Session: Rising Stars of Mechanical Engineering Celebration & Showcase

Paper Number: 148060

148060 - On the Mercurial Behavior of Ionic Liquids at Sliding Interfaces 

Ionic liquids (ILs) have attracted considerable attention in lubrication science owing to their unique properties (e.g., high thermal stability, negligible vapor pressure), which make them suitable for components working under extreme conditions, such as those found in engines and spacecraft. Despite the scientific and engineering relevance of published studies, a fundamental understanding of the nanoscale lubrication mechanism(s) of ILs is still lacking. Filling this knowledge gap requires shedding light on the chemical processes occurring at sliding interfaces in the presence of ILs.

Here, we used a multi-technique, multi-scale approach to evaluate the phenomena occurring at steel/steel sliding interfaces in the presence of two model classes of ILs, phosphonium phosphate ILs (pp-ILs) and alkylammonium orthoborate ILs (ao-ILs). In the case of pp-ILs, in situ nanotribological measurements and ex situ analyses demonstrated that the confined pp-IL molecules undergo a pressure-induced morphological change at an applied pressure up to 5.5 ± 0.3 GPa, which leads to the generation of a lubricious, solid-like interfacial layer. Increasing the pressure between 5.5 ± 0.3 GPa and 7.3 ± 0.4 GPa results in the removal of the solid-like layer formed by pp -ILs, leading to wear of the underpinning substrate. However, the surface adsorption of phosphate ions on metallic iron results in the formation of a densely-packed boundary layer that reduces nanoscale friction. In the case of ao-ILs, the combination of macroscale tribological experiments and surface-analytical results provided evidence for the dependence of the lubrication mechanism of ao-ILs on the IL structure. In the case of ao-ILs with asymmetric ammonium cations containing a long alkyl chain, no reaction layers were formed on iron surfaces, thus suggesting that the friction-reducing ability of these ILs originates from their pressure-induced morphological change at the sliding interface – in a similar manner to the nanoscale behavior of pp-ILs. In contrast, the higher friction response obtained with ao-ILs containing more symmetric ammonium cations and short alkyl chains is proposed to be due to the inability of this IL to create a transient interfacial layer owing to the reduced van der Waals interactions between the cationic alkyl chains. The resulting hard/hard contact leads to the degradation of cations and anions, whose products adsorb onto the iron surface.

The results of this work provide evidence for the existence of multiple mechanisms underpinning the promising lubricating properties of ILs in general, while also providing clues to strategies for rationally designing ILs with improved tribological response.

 

Presenting Author: Filippo Mangolini The University of Texas at Austin

Presenting Author Biography: Dr. Filippo Mangolini earned his Ph.D. in Materials Science at the Swiss Federal Institute of Technology (ETH Zurich, Zurich, Switzerland) in 2011, after graduating from Polytechnic University of Milan (Milan, Italy) in Materials Engineering with the highest honor in 2006. Upon completion of his Ph.D. in 2011, Dr. Mangolini performed postdoctoral research at the University of Pennsylvania and then at Ecole Centrale de Lyon (Lyon, France). His postdoctoral research was supported by the European Union through a Marie Curie International Outgoing Fellowship and by the Swiss National Science Foundation (SNSF) through an SNSF Postdoctoral Fellowship. After two years at the University of Leeds (Leeds, UK) as a University Academic Fellow and Marie Curie Fellow, Dr. Mangolini joined the faculty of the Walker Department of Mechanical Engineering at The University of Texas at Austin in Spring 2018 as an Assistant Professor in Materials Science and Engineering. In Fall 2020 he was appointed as a Walker Scholar.
Dr. Mangolini has received a number of international and national awards and honors for outstanding research and teaching achievements, including the 2022 American Society of Mechanical Engineers (ASME) Burt L. Newkirk Award, 2021 NSF CAREER Award, 2021 Society of Tribologists and Lubrication Engineers (STLE) Early Career Award, 2020 Dean’s Award for Outstanding Engineering Teaching by an Assistant Professor, 2020 Teaching award from the Walker Department of Mechanical Engineering, 2018 Ralph E. Powe Junior Faculty Enhancement Award, and the 2016 Mazzucotelli Award from the Italian Chemical Society.

Authors:

Filippo Mangolini The University of Texas at Austin