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On the basis of cylindrical roller thrust bearings, it was systematically examined to what extent PVD coatings are able to take over the function of extreme pressure and anti-wear lubricant additives. The bearings were tested under heavy-duty conditions in order to quickly distinguish the efficiency of different coating substrate systems. Close regard is paid to the structure of the coatings. Two different (Crx,Al1−x)N coatings, one with a columnar structure and one with a fine-grained structure, are tested concerning wear protection and friction reduction. It is shown that PVD coatings are able to reduce friction enormously on the example of cylindrical roller thrust bearings. Friction coefficients measured on a pin on disc tester could not be correlated to the FE8 roller bearing test rig, where the values for friction were not distinguishable. The FE8 test rig tests roller bearings with a rolling and sliding motion in contrast to the pin on disc test where sliding friction is produced. So the friction in the roller bearing test is more complex and the sliding contact does not make a major contribution to the frictional torque. Nevertheless, the lower sliding friction of contacts, which can be tested in the pin on disc test, reduces shear stresses in the slippage zones of roller bearings. Here, sensitive columnar and fine-grained coatings with high-friction coefficients [(Crx,Al1−x)N] failed earlier than coatings with low friction values (ZrCg). Additionally, the wear protection of (Crx,Al1−x)N coatings could be increased by a fine-grained coating microstructure. This coating architecture is more able to withstand shear loads. The conclusions of these tests are the need of a coating with fine-grained structure and friction-reducing properties in the FE8 roller bearing test rig.

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.