## 3 Results and Discussion  
### Stage 1: Limits of Operational Envelope of Neat P-SiSO  
The measured friction coefficient as a function of test duration for neat P-SiSO, the reference lubricant PFPE, as well as a GL-5 fully formulated gear oil (see SI for details) at test conditions V[100-300 N/25 \({}^{\circ}\)C/120 min] are shown in Figure 2. Two test runs of P-SiSO are shown to visualize repeatability. The GL-5 gear oil is seen to follow the typical behavior expected for traditional lubricants \begin{table}
\begin{tabular}{l c} \hline
**Test Stage and Objective** & **Test Code** \\ \hline Stage 1: Determine limit of operational envelope of neat P-SiSO & V[100–300 N/25 \({}^{\circ}\)C/120 min] \\  & V[150 N/40–100 \({}^{\circ}\)C/80 min] \\  & V[300 N/40–80 \({}^{\circ}\)C/60 min] \\ \hline Stage 2: Determine effect of additives in P-SiSO & C[150 N/25 \({}^{\circ}\)C/30 min] \\  & V[300 N/40–80 \({}^{\circ}\)C/60 min] \\  & C[300 N/80 \({}^{\circ}\)C/30 min] \\ \hline \end{tabular}
\end{table} 
Table 1: Test codes describing conditions (load, temperature, duration) corresponding to the different tribotest stages. containing EP and AW additives. EP additives are well-known to provide reduced friction at high load; however, this usually comes at the expense of increased wear [28]. In contrast, the anti-wear additives are normally active at lower loads, where they prevent wear, but generally increase friction [2]. The neat P-SiSO generates a low and steady friction coefficient that decreases with increased load. Furthermore, at the maximum load (corresponding to a Hertzian contact pressure of 3.0 GPa), the friction coefficient stabilizes around 0.90, which was the lowest friction recorded in the test. However, in contrast to the GL-5 lubricant, the neat P-SiSO also resulted in very low wear with \(W_{Hz}<1.15\), compared to 1.75 for GL-5. The PFPE reference fluid on the other hand, generated a high friction-coefficient (ca. 0.16) that deteriorated at the highest load, and the WSD was at the limit of what could be measured with the setup used, \(W_{Hz}>2.6\). This was also in line with the well-known poor boundary lubrication performance by PFPE that has often been seen in similar tests [15; 29; 30]. The results from the tribotest with neat P-SiSO, during conditions V[150 N/40-100 \({}^{\circ}\)C/80 min], are displayed in Figure 3. Two repeated tests are shown to illustrate the repeatability. Again for P-SiSO, friction and wear remained low for the whole duration of the test, and reached a minimum at the final stage. It can be clearly seen that there is an overshoot in temperature at each step increase. However, it should be noted that the temperature is measure in the heated disc support, so the effect in the actual contact can be expected to be lower. Still, an effect can be seen where the friction immediately rises before recovering and stabilizing at a level that is lower than seen before the increased temperature. The increase in temperature is likely to promote boundary film formation by providing energy for a chemical reaction between lubricant and surface. The friction coefficient appeared to level out at around 100 \({}^{\circ}\)C at 0.83. The wear parameter after the completed test was determined as \(W_{Hz}\approx 1.15\). When comparing the ball wear scar morphology to the ones obtained from tests at a higher load but lower temperature (V[100-300 N/25 \({}^{\circ}\)C/120 min]), a slight difference in intensity of the reflected light was seen. Images of the disc surfaces after testing at V[150 N/40-100 \({}^{\circ}\)C/80 min] are shown in Figure 4. Figure 4 was recorded from the optical interferometric profilometer using \(2.5\times\) optical magnification. The wear scar is barely distinguishable, and the measured surface roughness was approximately the same magnitude as the original lapped surface, i.e., around 50 nm \(R_{a}\). In the SEM image (Figure 4b), the wear scar can clearly be detected. When magnification was increased (Figure 4c), dark patches can be seen covering parts of the surface.