Surface analysis of epoxy nanocomposite insulator materials eroded by partial discharge

Effects of the addition of nano filler particles in the improvement of epoxy resin insulating characteristics were investigated and it has been clarified that surface erosion resulting from partial discharge (PD) could be substantially suppressed by the addition of several percent of nano filler content to epoxy composites. One insulating performance improvement mechanism resulting from nano filler addition could be related to the deposition of filler particles on the eroded surface. However, no analysis reports on PD-related surface erosion have been identified to date. In this study, epoxy resin degradation and epoxy composite materials were quantitatively evaluated and eroded surfaces resulting from PD were analyzed by energy dispersive X-ray spectrometry (EDX) and auger electron spectroscopy (AES). To facilitate our experiments, neat epoxy resin specimens consisting of 3 wt% micro-SiO₂ composites and 3 wt% nano-SiO₂ composites, were prepared for use. Accelerated PD tests were performed with an applied voltage of 4 kV_rms and a frequency of 600 Hz in an air atmosphere. In our experiments, erosion depths of all specimens were increased by lengthening the discharge time, and the addition of fillers to the epoxy resin suppressed the degree of the erosion, compared with the neat epoxy specimens. Out of these, nano filler composites were found to be particularly effective. The results of scanning electron microscopy (SEM) observation and EDX analysis of the eroded surfaces showed that the filler amounts deposited on the surface increased with lengthening discharge time in the composite specimens. After removing the deposited filler particles via ultrasonic cleaning, an increase in the SiO₂ surface concentration could still be detected for the nanocomposite materials via AES measurements. Through an analysis of AES depth profiles, the increased layer thickness of the SiO₂ surface contents was found to be from 10 to- 20 nanometers. This indicates that that the surface resin was selectively sputtered during PD. From the above experimental results, it can be concluded that one of the major mechanisms related to improving insulating performance via filler additions was the surface covering provided by filler deposition during the PD. In the case of nano filler composites, the outermost surface was more effectively and densely covered with filler particles during the PD.