## 3 Results and Discussion
### Mechanism of Silver Implantation
When irradiated with a nanosecond pulsed laser with a large pulse width (Figure 16(b2)), the entire dried dispersion film is heated, and its temperature increases. From the experimental results, surfaces irradiated with nanoseconds were melted as shown in Figure 8. Considering that the melting point of zirconia is about 2700 \({}^{\circ}\)C, both PVP and silver nanoparticles (boiling point of bulk silver is about 2160 \({}^{\circ}\)C [47]) become vaporized and leave the YSZ surface. Some vaporized silver cools down rapidly, and agglomerates in the air become tiny nanoparticles, which are again deposited on the YSZ surface. When heated in an electric furnace at 450 \({}^{\circ}\)C (Figure 16(b3)), a little higher than the decomposition temperature of PVP, large clumps of silver nanoparticles adhered to the YSZ surface. However, most of these clumps remained above the pores and did not penetrate the pores. This heating temperature was suitable for the decomposition and removal of PVP, but not for melting the silver and allowing it to flow onto the surface and into the pore for implantation. Moreover, the heat applied to the sample at this temperature for a considerably longer time than the laser irradiation caused a phase transformation of the YSZ, resulting in a large increase in monoclinic phases. Thus, the ultrashort laser pulse is considered to be suitable for removing only the surface layer of PVP and melting the silver to be embedded in the pore. Consider the difference in surface morphology depending on the laser irradiation parameters. When the scanning speed is too large, the number of pulses is insufficient and the PVP remains undecomposed. On the other hand, the smaller the scanning speed, the more the laser is repeatedly irradiated to the surface with nanopores once the surface-dried film is removed and the surface is bare. Then, the material remaining in the pore is easily removed by direct interaction with the laser beam. Since the threshold of the number of pulses that can decompose PVP changes with laser fluence, the suitable scanning speed also changes accordingly. Furthermore, at very low laser fluence, the increase in overlap rate is thought to promote the formation of LIPSS. At high fluence, the YSZ itself is processed, where the surface area is increased by nanopore formation and the ablation threshold becomes lower than the original YSZ, as shown in Figure 5.  
In addition to laser parameter control during PVP removal, there are other factors to consider to obtain a more uniform silver-embedded surface. First, by applying the nano-silver dispersion liquid evenly to YSZ to maintain a constant dried film thickness, it is thought that the dried film will be removed evenly during laser irradiation, and the adhesion of residual PVP debris can be suppressed. Furthermore, it is necessary to process the nanopores with high density and uniformity. In this study, laser irradiation was performed using a circular Gaussian beam, which creates gaps between pulses. Therefore, it is considered effective to use, for example, a square laser beam with a flat-top energy distribution, and a line beam laser. In this method, a commercially available nano-silver dispersion was used. According to the above discussion, the dispersant PVP is thought to be suitable for embedding silver nanoparticles in nanopores because it is water-soluble, easily wets and spreads silver on the YSZ surface, and decomposes at a relatively low temperature. Although silver nanoparticles of a few nanometers in diameter were used, methods for generating particles of various sizes have been reported in recent years [48], and changes in embedding phenomena with particle size also require investigation. Based on the above results, the advantage of the silver embedding method proposed in this study is summarized. Table 3 shows some of the methods for loading materials on the material surface.