# Metal Release and Cell Viability of 316L Stainless Steel Sputter-Coated with N-Doped a-C:H Coatings
## 3 Results and Discussion
### Corrosion Behavior
Such disappointing and unexpected results evidenced the need to consider the substrate/coating system as a whole. Metallic-based interlayers can actually induce galvanic coupling effects at reactive interfaces within the coating [41], deteriorating the corrosion resistance of the alloy with a resulting increase in the concentration of metallic ions in the electrolyte. In the current work, the increase in the overall release rate seems to be related to the doping element itself, when compared to the undoped CH_0N one. In fact, for the coatings with up to \(\sim\)9 at.% of N, more modest inhibition in the Ni release rates were achieved: only \(\sim\)20% (Figure 4), contrasting with the previously mentioned 70% for the undoped film. Similarly, the Fe release rates seem to follow Ni's, whereas a slight increase \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Coating** & **Condition** & **Pos(G)** & **Pos(D)** & **I\({}_{\text{D}}\)/I\({}_{\text{G}}\)** \\ \hline \multirow{2}{*}{**CH\_0N**} & As-deposited & 1552 & 1372 & 0.77 \\  & As-immersed & 1553 & 1374 & 0.76 \\  & As-deposited & 1553 & 1378 & 0.89 \\  & As-immersed & 1553 & 1378 & 0.89 \\  & As-deposited & 1560 & 1384 & 1.09 \\  & As-immersed & 1562 & 1385 & 1.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Raman curve fitting parameters of the as-deposited and as-immersed coatings. Figure 2: Evolution of the Ra and WCA with N doping content, for both (**a**) as deposited [13] and (**b**) post-immersion coatings, coupled with representative 3D AFM topographic maps (4 \(\times\) 4 \(\mu\)m\({}^{2}\)) and optical image of droplets. in Cr release was detected. Nitrogen addition clearly led to a lower corrosion inhibition ability of the DLC coating. According to the literature, N doping contents above \(\sim\)10 at.% are expected to increase the electrical conductivity of the a-C:H:N material [14], which is a serious disadvantage for protective coatings operating in corrosive/oxidizing environments. In fact, coatings with \(\sim\)11 at.% of N show improved electron transfer ability for redox reactions, which makes them suitable for other applications, such as biosensors [42]. However, for lower N contents, this effect has been reported to be minimal, and an improved corrosion resistance would be expected, by maintaining the threshold value of \(\sim\)10 at.%. This motivated the selection of this non-metallic element as the dopant in the current work. Surprisingly, the results were beyond expectations. While for the CH_9N, a deleterious effect of the doping level might occur due to the proximity with the threshold value, the opposite should occur for the CH_6N, in accordance with its better bacterial adhesion inhibition properties [13]. Overall, the high corrosion resistance of the 316L SS alloy can be further optimized by deposition a top compact well-adherent a-C:H film, with a WCA close to 80\({}^{\circ}\) and a promoting Ti-based adhesion layer. Figure 3: Raman results of the as-deposited and as-immersed coatings: (**a**) Raman spectra; and (**b**) obtained Pos(G), Pos(D), and I\({}_{\text{D}}\)/I\({}_{\text{G}}\) ratio upon curve fitting. A typical curve fitting for the as-deposited CH_0N coating into the G and D peaks is also presented.