23rd International Colloquium Tribology - January 2022 33 Novel Nanocomposite with Ionic Liquid and Graphene for Electroconductive Radial Plain Bearings Susanne Beyer-Faiss Dr. Tillwich GmbH Werner Stehr, Horb-Ahldorf, Germany Corresponding author: susanne.beyer.faiss@tillwich-stehr.com Dr. Yasmin Korth Dr. Tillwich GmbH Werner Stehr, Horb-Ahldorf, Germany Dr. Tobias Amann Fraunhofer-Institut für Werkstoffmechanik IWM, Freiburg, Germany Dr. Thomas Schubert IOLITEC Ionic Liquids Technologies GmbH, Heilbronn, Germany Dr. Sebastian Plebst IOLITEC Ionic Liquids Technologies GmbH, Heilbronn, Germany 1. Introduction Precision engineering has to meet the challenge to continuously improve performance and efficiency of precision sliding elements, although they are becoming smaller and smaller. The miniaturization of parts raises the demands on the lubricants to generate low friction and wear values even at higher specific pressures. This is a particular challenge, when systems are constructed with plastic materials, which are widely used in this field. Applications in practice show, that actual solutions come to their limits, when components have to deal with new surrounding conditions, for example with electrostatic charge going along with disruptive discharge, which harms materials as well as lubricants extremely up to damage. Plastic plain bearing material as well as the lubricant are insulators, and electrostatic charge is favoured. The idea is to create a plain bearing system, which allows to consistently deviate electric charge in any operating condition of the bearing even under hydrodynamic lubrication, so all parts of the plain bearing including the lubricant have to be equipped with a certain defined electric conductivity. In a joint research project an innovative concept for a sliding bearing system is being developed, consisting of a steel shaft combined with an optimized polymeric nanocomposite bearing material and an adapted electrically conductive lubricant, using ionic liquids (IL) and graphene (Project acronym: EPiG). 2. Approach 2.1 IL-Graphene-Compounds The compounds were developed in a multi-stage process. The selection criteria for the base polymer were favorable mechanical and tribological properties. The ILs were rated for their electrical conductivity, their corrosion resistance, and their long-term resistance towards the base polymers. Purity, price and compounding properties were used as evaluation criteria for the various types of graphene. The decision was made in favor of the base polymer PA66 in combination with IL P 666(14) BTA with a high electrical conductivity (114.8 µS / cm) and a reduced graphene oxide (rGO), which is produced in a wet chemical process using the Hummer method with an adjacent reduction. In addition, compounds with carbon nanotubes CNT were also produced in order to achieve higher electrical conductivity. 34 23rd International Colloquium Tribology - January 2022 Novel Nanocomposite with Ionic Liquid and Graphene for Electroconductive Radial Plain Bearings The content of P 666(14) BTA has been set on an identical level for all compounds, whereas the contents of graphene rGO and CNT has been varied: PA66 compounds Content IL Content rGO Content CNT Specific volume resistivity [ohm.cm] PA66 base polymer - - - 1.63E+15 4% rGO - 4 - 1.09E+15 IL+2% rGO fix 2 - 1.20E+15 IL+4% rGO fix 4 - 3.38E+14 IL+4,25% rGO fix 4.25 - 2.07E+12 IL+4,75% rGO fix 4.75 - 1.21E+11 IL+8% rGO fix 8 - 1.23E+09 IL+3% CNT fix - 3 1.79E+05 IL+4% CNT fix - 4 1.27E+05 IL+3% CNT+1% rGO fix 1 3 2.01E+05 Table 1: Composition of IL-graphene-compounds with specific volume resistivity according to IEC 60093. The percolation threshold of the PA66-IL-graphene compounds is higher than 4% graphene content. The electrical conductivity is very low overall. Compounds with CNT achieve an electrical conductivity, which is higher by ten orders of magnitude. 2.2 IL-Graphene-Lubricants Selection criteria for the lubricants were resistance to aging and oxidation, use at low temperatures down to -40 ° C, compatibility with the plastic material and the possibility of varying the viscosity. Solubility in the base oil, electrical conductivity, no corrosion with steel and stability under current flow were required for the IL [1]. The choice fell on PAO in combination with IL N 8881 BTA and the reduced graphene oxide, which is also used in the compounds. The major challenge was to prepare stable graphene- IL dispersions, which are at the same time electrically conductive. IL-graphene-dispersion D1311-90, which has been used for the tribological evaluation of the IL-graphene-compounds, has an electric conductivity of 1.96 µS/ cm and a viscosity of 64.2 mPas . s at 20°C. The stability of the dispersion lasts about 3 weeks. 3. Experimental and Results To characterize the friction and wear behavior of the modified polymeric bearing materials in contact with the adapted lubricant, tests are performed in tribological model systems under rotating motion. 3.1 Tests in Model System Sphere-on-Prism Friction and wear tests have been performed using the sphere-on-prism (ISO 7148-2) model system under unidirectional rotating motion using ½” spheres out of steel 1.3505 and the compounds listed in Table 1. Friction coefficients were monitored dependent on sliding speed (0-210 mm/ s) and load (1-3-6 N) at 25 °C (short term tests). Wear tests were done with v=28,2 mm/ s and 30N load at ambient temperature (100 hours long term tests). All IL-graphene compounds show a very good response to lubrication with the base oil PAO. Static COF are below 0.15 and sliding COF range around 0.05 and below. Some compounds even show static COF below 0.1. The wear reduction is very good. Lubrication with the PAO-IL-graphene-dispersion D1311-90 give in any case tested a higher static COF, in between 20 to 50 % for the graphene containing compounds and up to 100 % for the CNT containing compounds, followed by a steeper decline of COF when raising the sliding speed. This difference cannot be explained by the viscosity, since the difference in viscosity of both fluids is only 4 mPa . s. One effect could be the adhesion of the graphene, which is part of the PA66-compound as well as the IL-graphene-dispersion. D1311-90 also show a good response in terms of wear reduction, only in some cases wear is little higher compared to lubrication with the base oil. 3.2 Tests in Model System Plain Bearing on Shaft Based on the results from 2.1 two materials were injection moulded into plain bearings: PA66-IL-4.25% rGO compound, who shows favorable tribological behavior and a good response to lubrication combined with a certain conductivity. The other PA66-compound material combines IL- CNT and rGO. This compound exhibits a higher conductivity by seven orders of magnitude. Tests are performed in model system bearing-on-shaft (ISO 7148-2, alternating rotating motion). The lubricated, electrically conductive plain bearing systems are checked in long-term tests under application-related conditions in start-stop operation. For this purpose, a slide bearing test stand was converted for galvanic coupling [2] and for testing under current flow [3]. Tests are performed in sliding combination with a steel 1.3505 shaft (diameter 5 mm) and a bearing load of 50 N under an intermittently rotating movement with 1000 cycles, each with an increasing and decreasing profile of the 23rd International Colloquium Tribology - January 2022 35 Novel Nanocomposite with Ionic Liquid and Graphene for Electroconductive Radial Plain Bearings rotation speed from zero to max. 1600 rpm (continuously accelerated within 120s) and reverse (continuously reduced within 10s). In between the cycles a standstill period of 5s is established, symbolizing 1000 start-stop conditions. In addition, the linear wear of the bearing is monitored. Figure 1: Evaluation of friction behavior of novel electrically conductive nanocomposite bearing system, lubricated with an EC lubricant. 4. Conclusion The combination of ionic liquid with graphene has a potential for making a plain bearing system consisting of a modified PA66-compound and an adapted lubricant dispersion electrically conductive to such an extent, that electrostatic energy can be continuously dissipated during operation. The tribological behavior of the base polymer (friction and wear) is not negatively influenced and corresponds to that of established plastic plain bearing materials. The development of adapted lubricants with IL-graphenes with a defined low conductivity turns towards the right direction. However, the long-term stability of the IL-graphene dispersions has to be further improved, since it is important for maintaining the tribological function. References [1] Korth, Y.; Beyer-Faiß, S.: Untersuchungen von Ionischen Flüssigkeiten unter Stromfluss. GfT Jahrestagung 2021, 27.-29. September 2021, Online Konferenz. [2] Amann, T.; Gatti, F.; Oberle, N.; Kailer, A.; Rühe, J.: Galvanically induced potentials to enable minimal tribochemical wear of stainless steel lubricated with sodium chloride and ionic liquid aqueous solution, Friction 6 (2), 7, 2018. [3] Gatti, F.; Amann, T.; Kailer, A.; Baltes, N.; Rühe, J.; Gumbsch, P.: Towards programmable friction: control of lubrication with ionic liquid mixtures by automated electrical regulation, Scientific Reports 10 (1), 1-10, 2020. Acknowledgements The project, on which this report is based, is funded by the German Federal Ministry of Education and Research (BMBF) under the code 03XP0220 (term 01.05.2019 to 30.04.2022). The authors are responsible for the content of this publication.