High-performance chromium aluminium nitride PVD coatings on roller bearings

Abstract

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 (Cr x ,Al 1− 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 [(Cr x ,Al 1− x )N] failed earlier than coatings with low friction values (ZrC g). Additionally, the wear protection of (Cr x ,Al 1− 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.