# Influence of Carbon: Metal Ratio on Tribological Behavior of Mo-W-C Coating
## 3 Results
### Friction Behavior during lubricated Sliding
Figure 9 collates the SEM images of the tribolayer adhered to the steel counterparts of C2, C4 and C6 after lubricated sliding at 200 \({}^{\circ}\)C. For all three counterparts, the first SEM image shows the complete view of the wear scar on the ball surface, followed by two high magnification images of the adhered debris, which clearly point out positions 'a' and 'b' and their respective elemental composition via EDX analysis. Positon 'a' indicates the debris smearing over the ball surface, and position 'b' shows the debris being accumulated and thickened beside position 'a'. As observed from Figure 9a, the EDX spectra collected from both positions 'a' and 'b' of C2 counterpart contained Fe, Cr, C, W, Mo and S peaks. The Fe and Cr peaks were from the steel counterpart, whereas C peak could belong to both the coating and counterpart or to any of either. Specifically, W, Mo and S peaks indicated the formation of a tribolayer on the counterpart surface, where W and Mo came from the coating itself. In contrast to the C2 counterpart, a significant difference in the EDX spectra was observed when collected from the debris positions 'a' and 'b' for both C4 and C6 counterparts (Figure 9b,c, respectively). The EDX spectrum of position 'a' showed strong Fe, Cr and C peaks and weak W, Mo and S peaks, whereas only strong peaks of W, Mo and S were found in position 'b' indicating a clear difference in the tribolayer composition. This was particularly observed for low C/(Mo+W) ratios, and therefore Raman spectroscopy was used for further investigation. Figure 10 shows the Raman spectra collected from the abovementioned positions 'a' and 'b' of the steel counterparts sliding against C2, C4 and C6, respectively, at 200 \({}^{\circ}\)C. Each spectrum contains an image of the adhered debris at the inset, and the locations for spectrum collection are encircled. After deconvolution of the spectra, the position of dominant Raman peaks and the calculated I\({}_{\rm D}\)/I\({}_{\rm C}\) ratio are reported in Table 3 for all of the abovementioned conditions. The counterpart of C2 established the presence of sharp MoS\({}_{2}\) and WS\({}_{2}\) peaks at both positions 'a' and 'b', along with distinct D and G peaks only at position 'b' (Figure 10a). The I\({}_{\rm D}\)/I\({}_{\rm C}\) ratio was determined to be 2.14, representing the graphitic carbon debris particles within the tribolayer. On the other hand, the counterpart of C4 demonstrated the presence of WS\({}_{2}\) peak, along with dominant D and G peaks at position 'a', whereas position 'b' only contained sharp MoS\({}_{2}\) and WS\({}_{2}\) peaks (Figure 10b). An I\({}_{\rm D}\)/I\({}_{\rm C}\) ratio of 1.41 indicated the presence of amorphous carbon debris particles in the tribolayer. The counterpart of C6 showed Raman peaks almost similar to that of C4 at both positions 'a' and 'b' (Figure 10c), but with further reduction in I\({}_{\rm D}\)/I\({}_{\rm C}\) ratio to 1.3. Thus, the carbon debris particles of the formed tribolayer were more amorphous in nature for C6 than C4. A decrease in the C/(Mo+W) ratio showed conversion of graphitic-carbon debris into amorphous phase as a result of abrasion during sliding, but exhibited no significant  
Figure 8: Friction behavior of C2, C4 and C6 in lubricated condition against steel balls at 200 \({}^{\circ}\)C.  
influence on the MoS\({}_{2}\) and WS\({}_{2}\) phase formation. It should be noted that Mo peak was observed in the EDX spectra at position 'a' of both counterparts of C4 and C6; however, the presence of the MoS\({}_{2}\) peak was not detected via Raman analyses probably because of its tiny amount at the collection location. Figure 9: _Cont._  
The findings of EDX and Raman analyses together showed the tribolayer composition on the counterpart surface at 200 \({}^{\circ}\)C: (i) presence of MoS\({}_{2}\), WS\({}_{2}\) and graphitic carbon debris particles when sliding against C2; and (ii) presence of MoS\({}_{2}\), WS\({}_{2}\) and amorphous carbon debris particles when sliding against both C4 and C6.