the carbon top layers could not be differentiated in relation to their wear protection. The decreasing friction coefficient in the tests correlates with increasing values of wear protection. The carbon containing coatings showed a positive effect on friction properties with the lowest friction coefficients. As a consequence, firstly, the amorphous carbon coatings have an inert character and consequently, the adhesive reactions between the friction partners are very low. And secondly, high contact pressures might transform amorphous carbon into graphite, which is able to work as a solid lubricant. Material carryover from the carbonaceous coating to the steel surface was developed, more or less, by the Me-C:H-coatings during the tests. This mechanism was able to protect the un-coated rollers. Closer investigations were done with an ESMA analysis (electron beam micro range analysis) on ZrCg coatings. It could be seen that a reaction layer was formed on the rollers. This layer contained remarkable masses of zirconium and oxygen. As zirconium has a higher affinity to oxygen than to carbon, a chemical reaction of zirconium carbide with the oxygen to zirconium dioxide and carbon dioxide might be possible.