23rd International Colloquium Tribology - January 2022 71 Alternative lubricants in wind turbines to avoid WEC formation Dominik Kürten Fraunhofer Institute for Mechanics of Materials IWM, Germany Corresponding author: dominik.kuerten@IWM.fraunhofer.de Stefan Grundei Klüber Lubrication München SE & Co. KG, Germany Jörg Franke Schaeffler Technologies AG & Co. KG, Germany Sebastian Plebst IoLiTec Ionic Liquids Technologies GmbH, Germany Andreas Kailer Fraunhofer Institute for Mechanics of Materials IWM, Germany 1. Introduction Bearings in wind turbines must be protected against premature failures in order to increase their reliability and availability. Most of the bearings suffer from hydrogen embrittlement due to tribological loading, lubricant degradation, contamination and electrical interference. White etching cracks (WEC) are one of the most reported damages in rolling element bearings and still an open area of research. Alternative lubricants are urgently required to counteract this problem. Lubricants with significant improved conductivity were identified as a possible measure to prevent harmful chemical and electrical influences. 2. Results and Discussion Adding ionic liquids (IL) to non-polar lubricants is challenging because their solubility is limited. Moreover, impurities in the ILs can strongly affect the tribological behavior. Therefore, structural motifs of ILs were selected, for which sufficient solubility in lubricants can be expected to achieve sufficient electrical conductivity of the lubricant. Rolling contact fatigue (RCF) tests with thrust roller bearings offer the opportunity to characterize different lubricants concerning their affinity to hydrogen embrittlement in bearings. Various test series with different oils and IL and additive contents have so far shown that a significant improvement in running behavior is achieved by adding ILs. 2.1 Improvement of bearing life Figure 1: Comparison of the running time for different model lubricants. Figure 1 shows a comparison of the results of an RCF test for a gearbox oil with different IL contents. 3 tests were conducted for each lubricant sample. The pure gearbox oil leads to a bearing damage after a test duration of 80 - 95 hours in all cases. Adding a low content of IL to the gearbox oil improves the bearing performance. Maximum test durations of 150 hours were reached in 2 of 3 tests. With higher IL content no damage occurred within the test duration of 150 h. An XPS analysis of washers from bearing tests with pure gear box oil and with high IL content show the formation of a phosphate layer on the bearing surface due to additive reactions. Possibly formed by an anti-wear or friction modifier additive. In the case with a high IL content the formation of an additional, nitrogen and fluorine layer was detected on the surface as residues of the IL. 72 23rd International Colloquium Tribology - January 2022 Alternative lubricants in wind turbines to avoid WEC formation 2.2 Applying a Potential to the bearing A special test set-up was designed for carrying out rolling tests with superposed electrical potentials. The working electrode is positioned in the lubricant and the counter electrode is directly in contact with the stationary lower ring of the rolling bearing. Different electric voltages can be applied between the two electrodes, if the lubricant is electrically conductive. -1V +1V Figure 2: Photographs of rollers and cross-sectional optical micrographs of bearings test under different potentials To investigate electrochemical influences on friction, wear and hydrogen generation, tests can be carried out at anodic (oxidation) and cathodic (reduction) potential. RCF test with the gearbox oil + high IL content and different potentials show the ability to trigger the formation of WEC due to the applied potential. A cross-sectional analysis of the tested bearings is presented in Figure 2. Hydrogen was formed under an anodic potential during RCF testing. 2.3 Wear behaviour of ionic liquids The wear behavior of a base oil and additivated variants with IL additives in different concentrations was investigated in oscillating sliding test with an Optimol SRV tribometer. The COF was slightly influenced by these additives. In contrast, the wear behavior was strongly modified by the IL: The lubricant with a high IL content shows significantly lower wear compared to the pure base oil (Figure 3). Figure 3: SRV tests with a base oil and different IL contents, indicating a reduction of the wear volume with increasing IL content. 3. Conclusion On the basis of these results it is concluded that ILs as additives for fully formulated lubricants can help to increase lubricity and reduce their electrical resistance. Improving the conductivity of the lubricant helps to avoid bearing damage such as WEC and additionally improves the tribological properties of the lubricants. The wear resistance of tribological systems in particular is positively influenced. 4. Acknowledgement The Authors thank the Federal Ministry for Economic Affairs and Energy for funding of the project “WindPower-Life” grant number 0324208A-D.