# Controllable Friction of an Epoxy Composite via Thermal Treatment
## 3 Results and Discussion
### Friction and Wear Test of PHHD@MS@EP  
EP composites with different mass fractions of PHHD@MS were investigated to identify a possible optimum microcapsule concentration that would minimize the COF (Figure 8a,b) (the reciprocating friction stroke was 5 mm, sliding speed was 5 cm/s, and the test load was 1.5 N). As shown in Figure 8a, when the mass fraction of microcapsules increased from 0 wt% to 10 wt%, the lubricity of PHHD@MS@EP was enhanced, and the COF was reduced from 0.65 to 0.07. When the microcapsule content increased from 10 wt% to 15 wt%, the lubricity decreased slightly as the COF increased to 0.09. The average COF of the composites with 10 wt% and 15 wt% PHHD@MS showed excellent lubricating properties. However, as shown in Figure 8a, the former exhibited a relatively stable COF curve, which indicated a reliable improvement in the lubricating properties. Furthermore, the mechanical properties of PHHD@MS@EP samples with different mass fractions of PHHD@MS were investigated, as shown in Figure S3. PHHD@MS@EP with 15 wt% microcapsules had poorer mechanical properties than that with 10 wt%. The compressive stress of PHHD@MS@EP decreased from 65 MPa to 55 MPa, and the tensile stress decreased from 28 MPa to 23 MPa. The poor friction and mechanical properties were attributed to excessive microcapsules, leading to agglomeration and uneven dispersion in the composite, as shown in Figure S4, which clearly shows that some microcapsules were agglomerated [24].  
PHHD@MS@EP under different loads was also investigated (Figure 8b,d) (reciprocating friction stroke was 5 mm, sliding speed was 5 cm/s, and mass fraction of microcapsules was 10% in every composite). The EP composite exhibited a decrease in the COF (from 0.14 to 0.09), as the load increased from 0.3 N to 1.5 N, then, an increase to 0.67 when load increased from 1.5 N to 4.5 N. This phenomenon was related to the plastic deformation and wear debris of PHHD@MS@EP [28]. In polymers, plastic deformation tended to occur when the load increased to a certain value during friction tests. And as said above, PHHD@MS@EP showed less compressive strength compared with EP, which means that plastic formation was much easier for PHHD@MS@EP. At first, the pressure produced by the load (0.3 N) could not make the microcapsules release enough PHHD, therefore, a relatively large COF was obtained. As the load increased to 1.5 N, according to the Hertz contact theory, the Hertz contact pressure was approximately 80 MPa. Unobvious plastic deformation happened in the wear track. And as the load continued to increase, microcapsules released more PHHD. Therefore, the COF decreased. As the load increased to 3 N  
Figure 7: **(a)** COFs of PHHD@EP and PHHD@MS@EP under 1.5 N, 5 Hz, and 80 \({}^{\circ}\)C. **(b)** Optical images and XPS spectra of the two sides of PHHD@EP after test; **(c)** XPS spectra of F 1s at the bottom of PHHD@EP.  
and 4.5 N, obvious plastic deformation took place in the EP composite, which aggravated the development of abrasive and adhesive wear [27].  
In summary, selecting an appropriate mass fraction of microcapsules and external load could significantly improve the lubricating properties of EP composites. Introducing microcapsules could not only enhance the lubricating properties but also prolong the service time of EP composites.