## 3 Results and Discussion
### Stage 2: Effect of Additives in P-SiSO  
The friction traces of P-SiSO, PFPE, and the additivated P-SiSO samples evaluated at the conditions C(150 N/25 \({}^{\circ}\)C/30 min) are shown in Figure 7. Two repeated tests are shown for each lubricant sample. As seen in Figure 7a, the P-SiSO in neat form, as well as with all evaluated additive combinations performed significantly better than the reference PFPE in terms of friction. In Figure 7b, the vertical axis has been rescaled to focus on the additives. A trend of additive effectiveness can be discerned; the Oleylamine (OA) tends to increase friction in the two repeated tests; however, the other additives produced a slight decrease in average friction over the two repeated tests. The order of average friction coefficient is listed in the legend of Figure 7b. In Figure 7c, \(W_{Hz}\) was plotted against the average friction coefficient. The inset figures (also found in SI) display the wear scars of the balls lubricated with neat P-SiSO, reference PFPE, and P-SiSO + PNA + OPP, which was the additive combination that produced the lowest coefficient of friction. By comparing the wear scars, it was also clear that all P-SiSO samples performed significantly better than the reference PFPE fluid in terms of wear, which was in line with our previous results using similar RTILs [24]. Although the additives appear to influence the wear scar morphology (as seen from the change in wear scar light reflection in Figure 7c), it was difficult to quantify the change in tribological performance in terms of wear with high accuracy. The reason for this is that at these conditions, the neat P-SiSO displays \(W_{Hz}=1.11\), which is already close to the theoretically minimum value of \(W_{Hz}=1\), as given in Equation (1), and it is therefore difficult to improve upon. Figure 6: Analysis of disc surfaces from V[300 N/40–80 \({}^{\circ}\)C /60 min]. **(a)** and **(b)** display SEM images showing the tribofilms on the wear tracks for tests #2 and #1 respectively. **(c)** EDS spectra of representative regions of the wear scars shown in **(a)** and **(b)**. The results from the tribotests carried out at V[300 N/40-80 \({}^{\circ}\)C /60 min] and lubricated by the seven additivated samples are shown in Figure 8. In Figure 8a, all additives except OPP improved performance at 80 \({}^{\circ}\)C compared to the neat fluid. Figure 8b, shows repeated tests for best and worst performers from Figure 8a, i.e., P-SiSO with PNA or OPP. The repeated tests confirmed the poor performance of OPP and excellent performance of PNA. Figure 8: Results from the V[300 N/40–80 \({}^{\circ}\)C /60 min] tests. (**a**) Different additives in P-SiSO; and (**b**) repeated tests of best and worst performers. Figure 7: Results from the experiments at C[150 N/25 \({}^{\circ}\)C /30 min]. (**a**) Friction traces for the reference PFPE and P-SiSO with and without additives; (**b**) magnified friction traces of P-SiSO with and without additives; and (**c**) \(\mu\)-W chart with insets of ball wear scars for PFPE, neat P-SiSO and P-SiSO with PNA + OPP additives, respectively. The results from the harshest conditions, C(300 N/80 \({}^{\circ}\)C /30 min) are shown in Figure 9. As seen in Figure 9a, The neat P-SiSO generated steady friction traces around 0.125, which was similar to what could be seen after the wear mode transition of P-SiSO Neat test #1 and #3 at conditions V(300 N/40-80 \({}^{\circ}\)C/60 min). Additionally, after 30 min of operating at this wear mode, \(W_{Hz}=2.0\), which was almost identical to the measured wear of P-SiSO Neat test #3, which only operated for 5 min at the higher wear mode. This indicates that there was a high wear related to the transitioning between these two wear modes, and that the wear rate stabilized at a low rate after the transition. The additive combination PNA + OPP was not effective at these conditions, as it performed very similarly to the neat P-SiSO. The amine PNA was expected to reduce the effect of OPP by increasing its solubility [25].