1 Introduction There are more components and modern technologies that cover lubricants today. The electric vehicle transmission, specifically those with the drive motor and electronic parts within the transmission fluid, has the propensity to add elements not known to standard internal combustion engines. Other technologies are critical to assist with failure analysis at a more forensic level. A property having a major effect on the utility of a lubricant, even for electric vehicles, is its resistance to oxidation and the consequences of this toward copper depletion. The probability of exposed parts, either initially or over time, truly exist in everyday scenarios. The rate of change to the copper or other metallurgical properties is important to analyse. In addition to that aspect, the formation of dendrites and the ensuing deposits in close quarters can be problematic. This type of deposit formation causing arcing between wires, conductors or circuit board traces may result in catastrophic failure of the motor or other electrical components [1]. 1.1 Oxidation and Corrosion Testing There are numerous industry oxidation and corrosion tests that exist. Collectively these tests may prove to be informative when it comes to predicting the potential failure of fluids utilized in the electric vehicle (EV) realm. Scanning Electron Microscope (SEM) and the x-ray fluorescence (XRF) are two tools that are commonly utilized to assist with evaluating corrosion products and the effects thereof. These tools alone, however, are not comprehensive enough for modern, fill-for-life applications. Several candidate test methods are used to evaluate a fluids oxidative stability. Rotating Pressure Vessel Oxidation Test ASTM D2272 [2], the Aluminium Beaker Oxidation Ford Method BJ 110-04, the JIS K 2514 Indiana Stirring Oxidation Stability Test, and the CEC L-048 Oxidation Stability (DKA) all measure similar properties such as viscosity increase, change in acid number or differential IR and a sludge rating. The Aluminium Beaker Oxidation test also looks at the total copper or lead insoluble material after the test. Visual ratings, where implemented, qualitative, and others are discrete and quantitative. Research 45 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0033 Research and Development Utilizing the Conductive Layer Deposit and Wire Corrosion Bench Test Technology for Electric Vehicle Drivetrains Gregory Miiller, William VanBergen, Alexei Kurchan, David Gillespie, Gunther Mueller, Rico Pelz, Timothy Newcomb, Gregory Hunt* The electric vehicle technology is vacillating at remarkable rates. Thus, the enhancements of the associated fluids and their associated additives are also changing. Varying conditions, depending on the EV system and usage albeit passenger car or heavy duty, can cause the formation of conductive deposits and corrosion within drivetrains. Corrosion is a wellknown entity and tools to evaluate under these conditions are imperative. However, the formation of conducting layer deposits, a corrosion product, has also been identified as a failure mechanism for current electric motor designs. Different drivetrain base stocks along with additive formulations may have inherently different corrosion rates producing conducting layer deposits in both solution and vapor phases. To help resolve these issues, a group of industry experts involving both OEM’s and lubricant experts from across the globe, have developed bench tests to better predict these conductive and corrosion deposits. The apparatus’ developed are the conductive deposit test and the wire corrosion test. This paper covers several different aspects of the EV drivetrains and fluids performing in the CDT and WCT that show both field correlation and repeatability. Keywords fluids, additives, failure mechanism, additive formulation, CDT, WCT Abstract * Gregory Miiller, William VanBergen Savant Group Alexei Kurchan, David Gillespie Cargill Incorporated Gunther Mueller, Rico Pelz APL Landau Timothy Newcomb, Gregory Hunt The Lubrizol Corporation 2 Transmission Fluids with Electric Vehicles The performance requirements are changing for transmission fluids. The EV motors are more elegant in design and demanding because of the ability to have tight tolerances and the need for high torque and quick acceleration [5]. This is the opportunity to re-write requirements and develop new testing. Unlike internal combustion engines where engine testing and bench testing combined provide protection for the engines and enhance engine oil development, the EV technology lends itself more to strictly bench tests as the conditions are more readily reproduced in the laboratory. Understanding the conditions of each EV, BEV or other is important when developing these tests. Components, therefore, can be monitored individually as to performance characteristics such as foam, aeration, viscosity, thermal transfer, and electrical conductivity. However, it may be necessary in these cases to understand oxidation characteristics when testing various facets of the fluid. The results may be impacted in ways that could seriously modify the outcome thus more accurately depicting the actual outcome of the EV application. Oxidation properties can be recreated synthetically and in several diverse ways. Heating, airflow, oxygen flow, water, pressure and agitation are all methods to create the required conditions. Corrosion tests and deposit tests also have the potential to create predictive oxidation conditions. 2.1 Corrosion Testing Needs Corrosion testing has been shown necessary in both ICE and EV technologies. The degradation of metal surfaces through corrosion is a destructive process wherein the metal is electrochemically oxidized through interaction with its environment. Copper corrodes when exposed to a variety of aggressive agents including acids, salts, phosphates, oxygen, and sulphur compounds. When the environment is a formulated hydrocarbon solution, such as fuels and lubricants, the main concern is corrosion by sulphur compounds. The wire corrosion test (WCT) was developed specifically for the electric vehicle industry. With fluid materials in contact with various metals products, especially copper, this type of testing has become critical as these destructive conditions have been problematic globally [6]. This test was designed not to look at the level of corrosion that occurs such as the ASTM D130 copper corrosion test but was developed to analyze the rate of corrosion at a given temperature. This analysis is intrinsic for the industry with motors, motor windings, electrical connectors, electrical components, and circuitry all potentially exposed in the EV application. Research 46 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0033 The ASTM D5704 L-60 Engine Oxidation test takes another perspective for evaluation. Not only are the analytes from the other tests monitored, but the added gear carbon and varnish analyses are added to the dossier. This is valuable within the EV realm because of the agitation caused in the areas of concern. Final ratings for varnish are combined with a sludge rating from the pentane insoluble analysis and parts per million soluble copper measurement. However, none these tests were truly developed for the EV industry. Because of that they lack the depth for such an analysis needed for this sophisticated technology. How do actual corrosion tests such as the copper strip corrosion test ASTM D130 perform for these motors? 1.2 Limitations of ASTM D130 - Copper Strip Corrosion Test Despite being simple and straight-forward, it is clear the ASTM D130 method [3], is limited in precision due to the subjective color rating of the copper strip at the end of the test. However, that is not the only definitive factor of the limitations involved with this test. The method was first published in 1930 to analyze the free sulphur and corrosive compounds in gasoline. It was not until 1949 when the test was expanded to include the analysis of other petroleum products, including lubricating oils [4]. Since that time, the method has been utilized extensively by the petroleum industry, included several specifications. What has not changed is the variability that comes with the analysis. It is completely subjective, not including the exceptions where the tests are a definitive pass or fail. The concern is that fluids may become improperly tainted as being problematic when they may perform satisfactorily. However, electrification of the transportation industry, the incorporation of electronics and electric motors, has resulted in failure modes that require an understanding of corrosion that occurs under the actual operating temperatures, as well as potential damage from corrosion products on the application hardware [4]. There are two concerns in the end; one is that the test really is not definitive in nature. There are debates that can be argued in either direction. The other is that the test is subjective. Either way, the test is rooted in history. That means the tendency to remove such a test is difficult, because of the data history. That is not to mention the chemistries that have been formulated in conjunction with such a test and the history thereof. Modern technologies, however, should not have to be imperilled by such an antiquated test. surfaces require proper tribological performance causing an increase in rotational forces, surface wear and movement. However, conductive deposits create a destructive element that can be unpredictable. Therefore, a test has been developed to assist the industry with conductive deposit formation. This is the conductive deposit test, known as the CDT. The CDT was developed as a methodology to measure and monitor the formation of conductive deposits over time. The level of conductive deposits can be monitored by the reduction of resistance between traces on a circuit board with an applied voltage. The test utilizes circuit boards with conductive copper traces not to duplicate the failure of circuit boards, but to provide a small anulus Research 47 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0033 Figure 3: Motor winding destroyed The WCT test simulation takes place with a small diameter wire that is lowered into the test cell with one half in the fluid and the other in the vapor while being maintained at temperatures anywhere from 80 °C to 150 °C. With the introduction of a low voltage and amperage, the change in potential across the wire can be denoted as corrosion takes place. The corrosion can be destructive enough that the wire is eventually severed. The actual diameter change can be measured in angstroms over time. See Figure 1 for the test application and Figure 2 for the rate of change. As important monitoring corrosion has become within the industry, it cannot predict all types of oxidation failure modes or methodologies. One critical area is deposition. 2.2 Deposit Formation Concerns Exposure of various metals and other materials is inevitable in new EV systems. With moderately elevated temperatures, temperature cycling, aeration, mixing, water, and other contamination can cause oxidation deposit formation. When adding the corrosion of materials, it enhances the possibility of chemical reactions occurring thus forming carbonaceous and sludge conditions. Other products may also form from the additives or base stocks utilized in the fluid’s formula. There is evidence that a correlation may exist between the level of oxidative stress a lubricant endures and its propensity to create conductive deposits, but it has been observed that some lubricants can form conductive deposits well before any obvious oxidative indicators are evident. Components such as sulphur may initiate the corrosion, removal and even buildup of copper and other metal by-products. Potentially these deposits may prove to be conductive in nature within the confines of the electrical and mechanical areas of the EV transmission. There have been cases sited where the conductive deposits have become detrimental to the overall performance of field vehicles. Catastrophic failure has been documented [6]. Figure 3 depicts the failure of a motor stator. The hairpin, located at the top of the stator, was destroyed due to arcing caused by conductive deposit formation. 3 Conductive Deposit Test (CDT) Overview Corrosion and the ensuing by-products are synonymous in nature. However, it is the type of corrosion by-product or subsequent deposition that can be concerning. Not all deposit formations are created equal. Deposits may limit performance due to the destructive nature of moving Figure 1: WCT Diagram Figure 2: Wire diameter rate of change measured by a transformation equation measuring the summation of the areas between the expected passing result and the breakdown that may actually occur. The changes in resistance limits to failing fluids are determined based on the EV application. The summation, therefore, is recommended as it depicts the rate of change in Figure 6 which assists with the determination of a failure point. It has been shown that some known failing fluids have a conductive deposit factor, CDF, of over 2 CDF within the 500-hour testing period. However, the failure limit may be modified and set by specification groups based on the equipment being protected. As shown, there are poor field performing that fluids tran- Research 48 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0033 duplicating constricted areas in the EV where conductive deposits are formed as shown in Figure 4. Part of the circuit is in the test sample and the other is in the vapor phase. The fluid is maintained at 150 °C but may be modified to other desired temperatures. As these destructive deposits are formed, the resistance decreases between copper traces increases the potential for arcing. Propagation of these deposits over time form conductive bridges between copper traces and the resulting decrease in resistance is monitored as a function of time. Once arcing occurs, the traces may fail completely causing an open circuit. The test was originally designed to run out to 1000 hours. However, failures correlating to field failures have shown to occur far less than 500 hours. Even though the testing in this paper is 1000 hours, testing beyond 500 hours has been determined not to be critical in most cases. Figure 4: CDT Test Board in Fluid 4 Testing and Analysis The data collection involves monitoring the change in resistance of traces on the board in the liquid and vapor phase over time. As conductive deposits are formed in each phase, liquid and vapor, the total resistance of the decreases. The resulting graph is shown in Figure 5. A proven good fluid may run the entire 500 hours. Some failing fluids will come to the end of the test within 100 hours. Variability in resistance is expected as the conductive deposits form, copper is depleted through plating or solubility occurs. To exculpate data from these excursions, the data are filtered. It was still desirable to have a result that can be utilized as an indicator when the culmination of the reaction process is at a reasonable conversion of progression. This is ! ! " ! "" ! """ ! """" ! """"" ! """""" ! """"""" " ! "" #"" $"" %"" &"" '"" ("" )"" *"" ! """ ! "#$%&'(%)")*+#,%'-./ 0)1 2"0%'-345&)1 ! "#$%&'(&)*)+",-&'./ "$0' +,-./ 011,12.342 5367/ ./ 011,12.342 Figure 6: rate of change of deposits Figure 5 spire within the first 250 hours of the test. The conductive deposit factor will be referenced in this paper throughout. 5 Research Analysis using the CDT - Transmission Fluids used in EV The precision has been proven dependable for this test over the past few years of development. Because of the consistency of the test researchers have been active to investigate potential improvements to fluids performance. A study was conducted with existing EV transmission fluids. The transmission oils are in-industry fluids with known performance results. These fluids were first evaluated utilizing the CDT and kinematic analysis at 100 °C. The CDT results for ATF A and ATF B are shown below evaluating the resistance change over time. It is evident that these two transmission fluids have failure mechanisms, but in different capacities. The potential reasoning will be discussed later in this document. ATF A, Figure 7, had a propensity to fail in the vapor phase while leaving the oil phase stable over the entire 1000-hour period. Alternatively, ATF B in Figure 8, failed the oil phase with the vapor phase remaining unscathed from deposit. Thus, having two finished and marketed products with distinctively opposing failures was optimum for this study. 5.1 Material Reactions - Base Stock and Ester Dilution The goal for this experimental process was to determine the chemical relationships between these fluids and the dilution effects of various base stocks and additives. The first target was a wellknown group II base stock. The goal was to see if 20 % of this base stock or an ester would extend the result in hours. Understanding the relationships with enhanced materials is important. Esters are known for their excellent dielectric capabilities, and they are sulphur free. That make the EV application more inviting. Research 49 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0033 Figure 7: ATF A fresh ! ! " ! "" ! """ ! """" ! """"" ! """""" ! """"""" " #"" $"" %"" &"" ! """ ! #"" '()*+,-.+/ (/ 01)2+-3456/ 7 8(6+-39: ; ,/ 7 ! "#$ %&&'()*)+,-.(&/ 0,12) ! "#$%&''"'($)*( +),-%$%&''"'($)*( Figure 8: ATF B fresh ! " # $ % & ' ( ) * "! ! "! ! #! ! $! ! %! ! &! ! '! ! (! ! )! ! *! ! "! ! ! +,-./ 012345647,82159: 01,; 5<+69= >2? 45<@,/ ; 8= ! "#$ ! %%&'()%*+%,*-./ 0(12)%3)4*51(%,6'-7) A2B5C/ ? 5" D: 7,; 5C/ ? 5" ATF B is shown in Figure 11. The traction fluid had a significant impact on this fluid in the liquid phase, putting into the passing category. The interest here was the overall reaction to both materials. The traction reduction additive, in each case, extended the life of the transmission fluid significantly, see Figure 11. Both fluids went from failing early, or borderline failing, to easily passing at the solid performance level. From 200 hours to 800 and 1000 hours is significant as shown in Figure 12. Research 50 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0033 The reactions for the two original transmission fluids were more interesting than expected. Results are shown in Figure 9. ATF A, which had a failing oil phase failure at 268 hours remained unchanged. It was anticipated that if the interaction of the additives were causing the oil phase failures, then diluting it with 20 % of the group II base stock or the ester would improve the result. These tests were repeated to verify the accuracy of the results. The tests, not shown, were within a percent of one another. The reactivity of the vapor, created with without the dilution, was significant enough to not only destroy the copper but create significant levels of dendrites for conductive deposits. Vapor materials can be extremely destructive to metallic part [7, 8, 9]. ATF B, which failed the liquid phase at 506 hours, yielded some interesting results in both phases. Dilution using both the ester and group II base stocks did lead to improving the resistance to conductive deposit formation in the liquid phase, but had the opposite effect in the vapor phase. Instead of an oil failure of less than 200 hours the fluid did not have conductive deposit issues until beyond 700 hours, see Figure 10. However, there was enough of a mechanism change due to the base stock to cause the vapor to have conductive deposit issues, but only beyond 700 hours. Since the test will be anticipated to last only 500 hours to predict failures the 700 hours would not necessarily be concerning. To know that this change increased the probability of oil phase failure is interesting, nonetheless. The main take away is that ATF A, which failed in the vapor state, was unaffected by the dilution. The additives in the oil may be diluted or masked. 5.2 Materials - Traction reduction additive A traction reduction additive was selected that is a hydrocarbon synthetic based material that provides slip reduction in transmissions. The material is slightly higher in viscosity, to note, than the transmission fluid or ester utilized in this study. That is a fact to note for future reference. Once again, the traction reduction additive was added to the original EV driveline fluids. The purpose of this experiment was to investigate the level of reaction from the dilution of this material. The interesting result of ! " # $ % & ' ( ) * "! ! "! ! #! ! $! ! %! ! &! ! '! ! (! ! )! ! *! ! "! ! ! +,-./ 012345647,82159: 01,; 5<+69= >2? 45<@,/ ; 8= ! "#$%& '()*$+,-$./ 012$33$& 405617)(89$: 920; ()$<=)9$0>$4*=5? 9 ! "#$%&'$( )*+,-$%&'$( Figure 10: ATF B with 20 % base stock dilution ! " # $ % & ' ( ) * "! ! "! ! #! ! $! ! %! ! &! ! '! ! (! ! )! ! *! ! "! ! ! +,-./ 012345647,82159: 01,; 5<+69= >2? 45<@,/ ; 8= ! "#$%$&'()$*+,$"-./ ('01$#23'4$.14$ $*+,$5677$8 6.(9$0: $; ).1<9$5-.=) ! "#$%&'$( )*+,-$%&'$( Figure 11 Figure 9: Dilution table and response To evaluate this response, the viscosity of all the fluids, as mentioned above, were taken at 100 °C. The relative kinematic response was higher for the traction reduction additive. This led to an investigation of the boards. For in-fluid responses, the viscosity should not necessarily play a role in the improvement of the response. At least not from the authors’ perspective. Dilution, of course, is always a possibility but the rate of enhancement was significantly more than 20 %. However, for the vapor phase improvements there was noticeable changes on the actual board as well. There was a distinct line where the conductive deposits were formed. A capillary action is occurring for the lower viscosity fluids. For example, the transmission fluid had a KV100 at 7.33 cP with a capillary movement up the board of 6 mm. The fluid dilution of traction reduction additive had a KV100 of 10.74 cP and capillary movement of 0.3 mm. Thus, the board had an increase in conductive deposits on the upper portion of the board potentially from the capillary movement. The higher viscosity fluids did not have the capillary action. Not to mention the volatility level, potentially lower, could also play a part in the improvement. So, the level of deposits in the vapor phase area of the board were lower because of it, at least in this situation [10]. Further studies of these effects will be analysed for the next paper. 6 Conclusions Conductive deposits have proven to be problematic in the EV industry. Failures have occurred and resolutions are in place to help rectify the situation. The performance of the fluids utilized in the transmission have varying levels of quality. The conductive deposit test was developed to evaluate this attribute. The conductive deposit test has an electrified and nonelectrified circuit board with a tightly controlled space between them. At 150 °C the test will evaluate the resistance response in the oil and vapor phase simultaneously. As conductive deposits are formed, the resistance drops until potentially complete failure occurs. The test aligns with passing and failing field performing fluids. This paper evaluated the response of base stock, ester and traction reduction additives added to a driveline fluid that were poor performing. One in the vapor and the other in the oil phase. The base stock dilution had negligible effect where the ester had moderate improvement characteristics notably from the oxidation protection and polar responses. The largest improvement was from the dilution with the traction reduction additive. This synthetic fluid improved the propensity of both driveline fluids to form conductive deposits. 7 Future Work Continuing work in this area is important. Evaluation of the conductive deposit dendrites that are formed is necessary. Determination of appearance and composition of these deposits in various areas of the failures and the interaction of elements such as sulphur, oxygen and copper will be critical. Each of the documented scenarios from this paper will be evaluated. Scanning electron microscopy and inductively coupled plasma spectroscopy will be utilized for this process. Other additives will also be evaluated. 8 References [1] Han, Ji-Hung, “Over-limiting Current and Control of Dendritic Growth by Surface Conduction in Nanopores” [2] ASTM Method of Test D2272-02, Oxidative Stability of Steam Turbine Oils by Rotating Pressure Vessel (Reapproved 2011). ASTM 5 vol., pp. 846-864, 2010 and IP Method of Test 142/ 85 (92) [3] ASTM Method of Test D130-19: Standard Test Method for Corrosiveness to Copper from Petroleum Products by Copper Strip Test. ASTM Digital Volume D130 Section [4] Accepted for publication. Hunt, G., Choo, L., and Newcomb, T., “100 Years of Corrosion Testing—Is It Time to Move beyond the ASTM D130? The Wire Corrosion and Conductive Deposit Tests,” SAE Int. J. Fuels Lubr.17(1): 2024, doi: 10.4271/ 04-17-01-0002. [5] Chauhan, Saurabh “Motor Torque Calculations for Electric Vehicles” International Journal of Scientific & Technology Research Volume 4, Issue 8, August 2015. [6] Hunt, G., Javaid, R., Simon, J., Peplow, M. et al., “Understanding Conductive Layer Deposits: Test Method Development for Lubricant Performance Testing for Hybrid and Electric Vehicle Applications,” SAE Int. J. Elec. Veh. 12(2): 263-277, 2023, https: / / doi.org/ 10.4271/ 14-12-02- 0014. Research 51 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0033 Figure 12 rate. Journal of the Electrochemical Society 1988, 135 (8), 2027. https: / / doi.org/ 10.1149/ 1.2096201. [7] Hunt , G. New Perspectives on the Temperature Dependence of Lubricant Additives on Copper Corrosion SAE Int. J. Fuels Lubr. 10 2 2017 521 527 https: / / doi.org/ 10.4271/ 2017-01-0891 [8] Hunt , G.J. , Gahagan , M.P. , and Peplow , M.A. Wire Resistance Method for Measuring the Corrosion of Copper by Lubricating Fluids Lubrication Science 29 4 2017 279 290 https: / / doi.org/ 10.1002/ ls.1368 [9] Hunt, G. and Prengaman, C., “Understanding Vapor and Solution Phase Corrosion of Lubricants Used in Electrified Transmissions,” SAE Technical Paper 2020-01-0561, 2020, doi: 10.4271/ 2020-01-0561 [10] Forslund, M., “E-Mobility Fluid Testing for Heavy Truck Transmissions”, presented at the STLE EV Conference, San Antonio, TX, 2021. Research 52 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0033 [A] Barcroft, F.T., “A Technique for Investigating Reactions between E.P. Additives and Metal Surfaces at High Temperatures,” Wear 3, no. 6 (1960): 440-453 [B] Gahagan, M. and Hunt, G., “New Insights on the Impact of Automatic Transmission Fluid (ATF) Additives on Corrosion of Copper—The Application of a Wire Electrical Resistance Method,” International Journal of Automotive Engineering 7 (2016): 115-120. [C] Savant Group, USA and APL, Germany Zou, L. C.; Hunt, C., A new approach to investigate conductive anodic filament (CAF) formation. Soldering & Surface Mount Technology 2015. https: / / doi.org/ 10.1108/ SSMT-02-2014-0002. Vaughen, B.; Roth, J.; Steppan, J.; Hall, L.; Major, C., Electrolytic Copper Migration in Accelerated Tests: I. Polyethylene Glycol-400 Doped with Ammonium Perchlo-