## 3 Results
### Characterization of Lubricants
#### 3.1.2 Chemical Characterization of Lubricants  
The nature of these lubricants is presented by the ATR-FTIR study, as follows. Figure 2 shows the ATR-FTIR spectra of the water-based lubricants (L1, L2, and L3). As it can be observed from this figure, the base spectra of the three lubricants show traditional large and intense bands of water. A strong and broad peak at around 3300 cm\({}^{-1}\) due to the stretching vibrations of O-H, an intense peak at around 1630 cm\({}^{-1}\) related to the O-H-O scissors bending vibrations, and a small band at 2120 cm\({}^{-1}\) resulting from the coupling of the scissors bending signal and a broad liberation band can be observed in these spectra [21]. Despite these strong bands of water, it is possible to identify in this figure that the bands in the saline water (L2) spectrum have shifted slightly to the left, likely due to the presence of salts in water. On the other hand, the L3 spectrum shows additional bands at around 1394 cm\({}^{-1}\) and 1060 cm\({}^{-1}\) assigned to the C-OH and C-O stretching vibrations, respectively, which, along with others, overlap the bands at about 3410, 1720, and 1620 cm\({}^{-1}\), correspond to the nature of graphite [22; 23]. The ATR-FTIR spectra of the synthetic (L4) and mineral oils (L5) are shown in Figure 3. It can be seen that the spectrum of L4 exhibits the signals of polydimethylsiloxane. It has small peaks at about 2963 and 1410 cm\({}^{-1}\) due to asymmetric stretching and bending vibrations of the CH\({}_{3}\) group, an intense peak at 1259 cm\({}^{-1}\) associated to the CH\({}_{3}\) symmetric bending vibration in Si(CH\({}_{3}\))\({}_{2}\), a strong band between 1100 and 1000 cm\({}^{-1}\) as a result of Si-O stretching vibrations in Si-O-Si, and peaks at 865 and 784 cm\({}^{-1}\) attributed to the rocking vibration of CH\({}_{3}\) around Si-C and the stretching vibration of Si-C, respectively [24; 25]. As the signal located between 1100 and 1000 cm\({}^{-1}\) unfolds into two components, it is inferred that the oil is of a long-chain length and, therefore, a high molecular weight, which was reflected in its high viscosity [24]. On the other hand, the L5 spectrum shows strong bands at around 2922, 2851, 1459, and 1374 cm\({}^{-1}\) corresponding to the stretching and bending vibrations of hydrocarbons CH\({}_{2}\) and CH\({}_{3}\), and a small peak at 721 cm\({}^{-1}\) caused by the vibration of CH\({}_{2}\) that is characteristic of the degree of the refining process of a mineral oil [26]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Property at 25 \({}^{\circ}\)C**} & \multicolumn{8}{c}{**Lubricant**} \\ \cline{2-10}  & **L1** & **L2** & **L3** & **L4** & **L5** & **L6** & **L7** & **L8** \\ \hline \(\rho\) (Kg/m\({}^{3}\)) & 997 & 1027 & 1035 & 976 & 914 & 1126 & 1159 & 961 \\ \(\mu\) (\(\times 10^{-3}\) Pa-s) & 0.89 & 0.98 & 53 & 1093 & 260 & 798 & 883 & 691 \\ \(\upsilon\) (\(\times 10^{-6}\) m\({}^{2}\)/s) & 0.89 & 0.95 & 51 & 1120 & 285 & 709 & 762 & 719 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mass density (\(\rho\)), dynamic viscosity (\(\mu\)), and kinematic viscosity (\(\upsilon\)) of lubricants. Figure 2: Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR) spectra of water-based lubricants. The ATR-FTIR spectra of the natural-based lubricants (L6, L7, and L8) are shown in Figure 4. Natural oils are triglyceride esters of fatty acids, and this triglyceride nature can be corroborated by the intense bands at around 1743 cm\({}^{-1}\) that is the result of the stretching vibration of the ester group C = O. These spectra also show strong signals at 2924 and 2855 cm\({}^{-1}\) corresponding to CH\({}_{2}\) asymmetric and symmetric stretching vibrations, respectively. Additionally, each lubricant exhibits a different fingerprint region due to the type of natural oil and the other components present in the lubricants.