# Influence of Carbon: Metal Ratio on Tribological Behavior of Mo-W-C Coating
## 3 Results
### Friction Behavior during lubricated Sliding
The MoS\({}_{2}\) and WS\({}_{2}\) phases were in situ formed at the asperity contacts due to the tribo-chemical reactions that occurred between the Mo and W dopants of the coatings and sulfide-based EP additives present in the engine oil during sliding. The rate of these chemical reactions was further promoted at 200 \({}^{\circ}\)C, as evident from the high-intensity Raman peaks, and both MoS\({}_{2}\) and WS\({}_{2}\) played an important role in friction reduction. In addition to this, the severe abrasion during sliding drastically decreased the degree of graphitization of the as-deposited coatings, which further reduced the \(\upmu\). This was particularly evident for C4 and C6, where graphitic (sp\({}^{2}\) bonded) carbons present in the coatings were converted into the amorphous phase during abrasion and therefore led to a significant reduction of \(\upmu\) after the run-in period. A low C/(Mo+W) ratio was found beneficial for these excellent frictional properties at 200 \({}^{\circ}\)C, where the reduction in \(\upmu\) was simultaneously influenced by (i) in situ MoS\({}_{2}\) and WS\({}_{2}\) formation and (ii) conversion of graphitic-carbon-debris particles into the amorphous phase. The composition of the tribolayer formed at 200 \({}^{\circ}\)C was further studied from the debris adhered to the wear track of C2, C4 and C6, as shown in Figure 11. The locations for spectrum collection on the wear track are encircled and shown at the inset of each spectrum. After deconvolution of the spectra, the position of dominant Raman peaks and the calculated I\({}_{\text{D}}\)/I\({}_{\text{G}}\) ratio are reported in Table 3 for all of the abovementioned conditions. The wear track of C2 contained Mo\({}_{2}\)C phases similar to the as-deposited condition, but the I\({}_{\text{D}}\)/I\({}_{\text{G}}\) ratio of 1.36 indicated a conversion of graphitic carbon particles into the amorphous phase due to severe abrasion during sliding (Figure 11a). This is to note that the debris  
Figure 9: SEM images (taken at both low and high magnifications) of the tribolayer adhered to the steel counterpart and related EDX analysis on the debris positions ‘a’ and ‘b’ after lubricated sliding against (a) C2, (b) C4 and (c) C6 at 200 \({}^{\circ}\)C.  
adhered to the counterpart of C2 retains its graphitic nature (Figure 10a) in contrast to the debris accumulated within the wear track. In contrast to C2, dominant MoS2 and WS2 peaks were observed within the wear tracks of C4 and C6 (Figure 11b,c, respectively). The accumulated debris from the wear tracks of C4 and C6 showed I\({}_{\text{D}}\)/I\({}_{\text{G}}\) ratios of 1.55 and 1.49, respectively, indicating that the wear track contained amorphous carbon-debris particles (Figure 11b,c, respectively) similar to that observed on the counterpart surfaces (Figure 10b,c, respectively). Overall, the tribolayer composition was found identical from coatings with low C/(Mo+W) ratios when collected either from the counterpart surfaces or within the wear tracks. On the other hand, a high C/(Mo+W) ratio led to a noticeably different tribolayer composition within the wear track from the counterpart surface due to severe abrasion of the coating. The influence of this tribolayer on the wear behavior of C2, C4 and C6 was further studied and is reported in the next section. Figure 10: Raman spectra collected from the debris positions ‘a’ and ‘b’ (as mentioned in Figure 9) after lubricated sliding against (**a**) C2, (**b**) C4 and (**c**) C6 at 200 \({}^{\circ}\)C.