## 3 Results and Discussions
### Lubricity of PIL as a Neat Lubricant  
To be considered a good lubricant, one must ensure low friction and wear-reduction ability. In the present case, bis (2-hydroxyethyl)ammonium evacate demonstrated excellent lubricity at both temperatures of interest. Figure 8 shows the variation of the coefficient of friction during the test and its mean value. The results show very low and relatively stable friction. At the temperature of 30 \({}^{\circ}\)C, the friction was steady throughout the test period. However, at a higher test temperature, the variation pattern changed. There was a sharp rise at the test's onset, followed by a continuous decline throughout the test. This variation resulted in considerably lower friction observed at a higher test temperature. The reason behind that could be the temperature  
Figure 8: Variations of the coefficient of friction and its mean value observed during the PIL tribo-test. * friction coefficient values observed in our previous study [32]. Figure 7: Kinematic viscosity of pure and 1% of IL-DEE-loaded water as a function of temperature. induced viscosity change. At the lower viscosity, thinner interacting surfaces separating film was formed, resulting in less energy required to move interacting surfaces. A comparison with the results from our previous study was performed to determine the influence of fatty acid anions. In that case, bis (2-hydroxyethyl)ammonium oleate was used as a neat lubricant [32]. The combination of bis(2-hydroxyethyl)ammonium cation with erucic acid shows clear friction improvement compared to oleic acid. Furthermore, the increased chain length reduced the coefficient of friction by approximately 30 and 50% at the test temperatures of 30 and 80 \({}^{\circ}\)C, respectively (Figure 8). Erucis acid has a longer alkyl chain (C 22:1) than oleic acid (C 18:1). Therefore, a thicker and denser adsorbed layer could be formed, achieving better separation of interacting surfaces. The wear traces on the plate and wear scars on the ball are shown in Figure 9. The wear volume and wear scar diameter are listed in Table 2. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Ionic Liquids**} & \multicolumn{2}{c}{**Wear Volume, \(\mu\)m3 \(\times\) 103**} & \multicolumn{2}{c}{**Wear Scar Diameter, \(\mu\)m**} \\ \cline{2-5}  & **30 \({}^{\circ}\)C** & **80 \({}^{\circ}\)C** & **30 \({}^{\circ}\)C** & **80 \({}^{\circ}\)C** \\ \hline IL-DE & 7.52 & 12.21 & 121 & 115 \\ IL-DE & 23.1 * & 163.5 * & n.d. & n.d. \\ \hline \hline \end{tabular}  
\end{table} 
Table 2: Wear volume and wear scar diameter observed in IL-DEE tribe-test using it as a neat lubricant. Figure 9: Optical images and cross-section profiles of wear traces on the plate (**a,c**) and wear scars on the ball (**b,d**) observed after the IL-DEE tribo-test at 30 and 80 \({}^{\circ}\)C, respectively. P, Pm, and Pe are the cross-section profiles of the wear trace measured in particular locations. The arrows point out the lighter areas in the wear trace. The different worn surface morphology was observed during the tribo-test at different temperatures. At a temperature of 30 \({}^{\circ}\)C, the width and depth of the wear trace vary throughout its length--in the middle, it is narrower and shallow, while at both ends, it is deeper and broader. The profiles Pe and Pm shown in Figure 9 represent the cross-sections of the wear trace at the ends and in the middle, respectively. These profiles replicate the shape of the counter body because almost no wear occurs on the ball. Due to plastic deformation, part of the plates' material was pushed to the edges of the wear trace. These appear as uplifts on the sides of the cross-section profile. Of course, this material part is considered when wear volume is calculated. It is evident from these profiles that more intense wear appears at the ends of the wear trace. This phenomenon appears in a reciprocation motion when a high-viscosity lubricant is used. After the tribo-test at 30 \({}^{\circ}\)C, lighter areas were observed in the wear trace (Figure 9a).