# Surface Tracking of MgO/Epoxy Nanocomposites: Effect of Surface Hydrophobicity
## 1 Introduction  
Epoxy-based materials are widely employed for outdoor insulations in electrical devices because of the excellent insulating and mechanical properties [1; 2; 3; 4]. Outdoor insulating materials are exposed to complicated environments, including sunlight, dusts, moisture, etc. Dust accumulates, easily, on the surface of insulating materials. Conductivity of the dust increases when they are wetted from raining, fog, dew, etc. In this situation, discharge is likely to be induced on the dust surface, under an external electric field. Irreversible surface tracking is finally formed under continuous electrical stress, with an extensive deterioration of insulating properties. Permanent damage to the insulation and even serious accidents might be caused by surface tracking. It has been one of the important challenges for outdoor organic insulations.  
A number of studies were carried out to reveal the physical essence of surface tracking [5; 6; 7; 8; 9]. Nowadays, surface tracking is acknowledged as a surface degradation process of organicinsulations [5; 6]. The liquid contamination acts as a conduction pathway for the leakage current, under an external electric field. It is non-uniformly heated and evaporated due to the joule heat, leading to the formation of a dry-band zone. Most of the applied voltage is then withstood by the resistive dry-band zone. In consequence, an extremely high electric field consequently appears at the dry-band zone. Surface discharge finally takes place when the electric field is high enough. Finally, surface tracking is formed under the combined stresses from electricity, heat, particle collisions, ultraviolet light, etc.  
In recent years, researchers have gradually focused on the surface tracking processes of nanodielectrics. The science and technology of nanodielectrics, which are considered to be the next generation of insulating materials, have developed rapidly since the proposal of \"Nanometric Dielectric\" by [PERSON] [10], in 1994, and the redefinition of the term \"Nanodielectric\", by [PERSON] [11] in 2001. Remarkable thermal, mechanical, and dielectric properties have been achieved in nanodielectrics, due to the induced interfaces between the nanofillers and the polymer matrix [7; 8; 9; 12; 13; 14; 15; 16; 17; 18]. Improved tracking resistance has also been achieved in nanodielectrics, by enhancing thermal conductivity [9; 19], stability of the chains [7], surface charge dissipation [20], etc. Among various nanofillers, MgO was found to be an effective nanofiller for inhibiting the space charge [21], tweeing breakdown, and partial discharge [22]. Cross linked polyethylene (XLPE)/MgO nanocomposite material has been employed as the insulation for 500 kV DC cable, developed in Japan, due to its excellent electrical performance [23]. In our previous work, surface treeing, consisting of a series of segmented ablation holes has been observed in nano-MgO/epoxy composites [8]. It suggested that the scattering effect of nanoparticles on electrons plays an important role in the tracking process. Enhanced tracking resistance in the nanocomposites was partially attributed to the electron scattering by nanoparticles. However, besides the intrinsic properties of nanodielectrics, extrinsic surface properties were also found as important factors in our follow-up studies. Electrical properties are inevitably affected by surface conditions, including surface hydrophobicity [24; 25; 26; 27; 28]. In this paper, surface tracking measurements were conducted on nano-MgO/epoxy composite samples. Surface hydrophobicity was characterized by surface contact angle (SCA) [29], and its correlation with surface tracking has been discussed.