## 3 Results and Discussions
### Lubricity of PIL as a Neat Lubricant
Interestingly, these areas are located where the highest contact pressure is applied. EDS analysis shows that lighter areas have fewer tribo-film-forming elements, namely oxygen, carbon, and nitrogen (Table 3). At a temperature of 80 \({}^{\circ}\)C, the wear trace patterns have changed (Figure 9c). Moreover, according to its cross-section profile, it has the same size throughout its length. There are also a few minor scratches in the wear trace. According to the wear volume, the higher test temperature resulted in increased wear, which is not in line with the friction results. However, it may be that the majority of wear occurred during the running-in period. Therefore, this could explain why a high coefficient of friction was observed at the onset of the tribo-test. Figure 10 presents high-magnification SEM images of wear traces. It is evident from these images that the wear trace formed during the tribo-test at 30 \({}^{\circ}\)C is smoother and contains only short, tiny scratches, while at the temperature of 80 \({}^{\circ}\)C, more significant scratches occur. In both cases, the pushed-out material at the sides of the wear trace is seen, which was also evident in the cross-section profiles Pe, Pm, and P (Figure 9). According to EDS analysis, the worn surfaces after the tribo-test at 80 \({}^{\circ}\)C have similar amounts of active materials as the lighter areas in the tribo-test performed at 30 \({}^{\circ}\)C (Table 3). It suggests that the higher temperature induces tribo-film formation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Elements of** **Interest**} & \multicolumn{2}{c}{**After the Tribo-Test at 30 \({}^{\circ}\)C**} & \multicolumn{2}{c}{**After the**} & \multicolumn{1}{c}{**Outside the Wear**} \\ \cline{2-5}  & **Lighter Areas** & **Dark Areas** & **Tribo-Test at 80 \({}^{\circ}\)C** & **Trace** \\ \hline C & 0.58 & 0.69 & 0.39 & 0.18 \\ O & 0.02 & 0.18 & 0.05 & 0.01 \\ N & 4.00 & 4.78 & 3.80 & 6.74 \\ Fe, Cr, Al, Si & & Balance & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Atomic [%] composition of the wear traces on the plate. Figure 10: SEM images of segments of wear traces on the plate formed during the IL-DEE tribo-test performed at the temperatures of 30 \({}^{\circ}\)C (**a**) and 80 \({}^{\circ}\)C (**b**). Arrows indicate scratches, lines—boundaries of the wear trace. The heat in the frictional contact can be externally applied or occur due to the friction process. In the case of the tribo-test at the temperature of 80 \({}^{\circ}\)C, the specimens were externally heated and from the friction. On the other hand, during the low-temperature test, the heat was only generated due to friction. Consequently, the lighter areas in the wear trace (Figure 9a) were formed where the highest contact pressure occurs. Therefore, these areas possess similar composition. It must be noted that the entire plate surface, even outside the wear trace, also contains nitrogen. It means that ionic liquid is adsorbed on the surfaces without tribo-reactions. Similar adsorption phenomena were confirmed by [PERSON] et al., who studied ammonium-based ionic liquids [30]. The appearance of the wear scars on the ball is very similar for both test temperatures (Figure 9b,d). However, the measurements show that a slightly larger wear scar was formed at the lower test temperature, which is in contrast to the wear volume (Table 2). The reason behind that could be the width of the plate's wear trace, which varies throughout its length. Therefore, the wear scar on the ball represents the plate's widest section of wear trace. The observed wear volume was also compared with oleic acid anion containing PIL. It was found that erucic acid containing PIL had better wear reduction ability at both temperatures of interest (Table 2). The reason behind this could be a thicker adsorption layer that erucic acid with a longer alkyl chain could provide.