## 3 Experimental Results  
### Surface Comparison  
Surface roughness plays a fundamental role in antibacterial properties and for this reason, the surface morphologies were studied and compared before and after deposition. Figure 1 shows sample surfaces before and after laser cladding, a treatment which increased the surface roughness of all samples. The roughness of our LDPE plate prepared surface finishing was initially in the range of Ra = 0.30 \(\pm\) 0.02 \(\upmu\)m (Figure 1a). It increased to Ra = 0.81 \(\pm\) 0.28 \(\upmu\)m after treatment (Figure 1b). In the case of Ti6 Al4V, the initial surface roughness of Ra = 0.63 \(\pm\) 0.09 \(\upmu\)m (Figure 1c) increased to Ra = 11.23 \(\pm\) 1.11 \(\upmu\)m (Figure 1d), a value much higher than that measured on either ceramic substrate. This effect was caused by localized melting of the Ti6 Al4V substrate, which resulted in an alteration of the coating-substrate interface morphology and the formation of an alpha phase. In the case of ZTA, the roughness changed from Ra = 1.02 \(\pm\) 0.02 \(\upmu\)m (Figure 1e) to Ra = 4.59 \(\pm\) 1.29 \(\upmu\)m, due to the presence of partially melted particles. This is clearly visible in Figure 1f. Because the Y-TZP test bars were polished prior to laser cladding, their initial surface roughness was lower, Ra = 0.02 \(\pm\) 0.01 \(\upmu\)m (Figure 1g). This increased to Ra = 1.27 \(\pm\) 0.42 \(\upmu\)m after laser treatment (Figure 1h). On the LDPE substrate, the LDPE matrix under the Si\({}_{3}\)N\({}_{4}\) cladding (Figure 2a) and on the cross-section images (b), as well as the Si\({}_{3}\)N\({}_{4}\) particle penetration inside the LDPE matrix, are visible. Since the silicon nitride particles have a higher absorbance at 1064 nm [23] when compared to LDPE [44], the ceramic particles heat up and locally melt the polymeric substrate, resulting in the incorporation of silicon nitride into LDPE. As a result, at 225 V the heated particles melt LDPE (at about 120 \({}^{\circ}\)C) without causing visible decomposition, which occurs between 350 and 450 \({}^{\circ}\)C [45]. On the Ti6 Al4V substrate, the silicon nitride coating is more homogeneous, as can be observed in Figure 2c. The blue signal (titanium) is still present, but mixed with the other elements, it changes to a laverader color. On the cross-section of the Ti6 Al4V sample (Figure 2d), it is possible to clearly distinguish between the substrate and the cladded layers. Only occasional formation of Si-Ti intermetallics (see arrow) is evident. Moreover, the coating is clearly composed of two different phases: a silicon-rich (red) layer with a metallic appearance, and a layer with fine dispersion of Si\({}_{3}\)N\({}_{4}\) crystals (green) where the nitrogen signal is still intense. Laser cladding on ceramic substrates resulted in coatings with a similar morphology, composed of partially melted silicon grains (Figure 2e,f). Due to the high melting point and low surface roughness of the ZTA and Y-TZP substrates, the cladded layer did not strongly bond with the substrate, resulting in partial delamination. Previously, we observed an increase in bonding strength when the substrates were pre-scratched using a diamond tip pen [37]. The cross-section image (Figure 2g) of a Y-TZP pre-scratched substrate shows a more homogeneous layer when compared to both the LDPE and the Ti6 Al4V substrates. Figure 1: Laser microscope substrate surface images: untreated LDPE (**a**) and coated LDPE (**b**), untreated Ti6 Al4V (**c**) and coated Ti6 Al4V (**d**), untreated ZTA (**e**) and coated ZTA (**f**), and untreated Y-TZP (**g**) and coated Y-TZP (**h**). Figure 4: Raman imaging maps and Raman spectra of coated Ti6 Al4V substrate. Figure 5: Raman imaging maps of coated ZTA and Y-TZP substrate. Figure 3 shows the maps acquired on the LDPE substrate coated with silicon nitride powders. The first image presents the optical morphology of the surface, with clearly visible silicon nitride particles.