# Controllable Friction of an Epoxy Composite via Thermal Treatment
## 3 Results and Discussion
### Friction and Wear Tests of EP Composites  
Figure 5a-f present the tribological performances of various EP composites at RT or 80 \({}^{\circ}\)C (the mass fraction of microcapsules or PHHD was 10% in every composite). As shown in Figure 5a,b, the COF of EP and Shell@EP at RT increased rapidly to approximately 0.6. This shows that adding NPHHD@MS had no significant reductive effect on the friction. After adding PHHD, at RT, the COF of PHHD@EP and PHHD@MS@EP increased continuously to 0.6 in contrast with the EP resin. This means that adding PHHD could extend the duration of the running time. When the temperature increased to 80 \({}^{\circ}\)C, as shown in Figure 5d,e, PHHD@EP and PHHD@MS@EP showed a low and stable COF of approximately 0.13 over time, which means that filling PHHD could produce excellent lubricating properties at 80 \({}^{\circ}\)C.  
After the above friction tests, the wear scars on different composites at 80 \({}^{\circ}\)C were analyzed with a 3D white-light interferometer (Figure S1). The wear scars of EP and Shell@EP were obvious, suggesting that the composites were seriously worn during the friction tests. Conversely, after adding PHHD, the wear scars on PHHD@EP were greatly reduced. The method to calculate the wear rate value was described in Section 2.4 and analyzed as shown in Figure 4c,f. A high wear rate was observed in all EP composites, PHHD@EP (6.80 \(\times\) 10\({}^{-4}\) mm\({}^{3}\)/(N\(\cdot\)m), RT), PHHD@MS@EP (2.96 \(\times\) 10\({}^{-4}\) mm\({}^{3}\)/(N\(\cdot\)m), RT), EP (4.23 \(\times\) 10\({}^{-4}\) mm\({}^{3}\)/(N\(\cdot\)m), RT), and Shell@EP (5.23 \(\times\) 10\({}^{-5}\) mm\({}^{3}\)/(N\(\cdot\)m), RT). However, the wear resistance of PHHD@MS@EP and PHHD@EP was improved when the temperature increased to 80 \({}^{\circ}\)C, with the wear rate decreasing to 1.604 \(\times\) 10\({}^{-5}\) mm\({}^{3}\)/(N\(\cdot\)m) and 5.204 \(\times\) 10\({}^{-6}\) mm\({}^{3}\)/(N\(\cdot\)m), respectively. Moreover, the wear rate did not decrease for EP (3.89 \(\times\) 10\({}^{-4}\) mm\({}^{3}\)/(N\(\cdot\)m)) and Shell@EP (6.11 \(\times\) 10\({}^{-5}\) mm\({}^{3}\)/(N\(\cdot\)m)) at that temperature. Figure 5: COFs and wear rates of EP composites. (**a**,**b**) COFs of EP composites at RT; (**d**,**e**) COFs of EP composites at 80 \({}^{\circ}\)C; (**c**) wear rates of EP composites at RT; and (**f**) wear rates of EP composites at 80 \({}^{\circ}\)C. (**a**,**d**) was taken from one of three measurements of each sample, respectively. The average COFs in (**b**,**e**) were obtained from the three COFs of each sample. And every COF was calculated after the running-in process. )The above results suggest that adding PHHD to the EP composites provided a significantly higher wear resistance at 80 \({}^{\circ}\)C.  
To further verify the influence of PHHD on COF reduction and wear resistance, the worn surfaces of the EP composites were observed (Figure 6; the experiment was performed at 80 \({}^{\circ}\)C). The wear surfaces of EP resin and Shell@EP showed fatigue wear characterized by scale-like damage with cracks [24]. After adding PHHD, two-body abrasion was the main mechanism, and the wear track became smoother with several slight peels. The F element appeared according to the EDS result, indicating that PHHD appeared at the interface between the EP composite and the steel ball (Figure S2b). Furthermore, the wear track of PHHD@MS@EP was less coarse, and the worn surface was rather flat because the PHHD@MS was broken (Figure 6d). Moreover, the EDS result in Figure S2d confirms that PHHD was transferred to the friction interface. PHHD prevented direct contact between the ball and the polymer substrate and showed self-lubricating properties. The wear mode was changed to mild polishing. The mechanism behind the above phenomenon was also investigated. PHHD was in a solid state, when released to the friction interface at low temperatures. The reduction effect on the COF was not obvious due to the high viscosity and low fluidity of PHHD.