Understanding the electrification-driven lubrication mechanism of phosphonium ionic liquids to enhance the degradation resistance of electric vehicle components
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Authors
Rahman, Md Hafizur
Issue Date
2025
Type
Dissertation
Language
en_US
Keywords
Electric vehicle , Ionic liquids , Phosphonium , Synthesis , Tribology , Wear-corrosion
Alternative Title
Abstract
The electric vehicle (EV) market has been manifolded in the recent past, and therefore, enhancing the durability and efficiency of moving mechanical assemblies (MMAs) in EVs has become increasingly important. Despite the superior efficiency of EVs over internal combustion engine vehicles, more than half of their energy loss is still attributed to friction and wear in components such as gears and bearings. Moreover, electrified environments in EVs introduce stray currents that exacerbate material loss through arcing, challenging the performance of traditional lubricants. As an alternative, conductive lubricants, such as phosphonium based room temperature ionic liquids (PRTILs) could be utilized. However, the ionic nature of PRTILs could be detrimental to the surface due to corrosion, which can accelerate the overall material loss under sliding interaction. Till date, there is barely any study where the synergistic wear-corrosion mechanisms of PRTIL lubricated surfaces have been addressed. Furthermore, the impact of stray current could deteriorate the wear-corrosion synergism, which has a significant research gap.To address these research gaps, this dissertation investigates the development and performance of aromatic ring containing PRTILs as next-generation conductive lubricants for EV MMAs. Here, at first, PRTILs are synthesized, characterized, and studied for their unique ability to reduce friction and wear under tribological interaction in a ball-on-disk setup. Further, the best performing aromatic PRTIL is compared with a popular non-aromatic halogen-based PRTIL over a wide temperature range. Next, the corrosion behavior of both aromatic PRTIL and non-aromatic PRTILs are investigated using electrochemical techniques such as open circuit potential, potentiodynamic polarization, and cyclic potentiodynamic polarization. Furthermore, synergistic material loss rates are evaluated in a combined wear-corrosion setup facilitating sliding under electrochemical corrosion. Next, the effect of external electrification on tribological behavior of the PRTIL lubricated steel surfaces are investigated for the very first time in the electrified mini traction machine. Furthermore, the impact of external electrification on wear-corrosion synergism is revealed for PRTIL lubricated surfaces using a novel in-situ test-rig. In addition to these experiments, tribological, wear–corrosion, and wear–corrosion under electrification assessments are systematically performed on steel surfaces with varying surface textures and roughness levels. All tests are carried out under non-abrasive conditions to isolate and evaluate the influence of surface texture and roughness on the coefficient of friction (COF) of PRTIL lubricants. Finally, using machine learning algorithms, the physicochemical and electrochemical properties of PRTILs are utilized to correlate with the resulting COF and synergistic material loss rates.
Key findings in the dissertation demonstrate the critical roles of molecular structure, viscosity, and wettability in enhancing the lubrication performance of PRTILs over traditional lubricants. Double ring containing PRTILs, having higher viscosity, and lower wettability were beneficial to reduce friction, and wear than single aromatic ring containing PRTILs, or halogen-based PRTILs or traditional lubricants. Structure-property correlation for PRTIL lubricated surfaces were established using a structural feature parameter that showed significant correlation with the resulted friction, wear, and surface roughness after the tribological assessment. Further, double ring PRTIL exhibited lower corrosion rates than other PRTILs or lubricants due to their better surface repassivation ability. The wear-corrosion synergistic assessment revealed that the total material loss under the combined effect of wear and corrosion, increased significantly for each PRTIL, due to the corrosion accelerated wear mechanism. The double ring PRTILs, having least corrosion rates towards surface, were able to provide significantly less overall material loss rates than other PRTILs, or lubricants. Furthermore, the surface texture and roughness assessments revealed that parallel and randomly finished surfaces exhibited lower COF compared to perpendicular and 8-ground textures. Additionally, higher surface roughness led to an increase in COF overall. To connect the physicochemical and electrochemical properties of PRTILs with wear-corrosion synergistic material loss rates, machine learning based predictive modeling framework was employed that provided dimensionless parameters using principal component analysis. The first principal component demonstrated significant impact of the material loss rates overall. It was further revealed that a greater charge transfer resistance better protected the double ring PRTIL lubricated surfaces from corrosion accelerated wear than single ring aromatic PRTIL lubricated surfaces.
Additionally, the application of external current during wear revealed that a moderate ionic conductivity is ideal to limit the wear rate for PRTIL lubricated surfaces. The ionic conductivity was also crucial in modulating electric contact resistance, where higher conductivity reduced the resistance and caused surface damage through electric shock across the wear track. On the other hand, less conductive lubricants also exhibited significantly high material loss under electrification due to arcing. Finally, the wear-corrosion synergism under electrification further supports these findings where it was revealed that moderately conductive double ring PRTIL was able to withstand the combined effect of wear and corrosion under electrification, whereas PRTILs with the highest ionic conductivity experienced significantly higher material loss rates.
Overall, this work offers foundational knowledge for designing sustainable lubricants to meet the growing demands of electrified MMAs. The outcome of this research bridges the critical gaps in understanding the interfacial mechanisms of PRTIL lubricated electrified surfaces and paves the way for future research in EV tribology and PRTIL based lubrication technologies.