ADDITIVES
hBN-based additive as sulfur-free anti-wear and efficiency booster for e-driveline fluids
Stephan Wieber, Andreas Hees, Dmitriy Shakhvorostov - Evonik Operations GmbH Peter Moore, Evonik Oil Additives USA, Inc.
Solid lubricant materials have been documented in literature to have extraordinary tribological properties including very low friction in both boundary and mixed regime, anti-wear properties and pitting prevention by formation of a solid lubricant layer in the tribological contact [1,2] (general mechanism in figure 1). Additionally, solid lubricants such as hexagonal-boron nitride (hBN) are sulphur- and phosphorous-free and are electrically non-conductive. Therefore, they fulfill crucial prerequisites for additives to be applied in directly cooled e-transmissions having direct contact to copper and electrical componentry.
With viscosity levels targeting less than 4.5 cSt at 100°C, the demand for better sulphur-free friction modifiers and anti-wear additives is increasing. A shelf-stable additive that performs as an anti-wear and efficiency booster in the low-speed or high-torque regime has been developed based on hBN particles in the particle size diameter range of 300-400 nm. These are dispersed in base oil and are not classified as nano-materials.
Such tribological performance was shown by MTM (Mini Traction Machine, figure 2) measurements, FE8 (bearing test equipment, table 1) and e-drive transmission tests (figure 4). The formulations included base oil/additive blends as well as fully formulated e-drive fluids. Performance of the hBN-based additive was also studied in industrial gear oil (IGO) formulations. In selected ISO VG 68 IGO formulation cases, friction reduction in mixed and boundary regime exceeded the friction reduction of molybdenum-containing friction modifiers. This behaviour can be observed in lower viscosity e-drive formulations as well.
Figure 2: MTM measurements with 2h run-in, 50% slide-roll-ratio, 0.9 GPa (30 N) contact pressure, 100°C oil temperature of different industrial gear oil formulations whereas the Stribeck curves after 2h runtime is shown. (BF - Base Fluid (12 cSt@100 °C); FF - fully formulated; IGO - Industrial gear oil) Right: picture of the used Mini Traction Machine (MTM)
Figure 1: Shelf-stable hBN-based additive can improve wear protection, elongate pitting lifetime, and adjust friction performance due to protective tribo-layer Continued on page 42 LUBE MAGAZINE NO.176 AUGUST 2023 41
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