**5. Conclusions**

The tribological behavior of commercially available EM greases on hybrid bearing materials was characterized and evaluated. Results showed that EM grease products have notable differences in performance across different roughness and temperature conditions. These variations in performance have important implications for lubrication and design limitations. In general, greases whose performance is least affected by changing operating conditions will be more likely to meet the tribological needs of EMs.

Surface roughness has a significant impact on tribological properties. Rougher surfaces generally correspond to more friction and wear since they tend to have smaller local film thickness and higher pressure at asperity peaks [34,42,43], which result in high shear rates and stresses [12,42,43]. In contrast, surfaces with low roughness increase grease life and are less demanding in terms of lubrication ability since interacting smooth surfaces are more easily separated by lubricating films [8]. Yet, extremely smooth surfaces risk sudden seizure and asperities on rough surfaces may be useful for retaining lubricant [2]. Therefore, surface roughness plays a key role in determining the performance of grease lubricated systems.

Another important parameter for grease tribology is temperature, particularly for high-speed bearings. EM grease is known to be susceptible to thermo-oxidation degradation during high temperature bearing operation [9], which can lead to grease lubrication failure and, consequently, bearing failure [4,9–11]. Thus, grease formulations that do not compromise lubrication capabilities at high temperatures and resist thermal-oxidation are likely to be better for lubricating EM bearings. Temperature also influences film thickness through its effect on viscosity and the pressure-viscosity coefficient. Low temperature environments may be beneficial to achieving thicker lubricating grease films and reduce wear but will increase viscous friction. Further, excessively thick and viscous grease films may cause contact starvation from poor grease bleed and lack of reflow [16], which also leads to an increase in friction [22]. On the other hand, an increase in temperature can reduce

viscous friction and activate grease bleed, but, under high temperatures, film thickness can decrease [16,22,44] to levels that may promote harsher operating conditions detrimental to grease and bearing life. Consequently, EM greases will need to be optimized for high operating temperatures and formulated to have minimal temperature dependence.

Both temperature and surface roughness affect the lubrication regime, as quantified by the *λ* ratio. Ideal *λ* ratios during operation will be small enough to achieve low friction but not so small that there is a transition into mixed or boundary lubrication. Ideal *λ* ratios can also have a positive effect on bearing contact fatigue, prolong component life, and improve energy efficiency. Therefore, maintaining a consistent *λ* ratio across temperature and roughness conditions is a key factor in component design and grease selection.

EM grease formulations also need to be optimized for hybrid bearing materials, assuming the continued use of hybrid bearings to combat stray current. Umbrella grease type products might not capture all lubrication requirements [34] and, consequently, may jeopardize performance and system life. Further, non-traditional bearing material and material configurations can exhibit wear mechanisms distinct from those observed in traditional steel bearings. The 4-ball test results reported here indicate the ideal hybrid bearing configuration is ceramic rolling elements on steel races (NS3). The inverse bearing configuration, steel rolling elements on ceramic races (SN3), generated significantly larger and abnormal wear. Additionally, the NS<sup>3</sup> configuration was found to have better wear performance than traditional SS<sup>3</sup> bearings, which has positive implications for hybrid bearings and grease life.

The results of a comprehensive set of friction and wear tests, using 4-ball tests and ballon-disk measurements across a range of roughness and temperature conditions, showed that SL had the best overall performance under the conditions tested here (Figure 8). SL provided low wear at 40 nm Ra or less and consistently maintained low friction throughout both the full film and mixed lubrication regimes. When results were analyzed in terms of friction and wear separately, it was found that synthetic greases had the best friction behavior, while mineral greases had the best wear performance, with ML being best overall in terms of wear. However, ultimately grease selection will depend on the application. In the process of comparing four greases, this study also developed an approach for the *λ* ratio and the transition between lubrication regimes (Figure 7) that may be useful as a design tool more generally.

Going forward, the tribological performance of potential hybrid bearing materials combined with grease formulations for EMs need to be fully explored under conditions that resemble the environments of the target application. This is particularly important because tribology will play an important role enabling the electrification of the transportation industry, and, through tribological research, EM bearing lubrication can be optimized for EVs as it has been for ICEVs. In this context, the study reported here is a baseline and a template for further grease research in EM environments. Further, the present study demonstrates that market-available EM grease products can vary significantly in performance, providing insight into the effects of operating conditions and design criteria on grease behavior.

**Author Contributions:** Conceptualization, A.M. and D.S.G.; methodology, A.M. and D.S.G.; experiments, S.L. and D.S.G.; data curation, A.M. and D.S.G.; writing—original draft preparation, A.M. and D.S.G.; writing—review and editing, A.M., S.L. and D.S.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the National Lubricating Grease Institute (NLGI).

**Acknowledgments:** We appreciate the valuable input from our NLGI liaison throughout the project. We also acknowledge the UC Merced Instructional Lab Support Team and Instrumentation Foundry. Lastly, some of the 4-ball tests were performed by undergraduate students in the research group, Colin Cox, Alex McCollum, Jose Morales, and Eddie Santiago.

**Conflicts of Interest:** The authors declare no conflict of interest.
