Rheological Characterization and Tribological Evaluation of Water-Based Lubricants in AISI 52100 Bearing Steel

Abstract

This study explores the rheological and tribological behavior of water-based lubricants (WBLs) as potential alternatives for electric vehicle (EV) applications. As the transportation sector increasingly shifts towards EVs to reduce carbon emissions, the demand for efficient lubricants becomes critical. WBLs here are defined as fluids containing a minimum of 50 wt% water mixed with glycerol, ethylene glycol (MEG), polyethylene glycol (PEG), or one of two polyalkylene glycols (PAGs). Rheological properties were investigated and compared with traditional lubricating oils. Results demonstrate distinctive rheological characteristics in WBLs, with viscosity–temperature responses resembling traditional oils with lowered pressure–viscosity coefficients. Nevertheless, WBLs exhibit promising film-forming capabilities in highly loaded contacts. Additionally, in sliding contacts, WBLs generally display lower friction coefficients compared to traditional oils, with PEG exhibiting the lowest value near 0.1. These findings suggest that WBLs may offer advantages in reducing friction and energy loss in EV applications. Furthermore, sliding wear tests indicate low wear rates in WBLs such as PEG, PAG, and glycerol, supporting their potential as viable lubrication options. The study highlights the importance of high-pressure rheology and tribochemistry in wear mechanisms among the different lubricants.