Introduction
Ionic liquids (ILs) are molten salts containing organic or inorganic cations and anions which have lower boiling point than water (Battez et al., 2016; Khan et al., 2018; Khemchandani et al., 2014; Matczak et al., 2018). Their unique features such as high thermal stability, broad liquid range, low vapour pressure and non-flammability makes ionic liquids as versatile molecules for lubrication applications (Jimeenez and Bermudez, 2008; Pensado et al., 2008; Qu et al., 2006; Somers et al., 2013; Stolte et al., 2012; Yu et al., 2008). ILs offers some distinctive advantages in reducing friction and wear between surfaces in direct contact (Ichiro, 2009; Zhou et al., 2009). The ILs composed with ammonium, phosphonium, pyrrolidinium and imidazolium cations along with tetrafluoroborate, tosylate, halide and hexafluorophosphate anions were evaluated as lubricants and their interaction with metal surfaces was studied (Minami et al., 2010; Qu et al., 2006; Somers et al., 2012; Tiago et al., 2015; Zhou et al., 2009). These studies proved that the ILs act as neat lubricants and also as additives to the basestocks. It is believed that due to their inherent polarity ILs strongly adsorb on metal surfaces to form tribochemical film, this film enhances antiwear and antifriction performance (Liu et al., 2006; Mendonca et al., 2012; Mu et al., 2008; Qu et al., 2012; Ye et al., 2001). However, there is a limitation for using ILs as neat lubricants due to their higher cost and multistep reaction procedure involved in their synthesis compared to traditional base oils (Barnhill et al., 2014; Khan et al., 2018; Zhou et al., 2014) . However, with a minimum weight percent of ILs in mineral oil and other synthetic oils performed better than the neat base oils (Amiril et al., 2017; Ma et al., 2019; Otero et al., 2014). Batteze et al. detected the film forming capacity of phosphonium cation-based ILs at low concentrations in mineral oil and also evaluated the effect of IL concentration on film forming capacity and coefficient of friction under different test conditions. The choice of changing the combination of cation and anion is possible in ILs to attain specific properties (Ma et al., 2019; Mordukhovich et al., 2013; Salih et al., 2011). Recently some of the ILs gained much interest as lubricant additives such as dicationic bis(ammonium) and bis(imidazolium)-di[bis(salicylato)borate] ionic liquids (Gusain et al., 2014), phosphonium-alkylphosphate (Qu et al., 2015) and N,N’-dialkylimidazolium (Pejakovic et al., 2016) etc. These ILs when used as additives to lubricating oil remarkably enhanced its antiwear and antifriction properties. Zhang et al. (2017), studied the synergistic effect of polymeric IL additives with conventional additive ZDDP and found best synergism with 1:1 w/w of ZDDP and boron-containing polyisobutylene-based IL. Compared to ZDDP ILs are ash less and have stronger adsorption towards metallic surface, thus they provide better lubricity performance with low engine deposits (Mendonca et al., 2012; Oulego et al., 2018).
Currently, most of the ionic liquid based additives used for the tribological applications were synthesized with anions such as the halide [Fˉ, Cl‾, Br‾ or I‾], phosphate and sulfate these ILs exhaust high amount of toxic components to the environment. The ILs consisting of [PF6]‾ and [BF4]‾ shows severe corrosion effect due to the formation of hydrofluoric acid by vigorous reaction with water (Stolte et al., 2012; Swatloski et al., 2003). Kronberger et al. (2012), investigated the eco toxicity and bio degradability of lubricant additives with pyrrolidinium and quaternary ammonium based cations combined with CH3O4S‾, CH3O3S‾ and (CF3SO2)2N‾ as counter ions. The anion (CF3SO2)2N‾ being highly resistant to biodegradation and cannot be considered for applications with potentially high environmental exposure. Conventional IL additives containing halide, sulphate and PF6 anions are replaced with environmentally benign components like fatty acids and amino acids. Biodegradable and bio compatible behaviour of amino acid and fatty acid anions are alternatives to the hazardous anions. Liwen et al. synthesized two green ILs of choline with amino acids glycene and proline, these two ILs [choline][glycine] and [choline][L-proline] significantly improved the tribological properties of base oil (Mu et al., 2015). Minami et al. (2012), stated that the both tribological and thermo-oxidative stability depend on hydrophobicity of IL anion, as the hydrophobicity of anionic part increases results in improved performance. In recent years, the fatty acid derived ILs has been explored as environmentally benign lubricating oil additives. Khatri et al. (2018), reported the synthesis of tetramethylguanidinium ionic liquids with various fatty acid anions for steel/steel contact in grade I mineral oil. Long alkyl chain constituted ILs significantly reduced the friction and wear. The present work describes the synthesis of ricinoleate anion based ILs containing various cations and evaluation of their tribological performance by blending in base oil epoxy karanja 2-ethylhexyl esters (EKE) and dioctyl sebacate (DOS). The formation of tribofilms on surface with base oil and additive was investigated by SEM and EDX analysis was used to establish the elemental composition on the worn surfaces.