## 3 Results and Discussion
### Stage 1: Limits of Operational Envelope of Neat P-SiSO
Using low voltage (5 keV) EDS, the dark patches seen in Figure 4c was determined to contain Si and O, as seen in Figure 4d,e, respectively. Contrary to our initial expectation, P or S was not detected on this surface. Both P and S are commonly used in anti-wear or extreme pressure additives, and P is frequently reported as an active element in anti-wear  
Figure 2: Results from tribotests at conditions V[100–300 N/25 \({}^{\circ}\)C/120 min]. The friction traces are shown for two repeated tests with neat P-SiSO, compared to one test with PFPE and one test of GL-5. The wear scars corresponding to the friction traces are shown as color-coded insets in the right part of the figure. films of ionic liquids containing phosphorus [18; 31; 32; 33]. Instead, elevated levels of Si and O were found in the wear track. As seen in Figure 4c-e, Si and O are particularly associated with the dark patches (tribofilm) on the worn surface. Figure 5 shows the results for three repeated tribotests with neat P-SiSO at the conditions V[300 N/40-80 \({}^{\circ}\)C/60 min]. As can be seen in Figure 5a, the friction trace was steady at lower temperatures (averaging 0.89 at 40 \({}^{\circ}\)C), which was expected considering the results presented in Figure 2. However, an abrupt change in friction characteristics was observed at the highest test temperature of 80 \({}^{\circ}\)C. For tests #1 and #3, a step increase in friction occurred after 50 and 55 min, respectively. From the data in Figure 5a, test #2 appeared to maintain the low friction characteristics  
Figure 4: Disc surface after tribotesting at V[150 N/40-100 \({}^{\circ}\)C/80 min], seen from (**a**) optical profilometer; and (**b**) SEM. (**a**) the roughness inside wear scar can be seen to be of the same order of magnitude as the original lapped surface; (**b**) dark patches in the wear track indicate tribofilm; (**c**) SEM (scanning electron microscopy) image of a representative area of tribofilm at 5000x magnification; (**d,e**) EDS (energy dispersive X-ray spectroscopy) maps showing the distribution of the elements in the area shown in (**c**). Figure 3: Results of V[150 N/40–100 \({}^{\circ}\)C/80 min] for two repeated tests with neat P-SiSO. Ball wear scars are shown as insets to the right. throughout the test. However, each data point (Figure 5a) is the averaged friction coefficient over 15 s, and examining the friction trace at a higher resolution (15.4 Hz) (Figure 5b) revealed that test #2 also displayed a distinct friction spike during the final minute of the test, indicating a partial seizure. The corresponding ball wear scars are displayed in the insets of Figure 5a and the resulting wear parameters were determined as \(W_{Hz}\) = 1.96, 1.30, and 1.97, respectively. All wear scars showed significantly different wear scar morphologies compared to previous tests where low friction was maintained throughout: these ball surfaces were clearly abraded and showed higher than average surface roughness compared to P-SiSO ball wear scars shown in Figures 2 and 3. It should be noted that after the abrupt shift in friction characteristics, the friction coefficient quickly stabilized around 0.125, which indicates that the shift was not a complete loss of lubrication followed by seizure, but rather a transition to a less efficient, but seemingly stable lubrication mode. Figure 6 shows the SEM images and EDS spectra of the worn disc surfaces from the tribotest with neat P-SiSO at the conditions V[300 N/40-80 \({}^{\circ}\)C/60 min]. The combined SEM and EDS analysis revealed that the less worn surface, P-SiSO Neat test #2 (Figure 6a) has a composition similar to what was previously seen for the test run at V(150 N/40-100 \({}^{\circ}\)C/80 min); Si and O are found in the tribofilm; and in this case also S (Figure 6c). Regarding the more severely worn sample (Figure 6b), the boundary film composition found in the disc wear scar changed and P was clearly detected, together with Si, S, O, and C (Figure 6c). Figure 5: Results from V[300 N/40–80 \({}^{\circ}\)C /60 min].