# Porcelain Enamel Coatings for Building Facades
## 3 Results and Discussion  
### Coatings Characterization  
From the SEM micrographs of the enamel sample surfaces revealed in Figure 1, pigments seem to be uniformly spread with a homogenous distribution. White particles are visible on the surface of the red enamel coating (Figure 1a), which corresponds to CdO, added to the frit formulation to confer the desired color. Both enamel surfaces appear quite flat, even if roughness measurements revealed higher Ra values (average roughness) for the red enamel than for the white enamel (Table 3). However, the roughness of the enamel coatings is still lower than the outcomes measured on the corresponding painted samples, revealing a smoother morphology of the glassy layers. Surface roughness significantly impacts the aesthetic appearance of a coated surface. This is evident as both gloss and color perception alter in response to surface roughness. Beyond its aesthetic implications, surface roughness holds a pivotal role in the realm of building facades, particularly concerning cleaning convenience. When dealing with high surface roughness, the likelihood of the accumulation of pollutants and dirt increases, potentially rendering the surface more challenging to clean. Conversely, a smoother surface facilitates easier cleaning procedures. Figure 2 shows the scanning electron micrographs of the enamel cross-sections. The ground layer is equal for all the samples, with a measured thickness of about 35 \(\upmu\)m. The intermediate layer cannot be distinguished from the enamel covering layer. Sample red (Figure 2a) exhibits a structure with two enamel-covering layers, consisting of a first white enamel layer measuring approximately 125 \(\upmu\)m in thickness, followed by a second red layer with a thickness of around 320 \(\upmu\)m. Thus, the red enamel displays an overall thickness of 500 \(\upmu\)m. Instead, the white sample (Figure 2b) consists solely of a white enamel layer, which is about 200 \(\upmu\)m thick. The total thickness of the white sample is thus approximately 230 \(\upmu\)m. The presence of the additional layer in the sample red serves to enhance both the aesthetic properties and the durability of the coating. \begin{table}
\begin{tabular}{l l l} \hline
**Sample** & **Total Thickness [\(\upmu\)m]** & **Roughness Ra [\(\upmu\)m]** \\ \hline Red enamel & \(\approx\)500 & 0.26 \(\pm\) 0.03 \\ Red paint & \(\approx\)110 & 0.61 \(\pm\) 0.05 \\ White enamel & \(\approx\)230 & 0.12 \(\pm\) 0.02 \\ White paint & \(\approx\)110 & 0.42 \(\pm\) 0.07 \\ \hline \end{tabular}
\end{table} 
Table 3: Thickness and roughness of the coatings. Figure 1: Scanning electron micrographs of samples surface of the (**a**) red and (**b**) white enamel coatings. The impact test was carried out to investigate the adhesion of the coating to the substrate. While the paint coatings display excellent adherence with the metal substrate, with the appearance of very small cracks on the coating layer (Figure 3c,d), the impacted zone in the enamel-coated samples is characterized by the loss of the coating after deformation. Paint shows a different behavior compared to enamel. In fact, paint is deformable, and thus, in the case of good adhesion, it follows the deformation of the substrate caused by the impact. The ASTM B916-01 [27] standard classifies the adherence of porcelain enamels depending on the amount of coating that remains after the deformation and the presence of substrate oxides. Accordingly, the white enamel shows very good adherence with a still visible oxide layer, which may be classified as grade 4 (Figure 3b). The adhesion between the red coating and the metal substrate seems to be weaker, with a broader impacted zone (Figure 3a). Since dendrites are clearly present at the steel-enamel interface, as shown in Figure 2[21], and comparable in the red and white enamels, the different quality of adhesion is not explained by differences at the dendrite level.