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

**Daniel Sanchez Garrido, Samuel Leventini and Ashlie Martini \***

> Department of Mechanical Engineering, University of California Merced, 5200 N. Lake Rd., Merced, CA 95343, USA; dsanchezgarrido@ucmerced.edu (D.S.G.); sleventini@ucmerced.edu (S.L.) **\*** Correspondence: amartini@ucmerced.edu

**Abstract:** 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.

**Keywords:** grease; hybrid bearings; electric vehicles; electric motors; friction and wear

**Citation:** Sanchez Garrido, D.; Leventini, S.; Martini, A. Effect of Temperature and Surface Roughness on the Tribological Behavior of Electric Motor Greases for Hybrid Bearing Materials. *Lubricants* **2021**, *9*, 59. https://doi.org/10.3390/ lubricants9060059

Received: 5 April 2021 Accepted: 5 May 2021 Published: 24 May 2021

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