Surface chemistry of fluorine containing ionic liquids on steel substrates at elevated temperature using Mössbauer spectroscopy

Ionic liquids have properties that make them attractive as solvents for many chemical synthesis and catalysis reactions. Consequently, research has focused on their application as advanced solvents. Recently, ionic liquids were shown to have promise as a lubricant due to many of the same properties that make them useful as solvents. The focus of this paper is to study the surface chemistry of ionic liquid lubricated steel in sliding contact to temperatures from room to 300 °C. Tribological properties were evaluated using a pin on disk tribometer with high temperature capability (up to 800 °C). Chemistry was studied using Mössbauer spectroscopy and X-ray photoelectron spectroscopy. Samples used for tribological evaluation were 1 inch diameter polished M50 disks. Samples used for studying the surface chemistry were enriched ⁵⁷Fe grown via thermal evaporation. Some ⁵⁷Fe samples were oxidized to Fe₂O₃ and Fe₃O₄ prior to treatment with ionic liquids. The metallic and oxidized ⁵⁷Fe samples were then reacted with ionic liquids at elevated temperatures. Three ionic liquids were used in this study; 1-n-ethyl-3-methylimidazolium tetrafluoroborate (BF₄), 1,2-di-methyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide (TFMS), and 1,2-di-methyl-3-butylimidazolium hexafluorophosphate (PF₆). This study was focused on understanding the high temperature stability of the liquids in contact with metal and under tribological stress. Therefore, the friction data was collected in the boundary (or mixed boundary/EHL) lubrication region to enhance surface contact. BF₄ provided a friction coefficient of 0.04 for both the room and 100 °C tests and varied between 0.07 and 0.2 for the 300 °C test. The results from TFMS lubrication showed a friction coefficient of 0.025 at room temperature and 0.1 at 100 °C. The 300 °C test friction coefficient ranged between 0.1 and 0.3. Chemical analysis of the surface revealed corrosion of the surface due to reaction between the ionic liquids and steel/iron substrates.