Tribological behavior of low viscosity synthetic ester base oils for Electric Vehicles (EVs) under pure sliding electrified conditions
The current state of the automotive industry reflects a progressive transition from conventional internal combustion engine vehicles (ICEVs), which are associated with significant environmental impacts, to electric vehicles (EVs), a more energy efficient and potentially more sustainable alternative that is becoming increasingly established. This shift has accelerated notably in recent years, with reports indicating that in 2024 more than 20% of all new cars sold worldwide were electric vehicles.
Electric vehicles operate under more demanding conditions than internal combustion engine (ICE) systems, as their electric drive units are subjected to high torques, elevated temperatures, and rotational speeds that can reach up to 30,000 rpm. Moreover, EV powertrains are exposed to stray currents coming from the inverter, which tend to concentrate on the roller bearings of the rotor. These parasitic currents can induce electrical discharge events, accelerating bearing degradation and ultimately leading to electrical breakdown of the lubricating film. Therefore, the approach for formulating lubricants as well as the lubrication strategies for EVs whose roller bearings are subjected to combined mechanical and electrical stresses are quite different to the conventional ones.
The new main requirements for lubricants in EVs (E-Fluids) must combine appropriate electrical properties (such as controlled electrical conductivity and sufficient dielectric strength to prevent dielectric breakdown) with suitable rheological characteristics, including low viscosity, adequate thermal conductivity and copper compatibility among others. For this investigation, three synthetic ester base oils were selected due to their low viscosity and electrical properties suitable for the operating conditions characteristic of EV drivetrains, as well as being a biodegradable alternative. A PAO base oil commonly used in EV Drive Fluids was also tested as a comparative reference for the results obtained.
As no standardised testing procedure is yet available, an internal procedure has been developed to evaluate the tribological behavior (friction and wear) and the relationship with the rheological and electrical properties of different lubricants for EVs under no electrification (0 A) and DC electrified conditions (3 A). Tribological tests have been performed in a unidirectional tribometer with a ball-on-disc configuration using AISI 52100 steel (100Cr6) discs (Ø=24mm, t=8mm) and balls (10 mm radius) as the counterface. The tribometer was coupled to a power supply for the application of a stable current during tests, forming a two-electrode based cell. The wear profile and volume were determined for the discs and balls using 3D optical profilometry.
The preliminary conclusions indicate that both the synthetic ester base oils and PAO8 exhibit an increase in CoF values when tested under current flow. This behavior is attributed to the passage of electrical current and to the viscosity reduction caused by local thermal effects within the tribological contact. Wear is clearly affected as well, with electrified tests showing deeper wear tracks than nonelectrified conditions, along with signs of electrical discharge damage (EDD) of varying intensity depending on the dielectric and rheological properties of each oil.