24th International Colloquium Tribology - January 2024 47 Performance Enhancement of Molybdenum-Based Friction Modifiers David Boudreau Sr 1* , Brian Casey 1 1 Vanderbilt Chemicals LLC, Norwalk, CT, USA * dboudreau@vanderbiltchemicals.com 1. Introduction Organomolybdenum friction modifiers reduce friction and enhance wear protection in lubrication formulations through the in-situ formation of molybdenum disulfide tribofilms (MoS 2 ). While these additives were initially developed for a traditional ICE architecture, a shift to hybrid and battery EV brings new challenges. This work investigates additive combinations of three traditional organometallic friction modifiers in EV-focused low viscosity fluids. These friction modifiers are based on molybdenum and boron and have been used extensively in traditional drivetrains. Each has unique benefits and challenges in traditional drivetrain applications. Development of new fluids for electric vehicles includes some of the same challenges as with fluids for ICE, but there are significant differences. First, the average overall operating temperature of a lubricant in a plug-in hybrid engine is lower than that of an ICE, by as much as 25% [1]. In the context of traditional molybdenum and boron additives, that often require a high activation temperature to be functional, lower operating temperatures may pose a significant barrier to their effectiveness. An additional concern is yellow metal corrosion. In the context of EV, there is a significant increase of the possibility of contact of the lubricating fluid with copper and electronics. As such, the issue of fluid conductivity comes into play in EV-based systems. A high conductivity fluid can promote short circuiting and current leaks. A low conductivity fluid in turn acts as an insulator and or capacitor. This can result in the build-up of a large charge gradient, which will eventually equilibrate through electrical discharge [2]. For electrical properties, the use of organo-metallic ligand chemistry is in its relative infancy. Some studies indicate that organoborates may possess very high conductivity [3]. The use of borate-capped dispersants also shows a large contribution to electrical conductivity. In contrast, molybdenum dithiocarbamates might contribute only a small increase to the overall conductivity of a formulation [4]. In this experiment combinations of three friction modifiers, based on molybdenum and boron, are explored to assess their effects on low friction onset, wear, corrosion, and conductivity in an EV style formulation. 2. Experimental A base formulation was prepared using a Group IV PAO 4cSt base stock. The formulation contained additives typically found in an EV or automatic transmission fluid. This formulation was topped with combinations of three friction modifiers as described in [Table 1]. Each additive is charged within a typical concentration range used in ICE, with total molybdenum concentration not exceeding 320 ppm Mo. Table 1: Friction modifiers and concentrations Additive ID Description Concentration Range (ppm metal) Mo-FM1 Molybdenum dithiocarbamate 0-0.3%wt (0-300 ppm Mo) Mo-FM2 Molybdate Ester/ Amide 0-0.3%wt (0-320 ppm Mo) B-FM Borate Ester/ Amide 0-1%wt (0-90 ppm B) The ternary combinations are shown in [Figure 1], with minimum and maximum values of each additive representing the range of 0% to 100% charge. Figure 1: Composition of ternary mixtures In addition to testing freshly prepared samples, lubricant samples were also aged by a bulk oxidation method of 48 hours at 160-°C under an oxygen bubble. This was done to simulate oil after an extended in-use period. Extended Copper Corrosion testing was performed on fresh oils in a modified ASTM D130 test, with separate tests running at 24, 168, and 336 hours at 150-°C, and including ICP analysis of solvated copper from each run. Electrical conductivity was measured using an Epsilon+ dielectric meter in a temperature range of 40-160-°C. Mini-Traction Machine (MTM) Stribeck cuves were obtained using a 35 N load, 50% SRR, and 3-3000 mm/ s rolling speed. Three curves were averaged at temperatures from 40-140-°C, in 20-°C increments. 48 24th International Colloquium Tribology - January 2024 Performance Enhancement of Molybdenum-Based Friction Modifiers 3. Results and Discussion 3.1 Extended Cu Corrosion (Modified ASTM D130) The ICP results of the extended copper corrosion [Figure 2] show the increased solubilized Cu with increasing time. Copper leaching is relatively low (max 136 ppm Cu, average 85 ppm Cu at 336 hours across all formulations). Copper strip ratings at 336 hours range from 1a to 3a. The lowest corrosion being Formula 4, containing only Mo-FM2. Figure 2: Cu Corrosion (Extended D130 @ 150-°C) 3.2 Electrical Conductivity All formulations, fresh and aged, yield electrical conductivities in a range similar to current EV specific fluid technology [5]. Due to the total number of curves, only selected low and high conductivity formulations are shown in FIGURE 3. Figure 3: Electrical Conductivity All aged oils show some increase in conductivity, relative to the fresh oils, including baseline F2. Formula F8 shows the least deviation from baseline in both fresh and aged oils, indicating it may be the best additive combination for longterm/ fill-for-life applications. 3.3 Friction Coefficient (via MTM) While measurements were taken on both fresh and aged oils, only aged data is shown below in FIGURE 4. Values represent the boundary friction obtained from 3 individual Stribeck curves at each temperature observed. For clarity, only select low and high CoF formulations are shown. Fresh oil data generally trended to have slightly lower CoF values than aged oil, independent of formulation. Figure 4: MTM Boundary Friction at Temperature Most formulations of aged oils trended like F11 and showed an increase in friction with ageing relative to baseline F2. F8 was a clear exception, showing a decrease in friction. For F8, friction reduction was not only maintained after ageing, but actually improved. 4. Conclusion In this initial investigation, combinations of B-FM, Mo-FM1 and Mo-FM2 were shown to be effectively used in an EV based fluid without significant detriment to critical concerns such as conductivity and corrosion. Of particular merit was a combination of molybdenum dithiocarbamate and borate ester. In terms of extended oil life, conductivity and friction remained low with this combination. Copper corrosion of this formulation was not the lowest in the series, but all formulations including this combination performed very well in this extended test - giving good coupon ratings and relatively low solvated copper. Future investigations will focus on the effects of these additives on other properties such as extreme pressure, scuffing, and wear. References [1] Chevron Oronite (2023), Hybrid Vehicles are a Bridge to the Future - A Natural Step in the Evolution from Internal Combustion Engines (ICEs) to Full Battery Electric Vehicles (BEVs), https: / / www.oronite.com/ about/ news-activities/ hybrid-solutions.html [2] Lubes n‘ Greases (Jul 01, 2021), EV Fluid Development Challenges, https: / / www.lubesngreases.com/ electric-vehicles/ article/ ev-fluid-development-challenges/ [3] M. Özcan, C. Kaya, F. Kaya (12 July 2023), An Optimization Study for the Electrospun Borate Ester Nanofibers as Light-Weight, Flexible, and Affordable Neutron Shields for Personal Protection, https: / / doi. org/ 10.1002/ mame.202300150] [4] Gao et al (2018) US Patent Application No. 2018/ 0100114 A1 [5] A. Eastwood, S. Patterson, “e-Fluid Technology for Electrified Drivetrains”, LUBRIZOL 360 Webinar Series, Sept. 21, 2021, https: / / 360.lubrizol.com/ Resources/ Webinars