**1.2. Ionic liquids**

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].

for different metalworking applications [20]. Halogen-containing anions such as [PF6] or [BF4] contain rich fluorine compound that is moisture-sensitive [20, 40]. They can cause corrosion on the steel surface under humid conditions, thus may release toxic and corrosive hydrogen halides to the environment. In this work, a biocompatible with low-toxic level of phosphoniumbased IL, trihexyl (tetradecyl) phosphonium bis (2,4,4-trimethylpentyl) phosphinate, [P66614] [(iC8)2PO2] (PIL) was investigated as an oil-miscible IL in polar base vegetable-based lubricants. The application of 1 wt. % of PIL is anticipated to be adequately sufficient in improving the lubrication performance of the base oil when used on the metal sliding surfaces [30, 31].

Tribological Interaction of Bio-Based Metalworking Fluids in Machining Process

http://dx.doi.org/10.5772/intechopen.72511

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The crude vegetable oils were modified to enhance certain limitations such as low thermal and oxidative stability due to unsaturation in oil molecule. There are various methods of modification that have been identified by Shashidhara and Jayaram [41] which included reformulation of additives, chemical modification, and genetic modification of oilseed. Prior to this experiment, fatty acid methyl esters (FAMEs) from Jatropha oil and RBD palm olein were chemically modified through transesterification process to develop modified Jatropha oil (MJO) and modified RBD palm olein (MRPO) [18, 31]. They are the product of the transesterification process (ester) between FAMEs from the vegetable oils with a polyol of trimethylolpropane (TMP) and better known as the TMP triester. After the transesterification process, the absence of hydrogen atom at carbon-β in the structure of the ester oil has enhanced the thermal and oxidative stability [42]. Both MJO and MRPO were mixed with two different types of additives; hexagonal boron nitride (hBN) and phosphonium-based ionic liquid (PIL). A small amount of 0.05 wt. % of hBN particles were blended in the base oils using a magnetic follower at a temperature of 60°C for 30 min. Meanwhile, 1 wt. % of PIL was heated at 70°C to reduce its viscosity prior to the mixing procedure with the base oils. Next, the preheated PIL was poured into the base oil and heated at 60°C and stirred rigorously by using the magnetic follower for 30 min. The modified oils were compared with a commercial synthetic ester (SE, Unicut Jinen MQL) as a reference oil.

The rheological properties were determined through kinematic viscosity (ASTM D445) and viscosity index, VI (ASTMD2270). The kinematic viscosity was measured using a viscometer at 40 and 100°C. It was calculated from the ratio of dynamic viscosity over density at the same temperature. The correlation between viscosity and temperature was further associated with

Tapping torque tests (ASTM D5619) were carried out on a CNC machine, installed with a tapping torque set up, as shown in **Figure 1**. The tests were conducted using AISI 1215 cylindrical low carbon steels at the machining speed of 400 rpm as shown in **Table 1**. The workpieces

VI. The testing was repeated for three times and the average value was recorded.

**2. Methodology**

**2.1. Lubricant preparation**

**2.2. Rheological properties**

**2.3. Tapping torque test**

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

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 additive of a 0.5 wt. % of [BMIM-PF<sup>6</sup> ] IL when cutting titanium round bars using MQL system [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 ILs ([BMIM]<sup>+</sup> with [PF<sup>6</sup> ] − & [BF4 ] − ) have significantly affected the tribological conditions of the milling process by reducing the peak and mean machining forces in finish as well as rough machining conditions.

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

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 for different metalworking applications [20]. Halogen-containing anions such as [PF6] or [BF4] contain rich fluorine compound that is moisture-sensitive [20, 40]. They can cause corrosion on the steel surface under humid conditions, thus may release toxic and corrosive hydrogen halides to the environment. In this work, a biocompatible with low-toxic level of phosphoniumbased IL, trihexyl (tetradecyl) phosphonium bis (2,4,4-trimethylpentyl) phosphinate, [P66614] [(iC8)2PO2] (PIL) was investigated as an oil-miscible IL in polar base vegetable-based lubricants. The application of 1 wt. % of PIL is anticipated to be adequately sufficient in improving the lubrication performance of the base oil when used on the metal sliding surfaces [30, 31].
