## 3 Results and Discussion
By calculating the pressure-viscosity coefficient of the lubricants at the measured temperatures by use of an analytical film thickness equation gives pressure-viscosity values for the IL that is significantly lower than the paraffin. The values are presented in Table 5. It should be kept in mind that assessment of pressure-viscosity coefficients from film thickness measurements may not be the most precise method, and could also be erroneous should inlet shear thinning occur [49; 50]. However, the pressure-viscosity values presented in Table 3 that are based on the falling cylinder viscometer results at high pressure confirms that the [Choline] [Proline] has a much lower pressure-viscosity than conventional hydrocarbon oils and that this is the reason to the comparatively low film thickness results shown in Figure 6.  
Figure 5: Water content increase in [Choline] [Proline] at relative humidity of 15%. The results from the friction measurements at the lower pressure of 1.25 GPa are shown in Figure 7. The friction coefficient for the IL is substantially lower than the friction coefficient for the paraffin oil at both entrainment speeds. In Figure 8 the results from the friction measurements performed at 1.95 GPa of pressure are shown. Even here the friction coefficient for the IL is considerable lower than the friction coefficient for the paraffin oil at both entrainment speeds and both pressure levels. \begin{table}
\begin{tabular}{l c c} \hline \hline
**Lubricant** & **Paraffin** & **Ionic Liquid** \\ \hline Refractive index & 1.47 & 1.46 \\ Dynamic viscosity @ 25 \({}^{\circ}\)C [mPas] & 93 & 227 \\ Dynamic viscosity @ 40 \({}^{\circ}\)C [mPas] & 46 & 92 \\ Dynamic viscosity @ 60 \({}^{\circ}\)C [mPas] & 21 & 37 \\ \(\alpha^{*}\)@ 25 \({}^{\circ}\)C [GPa\({}^{-1}\)] & 20.2 & 11.2 \\ \(\alpha^{*}\)@ 60 \({}^{\circ}\)C [GPa\({}^{-1}\)] & 17.6 & 9.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Data for test lubricants. Figure 6: Central film thickness measurements for Paraffin and [Choline] [Proline] at 25 and 60 \({}^{\circ}\)C at 0.5 GPa of pressure. [Choline] [Proline] had a water content of slightly below 15 wt %. Figure 7: Friction measurements for Paraffin and [Choline] [Proline] at 40 \({}^{\circ}\)C and 1.25 GPa. [Choline] [Proline] had a water content of slightly below 15 wt %. The Hamrock & Dowson central and minimum film thickness was calculated for the conditions of the friction tests with the lubricant parameters given in Table 5, and pressure-viscosity coefficients linearly interpolated to 19.04 GPa\({}^{-1}\) for paraffin and 10.28 GPa\({}^{-1}\) for the IL. The results are shown in Table 6.  
Evidently, all tests were performed in full film lubrication since the minimum film thickness for both lubricants at all combinations of speeds and pressures were significantly higher than the combined roughness of the specimens. It is however remarkable that the difference in friction between the IL and the paraffin is significant while the difference in film thickness is not that pronounced. At the pressure of 1.25 GPa and 1 m/s in entrainment speed, the friction coefficient is up to 70% lower for the IL compared to the paraffin oil. At the same entrainment speed, the calculated difference in film thickness is approximately 11%. Another hygroscopic lubricant that generated low friction in full film elastohydrodynamic lubrication is glycerol and its aqueous solutions [51]. Here, it was seen that increasing water content would reduce friction up to a certain level where the friction coefficient started to increase with even higher water content. It was hypothesized that the increase in water content would reduce viscosity in the high pressure region of the contact and this reduce friction. At the same time, increased water content would also reduce film thickness due to lower viscosity in the inlet region of the contact. This would lead to higher risk of asperity interactions and thus increase friction when the film is no longer thick enough to fully separate the surfaces.