## 3 Results and Discussion  
### Surface Morphology Analysis  
Figure 2 presents the surface morphologies of DLC coatings prepared under varied gas flow ratios. All samples exhibit similar surface morphologies with some cracks. Figure 2a shows the SEM image at a nitrogen flow rate of 0 (sample C2N0), Figure 2b shows the surface morphology at a nitrogen flow rate of 4 sccm (sample C1N4), and Figure 2c shows the surface morphology at a nitrogen flow rate of 6 sccm (sample C1N6). It can be observed that after introducing nitrogen, the number of cracks on the surface of the film decreases. Overall, the addition of nitrogen effectively enhances the compactness of the DLC films. Figure 3 shows the variation in diamond-like carbon films under different nitrogen flow rates. This phenomenon is attributed to a greater incorporation of nitrogen into the amorphous carbon network, which reduces the internal stress at the interface and, thus, promotes film growth [25]. However, as the methane flow rate decreases and relatively excessive nitrogen flow prevails, the film growth rate diminishes.  
Figure 4 displays AFM images of N-DLC thin films, denoted as (a-e) corresponding to compositions C1N8, C1N6, C1N4, C1N2, and C2N0, respectively. Examination of the images reveals the presence of fine nanoscale particles dispersed across the surfaces of all film samples. Notably, at lower nitrogen flow rates, these nanoparticles tend to agglomerate, resulting in a non-uniform distribution on the film surface. In contrast, as the nitrogen flow  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **The Gas Flow** & **The Gas Flow** & **Substrate** & **RF** & **Deposition** & **Deposition** \\  & **of CH\({}_{4}\)** & **of N\({}_{2}\)** & **Temperature** & **Power** & **Time** & **Pressure** \\  & **(sccm)** & **(sccm)** & **(\({}^{\circ}\)C)** & **(W)** & **(min)** & **(Pa)** \\ \hline C2N0 & 20 & 0 & & & & \\ C1N2 & 18 & 2 & & & & \\ C1N4 & 16 & 4 & 25 & 100 & 75 & 18 \\ C1N6 & 14 & 6 & & & & \\ C1N8 & 12 & 8 & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Deposition parameters of DLC films for each sample.  
Figure 2: Surface and cross-sectional morphology of films at different gas flow ratios. (**a**) simple C2N0; (**b**) simple C1N4; (**c**) simple C1N6; (**d**) cross-sectional of sample C1N6.  
rate increases, the nanoparticles disperse more homogeneously and gradually diminish in size, ultimately leading to a uniform surface morphology. Consequently, the N-DLC thin films exhibit smoother and more compact surfaces.  
Figure 5 presents the surface roughness variations in the N-DLC thin films. It is evident that Ra initially increases and then decreases with an increasing nitrogen flow rate in the introduced gas. The fluctuations in roughness can be elucidated through the underlying principles governing the thin film deposition process [26]. Upon arrival at the substrate surface, gas-phase atoms experience reflection due to their higher energy levels, while another fraction is adsorbed onto the substrate surface. Adsorbed gas-phase atoms undergo diffusion and migration across the surface, resulting in collisions and coalescence to form  
Figure 4: AFM images of DLC films: (**a**) C1N8, N2 flow ratio 40%; (**b**) C1N6, N2 flow ratio 30%; (**c**) C1N4, N2 flow ratio 20%; (**d**) C1N2, N2 flow ratio 10%; (**e**) C2N0, N2 flow ratio 0%.  
Figure 3: Variation in film thickness with gas flow ratio.  
small atomic clusters that subsequently condense into clusters on the substrate, leading to increased surface roughness. We can see that when the nitrogen flow rate accounts for 20% of the total gas flow rate, the surface roughness of the sample is the highest. Further increasing the nitrogen flow rate reduces the roughness of the sample, and the addition of more nitrogen elements makes the film smoother.