24th International Colloquium Tribology - January 2024 237 Oxidation Effects on the Rheology and Tribology of Sustainable Lubricants for Electromechanical Drive Systems Didem Cansu Güney 1* , Joachim Albrecht 1 , Katharina Weber 1 1 Research Institute for Innovative Surfaces FINO, Aalen University, Beethovenstr. 1, D-73430 Aalen, Germany * Corresponding author: didem_cansu.gueney@hs-aalen.de 1. Introduction Lubricants are essential for improving the performance of vehicles and construction equipment, particularly in the automotive industry, to optimise efficiency [1]. Due to leakage and disposal, sustainable and eco-friendly alternatives are essential [2]. The rise of electric vehicles poses new challenges, including high speed operation in excess of 20,000-rpm at elevated temperatures [3]. A current challenge is scarcity of commercially available lubricants that are sustainable, biodegradable, non-toxic, and performance-enhancing for electromechanical drives [4]. Gear oils are vital for reducing friction, wear, noise and corrosion [5]. Sustainable gear oils aim to extend change intervals and achieve a 0.5-% powertrain efficiency increase while reducing environmental impact. This aligns with the standard requiring 25-% renewable content and significant biodegradability for biolubricants [6, 7, 8]. This study compares sustainable lubricants for electromechanical drives derived from native oil (“Native”), synthetic ester (“Synth”) and polyalkylene glycol (“PAG”) with a conventional mineral oil-based lubricant (“Mineral”), all with the same 40-°C kinematic viscosity. The effect of ageing on these lubricants is investigated by assessing their rheological, tribological and electrochemical properties in aged and non-aged conditions, with the aim of improving sustainability without sacrificing technical suitability. 2. Experimental details and results The ageing of the lubricants was carried out using the Turbin Oxidation Stability Test method [9]. 100 ml of oil was heated at 95-°C for 13 days in a dark environment and subjected to of compressed air. 2.1 Rheological Investigation 2.1.1 Temperature-dependent behaviour Dynamic viscosity is determined using a rotational rheometer in a cone-plate system. Samples are sheared at a constant shear rate of and at temperatures ranging from 25-°C to 100-°C. To calculate kinematic viscosity, the obtained dynamic viscosity is divided by the previously determined density. The kinematic viscosity is then used to create Ubbelohde-Walther diagrams, as shown in Figure 1a and 1b for non-oxidised and oxidised lubricants [10]. The linear slope represents the temperature gradient m, which is related to the viscosity index. Low m values indicate a higher viscosity index. A high viscosity index with low viscosity values is desirable. Fig. 1: Ubbelohde-Walther diagrams of the a) non-oxidised; b) oxidised lubricants with the respective gradients. In black: mineral oil, in red: native oil, in green: synthetic ester, in blue: polyalkylene glycol. “Native” shows dramatically increased viscosity values after oxidation and thus poor oxidation stability. The viscosity values of “PAG” and “Synth” are in the range of the values of “Mineral”. After ageing, the viscosity index of all samples, except “PAG”, increases. However, “Mineral” has the highest m-value, which means that “Synth” and “PAG” show a lower sensitivity to temperature fluctuations in contrast to “Mineral”. The results show that “PAG” and “Synth” can be considered as gear oils, while “Native” appears unsuitable due to its strong viscosity change after oxidation. 2.1.2 Jump test The jump test is used to investigate the reaction of the lubricant to a sudden change in load or deformation. The lubricant is first subjected to a low shear rate of , which is then abruptly changed to and finally returned to the initial value. The relaxation time is found by measuring how long it takes for the lubricant´s dynamic viscosity to return to its original state after a sudden change. Figure 2 shows the jump tests of the lubricants in the non-oxidised and oxidised states. The jump test results of the oxidized “Native” sample are not shown due to its strong increase in viscosity upon oxidation. “Mineral” and “Synth” do not regenerate during the entire test period. “Native” also does not regenerate in the non-oxidised state. In the oxidised state, however, regeneration occurs after 122.4-s (not shown in Figure 2). “PAG” regenerates after 32.4-s in the non-oxidised state and after 14.4 s in the oxidised state. The relaxation time shortens as the oil ages. “Native” is no longer considered in the following measurements due to the very high viscosity after oxidation. 238 24th International Colloquium Tribology - January 2024 Oxidation Effects on the Rheology and Tribology of Sustainable Lubricants for Electromechanical Drive Systems Fig. 2: Jump test of the non-oxidised and oxidised lubricants. In black: mineral oil, in red: synthetic ester, in green: native oil, in blue: polyalkylene glycol 2.2 Tribological Investigation An oscillating tribometer was used to determine the coefficient of friction according to DIN 51834-2 [11]. A spherical counter body, here 100Cr6, acts with a constant force of F-=-50-N perpendicularly on the surface (100Cr6) lubricated with 0.3 ml oil. The lubricated surface moves simultaneously with a certain path length of s-=-1 mm and a frequency of f-=-50-Hz in a linear oscillating motion for a duration of t-=-120 min. Figure 4 shows the coefficients of friction of the non-oxidised and oxidised lubricants. Fig. 4: Coefficient of friction of the lubricants over time. In black: mineral oil, in red: synthetic ester, in blue: polyalkylene glycol. The coefficients of friction of the lubricants are all between 0.08 and 0.12. Since the tribological contact is in the partially lubricated state, there is always solid contact between the interacting components, which explains the observed coefficients of friction. Differences can be seen between the different oils and between the non-oxidised and oxidised states. “PAG” shows the lowest coefficients of friction. “Synth” shows the highest friction values. It is noticeable that the coefficients of friction of the lubricants decrease upon oxidation. This is possibly due to the increasing viscosity after ageing which increases the film thickness and thus reduces the solid body interaction in the partially lubricated state. 2.3 Electrochemical Investigation The electrical properties of “Synth” and “PAG” were characterised by electrochemical impedance spectroscopy in the frequency range of 10 5 - 0.1 Hz. The results of the non-oxidised and oxidised samples are shown in Nyquist plots, Figure 5 and Figure 6. Fig. 5: Nyquist plot of the non-oxidised and oxidised synthetic ester. Fig. 6: Nyquist plot of the non-oxidised and oxidised po-lyalkyleneglycol-based oil. The real part of the impedance is plotted on the x-axis of the Nyquist diagram and the imaginary part on the y-axis. The real part corresponds to the ohmic resistance. For both samples, the oxidised lubricant has a lower resistance than the non-oxidised lubricant. The oxidised samples therefore have a higher electrical conductivity than the non-oxidised samples. When comparing the two lubricants, it is noticeable that “Synth” has a significantly higher resistance (GΩ range) than “PAG” (MΩ range). 3. Conclusion In this study, sustainable lubricants were compared with a mineral oil-based lubricant in terms of various performance parameters. The results highlight the potential advantages of “PAG” and “Synth” in certain applications, whereas “Native” is severely impacted by ageing. References [1] M. Woydt, Wear, 2022, 488-489 (1), 204134. [2] N. Salih et al., Biointerface Res. Appl. Chem., 2021, 11-(5), 13303. [3] L. I. Farfan-Cabrera, Tribol. Int., 2019, 138 (40), 473. [4] M. Beyer et al., Tribol. Online, 2019, 14 (5), 428. [5] M. Arca et al., Int. J. Sustain. Eng., 2013, 6 (4), 326. [6] W. J. Bartz, Proc. Inst. Mech. Eng. D: J. Automob. Eng., 2000, 214 (2), 189. [7] DIN SPEC 51523 (2011). [8] R. Luther, Lubricants and Lubrication, Wiley-VCH Verlag, Weinheim, Germany, 2017. [9] DIN EN ISO 4263-4 (2006). [10] D. C. Güney et al., Materialwiss. Werksttech., 2023, accepted for publication. [11] DIN 51834-2 (2017).