# Controllable Friction of an Epoxy Composite via Thermal Treatment
## 3 Results and Discussion
### Friction-Switching Experiments Based on Thermal Treatment  
Scheme 3 illustrates the mechanism of the smart friction response to temperature. The key to realize \"real-time\" COF conversion was to change the contact state between the two interfaces. Below 80 \({}^{\circ}\)C, PHHD was released under mechanical forces, but due to the high viscosity and poor fluidity of PHHD, it could not easily flow to the frictional interface to form a stable film. At that point, the contact state was \"solid-solid\". As a result, a high COF was obtained. After heating above 80 \({}^{\circ}\)C, PHHD melted into a liquid with low viscosity and great fluidity. Microcapsules in the composite broke, and liquid PHHD transferred to the frictional interface [24; 29; 30; 31]. The contact state between the ball and the sample changed from \"solid-solid\" to \"solid-liquid\". Consequently, a low COF was obtained. When the temperature increased to 110 \({}^{\circ}\)C (below the melting temperature of the microcapsule shell [25]), the EP became soft (the melting point of EP was 128 \({}^{\circ}\)C, according to Figure 9b), which reduced its mechanical properties, making the working conditions unstable. The friction test of pure EP at 110 \({}^{\circ}\)C was also performed as shown in Figure S5, the COF of which slowly increased to approximately 0.7. This result confirmed the above conclusion. Therefore, a rest period of approximately 2-3 min before each cycle was necessary.  
As shown in Figure 9a, the changes in the COF at different temperatures were recorded. At first, the COF stabilized at approximately 0.65 at RT after the running-in process because the contact state was \"solid-solid\". As the temperature gradually rose to 60 \({}^{\circ}\)C, the increased fluidity and reduced viscosity of PHHD resulted in a lower COF of approximately 0.35. When the temperature continued to rise to 80 \({}^{\circ}\)C, the contact state changed totally from \"solid-solid\" to \"solid-liquid\", and the COF significantly decreased to approximately 0.1. However, when the temperature continued to rise to 110 \({}^{\circ}\)C, the EP became soft, and the COF suddenly rose from 0.12 to 0.68. These results suggest that \"real-time\" friction conversion could be realized via heating.  
Figure 9: (**a**) The change in the COF of PHHD@MS@EP at different temperatures; (**b**) DSC curve of EP; (**c**) variation in COF under repeated temperature rising and dropping; and (**d**) changes in the COF of PHHD@MS@EP after being subjected to UV irradiation.  
Moreover, the switching of the frictional states could be repeated at least five times, as shown in Figure 9c. This result shows that PHHD@MS@EP could be used to establish smart friction.  
In practical industrial applications, the long-term stability of PHHD@MS@EP is vital for achieving friction control. Conditions imitating a real environment were used to check the stability of PHHD@MS@EP. The methods were described in Section 2.5, and the results are shown in Figure 9d. The results show that PHHD@MS@EP retained the ability to alter friction for at least 24 h in a harsh environment, suggesting that it possessed excellent stability. The stability of PHHD@MS@EP was attributed to the protective effect of the microcapsules for PHHD against the environment. The shells of the microcapsules provided an interface between the encapsulated PHHD and the external environment.