23rd International Colloquium Tribology - January 2022 183 Impact of Lubricant Formulation on Surface Damage in Electric Vehicle Transmissions Alexander MacLaren Imperial College London, London, UK Corresponding author: dam216@imperial.ac.uk Amir Kadiric Imperial College London, London, UK 1. Introduction Power losses in electric vehicle (EV) drivetrain transmissions are responsible for a much larger portion of the overall energy loss than in internal combustion engine powered vehicles [1]. The electrification of the fleet of passenger cars worldwide, as part of the global drive to reduce CO 2 emissions, requires tackling consumer range anxiety, and hence maximising driving range and battery duty cycle [2], both of which depend on mechanical transmission efficiency. Drivetrain losses can be divided into load-dependent (traction) and load-independent (churning, windage) components, which depend primarily on gearbox architecture and oil rheology [3]. Owing to the relatively high speeds in EV gearboxes, oil churning losses caused by rotating gears and bearings are a major contributor to drivetrain loss. The current trend for reducing gearbox oil viscosity has the effect of mitigating these viscous losses. However, transmission lubricants must also prevent surface damage by providing a hydrodynamic film, even under the aggressive conditions of high motor torque and low entrainment speed, for which high lubricant viscosity is required. To ensure the transmission is reliably efficient over lifetime, the gearbox design and lubricant formulation must reconcile these competing requirements to control friction, wear, and surface fatigue damage. To achieve this, efficiency and reliability must be considered together to be optimised through drivetrain design and lubricant formulation. In this study, a selection of custom made and commercial EV transmission lubricants are assessed in terms of their ability to resist surface damage modes in EV transmissions, including scuffing and micropitting. The formulations tested include a water-based lubricant, an automatic transmission fluid, a gear oil, and a common base oil. Scuffing performance is evaluated in a ball-on-disc tribometer under EV-representative conditions using a recently developed test method based on contrarotation which improves test repeatability and allows the effect of lubricant viscosity and chemical formulation to be examined independently [4]. Micropitting is studied using a proven methodology employing a triple-disc machine, enabling close control of contact conditions, and allowing the evolution of micropitting with contact cycles to be monitored [5]. Results are discussed in terms of lubricant chemistry, tribofilm evolution, asperity contact stresses and wear during the test, with a particular focus on the water-based fluid. 2. Experimental Procedure Two sets of experiments were conducted, one to investigate micropitting, and the other for scuffing, two primary damage modes of gears and bearings in EV helical reduction gearboxes. The contact conditions were chosen to replicate those experienced by the motor pinion tooth contacts during rapid acceleration, based on the gear geometry of a drive unit from a popular EV currently on the market. 2.1 Micropitting The triple-disc rolling contact fatigue machine (MPR by PCS Instruments), shown in Figure 1, was employed to examine the micropitting performance of each fluid under the conditions given in Table 1. AISI 52100 specimens and line contact conditions were used in all tests, maintaining a lambda ratio of 0.4 to enable a representative comparison in the boundary regime. Table 1: Contact conditions for MPR tests Max. Hertz Pressure p 0 1.5 GPa Entrainment Speed U e 3.902 m/ s Slide-Roll Ratio SRR -5 % Lambda Ratio Ʌ 0.4 ± 0.02 The MPR allows independent control of disc speed ωdiscs and roller speed ωroller, hence any entrainment speed Ue = (Udiscs+Uroller) and slide-roll ratio SRR = × 100 % is possible up to tangential speeds of 4 m/ s. The upper disc load P was controlled and the roller drive torque measured, allowing traction coefficient μ to be calculated. 184 23rd International Colloquium Tribology - January 2022 Impact of Lubricant Formulation on Surface Damage in Electric Vehicle Transmissions Figure 1: Configuration of discs and roller in PCS MPR test The roller and disc surfaces were inspected at intervals during the test, according to the schedule in Table 2. Inspections were conducted by optical microscopy of the roller surface, and stylus profilometry of the three discs and the roller, at 4 positions, 90° apart, on each. Table 2: Inspection Intervals for Micropitting Test Step 10 3 Cycles Cumulative Roller Inspection Disc Inspection 1 50 50k 2 50 100k 3 450 550k 4 450 1M 5 1000 2M - 6 1000 3M - 7 2000 5M - 8 2000 7M - 9 4000 11M 2.2 Scuffing Scuffing performance was evaluated using a ball-on-disc tribometer from PCS instruments, the ETM, capable of independent control of ball and disc tangential speeds up to 4 m/ s, traction force measurement, and contact pressure up to 3.5 GPa for AISI 52100 steel specimens. The Spacer Layer Imaging Method (SLIM) was used to examine the ball surface at set intervals to establish tribofilm thickness and qualitatively monitor wear on the ball surface. The test method was based on that previously developed by Peng et al [4]. An initial running-in step at low speed and SRR was employed to achieve maximally comparable contact conditions. Successive increases in SRR in contrarotation (SRR >200 %) at constant entrainment speed were then conducted, interspersed by a rest step to allow thermal equilibration, during which a SLIM image was taken. The test was stopped at the onset of scuffing, characterised by a rapid and irreversible increase in traction coefficient, accompanied by appreciable noise and vibration from the ETM. 3. Results Progression of roller material loss was evaluated via stylus profilometry, a typical example is given in Figure 2 . The evolution of the roller surface throughout the micropitting test allows the progress and nature of the damage to be monitored, as shown in Figure 3. These and other results will be presented in the talk. Figure 2: Example evolution of micropitting wear on the roller profile during test on gear oil Figure 3: Example optical microscope images of roller surface evolution throughout micropitting test on gear oil 23rd International Colloquium Tribology - January 2022 185 Impact of Lubricant Formulation on Surface Damage in Electric Vehicle Transmissions 4. Conclusion Micropitting, wear and scuffing failure were successfully generated on steel specimens across a representative range of candidate EV transmission lubricants, allowing a comparison of their performance under conditions experienced in EV transmission components. References [1] Kadiric, A., & Shore, J. (2021). Prediction of Power Losses in Electric Vehicle Transmissions. STLE Virtual Annual Meeting & Exhibition. Online. [2] Pevec, D., Babic, J., Carvalho, A., Ghiassi-Farrokhfal, Y., Ketter, W., & Podobnik, V. (2019). Electric vehicle range anxiety: An obstacle for the personal transportation (r)evolution? . 4th International Conference on Smart and Sustainable Technologies, SpliTech 2019. [3] Christodoulias, A. I., Olver, A. V., Kadiric, A., Sworski, A. E., Kolekar, A., & Lockwood, F. E. (2014). The efficiency of a simple spur gearbox - A thermally coupled lubrication model. American Gear Manufacturers Association Fall Technical Meeting 2014, AGAM FTM 2014, 81-98. [4] Peng, B., Spikes, H., & Kadiric, A. (2019). The Development and Application of a Scuffing Test Based on Contra-rotation. Tribology Letters, 67(2), 37. [5] Wainwright, B., & Kadiric, A. (2021). The Effect of Surface Roughness and the Λ Ratio on the Initiation and Progression of Micropitting Damage. STLE Virtual Annual Meeting & Exhibition. Online.