# Influence of Carbon: Metal Ratio on Tribological Behavior of Mo-W-C Coating
## 3 Results  
### Coating Microstructure and Composition  
Figure 2 shows the surface morphology, cross-sectional microstructure and coating thickness of C2, C4 and C6. The nucleation during the coating growth resulted in little spherical grains with a few tens of nm diameter. Competitive growth of these small grains led to the formation of large grains of few \(\upmu\)m diameter as observed in Figure 2a,c,e. The cross-sectional micrographs (Figure 2b,d,f) revealed the coating architecture consisting of a thin base layer (in the range of \(\sim\)107-150 nm), followed by a dense columnar Mo-W-C structure with domed tops. C2 showed fine columnar grains, which then coalesced and formed larger clusters in C4 as a result of increased target power to 4 KW. With a further increase in the target power to 6 KW, the high adatom mobility led to densification of the fine columnar microstructure with no clear detectable column boundaries in C6. The sputtering yield of the compound target was also increased with an increase in the operating power, eventually leading to an increase in coating thickness. The overall coating thickness was observed as \(\sim\)1.89, \(-\)2.24 and \(\sim\)2.79 \(\upmu\)m for C2, C4 and C6, respectively. Figure 3 shows the elemental distribution of C2, C4 and C6, as understood from the GDOES analysis. Elements such as C, Mo and W represented the Mo-W-C coating, whereas Fe indicated the steel substrate underneath. During the depth profiling (Figure 3a-c), all three coatings were observed as being mainly carbon-rich (\(\sim\)65-78%), whereas respective base layers were found to be predominantly metal-rich (Mo+W \(-\)40-50%). Figure 3d summarizes the elemental composition of C2, C4 and C6 from the depth-profiling data. As expected, the percentage of individual metal dopants (Mo, W), and therefore the total metallic content (Mo+W), was gradually increased with an increase in the target power from 2 to 6 KW. The Mo+W content was found to be \(\sim\)15.44%, \(-\)24.38% and \(-\)29% in C2, C4 and C6, respectively, which corresponded to the C/(Mo+W) ratios of 5:1, 2.8:1 and 2.2:1, as detailed in Table 2. It should be noted that C4 and C6 were deposited at two- and three-times higher target power respectively than C2, but the complex sputtering rate of the compound metallic target did not lead to the likewise increase in the Mo+W content. C4 and C6 contained \(\sim\)1.6 and \(\sim\)1.9 times higher Mo+W content than C2, thus resulting in relatively low C/(Mo+W) ratios. The SEM images in Figure 2 further confirmed the linear relationship between the increase in coating thickness and Mo+W content. Therefore, a decrease in the C/(Mo+W) ratio indicated an increase in the coating thickness. Surface MorphologyCross-sectional microstructure Figure 4a-c shows the adhesive strength of C2, C4 and C6 in terms of critical load L\({}_{\text{C3}}\), and Figure 4d-f shows the condition of respective scratch track ends for 100 N applied load. Once the critical load is reached, the adhesive failure leads to substrate exposure within the scratch track. C2 showed the lowest L\({}_{\text{C3}}\) (\(\sim\)34.6 N) when compared to C4 and C6 (L\({}_{\text{C3}}\) of 80.8 and 85 N, respectively), and therefore a decrease in the C/(Mo+W) ratio indicated an increase in the adhesion strength. This high adhesion strength of C4 and C6 was specifically attributed to their high target power during coating deposition. An operating target power in the range of 4-6 KW generated a significant amount of highly energetic metal ions, which replaced the substrate atoms during the ion etching step, followed by thorough bonding between the identical elements across the interface of coating and substrate. This bonding \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Coating**} & **Carbon Content** & **Total Metallic** & **C/(Mo+W)** \\  & **(Atomic \%)** & **Content (Atomic \%)** & **Ratio** \\ \hline C2 & 77.95 & 15.44 & 5:1 \\ C4 & 69.07 & 24.38 & 2.8:1 \\ C6 & 64.47 & 29 & 2.2:1 \\ \hline \hline \end{tabular}
\end{table}