## 3 Results and Discussion
### Surface Morphology
#### 3.2.1 Effect of Laser Fluence and Scanning Speed
When the scanning speed increased, the removed area also decreased due to the decrease in \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **(a) 0.4 J/cm\({}^{2}\)** & **(b) 0.5 J/cm\({}^{2}\)** & **(c) 0.7 J/cm\({}^{2}\)** \\ \hline Average pore diameter [nm] & 125 \(\pm\) 21 & 122 \(\pm\) 16 & 128 \(\pm\) 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Pore size on irradiated YSZ surfaces in Figure 5.  
Figure 5: SEM images of the irradiated surface at a scanning speed of 10 mm/s with varying laser fluence: (**a**) 0.4 J/cm\({}^{2}\), (**b**) 0.5 J/cm\({}^{2}\), (**c**) 0.7 J/cm\({}^{2}\); (**a1–c1**) are low-magnification images, and (**a2–c2**) are high-magnification images. laser pulse overlap (Figure 6b). At high scanning speed, the dried film swelled into a tunnel-like shape, some top parts partially remained, and some were peeled off (Figure 6c). Under the swelled film, nanopores with embedded materials were observed. It is considered that the surface was expanded due to the heating effect of laser irradiation since the dried film was mainly PVP polymer containing silver nanoparticles. It was reported that when the resin surface was irradiated by a femtosecond pulsed laser, the resin expanded and decomposed near the surface, and then a cavity was formed [36]. It has also been reported that increasing the fluence in laser ablation causes the thermal stress difference between the substrate and the coating film to be greater than the adhesive force between the two, causing the coating film to delaminate [37]. Thus, in this study, the film expanded and decomposed, and the internal pressure between the surface film and the YSZ substrate increased, which is thought to have caused the top surface of the film to delaminate. To investigate the elemental component of the irradiated surface, elemental mapping by EDX was performed. The sample was prepared by area irradiation with a laser fluence of 0.5 J/cm\({}^{2}\), a scanning speed of 200 mm/s, and a scan pitch of 8 \(\upmu\)m. Figure 7 shows an SEM image and elemental map of the nanopores. Many yellow dots indicating silver (Ag) were observed on the surface corresponding to the area where the pore formed. In addition, microscale debris adhered around the pores also showed silver and carbon (C). It is considered that the carbon was derived from the PVP in nano-silver dispersion, indicating the microscale debris was dried PVP containing silver nanoparticles. By extracting PVP microscale debris adhering to the YSZ surface from the SEM image after laser irradiation using image analysis, the PVP removal rate, i.e., the area ratio to the irradiated surface, was calculated. The PVP debris was found to be only about 1% of the YSZ surface area, indicating that most of the PVP was removed. To effectively remove residual PVP, the pitch spacing at which the laser scans is considered to be important. If the pitch spacing between each irradiated line is too wide, unirradiated areas may occur and PVP may not be fully removed. To clarify the positional relationship between the distribution of silver and the nanopores on the YSZ surface, a distribution map was made by image analysis software, as shown in Figure 7c. The orange color represents the nanopores extracted from the SEM image (Figure 7a), and the blue color represents the distribution of silver detected in Figure 7b. The nanopores were densely distributed in a circular region corresponding to the laser pulse shape. Silver was also distributed corresponding to the pulse shape where the pores were concentrated, and was almost uniformly distributed within the circular region. There was almost no silver adhered to the gaps between the pulses where almost no pores were formed, i.e., the unprocessed area. This suggests that silver is uniformly distributed in the pore-generated area and that the pores act as anchors to hold the silver. Hence, according to the above results, it was found that silver nanoparticles were successfully embedded and remained in the nanopores after laser irradiation.