23rd International Colloquium Tribology - January 2022 375 Conductive Layer Deposits and the Development of Bench Test Technology for Electric Vehicle Drivetrains Greg Miiller, John Bucci Savant Group, Midland, MI, United States Gunther Mueller, Rico Pelz APL, Landau, Germany Timothy Newcomb Lubrizol, Wickliffe, OH, United States 1. Introduction The EV industry is continuously evolving with new technologies leading the way for improved lubricants. Both electric and hybrid vehicles, with various types of drivetrain designs, are operating around the world under diverse conditions. As real-world experience accumulates, opportunities to increase efficiency and improve reliability are being revealed. One such opportunity to improve reliability involves enhanced protection of the electric motor and electronics from corrosion. One way to accomplish this is through enhancing the protection provided by the lubricant. However, the current tests employed to assess this key lubricant characteristic were designed with mechanical parts in mind, not electric motors and electronics. To ensure improved reliability of electrified hardware, new methods are needed. This paper describes the development of two such tests designed to address this challenge. 1.1 Deposit Tests Electric motor burn out has occurred from time to time in electrified vehicles, some of which have been caused by the accumulation of conductive corrosion byproducts formed around the motor coils. In addition, electric sensors occasionally malfunction due to corrosion of wires or contacts. In these cases, the lubricants in use were believed appropriate to the application as all performed well in versions of the ASTM D130 “Standard Test Method for Corrosiveness to Copper from Petroleum Products by Copper Strip Test.” Clearly a more robust assessment of the corrosion protection, more attuned to electrified hardware, is needed. To help resolve this, a group of industry experts gathered from across the globe to develop a bench test to predict the potential to form conductive deposits. Corrosion is the chemical process that converts metal into oxides, hydroxides, carbonates or sulfides. Though the copper wires of the electric motor windings are insulated with a polymer film, the integrity of the insulation can be compromised allowing contact with the lubricant. In addition, not all metal connections from the electric motor to the power supply are insulated, nor are all the electrical connections within other electrical devices. Corrosion can lead to the removal of metal leading to an open circuit. The corrosion process itself can vary nonuniformly with temperature. A test has been developed, the wire corrosion test (WCT), to determine the rate of corrosion and copper depletion over time in both solution and vapor states [1]. Whereas removal of copper can lead to a failure through the creation of an open circuit, a failure caused by the formation of solid corrosion byproducts, conductive deposits, can be a bit more traumatic. These deposits can provide a new electrical path, essentially enabling the shorting of the electric motor leading to thermal runaway ending in motor burn out. A test to specifically determine this risk is needed. Several examples of such tests have been presented at various industry meetings. The conductive deposit test (CDT) described here assesses the formation of conductive deposits forming from the chemical reaction of the lubricating fluid and copper. It operates at elevated temperatures under low voltage electrified conditions and evaluates the tendency of deposits to form in both the solution and vapor states. In actual electric drive units (EDUs), where the lubricant led to failures attributed to electrical short circuits, deposit formation started within smaller spaces, typically within crevices around the mounting sections and gaps around the electric motors. To mimic these confined spaces, the test uses stacked uncoated circuit boards with repeatable spacing between the copper traces. A voltage of 5 vdc is placed onto the traces and monitored, via resistance, over time. Because the reaction with the lubricant and ensuing deposit formation is different between the solution and vapor in the powertrain, they are also separated in this test. Each are tracked independently ensuring the capture of a failing condition. A diagram of the test board is shown in Figure 1. To ensure the test was applicable to EDUs in real world operation, two lubricants associated with real field fail- 376 23rd International Colloquium Tribology - January 2022 Conductive Layer Deposits and the Development of Bench Test Technology for Electric Vehicle Drivetrains ure were selected as bad references. Lubricants used to replace these, that is correct the field problems, were selected as good references. Though 150°C was selected as the standard setpoint to encourage failure, consideration has been taken so that this equipment can ramp potentially from a lower temperature around 130°C and include temperature spikes up to 160°C if desired (full range is room temperature to 180°C). Figure 1 Figure 2: Field Failing Fluids The test results are shown in Ohms, which is a measurement of the resistance naturally occurring within the copper traces. If the traces on the test board are bridged by conductive deposits, the resistance will decrease, ultimately resulting in a short circuit in the extreme case. Figure 2 shows the results of the two bad reference lubricants. The results are indicative of what occurred within the motor windings and ensuing failure. These have been reproduced on multiple units with several laboratory operators. Passing oils show no such degradation. The test can potentially operate for as long as 1000 hours but efforts to shorten this timeframe are showing promise with failures routinely observed at 650 hours or less. Repeatability for the apparatus has been less than 16% of the mean and in most cases less than 8% of the mean. 2. Conclusion Failures have been shown in EV motors relating to corrosion and the formation of conductive deposits. Tests have been developed in the form of a conductive deposit test (CDT) and a wire corrosion test (WCT). Research shows that the mechanism for failure between the lubricant and vapor states can be different. Therefore, both lubricant and vapor states are assessed within these tests for accuracy reasons. Both tests have proven to be effective in predicting these failure modes. The separation between passing and failing fluids is evident and proven. References [1] Hunt, G., “New Perspectives on Lubricant Additive Corrosion: Comparison of Methods and Metallurgy”. SAE Technical Paper 2018-01-0656, 2018.