**2. Methodology**

**1.2. Ionic liquids**

48 Lubrication - Tribology, Lubricants and Additives

and oxidative stability, pour point) [20, 31].

tive of a 0.5 wt. % of [BMIM-PF<sup>6</sup>

with [PF<sup>6</sup>

] − & [BF4 ] −

using lubricant mixtures on different material sliding pairs [36–39].

ILs ([BMIM]<sup>+</sup>

machining conditions.

Ionic liquids are chemical compounds composed of cations and anions that have melting points lower than 100°C. The cations are usually organic compounds such as nitrogen or phosphorus, and the anions are the weakly coordinating compounds like bis(trifluoromethylsulfonyl)imide or hexafluorophosphate [19–21]. For many years since the first ionic liquid (IL) was reported in 1982, the research and development of ILs have been rapidly evolving in the research works and in various industrial applications [19, 21–24]. IL lubricants are found in lubricant industries as neat lubricant or lubricant additives for various mechanical lubrication purposes [20, 25, 26]. ILs exhibit remarkable properties such as nonflammable, nonvolatile, low melting point, high thermal stability, highly miscible with organic compound, and better intrinsic properties [27, 28]. Use of ILs as lubricant additives may eliminate further requirements of using detergents, defoamers, antioxidants, or even antiwear and antifriction additives in enhancing the performance of the conventional lubricant in current additive formulation processes [27–29]. Thanks to the abovementioned advanced characteristics of ILs, they have been proven to not only improve the tribological properties (friction and wear) [28–30] of different polar and nonpolar base oils, but also enhanced their physicochemical properties (viscosity, thermal

Pham et al. in 2014 examined the effect of two imidazolium-based ILs ([EMIM] [TFSI] & [BMIM] [I]) as neat lubricants in micro end milling [32]. They suggested the potential use of these ILs as green lubricants that exhibit extremely low volatile organic compounds as well as for the use in MQL systems. A study by Davis et al. in 2015 uses water-based lubricant with an addi-

[33]. They found out that the lubricant mixture has effectively reduced the tool wear by 60% when compared to dry cutting and 15% more than MQL without the IL. Goindi et al. [34] have recently proposed the use of imidazolium-based ILs with two different anions in minute quantity being mixed in a canola vegetable oil during orthogonal milling of a plain medium carbon steel via MQL method. They reported that the small quantities of the two imidazolium-based

milling process by reducing the peak and mean machining forces in finish as well as rough

Somers et al. [35] tested the application of various imidazolium-, phosphonium-, and pyrrolidinium-based ILs as lubricant additives in different polar and nonpolar base oils including vegetable oil, polyolesters, mineral oil, and polyalphaolefin and found that the miscibility of ILs in these base oils depends highly on the molecular structures of the ILs used. High miscibility in both polar and nonpolar base oils is apparent for ILs that comprise quaternary structures with relatively long hydrocarbon chains of the cations and anions [20, 30]. Several recent studies have confirmed this finding and provided reports on their tribological investigations

To date, tailor-made ILs investigated for the application as lubricants and/or lubricant additives have known to play an important role in enhancing tribological interactions between sliding materials. The application of IL-based MQL machining may be explored for other nontoxic, fully miscible, and biocompatible ILs as neat as well as lubricant additives in various base oils

] IL when cutting titanium round bars using MQL system

) have significantly affected the tribological conditions of the
