23rd International Colloquium Tribology - January 2022 457 Polymer-coated plain bearings during start-stop operation - an experimental and numerical assessment Florian König Institute for Machine Elements and Systems Engineering, RWTH Aachen University, Aachen, Germany Corresponding author: florian.koenig@imse.rwth-aachen.de Georg Jacobs Institute for Machine Elements and Systems Engineering, RWTH Aachen University, Aachen, Germany 1. Introduction Plain bearing applications such as gearboxes in wind turbines and operating strategies such as automatic startstop systems lead to an increased proportion of mixedfriction operation. The resulting wear limits the service life of a plain bearing, which makes wear resistance utterly important. In addition to wear resistance, high demands are placed on the efficiency of a plain bearing. Low friction losses are thus important for the selection of a plain bearing material. In order to ensure low wear and low friction, even under mixed-friction conditions, self-lubricating materials are often used in the bearings including metal backing composite materials, polymer and filled polymer composite materials. The use of anti-friction coatings for plain bearings, such as for example polyamide-imide (PAI) [1], has emerged, e.g. for automotive crank train bearings, due to their promising anti-friction performance [2]. In a direct comparison to conventional bimetallic bearings, anti-friction coatings show lower wear during running-in and during the subsequent start-stop cycles [2]. However, high friction losses may be found in the first few start-stop cycles, which can be attributed to the adhesive forces of the polymer/ steel interface. The question arises, whether running-in leads to reduced frictional losses in subsequent star-stop cycles. Therefore, the aim of this study is to experimentally investigate the tribological behaviour of PAI-coated bearings during start-stop operation. Additionally, the behaviour during start-stop operation was studied in a mixed-elasto-hydrodynamic lubrication (mixed-EHL) simulation model for further elucidation of running-in effects. 2. Materials and methods 2.1 Polymer-coated plain bearing experiments In this study, plain bearings from bronze CuSn12Ni2-C were coated with a f 10 µm thick layer of polyamide-imide (PAI). These bearings were subjected to varying number of start-stop cycles under stationary load in a plain bearing test rig. The aim was to study the frictional behaviour and the wear caused by starting and stopping. The PAI-coated plain bearings, test rig and test method are shown in Figure 2-1. In this study, the bearing housing was heated to a temperature of 90 °C with circumferentially positioned heating cartridges to ensure isothermal conditions. The oil supply temperature was set to 80 °C with an electrically heated hose. After this steady state was reached, the bearings were subjected to 5,000 start-stop cycles at specific pressures of 2 MPa, where the radial load was applied to the bearing housing using a flexible load unit. Afterwards the bearing was removed from the test rig and the contour and roughness was analysed. The speed profile is shown in Figure 2-1. It consists of start-up ramp (2 seconds) to the nominal speed, a plateau phase of 2 seconds and a stopping ramp of 3 seconds to standstill. The standstill period between two cycles was 10 seconds to ensure the displacement of oil between the shaft and the bearing. A friction gauge, which is connected directly to the housing of the bearing, was used to determine the frictional force and friction coefficient. Further details on the test rig can be found in [3,4]. 458 23rd International Colloquium Tribology - January 2022 Polymer-coated plain bearings during start-stop operation - an experimental and numerical assessment Figure 2-1: A PAI-coated plain bearing, schematic presentation of the plain bearing test rig and test method 2.2 Numerical simulation The contact conditions in plain bearings during startstop operation were calculated using a transient mixed-elasto-hydrodynamic lubrication (mixed-EHL) simulation model, which was previous generated in AVL Excite Power Unit for metallic bearings [4]. However, for PAI-coated bearings with a surface layer with a low Young’s modulus, the existing transient elastohydrodynamic simulation model was extended. In this study, the elastic and frictional behaviour of the polymer coating was considered according to the approach by O ffner and K naus [5]. 3. Results In screening tests, PAI-coated plain bearings with further filler materials have shown a superior wear performance during start-stop operation, when compared to non-coated bearing systems [4]. However, high friction losses were found in the first few start-up procedures, which can be attributed to the adhesive forces of the polymer/ steel interface. With increasing number of start-stop cycles, running-in of the coated bearing shells was observed which lead to a slightly improved frictional performance as shown in Figure 2-2. Figure 2-2: Experimental frictional performance of a bearing during 5,000 start-stop cycles The black line at a CoF of 0.02 marks the expected transition between mixed lubrication and hydrodynamic lubrication. Thus, slight running-in effects in terms of transition speed can be observed during the operation at 2 MPa. However, due to the missing wearing-in effects, the frictional losses in terms of CoF remained nearly constant at values of 0.2 during starting and stopping. After the experiment, the bearing was taken from the test rig and wear analysis was performed. Very low wear losses in terms of profile change were observed. However, the anti-friction coating initially showed Rz and Ra values in the range of 4…7 µm and 0.7…1.2 µm, respectively. Due to the starting and stopping, a reduction of Rz and Ra to values of 4 µm and 0.6 µm was overserved, respectively. To further study the tribological performance of plain bearings with PAI-coatings during start-stop operation, the new and worn system with reduced roughness were compared in a mixed-EHL simulation. The results are shown in Figure 2-3. Figure 2-3: Simulated frictional performance of a bearing during the 1 st and 5’000 th start-stop cycle The direct comparison between experimental and numerical results shows a good agreement in terms of frictional performance. 4. Summary and conclusion The aim of this study was to elucidate the tribological performance of plain bearings with anti-friction coatings made out of polyamide-imide (PAI) during start-stop operation. Based upon the experimental results of the friction and wear behavior and the simulation results, the following conclusions can be drawn: At low speeds, high friction losses were observed in the starting and stopping phase, which can be attributed to the adhesive forces of the polymer/ steel interface. The PAI coating showed a high wear resistance with negligible wear losses during 5,000 start-stop cycles. Consequently, negligible wearing-in was observed in terms of profile change. Nonetheless, the roughness of the PAI coating reduced significantly, which was further considered in a mixed-EHL simulation model of the startstop cycle. The simulation model demonstrates that it is important to ensure wearing-in of anti-friction coatings at increased loads to achieve low friction losses during start-stop operation throughout a bearings service life as 23rd International Colloquium Tribology - January 2022 459 Polymer-coated plain bearings during start-stop operation - an experimental and numerical assessment well as move the transition speed between the mixed and hydrodynamic lubrication regime towards lower speeds. Modelling the tribological behavior of such a coated bearing still continues to be a major challenge that should be tackled in the future. 5. Acknowledgement Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - GRK 1856. References [1] Ligier J-L, Noel B. Friction Reduction and Reliability for Engines Bearings. Lubricants. 2015; 3(3): 569-596. https: / / doi.org/ 10.3390/ lubricants3030569 [2] Gudin D, Mian O, Sanders S. Experimental measurement and modelling of plain bearing wear in start-stop applications. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology. 2013; 227(5): 433-446. doi: 10.1177/ 1350650112471287 [3] König F., Ouald Chaib A., Jacobs G., Sous C.: A multiscale-approach for wear prediction in journal bearing systems - from wearing-in towards steadystate wear. Wear 2019; 426-427: 1203-11. https: / / doi.org/ 10.1016/ j.wear.2019.01.036. [4] König, F., Sous, C., Jacobs, G. Numerical prediction of the frictional losses in sliding bearings during start-stop operation. Friction 9, 583-597 (2021). https: / / doi.org/ 10.1007/ s40544-020-0417- 9 [5] Offner G, Knaus O. A Generic Friction Model for Radial Slider Bearing Simulation Considering Elastic and Plastic Deformation. Lubricants. 2015; 3(3): 522-538. https: / / doi.org/ 10.3390/ lubricants3030522