# Metal Release and Cell Viability of 316L Stainless Steel Sputter-Coated with N-Doped a-C:H Coatings
## 3 Results and Discussion
### Corrosion Behavior  
Static immersion tests are economic and simple methods for studying the corrosion behavior of materials for extended time periods, simulating as closely as possible what might happen in real-life situations, namely in orthodontics. After the 30-day immersion test in acidic artificial saliva with pH 2.3, no coating damages were identified. As shown in Figure 2, the coatings' Ra systematically increased upon immersion (more than doubles), with more pronounced surface morphological features as seen in the 3D AFM topographic maps. Similarly, the wettability of the coatings increased, as seen by the opposite trend in the recorded WCA values (Figure 2), which may be justified by the variation of surface roughness. Indeed, and according to [PERSON]'s equation [28; 29], if the surface is in the hydrophilic regime (WCA \(<\) 90\({}^{\circ}\), which is the case of the coatings under study), an increase in surface roughness will lead to a decrease in the WCA, as previously reported [30]. Nonetheless, the undoped CH_0N was the most stable coating, while the doped CH_6N and CH_9N showed higher variations in the WCA. The corrosion-promoting environment can also foster the formation of oxygen-containing hydrophilic groups, accompanied by a reduction in the number of hydrocarbon groups [31], with a possible effect on the WCA of the aged coatings. Other more significant alterations, such as discoloration, detachment, or cracking detected by other researchers [32; 33; 34] were unnoticeable in the present study. The C-bonding configuration of the coatings was evaluated by Raman analysis, Figure 3. As can be seen, Raman spectra of CH_0N and CH_6N coatings almost overlap, indicating that the immersion did not significantly affect its as-deposited C-bonding configurations, Figure 3a. The evolution of the Raman features upon curve fitting is summarized in Table 2 and depicted in Figure 3b, namely regarding the D and G peak positions--Pos(D) and Pos(G) -, and the intensity ratio between both bands--I\({}_{\text{D}}\)/I\({}_{\text{G}}\). Nevertheless, results from the coating with the highest N content (CH_9N)--which was already the one with the highest as-deposited total sp\({}^{2}\)/sp\({}^{3}\) content [13]--suggest that it underwent graphitization upon the 30-day immersion. This is noticeable in Figure 3b, where both Pos(G) and I\({}_{\text{D}}\)/I\({}_{\text{G}}\) ratio rise, being indicative of a less disorderly structure with an increase in the size and/or number of sp\({}^{2}\) clusters [14]. Figure 1: Cross-section and surface SEM micrographs coupled with the corresponding EDS elemental distribution maps of the as-deposited (**a**) CH_0N and (**b**) CH_9N coatings deposited onto Si substrates. The N-induced decrease in the sp\({}^{3}\) content, along with the higher surface hydrophilicity [35], may help in explaining the worse corrosion behavior of the doped films. The average release rates of the three main elements in the SS nominal composition--Ni, Cr, and Fe--are compiled in Figure 4.  
The analysis of each metal in the solution revealed a substantial reduction in nickel and iron average release rates, by approximately 70%, for the steel substrates coated with undoped a-C:H, that is, CH_0N (Figure 4), while Cr release rate remained approximately constant. Accordingly, it is possible to conclude that the corrosion resistance of the medical-grade 316L SS in the acidic solution was improved by depositing the dense amorphous C-based coating with a Ti-based adhesion-promoting interlayer. Other authors also reported a significant decrease in the commonly focused Ni element from several metallic alloys [36; 37; 38; 39; 40], with an overall biocompatibility enhancement, by using DLC protective coatings. Nonetheless, in a previous study concerning the protection of SS against corrosion by coating with a-C:H-based coatings, considerably higher Ni, Cr, and Fe release rates from coated substrates were found when compared with the bare SS ones [18].