Marius Kuhn

Wear and friction characteristics of PVD-coated roller bearings

Abstract 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.

Wear protection and low friction in roller bearings by different PVD-coating systems

To what extent PVD-coatings are able to take over the function of EP/AW-additives was systematically examined on the basis of cylindrical roller thrust bearings. The bearings were tested under heavy-duty conditions in order to distinguish the efficiency of different coating-substrate-systems quickly. Five Me-C:H-coatings showed the best performance of the investigated ones and fulfilled the required criteria for roller bearings in the boundary friction. The least wear took place, if only the bearing washers were coated. Material carry-over from the carbonaceous coating onto the steel surface developed during the test. To investigate the excellent wear protection mechanism of carbonaceous coatings running on un-coated counterparts ESMA analysis was implemented. A tribolayer based on a process of material carry-over could be found.

INFLUENCES OF ADDITIVES ON THE WEAR PROTECTION OF PVD-COATED AND ESTER LUBRICATED ROLLER THRUST BEARINGS

Mixed friction cannot always be avoided in rolling bearings. It is therefore necessary to protect the bearings against wear. Nowadays, extreme-pressure- and anti-wear-additives fulfill this function. As these additives are harmful to the environment it was systematically examined to what extent PVD-coatings are able to take over their function. The effectiveness of the coatings was tested in the FE8 lubricant roller bearing test rig on the basis of cylindrical roller thrust bearings. With the help of this world wide established test rig the roller bearings are tested on boundary friction conditions (κ ≪ 1).Copyright