**4. Conclusions**

The presented paper illustrated the role of fumed silica and ground silica fillers in suppressing the DC erosion of SiR through a novel framework. The erosion performance outcomes sugges<sup>t</sup> that fumed silica and its interaction with SiR were effective in promoting the formation of a coherent shielding residue, which resulted in suppressing the DC erosion of SiR. This was found despite the higher composite thermal conductivity of the ground silica filled composite, which further supports the influence of the filler's interface interactions over enhancements in the thermal conductivity on suppressing DC erosion. Simultaneous TGA–DTA analysis shows the significant influence of fumed silica in suppressing depolymerization and promoting radical-based crosslinking at high temperatures in SiR as a result of its favorable interaction with the siloxane chains of the polymer tethering their flexibility and mobility during depolymerization. This results in the formation of a much higher additional residue with the fumed silica filled composite, despite being filled at one sixth of the loading level of the ground silica filled composite. The formation of a higher additional residue could result in a higher carbon content, which still would not be enough to promote a tracking failure during the IPT. The microscopy conducted on the eroded composites from the dry-arc resistance test shows coherency and low surface fracture in the residue of the fumed silica filled composite. This could also explain the better erosion performance of the composite as a result of the shielding effect of the coherent residue preventing progressive erosion. Moreover, the surface morphology outcomes of the dry-arc tested composites are consistent with those of the IPT, which validates the use of the dry-arc test as part of this framework. The surface roughness outcomes show a rougher surface waviness and higher values of *Ra* for the ground silica filled composite, which could further indicate the weakness and porosity of the residue, leading to an inferior performance under the IPT. The overall conclusion of the study suggests a significant role for the silica filler size in suppressing the erosion of SiR under DC voltage as a result of its influence on the eroded residue characteristics.

**Author Contributions:** Conceptualization, A.Y.A., R.A.G. and E.D.; methodology, A.Y.A., R.A.G. and E.D.; software, A.Y.A.; validation, R.A.G. and E.D.; formal analysis, A.Y.A., R.A.G. and E.D.; investigation, A.Y.A.; resources, R.A.G. and E.D.; data curation, A.Y.A.; writing—original draft preparation, A.Y.A.; writing—review and editing, A.Y.A., R.A.G. and E.D.; visualization, A.Y.A.; supervision, R.A.G. and E.D.; project administration, R.A.G. and E.D.; funding acquisition, R.A.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors of the paper would like to thank the Natural Sciences and Engineering Research of Canada (NSERC) and the National Research Council Canada (NRC) for their financial support.

**Acknowledgments:** The authors would like to thank Souheil-Antoine Tahan, Simon Laflamme, Mohammad Saadati and Joel Grignon for providing the permission, technical help and advice needed to complete the microscopic part of this study.

**Conflicts of Interest:** The authors declare no conflict of interest.
