Methyl-imidazolium based ionic liquids have good tribological properties, because of their ability to form a chemically reacted film on the surface. This paper mainly compares the tribological properties of two ionic liquids, namely, the chloride-based imidazolium ionic liquid (IL1) and the tetrafluoroborate-based methyl-imidazolium ionic liquid (IL2). The two ionic liquids were synthesized in the lab and blended with mineral base oil at various proportions, i.e. 0.5 %, 1 % and 1.5 % by weight. The tribological properties of the oil samples were tested for 1hr on a reciprocating wear testing machine using a ball-on-flat configuration. The flat is made of hardened AISI 52100 steel, while the bearing ball made of the same material serves as the counter facing surface. The results show that IL2 exhibited a nearly 40% smaller friction coefficient than IL1 at 100°C; the IL1 was found to corrode the steel specimens at 100°C; the AFM images showed the formation of chemically reacted tribofilms on the surface of samples tested with IL2; SEM and EDS results proved the presence of chlorine, boron, and fluorine on the respective wear tracks.
The chemical and physical characteristics of ionic liquids (IL) suggest that ILs when added as additives in lubricants offer better tribological properties than other conventional additives. However, the use of ionic liquids as such as a lubricant is not feasible. Phosphonium based ILs exhibit high miscibility and has been proved to be less corrosive when used as additives in small concentration. This study focuses on the behavior of Trihexyltetradecyl phosphonium bis (2,4,4-trimethylpentyl) phosphonate when blended with mineral base oil and engine oil. A comparison of its performance when used with ZDDP in diesel engine lubricant is also included. Tribological tests were carried out in a reciprocating wear test setup on AISI 52100 steel surfaces with a ball-on-flat geometry. Atomic force microscopy of the worn surfaces revealed the formation of a stable film with 3 % IL and 1 % ZDDP blend in base oil. Surface films formed with 1 % and 5 % IL and 1 % ZDDP was observed to be severely worn due to unstable film formation and corrosive nature of the IL at 5% concentration.
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