In order to characterize different lubricants concerning the affinity to cause hydrogen embrittlement in bearings, rolling contact fatigue tests were performed. Hydrogen analyses in the rolling elements were carried out before and after the tests as well in order to quantify the hydrogen concentration. In general, Lubricant degradation is considered to be the primary cause for brittle flaking and white etching cracks (WEC) in bearings. Henceforth, the tested lubricants were as well characterized by FTIR, XPS and viscosity measurements in rheometer.
The rolling contact fatigue tests were designed and developed for thrust roller bearings at Fraunhofer IWM. The bearing tests were performed with a FAG 81104 thrust roller bearing. The rolling tests were done with a load of 8kN and a rotational speed of 700 rpm at a temperature of 100°C. The bearings were dipped lubricated during the test. The tests were performed with commercial gearbox lubricants. The hydrogen analyses of the bearings were done with hot gas extraction. Degradation reactions of the lubricants were detected by using FTIR, XPS and Rheometer.
Brittle flaking and WEC is considered to be hydrogen driven fatigue effect. The formation of WEC and brittle flaking in bearings is caused by an increased hydrogen concentration of the bearing steel. WEC are mostly formed along subsurface cracks in highly stressed vicinity. Furthermore, the grain structures near the WEC region are altered with respect to the hydrogen evolution. Hydrogen evolution in a rolling contact is associated with lubricant degradation. Oxidation fragmentation reactions of the lubricant are considered to be one of the sources. In commercial lubricants additives and contaminations are another possibility for a hydrogen evolution of the lubricant.