Effect of Temperature and Surface Roughness on the Tribological Behavior of Electric Motor Greases for Hybrid Bearing Materials

: Greased bearings in electric motors (EMs) are subject to a wide range of operational requirements and corresponding micro-environments. Consequently, greases must function effectively in these conditions. Here, the tribological performance of four market-available EM greases was characterized by measuring friction and wear of silicon nitride sliding on hardened 52100 steel. The EM greases evaluated had similar viscosity grades but different combinations of polyurea or lithium thickener with mineral or synthetic base oil. Measurements were performed at a range of temperature and surface roughness conditions to capture behavior in multiple lubrication regimes. Results enabled direct comparison of market-available products across different application-relevant metrics, and the analysis methods developed can be used as a baseline for future studies of EM grease performance.

dation and melting of material that leads to wear. Often, silicon nitride is used in hybrid bearings that consist of 48 ceramic rolling elements and traditional steel raceways [26]. Hybrid bearings have been found to last longer than 49 predicted based on the Lundberg-Palmgren theory [30] and grease life with hybrid bearings was found to be up to 50 four times longer than with traditional all steel bearings [26]. However, there are issues associated with the use of 51 ceramic bearing elements, particularly related to lubricant additives. For example, phosphorus-based additives were 52 found to not react with silicon nitride as they would with steel so the tribofilms formed were not effective in improving 53 silicon nitride bearing life [31,32,33]. Another potential issue is that hybrid bearings experience higher contact stress 54 than all steel bearings under the same applied load [30]. Ceramic materials have greater hardness than steel and thus 55 ceramic deforms less at the contact, resulting in a smaller contact area than an all steel configuration. A load applied 56 to a smaller contact area will result in greater contact stress than the same load applied to a larger contact area. 57 The above mentioned challenges with grease lubrication in EMs can be partially addressed through design or formulation is crucial because grease behavior is extremely application dependent [22]. 66 Grease optimization often focuses on identifying the best combination of base oil and thickener for EM applica-

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Based on the current findings, it is evident that both base oil and thickener affect grease performance and that 79 optimizing this performance for EM applications requires characterization at the conditions in which the motor will 80 operate. Specifically, EM greases are subject to higher temperatures and may be required to function with different 81 bearing materials than traditional applications. Here, we tested the tribological performance of four commercially 82 available greases with formulations/additives designed for EM applications, with different combinations of mineral 83 or synthetic base oil with lithium (complex) or urea thickeners. The study focused on lubrication of silicon nitride 84 sliding on steel across a range of temperature and surface roughness conditions. The tribological performance of these 85 greases and bearing materials was quantified in terms of friction and wear. Characterization included both ball-on-disk 86 and 4-ball tests as well as an analysis of the results in terms of lubrication regimes. Finally, the four greases were 87 evaluated based on a ranking system that emphasized priorities for EM applications.  • One silicon nitride ceramic rotating element on three steel stationary elements (NS 3 ) • One steel rotating element on three silicon nitride ceramic stationary elements (SN 3 ) 120 The SS 3 case resembled a traditional all steel bearing assembly and the NS 3 case resembled a hybrid bearing assembly.

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The SN 3 resembled an inverted hybrid bearing assembly, meaning that material typically used for the races was used 122 as the rolling element and vice versa. Tests were run at a load of 392 ± 2 N, which corresponds to a maximum Hertz 123 contact pressure of 4.6 GPa for the SS 3 configuration and 5.2 GPa for the NS 3 /SN 3 configurations. The speed was 124 1200 ± 60 revolutions per minute for 60 minutes and the temperature was held at 75 • C ± 2 • C. All four greases were 125 tested twice for each material configuration.  Also, for these testing parameters, greases with mineral base oil have lower wear rate than the synthetic base greases.

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The sensitivity of wear rate to changes in roughness was quantified as the slope of a linear fit to the data. Although greases. The lowest wear rate at 100 • C is observed for the ML grease and, at 150 • C, is found for the SL grease.

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Additionally, at 100 and 150 • C, for the greases tested here, lithium thickened greases have a lower wear rate than 146 their polyurea counterparts.

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The temperature dependence of the wear rate is very different for synthetic vs. mineral based greases. Specifically, 148 the wear rate increases nearly linearly between 100 and 150 • C for the mineral greases, but is nearly constant for the 149 synthetics. Due to this behavior, the linear approximation cannot be used to quantify the change of wear rate with 150 temperature for the synthetic greases. However, the linear fit was performed for the mineral greases as shown in 151 Fig. 1d. The wear rate is less dependent on temperature for the ML grease than the MP grease. greases (see Fig. 2a). Also, on most surfaces, friction was lowest for the SL grease. For the rougher surfaces, the ML 155 also exhibited low friction behavior. For these tests, the lithium based greases had lower friction than the polyurea 156 greases, except on the smoothest surfaces where the friction coefficient was below 0.08 for all greases.

Four-Ball Test
Results from the 4-ball tests are shown in Fig. 3. ML had the lowest wear across all three bearing configurations.

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The performance of ML might be attributable to thicker lubricating films that provide more separation between inter-169 acting surfaces or better anti-wear film formation. For the SS 3 and NS 3 configurations, average wear increased as ML 170 < MP < SP < SL. For the SN 3 configuration, wear was high for all four greases and large error bars precluded direct 171 comparison between the greases.

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Comparing the different material combinations, for all greases, the lowest average wear was observed for NS 3 , 173 followed by SS 3 and then SN 3 . The observation that wear for NS 3 was lower than that for SS 3 is consistent with 174 previous reports that grease life with hybrid bearings is longer than with standard bearings [26]. Lower wear for 175 NS 3 is also consistent with experimental and anecdotal observations that suggest longer lives for hybrid bearings than 176 estimated by the Lundberg-Palmgren equations [30].

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In contrast, the SN 3 configuration consistently had very high wear. This configuration also exhibited qualitatively 178 very different behavior than the other two material pairs. As shown in the insets to  The friction results shown in Fig. 2 suggested that changing either roughness or temperature caused a transition 188 between lubrication regimes. The lubrication regime can be determined by the lambda ratio: where h is the film thickness, R a,ball is the average roughness of the ball and R a,disk is the average roughness of the lubrication [40]. 193 It is known that the film thickness of a grease may be larger or smaller than the film thickness for its base oil, grease film thickness that is applicable for all conditions. Therefore, as a first order approximation, we calculated film 196 thickness using the Hamrock and Dowson equation [2] for central film thickness with parameters for the base oil: where, U is the speed, R is effective radius, E is effective elastic modulus, α is the pressure-viscosity coefficient, η is   where friction increases with λ.

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The greases clearly exhibit full film above lambda ratios of 1. In this regime, the lowest friction was exhibited 208 by the SP and SL (synthetic greases). The mixed regime is clearly observed at λ ratios below 1. Here, as composite 209 roughness increases, λ values decrease, and friction tends to increase. In mixed lubrication, the lowest friction was 210 observed for the ML and SL (greases with lithium thickener). Across most of the lubrication regimes measured, SL 211 had the best friction performance.

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The transition between the full film and mixed lubrication regimes is important because both friction and wear 213 are much higher in the mixed regime due to asperity contacts in the interface. Therefore, it is desirable to remain in 214 the full film regime as long as possible. To identify the λ ratio at which the full film-mixed transition occurs for each 215 grease, we found the intersection of a linear fit to the data in the mixed regime and a linear fit to the data in the full 216 film regime. The two largest λ ratios for each grease were fit for full film and the three smallest λ ratios were fit for 217 the mixed regime. The transition lambda (λ t ) values for each grease were found to be: SP at λ t = 0.48, MP at λ t = 218 0.47, ML at λ t = 0.37, and SL at λ t = 0.58.

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The ML grease had the lowest λ t , indicating that the interface would remain in the full film regime the longest 220 with increasing temperature or roughness. However, it is important to note that ML also has higher friction in this 221 transition region. So, ML's lower λ t suggests the lubricant is able to maintain a thicker lubrication film than the other 222 greases but this comes at a cost of higher viscous friction. On the other hand, SL has a larger λ t value but considerably 223 lower friction than the rest of the tested greases in this transition range. In fact, despite having a larger λ t value, SL 224 maintained lower friction at most test conditions. This analysis shows there is a compromise between low friction in 225 full film lubrication and how long the interface will remain in that regime before the onset of mixed lubrication.

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For bearings, the λ ratio also affects contact fatigue life. Low λ ratios are associated with surface deformation and   distress [2]. In the context of the conditions studied here, small surface roughness and low temperature conditions that 228 correspond to higher λ ratios will have lower contact fatigue and longer life.

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The λ ratio determines lubrication regime as well as contact fatigue. In our study, this critical ratio is determined 231 by surface roughness, grease properties and temperature. So, for a given grease, roughness and temperature, the λ 232 value can be calculated and the conditions at which the lubrication regime transitions to mixed can be predicted.  horizontal planes corresponding to λ t , the ratio at the transition between mixed and full film lubrication, calculated 241 from the friction data for each grease in Fig. 4. The intersection between this plane and the surface predicted as 242 described above indicates the temperature and surface roughness at which the interface will transition from full film to 243 mixed lubrication. Such an approach can be used as part of the design process to guide selection of a grease, surface 244 roughness specifications or prescribed limits on operating conditions. The four greases evaluated in this study exhibited varying levels of performance at different surface roughness and 247 temperature conditions. These observations are summarized briefly here.

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As observed in Fig. 1a, MP had the lowest wear rate on smooth surfaces while ML had the lowest wear on rough 249 surfaces. ML was also found to exhibit the least dependence of wear rate on surface roughness. On average, greases 250 with mineral base oil had lower wear rate and roughness dependence than the synthetic base greases. In high tem-251 perature tests (150 • C), the lowest wear rate was found for the SL grease. Also, the wear rate of the synthetic greases 252 did not change with temperature at high temperatures, while an increase in wear rate with temperature was observed for the mineral greases. In the 4-ball tests, ML had the lowest wear across all the various bearing configurations.

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For the SS 3 and NS 3 configurations, average wear increased as ML < MP < SP < SL. Larger wear rates increase 255 material debris which can have implications such as artificial surface roughness, reduced lubricating capabilities and 256 abrasion/erosion, so low wear is extremely important.

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In terms of friction, on most surfaces, friction was lowest for the SL grease. For rougher surfaces, the ML 258 also exhibited low friction behavior. Among the greases tested here, the lithium based greases had lower friction 259 than the polyurea greases, except on the smoothest surfaces. At 40 • C, the lowest friction was exhibited by the SP 260 grease whereas, at 100 • C, the SL grease had the lowest friction. At both 40 and 100 • C, the friction was lower for 261 synthetic greases than their mineral counterparts. The friction data was also used to determine transitions between 262 the full film and mixed lubrication regime. This analysis showed that under these testing parameters, ML had the 263 lowest λ t , indicating that the interface would remain in the full film regime the longest with increasing temperature or 264 roughness. However, ML also had higher friction in this transition region indicating that the lubricant maintained a 265 thicker lubrication film at a cost of higher viscous friction. In contrast, SL had a larger λ t ratio but considerably lower 266 friction than the rest of the tested greases in this range.

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While the comparisons between greases in terms of individual performance metrics are valuable, they need to be 268 combined to determine which grease is best for a given application. Therefore, it is necessary to develop a grease 269 evaluation and comparison method to assess these commercially available greases. Performance metrics included are The results are shown as radar plots in Fig. 6.

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The individual rankings for each grease were also summed to give an overall score, shown next to the radar plots 277 in Fig. 6. Based on the overall score, under the testing parameters used here, the two lithium greases outperformed the and design limitations. In general, greases whose performance is least affected by changing operating conditions will 289 be more likely to meet the tribological needs of EMs. (NS 3 ). The inverse bearing configuration, steel rolling elements on ceramic races (SN 3 ), generated significantly larger 320 and abnormal wear. Additionally, the NS 3 configuration was found to have better wear performance than traditional 321 SS 3 bearings, which has positive implications for hybrid bearings and grease life.

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The results of a comprehensive set of friction and wear tests, using 4-ball tests and ball-on-disk measurements across a range of roughness and temperature conditions, showed that SL had the best overall performance under the 324 conditions tested here (Fig. 6). SL provided low wear at 40 nm Ra or less and consistently maintained low friction 325 throughout both the full film and mixed lubrication regimes. However, ultimately grease selection will depend on the 326 application. In the process of comparing four greases, this study also developed an approach for the λ ratio and the 327 transition between lubrication regimes (Fig. 5) that may be useful as a design tool more generally.

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Going forward, the tribological performance of potential hybrid bearing materials combined with grease formula-329 tions for EMs need to be fully explored under conditions that resemble the environments of the target application. This 330 is particularly important because tribology will play an important role enabling the electrification of the transportation 331 industry and, through tribological research, EM bearing lubrication can be optimized for EVs as it has been for ICE 332 vehicles. In this context, the study reported here is a baseline and a template for further grease research in EM envi-333 ronments. Further, the present study demonstrates that market-available EM grease products can vary significantly in 334 performance, giving us insight into the effects of operating conditions and design limitations.