# Surface Tracking of MgO/Epoxy Nanocomposites: Effect of Surface Hydrophobicity
## 3 Results and Discussions  
It is acknowledged that nano-particles are easy to agglomerate, due to their large specific surface area. Therefore, samples were fractured in liquid nitrogen and SEM pictures were taken on their cross-sections, which are shown in Figure 2. A smooth surface can be observed in Figure 2a, since no nano-particles were doped into it. On the contrary, white dots, which are marked by yellow circles in each picture, were found in the nanocomposites. Cracks first appeared at the interface between nano-particles and the epoxy matrix in them, so it can be inferred that the white dots refer to the nano-MgO particles. It can be seen from Figure 2b-e that the nano-particles were uniformly dispersed into the epoxy matrix when the MgO content was less than 6 wt%. A slight conglomeration was found in Figure 2f when it was over-doped (samples with an addition of 10 wt% nano-MgO particles). Figure 1: Schematic diagram of surface tracking measurement and electric field distribution near the electrodes. Figure 3 shows the anti-tracking properties of the nanocomposite samples. The typical ablation morphology of the samples is shown in Figure 3a, in which three ablation regions can be observed. As shown in the enlarged SEM pictures, the smooth surface around the electrode/sample contacts was detected in Region A. A \"tank\" ablation in Region B, was located between the two halves of Region A. The most severe ablation area appeared in Region C. In this region, the surface trees started from the end of Region A and extended along the electric field, leading to the crossed ablation trees. In addition, enhanced tracking resistance was found in the nanocomposite samples. As shown in Figure 3b, CTI increased from 400 V of neat epoxy samples to 475 V of samples with a 10 wt% addition of nano-MgO particles. Considering the relatively low precision (25 V) of CTI, ablation area of the samples under a fixed voltage (425 V), was also measured, as shown in Figure 3c, to evaluate the tracking resistance. In accordance with the increase of CTI, ablation area decreased from 308.72 mm\({}^{2}\) of neat epoxy samples to 129.05 mm\({}^{2}\) of 10 wt% nano-MgO-doped samples. All these results indicated an improved tracking resistance in nanocomposites. An interesting phenomenon, as shown in Figure 4, was that the dependence of the SCA on the addition of nano-MgO particles, presented a similar tendency with that of the CTI. SCA for neat epoxy  
Figure 3: Typical surface morphology of the epoxy samples, after the tracking measurement, with enlarged SEM pictures for the tree regions (**a**). The corresponding comparative tracking index (CTI) (**b**) and ablation area after 50 droplets, at a fixed voltage of 425 V (**c**) for samples with different nanofiller contents. Figure 2: SEM pictures of the cross-sections of the samples with different additions of nano-MgO particles. (**a**) Neat epoxy, (**b**) 0.1 wt%, (**c**) 1 wt%, (**d**) 2 wt%, (**e**) 6 wt%, and (**f**) 10 wt%. The white dots marked by the yellow circles refer to the nano-MgO particles. samples were about 80 degree, which was similar to other reports [31]. With an increase of the MgO content, both the SCA and the CTI increased remarkably. It is, thus, reasonable to deduce that surface hydrophobicity is closely correlated with tracking resistance characteristics in nano-MgO/epoxy composites. The reason for this remarkable similarity could be due to the better hydrophobicity of the sample; the smaller water film area deposited on the surface of sample. The test solution tended to be cohesive, when SCA was relatively high. As a result, the formation of the drying region was probably inhibited. Accompanying this was the inhibited, non-uniform electric field distribution, which arose from the evaporation of the solution, by joule heat. Consequently, the probability of forming a continuous conduction path was considerably reduced, resulting in an increased tracking resistance.