# Surface Functionalization of Polyethylene by Silicon Nitride Laser Cladding
## 3 Experimental Results  
### Surface Characterizations of Pristine and Coated Substrates  
Laser microscopy images and their relative three-dimensional plots are shown in Figure 1. The surface of the bulk Si\({}_{3}\)N\({}_{4}\) sample (Figure 1a) appeared quite smooth (Ra = 0.011 \(\pm\) 0.001 \(\upmu\)m) due to polishing [19]. A higher surface roughness was found for the PE bulk sample (Figure 1b) as a consequence of its preparation process (i.e., in particular, the surface roughness of the silicon rubber mold), resulting in a Ra = 0.297 \(\pm\) 0.022 \(\upmu\)m. The laser-cladded polyethylene (Figure 1c) showed the highest roughness (Ra = 0.602 \(\pm\) 0.138 \(\upmu\)m) among the investigated samples due to the presence of the Si\({}_{3}\)N\({}_{4}\) particles embedded in the PE surface and the irregular re-melting of the polymer.  
SEM images of the surface at higher magnification are provided in Figure 2. The surface of the untreated polyethylene (Figure 2a) appeared relatively smooth at low magnification, but revealed a lamellar structure with an average thickness of 3.5 \(\pm\) 0.6 \(\upmu\)m when observed under higher resolution. The morphology of the PE surface modified with Si\({}_{3}\)N\({}_{4}\) cladding (Figure 2b) was characterized by the presence of Si\({}_{3}\)N\({}_{4}\) particles partially embedded in the PE matrix. Micro-sized porosities can be observed on the surface due to the cleaning process: some silicon nitride particles, not completely embedded into the matrix, were removed from the treated surface during washing. The dimension of the particles was inhomogeneous (diameter between 5 and 25 \(\upmu\)m) and the distribution of the Si\({}_{3}\)N\({}_{4}\) particles was non-uniform across the surface.  
A colored EDS map obtained at 1000\(\times\) magnification (Figure 3) was used to check the distribution of carbon (green) as a marker for the presence of polyethylene matrix. As expected, the silicon signal (red) was concentrated at the ceramic particles with only limited overlap with the carbon signal in the regions where the particles were embedded in the matrix (_cf._ surface analysis on the left side of Figure 3). In a cross-section analysis (_cf._ on the right side of Figure 3), it was observed that the Si\({}_{3}\)N\({}_{4}\) particles did not penetrate deeply into the PE matrix. It was also observed that most of the particles  
Incorporation of Si\({}_{3}\)N\({}_{4}\) onto the PE surface was confirmed by Raman spectroscopy. Figure 4 shows the Raman spectra of the different samples: (a) bulk Si\({}_{3}\)N\({}_{4}\) (reference material), (b) low density PE, and (c) laser-cladded PE. Table 1 shows the band assignment relative to both raw materials. The strong Si\({}_{3}\)N\({}_{4}\) bands at about 200 cm\({}^{-1}\) in (c) confirm the presence of the \(\beta\)-Si\({}_{3}\)N\({}_{4}\) phase, but also other weaker vibrations, in particular, those related to the E\({}_{2g}\) mode are still clearly visible [20].  
Figure 4: Raman spectra of 3 different samples: (**a**) bulk Si\({}_{3}\)N\({}_{4}\), (**b**) PE substrate, and (**c**) Si\({}_{3}\)N\({}_{4}\)-coated PE substrate.  
Figure 3: EDS maps of the Si\({}_{3}\)N\({}_{4}\)-coated PE substrate; surface analysis and cross section analysis on the left and right side, respectively.  
Low density polyethylene Raman spectra (b) shows three main bands at 1062, 1081 and 1129 cm\({}^{-1}\) that represents the C-C stretching; additional bands (from 1170 to 1460 cm\({}^{-1}\)) represent different bending modes of the CH\({}_{2}\) bonds [21]. In the Raman spectrum of cladded PE, polymer-related bands were weaker due to the presence of the ceramic layer on the surface.