# Influence of Carbon: Metal Ratio on Tribological Behavior of Mo-W-C Coating
## 4 Discussion  
The C/(Mo+W) ratio is a key to understand the microstructural characteristics of Mo-W-C coatings, which in turn influences the tribological behavior irrespective of the test conditions. As-deposited C2 contains a high C/(Mo+W) ratio of 5:1, whereas both C4 and C6 maintain low C/(Mo+W) ratios of 2.8:1 and 2.2:1, respectively (see Table 2). Raman analysis of these as-deposited coatings indicate the existence of both metal carbide phases (WC, W\({}_{2}\)C and Mo\({}_{2}\)C) and sp\({}^{2}\)-bonded carbon content representing their graphitic nature. Higher is the C/(Mo+W) ratio, more sp\({}^{2}\) bonded carbon content present in the coating increases the I\({}_{\rm D}\)/I\({}_{\rm C}\) ratio. So, a decrease in C/(Mo+W) ratio indicates a decrease in the I\({}_{\rm D}\)/I\({}_{\rm C}\) ratio and therefore the degree of graphitization in as-deposited coatings (see Figure 7a-c). This decrease in C/(Mo+W) ratio can be further correlated with an increase in the coating thickness, critical load for adhesion, hardness and elastic modulus values (see Table 3). A low C/(Mo+W) ratio indicates (i) the generation of high metal ion fraction during the ion etching step, which drastically increases the adhesion strength; and (ii) the presence of a significantly high Mo+W percentage, and therefore more hard metal carbide phases formed during coating deposition step, which straightaway increases the thickness, average hardness and elastic modulus of C4 and C6. These microstructural properties benefit the tribological behavior at both ambient temperature and 200 \({}^{\circ}\)C during lubricated sliding. Irrespective of the C/(Mo+W) ratios and the test temperatures, the abrasive wear mechanism is present throughout the sliding distance, whereas tribo-chemical wear mechanism is appeared to be particularly dominant during the steady-state period. These two simultaneous events, depending on the test temperature, influence the friction and wear behavior of Mo-W-C coatings during lubricated sliding:  
1. The abrasion dominates the initial run-in period, where the disorder of carbon-carbon bonds in as-deposited coatings is increased due to sliding. At ambient temperature, this disorder leads to decreased graphitic cluster size, which in turn increases the breathing mode of six-fold rings, resulting in high I\({}_{\rm D}\)/I\({}_{\rm C}\) ratios. Although the carbon debris particles retain their graphitic nature (Figure 7d-f), maintaining Tuinstra and Koenig (TK) relationship [31], they eventually weaken the material, leading to a very low \(\mu\) (Figure 6). These rings are further distorted during lubricated sliding at 200 \({}^{\circ}\)C, when they open up and form a chain, leading to conversion of graphitic debris particles towards amorphous phase, where TK relationship is no longer valid [31]. This conversion is evident from the significantly low I\({}_{\rm D}\)/I\({}_{\rm G}\) ratios (Figures 10 and 11). These amorphous carbon debris particles cause significant wear of the counterparts as compared to the coatings, but eventually helps in achieving friction reduction during steady-state period (Figure 8). 2. The tribo-chemical reactions take place between both Mo and W dopants of the coatings and the sulfide-based EP additives of the engine oil at the asperity contacts and form an in situ lubricious tribolayer containing MoS\({}_{2}\) and WS\({}_{2}\). The high flash temperature generated at the asperity contacts promotes these chemical reactions during sliding at ambient temperature, and then it is further accelerated when the test temperature is increased to 200 \({}^{\circ}\)C. The formed tribolayer contains mainly WS\({}_{2}\) at ambient temperature (evident from Raman spectra in Figure 7d-f), but both MoS\({}_{2}\) and WS\({}_{2}\) at 200 \({}^{\circ}\)C (evident from both the Raman and EDX analyses in Figures 9-11). This protective tribolayer leads to negligible wear at ambient temperature (Figure 12) and slightly higher wear at 200 \({}^{\circ}\)C (Figure 13), but without any substrate exposure.