## 3 Results and Discussion  
### Self-Healing Performance  
#### 3.1.1 Repair Effect of Mechanical Damage and Breakdown Strength  
Breakdown strength is one of the most important macroscopic performances of insulating materials, which can be affected by damage defects. The scratch defect can be equivalent to mechanical structural damage in polyethylene insulating materials. Therefore, the self-healing ability was visually represented by the AC breakdown strength of the sample before and after repairing damage. The variation in breakdown strength is shown in Figure 2.  
Figure 2: Alternating current (AC) breakdown strength of polyethylene samples. Although the breakdown strength of pure polyethylene decreases slightly after modifying the process, the change trend of its breakdown strength after damage is the same as that of the original polyethylene. In addition, this paper focuses on the effect of microcapsules on the typical electrical performances of polyethylene. Therefore, this paper only gives the mechanical damage and breakdown strength results of pure polyethylene with a modified process, which was the comparison with the composite. In Figure 2, the insulation strength of polyethylene can be reduced by scratch damage. For pure polyethylene, different heating temperatures and times can only make the breakdown strength fluctuate slightly. In other words, the heating conditions in this paper had almost no effect on the insulation performance of pure polyethylene. For the composite, when heated for 10 min at different temperatures, the breakdown strength was clearly recovered, and the recovery range was basically the same. However, when heated for 30 min, the breakdown strength of the samples at 60 \({}^{\circ}\)C and 90 \({}^{\circ}\)C can be greatly increased, while that of the sample at 45 \({}^{\circ}\)C was smaller. When heated for 60 min, the breakdown strength of all samples tended to change smoothly. It can be concluded that when the temperature was higher than 45 \({}^{\circ}\)C, the DCPD could rapidly initiate the ring-opening shift polymerization reaction, thus filling the damage defects and restoring the material performance, to a certain extent. However, when the temperature was higher than 60 \({}^{\circ}\)C, the polymerization product could be cured better, and the properties of the repair product could be improved, so as to further repair the performance of the matrix. The curing time was 10-30 minutes. Therefore, the suitable repair condition selected in this paper was 60 \({}^{\circ}\)C/30 min, which fully meets the operating conditions of polyethylene insulating material. In these conditions, the insulation performance of the polyethylene matrix hardly changed, but the self-healing effect was obvious. This paper focuses on the study of self-healing behavior in the appropriate repair conditions (such as 60 \({}^{\circ}\)C/30 min). Figure 3 shows the OM results of the scratch test. In pure LDPE, the scratch morphology hardly changed after heating. Scratches not only destroy the physical structure of polyethylene, but also lead to a decrease in breakdown strength. This is mainly caused by the distortion effect of a defective structure on electric field [14]. After heating,  
Figure 3: Morphology of the scratch. (**a**) Unheated pure LDPE. (**b**) Heated pure LDPE. (**c**) Unheated composite. (**d**) Heated composite. the breakdown strength of pure polyethylene showed no obvious recovery. Thus, structural damage is irreversible in pure polyethylene, which can directly reduce the insulation strength of polyethylene. Doping microcapsules can reduce the insulation strength of polyethylene to a certain extent, which is mainly related to the introduction of impurities and interface defects. In the process of doping microcapsules, many impurities, such as polar groups, can be introduced, which can be ionized in the electrical field, increasing carrier concentration.