# Influence of Carbon: Metal Ratio on Tribological Behavior of Mo-W-C Coating
## 3 Results
### Friction Behavior during lubricated Sliding  
The most extensively researched coating for enhanced tribological properties during room-temperature lubricated sliding is Cr-DLC. When a combination of different lubricants was attempted to understand the friction performance of hybrid ion beam/unbalanced magnetron-sputtered Cr-DLCs, the mean friction coefficient (\(\upmu\)) of \(\sim\)0.1 was observed as being almost stable irrespective of the Cr content and different lubricant combinations for 2400 m of sliding distance [29]. Thus, a combined steered cathodic arc/unbalanced magnetron-sputtered Cr/C coating was included in this study to set up a reference for the tribological performance of Mo-W-C coatings. Figure 6 shows the friction behavior of C2, C4 and C6 in lubricated conditions, along with Cr/C coating at ambient temperature. All coatings showed a very short run-in period, followed by an extended steady-state period through which \(\upmu\) remained almost stable. Cr/C coating showed \(\upmu\)-0.12, which was \(\sim\)2-3 times higher as compared to the Mo\(-\)W\(-\)C coatings (C2, C4 and C6 showed \(\upmu\)-0.047,  
Figure 5: Nano-hardness and elastic modulus of C2, C4 and C6. -0.033 and -0.052, respectively). The low \(\mu\) of Cr/C coating was attributed to the formation of a little amount of lubricious CrCl\({}_{3}\) during sliding [26], whereas Raman spectroscopy was used to investigate the reasons for Mo-W-C coatings and the influence of the C/(Mo+W) ratio on such friction reduction. Figure 7a-c shows the Raman spectra collected from C2, C4 and C6 in as-deposited condition, and Figure 7d-f shows the same when collected within the wear tracks after lubricated sliding at ambient temperature. After deconvolution of the spectra, the positions of both disordered (D band) and sp\({}^{2}\)-bonded graphitic carbon (G band) peaks, as well as the calculated I\({}_{\rm D}\)/I\({}_{\rm G}\) ratio, are reported in Table 3 for all of the abovementioned conditions. The other deconvoluted Raman peaks are not discussed in this work unless related to the tribological properties (if interested, readers can look into the details of all other Raman peaks here [30]). In Figure 7a-c, an I\({}_{\rm D}\)/I\({}_{\rm G}\) ratio of 2.51 clearly indicated the graphitic nature of C2, along with the presence of Mo\({}_{2}\)C peaks, whereas significantly high Mo+W content of C4 and C6 led to the presence of less sp\({}^{2}\) bonded carbon and therefore a reduction in I\({}_{\rm D}\)/I\({}_{\rm G}\) ratio (1.96 and 1.85 respectively). Thus, a decrease in the C/(Mo+W) ratio indicated a decrease in the degree of graphitization in the as-deposited coatings. Mo\({}_{2}\)C peaks were also identified in C4, but could not be clearly observed in C6, due to peak shift towards lower wave number and associated peak broadening effect. When Raman spectra were collected within the wear tracks of these coatings, Figure 7d-f showed the presence of a sharp WS\({}_{2}\) peak (apart from Mo\({}_{2}\)C and WC peaks) and a noticeable increase in the I\({}_{\rm D}\)/I\({}_{\rm G}\) ratio (2.69, 2.72 and 2.46 for C2, C4 and C6 respectively). During lubricated sliding, two simultaneous events took place: (i) the high flash temperature at the asperity contacts promoted chemical reactions between the engine oil and coating resulting in an in situ formation of lubricious WS\({}_{2}\) within the wear track; and (ii) the sliding increased the carbon-carbon bond disorder in the coatings, leading to the formation of debris of more graphitic in nature (i.e., higher sp\({}^{2}\) content), which was represented by the high I\({}_{\rm D}\)/I\({}_{\rm G}\) ratio. Both of these events significantly reduced the \(\mu\) at ambient temperature [23].