Wettability, Surface Morphology, and Wet Flashover Response of SiO₂-Graphene-Coated 150 kV Ceramic Insulators Under Natural Contamination

Outdoor high-voltage insulators are essential components in power transmission systems because they maintain electrical insulation while mechanically supporting energized conductors. However, the long-term reliability of these components is strongly influenced by environmental exposure, particularly surface contamination and moisture accumulation. Under polluted and humid conditions, contaminants deposited on insulator surfaces can interact with moisture to form conductive films that increase leakage current and may eventually initiate surface flashover [1, 2]. In tropical environments, biological contaminants such as moss, algae, and organic deposits are frequently observed on ceramic insulators. These biological pollutants can retain moisture for extended periods, intensify surface conductivity, and increase the probability of electrical discharge initiation.

Surface wettability plays a key role in determining the electrical behavior of outdoor insulation systems. Hydrophilic surfaces tend to promote continuous water-film formation, which facilitates leakage current flow and increases flashover susceptibility under wet conditions, whereas hydrophobic coatings can reduce water spreading and improve surface insulation performance [3-5]. Conventional ceramic insulators inherently exhibit relatively high surface energy and microstructural porosity, making them prone to moisture spreading and contaminant accumulation [1, 2]. Consequently, surface modification techniques have been widely investigated to enhance hydrophobicity and improve the environmental durability of ceramic insulators without altering their intrinsic dielectric properties.

Among the various modification strategies, silica-based coatings have attracted considerable attention because of their chemical stability, adjustable surface morphology, and compatibility with scalable deposition techniques such as spray coating [4]. The incorporation of nanoscale SiO₂ particles can generate micro/nano surface structures that enhance water repellency by suppressing continuous liquid-film formation on the insulator surface. In addition, graphene-based materials have emerged as promising components for functional coatings owing to their two-dimensional morphology, low surface energy, and barrier characteristics, which are beneficial for reducing moisture penetration and surface degradation [6, 7].

Recent studies indicate that hybrid coatings combining inorganic nanoparticles and carbon-based nanomaterials can provide advantages in controlling surface wetting behavior. In such systems, both surface morphology and surface chemistry are relevant to the stability of hydrophobic performance [6-8]. The effectiveness of hybrid coatings depends not only on surface roughness or chemical composition independently, but also on the interaction between coating morphology and interfacial chemistry. These coatings can modify surface roughness, reduce moisture spreading, and may delay the development of leakage current and dry-band arcing phenomena under contaminated conditions.

Despite these advances, the relationship between coating-induced wettability enhancement and the resulting electrical flashover behavior remains insufficiently understood. Many previous studies primarily emphasize improvements in contact angle or hydrophobic performance, while the electrical response of coated insulators under severe contamination conditions has received comparatively less attention [1, 2, 8]. In particular, natural biological contamination may alter the wetting mechanism by promoting persistent moisture retention and conductive film formation on the insulator surface, thereby influencing the effectiveness of hydrophobic coatings. Since flashover processes are strongly influenced by localized wet-film continuity and electric-field intensification along contaminated surfaces, a deeper understanding of how coating morphology and surface chemistry jointly affect electrical behavior is required.

Against this background, three coating routes (SiO₂, graphene, and hybrid SiO₂-graphene) were evaluated on naturally contaminated 150 kV ceramic insulators by combining contact-angle measurement, FE-SEM/EDS observation, FTIR spectra, wet flashover tests, and leakage-current measurement. Rather than treating hydrophobicity improvement as an automatic proxy for electrical improvement, the analysis compares the surface observations with the actual electrical-test results.

Three research questions guide the analysis: (1) how strongly do the three coatings change the measured contact angle of contaminated ceramic surfaces; (2) do FE-SEM/EDS and FTIR observations support a more compact and chemically modified coated surface, especially for the SiO₂-graphene system; and (3) does higher static hydrophobicity coincide with better wet flashover performance under natural biological contamination? The working hypothesis is that the hybrid coating improves surface wettability and apparent surface coverage, but wet flashover remains controlled by contamination-assisted conductive paths, local non-uniformity, and defect-triggered discharge initiation.

3.1 Wettability performance

The contact-angle results show a consistent upward shift after coating for all three systems, but the magnitude and statistical strength of the change differ among them. For the SiO₂ specimens, the average contact angle increased from 72.57 ± 24.59° to 110.58 ± 3.09°. For graphene, it increased from 56.73 ± 7.31° to 101.79 ± 3.44°. For the hybrid SiO₂-graphene specimens, it increased from 21.32 ± 2.71° to 105.88 ± 1.95°.

The detailed contact-angle data before and after coating are summarized in Table 4, which provides the basis for comparing the wettability improvement among the SiO₂, graphene, and SiO²-graphene coating systems.

Welch’s t-test indicates that the improvement is statistically significant for graphene (p = 0.0029) and especially for the hybrid system (p < 0.001), whereas the SiO₂-only system shows a strong numerical increase but does not reach p < 0.05 (p = 0.114) because one pre-coating value remained relatively high and the dataset is small (n = 3 per condition). Thus, the strongest evidence-based statement is that all coatings increase the measured contact angle, while the hybrid surface provides the narrowest post-coating dispersion.

Table 4. Contact angle measurements before and after coating

This is more informative than simply calling the coating ‘hydrophobic.’ The hybrid system transforms a severely wettable moss-contaminated surface (21.32 ± 2.71°) into a uniformly water-repellent one (105.88 ± 1.95°), which is a much stronger surface-state change than that observed from the SiO₂-only route.

At the same time, these wettability data should not be overextended. Static contact angle captures a local near-surface wetting response under a small droplet, whereas flashover under contaminated wet conditions depends on the continuity of conductive pathways over a much larger stressed surface. For this reason, contact-angle enhancement is not treated as direct proof of improved wet flashover strength.

3.2 FE-SEM analysis of coating morphology

The FE-SEM observations are interpreted qualitatively. The uncoated ceramic surface shows irregular topography, visible pores, and surface defects that are compatible with easier moisture retention. After SiO₂ coating, granular surface features become more apparent, suggesting partial coverage by inorganic particles. The graphene-coated surface shows broader plate-like regions, while the hybrid surface appears more continuously covered with fewer visibly open defective regions.

These observations support an apparent morphological progression from heterogeneous exposed ceramic to a more covered composite-like surface. However, because no roughness measurement, particle-size statistics, or porosity image analysis was performed, terms such as ‘hierarchical’ and ‘more continuous’ are now used as qualitative descriptors tied to the SEM images rather than quantified structural metrics.

Practically, this morphological change suggests that the hybrid coating can reduce exposed high-energy ceramic sites and moderate local wetting heterogeneity. That interpretation remains consistent with the narrower post-coating contact-angle dispersion, but it is presented as a surface observation rather than as direct electrical proof. Figure 1 presents the FE-SEM surface morphology and FTIR spectra of the uncoated, SiO₂-coated, and SiO₂-graphene-coated 150 kV ceramic insulators. The figure supports the interpretation that the hybrid coating produces a more continuous surface coverage and provides qualitative evidence of chemical features associated with the coating layer.

(a) After 

(b) After

Figure 1. (a, b) FE-SEM surface morphology and FTIR spectra of uncoated, SiO₂-coated, and SiO₂-graphene-coated 150 kV ceramic insulators

3.3 FTIR analysis and chemical interaction

The FTIR spectra are interpreted as qualitative evidence of coating-related chemical features, not as definitive proof of a specific bond architecture. The spectra indicate silica-related absorption bands for the SiO₂-containing coatings and carbon-related features after graphene incorporation.

This interpretation is strengthened when the spectra are read together with the EDS results from the raw experiment file: for the hybrid sample, carbon appears after coating at 13.45 mass% and silicon increases to 26.65 mass%, whereas the uncoated counterpart did not show measurable carbon and contained only 10.73 mass% silicon.

Taken together, the data support successful surface deposition and a modified surface chemical environment, rather than proving that FTIR alone establishes strong morphology-chemistry coupling.

Overall, the evidence remains convergent but qualitative: FE-SEM indicates a more covered surface, EDS indicates changed elemental composition, and FTIR indicates coating-related functional features, as shown in Figure 2. Figure 2 presents the FTIR spectra of the SiO₂-coated and SiO₂-graphene composite coatings, supporting the qualitative interpretation of silica-related and carbon-related features after coating.

Figure 2. FTIR spectra of SiO₂-coated and SiO₂-graphene composite coatings on 150 kV ceramic insulators

A stronger claim about interfacial bonding would require XPS or additional spectroscopy, which were not part of the present dataset. This limitation is stated explicitly to keep the chemical interpretation fully aligned with the evidence level.

The corresponding elemental composition obtained from EDS analysis is described in the text, while the surface morphology before and after coating is shown in Figure 1.

3.4 Structure property relationship and electrical relevance

Overall, the dataset supports an observed relationship among surface coverage, wettability response, and electrical behavior, but not a fully quantified predictive coupling model. The hybrid coating shows the clearest surface-state improvement in terms of post-coating contact angle and spatial uniformity, and the FE-SEM/EDS/FTIR evidence is consistent with a more effectively modified outer surface.

However, the electrical results show that this surface-state improvement does not automatically suppress wet flashover. The interpretation separates two levels of behavior: (i) local wetting behavior measured by static contact angle and (ii) long-path electrical breakdown behavior controlled by conductive film continuity, field intensification, contamination heterogeneity, and local coating defects.

The main contribution is therefore not only material-based but also interpretive: the dataset shows how a very favorable hydrophobic response can still coincide with weak or even adverse wet flashover outcomes under biological contamination.

This distinction is more precise than simply labeling the hybrid coating as either ‘effective’ or ‘ineffective.’ Thus, the mismatch between surface-state improvement and electrical outcome is treated as the main scientific result, not as a contradiction that should be hidden.

A future predictive model would require thickness-resolved coating characterization, quantified contamination severity, and real-time wet-path diagnostics, none of which were available in the present dataset. Within these limits, the work still provides a useful surface-to-electrical interpretation grounded in the actual measurements. The flashover-voltage and leakage-current responses under dry and wet conditions are summarized in Tables 5 and 6 to clarify the electrical performance of each coating system.

3.5 Electrical performance evaluation

The wet electrical results do not mirror the contact-angle improvement. For the DF4 series, the wet flashover voltage decreased from 15.33 ± 0.40 kV to 13.83 ± 0.38 kV after hybrid coating, corresponding to a 9.8% reduction. For the L3 series, it decreased from 18.50 ± 0.51 kV to 14.00 ± 0.39 kV, corresponding to a 24.3% reduction. By contrast, the available leakage-current readings at moderate stress remain low, within 0.014-0.020 mA.

These results support a cautious conclusion: under the present natural-contamination conditions, improved static hydrophobicity did not translate into higher wet flashover voltage. The most plausible explanation is that moisture-retaining biological deposits, together with local coating non-uniformity or defect sites, still maintained electrically continuous wet paths that dominated breakdown initiation. This interpretation is consistent with prior work showing that surface wetting, contamination, and coating durability can strongly influence leakage-current and flashover behavior [1, 2, 8, 15, 16]. urther studies on coated and contaminated insulators similarly indicate that coating damage, contamination distribution, and leakage-current features must be considered when interpreting flashover behavior and insulator condition [17-20]. More recent studies on hydrophobic surface-energy control, nano-TiO₂ coatings, porcelain-insulator coating performance, and condensation-related flashover behavior also emphasize that coating chemistry, surface energy, and moisture dynamics can affect leakage current and flashover resistance [21-25].

Other explanations are also possible. Besides contamination-assisted conductive films, the flashover decrease may also be influenced by local thickness non-uniformity, imperfect coating adhesion, or changes in surface resistance under high-field wet conditions. Figure 3 compares the wet flashover voltage before and after SiO₂-graphene coating, while Table 5 summarizes the dry and wet flashover-voltage data.

Table 5. Flashover voltage comparison (Dry and Wet conditions)

Table 6. Leakage current at 50% flashover voltage

Figure 3. Comparison of wet flashover voltage of ceramic insulators before and after SiO₂-graphene coating

Because the complete raw replicate series for flashover and leakage current are not preserved in the manuscript archive, these electrical results are discussed descriptively rather than inferentially. Figure 4 presents the leakage-current response measured at 50% of the flashover voltage, and Table 6 summarizes the corresponding test-voltage and leakage-current values. This descriptive treatment keeps the argument aligned with the available evidence and directly addresses the reviewer concern about over-interpretation.

Figure 4. Leakage current of ceramic insulators at 50% flashover voltage before and after SiO₂-graphene coating

The decrease in wet flashover voltage is interpreted as an observed outcome that requires cautious mechanism discussion, not as final proof that the coating is electrically detrimental under all service conditions.

3.6 Electrical field and discharge mechanism interpretation

Under wet contaminated conditions, the insulator surface may contain a discontinuous mixture of droplets, thin films, biological residue, and coated regions. When these regions become electrically connected along a stressed path, local current concentration and dry-band formation can occur even if the static contact angle measured at selected points remains high. Figure 5 illustrates the proposed wet flashover mechanism under contaminated and humid surface conditions.

The proposed mechanism is consistent with the present data, but it should be read as a probable mechanism rather than as a uniquely verified one, because the study did not directly image the evolving wet conductive path during breakdown.

From an engineering perspective, coating development for outdoor insulators should prioritize not only hydrophobicity, but also coverage continuity, durability under biological fouling, and resistance to localized conductive bridge formation.

For field application, these targets are more useful than contact-angle magnitude alone. The discussion therefore distinguishes direct observation, engineering implication, and mechanism inference more clearly.

Figure 5. Mechanism of wet flashover