Comparing the Tribological Properties of Chloride-Based and Tetra Fluoroborate-Based Ionic Liquids Shirke Saurabh Dyaneshwar 1 , Shah Aditya Manoj 1 , Dulange Amit Gangadhar 1 , Kamlesh Rudreshwar Balinge 2 , Anil Payyappalli Mana 1* , Pundlik Rambhau Bhagat 3 1 School of Mechanical Engineering, Vellore Institute of Technology, Vellore 632014, India 2 Department of Science and Humanities, Saveetha School of Engineering, Chennai 602107, India 3 Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Vellore 632014, India Corresponding Author Email: pmanil@vit.ac.in https://doi.org/10.18280/acsm.430507 ABSTRACT Received: 16 July 2019 Accepted: 27 August 2019 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. Keywords: tribological properties, friction, lubricant, ionic liquids, surface characterization 1. INTRODUCTION The limitations of conventional lubricants are that they are suitable for one particular pair of materials. It may not work for another pair. Also, the various kinds of additives added in fully formulated oils may not be compatible with all types of materials [1]. Whereas ionic liquids are not so when added as additives. Ionic liquids are positively and negatively charged ions of compounds. They are neither acidic nor basic. Most of these compounds are in the liquid state. However semi-solid ionic liquids also exist. The major characteristics of ionic liquids are low volatility, low melting point, non-flammability, thermal stability and good thermal conductivity. The major disadvantage of using phosphonium and tungstate based ILs are that they corrode the metal at high concentration so the amount of the ionic liquid in lubricating oil should be controlled in such a way that they do not cause any harm to the surfaces in contact [1]. The first article that brought about the use of ionic liquids as lubricants was published in the year 1961 [2]. A mixed salt of LiF, BeF2, and UF4 melted at 460 °C was tested at 650- 815 °C. However, its use was reported as a nonconventional class of solvents [3-4]. Room temperature ionic liquids were still not explored as lubricants until the year 2001. In 2001, Ye et al. [5], reported that alkyl imidazolium tetrafluoroborates at room temperatures have good antiwear properties and exhibited a low coefficient of friction when tested with various material pairs such as steel/steel, steel/ aluminum etc. [5]. Imidazolium-based ionic liquids have been the most investigated for their lubricating properties [5-12]. The superior tribological performance of ionic liquids is attributed to the dipolar structure and their ability in adsorbing on to the surface of the materials and subsequent formation of an antiwear film [5]. Even though some of the hydrophilic ionic liquids have been proved to be corrosive against steel [13-19], ionic liquids can be subjected to very severe load conditions [5, 7] and exhibit very high thermal decomposition temperatures [7]. A comparative study was reported between tribological properties of imidazolium-based ionic liquids under steadily advancing loads in alloys that were applied with ionic liquids versus uncoated alloys. The results proved that the alloys not coated with ILs did not show good wear resistance. The SEM analysis also showed that there was the formation of a phosphate-based reaction film which reduced the friction [9]. Ionic liquids applied with two layers on a silicon substrate; among them, one layer acting as an anchor layer proved to have better tribological properties than applied as a single layer [10]. Furthermore, carbon chain length on the imidazole ring of certain halogen-free ionic liquids was reported to influence the tribological properties when used in steel-copper contacts [11]. Load carrying capacity has been proved to be much better than conventional ZDDP based lubricants [7]. Environmental friendly ionic liquids that are free from halogens, phosphorous and sulfur have also been proved to have good tribological properties. These bis borate anion based and imidazolium/ammonium cation ionic liquids exhibited high viscosity, hydrophobicity and good miscibility with base stocks [20]. But Pyridinium based ionic liquids with methyl sulfate anions have been found to be highly corrosive. Dicationic structures exhibited better performance compared to mono cationic structures [21]. Much interest was observed among the researchers in Annales de Chimie - Science des Matériaux Vol. 43, No. 5, October, 2019, pp. 317-327 Journal homepage: http://iieta.org/journals/acsm 317