# Influence of Carbon: Metal Ratio on Tribological Behavior of Mo-W-C Coating
## 2 Materials and Methods
### Coating Characterization  
The microstructures and imaging related to the coating properties of C2, C4 and C6 were studied by using an FEI NOVA NANOSEM 200 coupled with an Energy Dispersive X-ray (EDX) module. The elemental composition was examined by using a GD-Profiler 2\({}^{\mathrm{TM}}\) glow discharge optical emission spectroscopy (GDOES) equipped with an RF source, and the depth profiling was performed up to 8 \(\upmu\)m from the coating surface. The C/(Mo+W) ratio was calculated from the depth-profile data. An automatic REVETEST scratch tester from CSM instruments was used to assess the critical load (Lc\({}_{3}\)) for coating delamination, and therefore the adhesion strength between coating and substrate. A cone-shaped (120\({}^{\circ}\)) diamond indenter with a preset load of 10 N was allowed to scratch about 10 mm length till the normal force gradually increases to 100 N. The nano-hardness and elastic modulus values were measured by using a CSM nano-indentation tester with a Berkovich diamond indenter. Then 20-25 repetitions with a preset load of 10 mN were carefully performed on the coating surfaces in order to avoid any rough surface features that can impact the result. The resultant load-displacement curves were used to calculate the hardness and modulus values, using the Oliver and [PERSON] method, and then their average was plotted. The associated software measured both the hardness and modulus values in GPa unit, but later the hardness values were converted to Vickers units (HV). The tribological behavior in boundary lubricated condition was studied by using CSM pin-on-disc tribometer at both ambient temperature condition (~30 \({}^{\circ}\)C and relative humidity of ~30%) and at 200 \({}^{\circ}\)C. The lubricant was Mobil1 Extended life\({}^{\mathrm{TM}}\) 10 W-60 engine oil. It has a flashpoint of 234 \({}^{\circ}\)C and contains both anti-wear (AW) and sulfide-based extreme pressure (EP) additives, but no friction modifiers (i.e., MoDTC) [25]. Therefore the friction behavior in this work is not influenced due to the presence of MoDTC as previously reported for several metal-doped DLC coatings [14; 15; 16; 17]. The sliding tests were performed under 5 N static load against uncoated 100 Cr6 steel balls (average hardness of ~260 HV) of 6 mm diameter. The sliding distances were fixed to 7500 m for ambient temperature tests and 1800 m for 200 \({}^{\circ}\)C tests. The coefficient of friction was calculated as \(\mu=F_{T}/F_{N}\), where \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Process Steps**} & \multicolumn{3}{c}{**Process Parameters**} \\ \cline{2-3}  & **Chamber Pressure (mbar)** & **Temperature (\({}^{\circ}\)C)** & **Other Operational Parameters** \\ \hline \multirow{2}{*}{Target cleaning} & \multirow{2}{*}{\(2\times 10^{-3}\)} & \multirow{2}{*}{150} & Graphite targets (UBMS mode, 1–3 KW) \\ \cline{3-3}  & & & Mo-WC target (HIPIMS mode, 900–1250 V) \\ \hline \multirow{4}{*}{HIPIMS Ion etching} & \multirow{4}{*}{\(1\times 10^{-3}\)} & \multirow{4}{*}{150} & HIPIMS ion etching at 1300 V with 40 min step time \\ \cline{3-3}  & & & Graphite targets (UBMS mode) OFF \\ \cline{3-3}  & & & Ar flow rate: 80 sccm \\ \cline{3-3}  & & & \(-\)1200 V substrate bias \\ \hline \multirow{4}{*}{Base-layer deposition} & \multirow{4}{*}{\(3.5\times 10^{-3}\)} & \multirow{4}{*}{250} & 30 min step time \\ \cline{3-3}  & & & Ar and N2 flow rate: 200 sccm (each) \\ \cline{3-3}  & & & \(-\)65 V substrate bias \\ \cline{3-3}  & & & Graphite targets (UBMS mode, 0.5 KW) \\ \cline{3-3}  & & & Mo-WC target (HIPIMS mode, 1000 V) \\ \hline \multirow{4}{*}{Mo-W-C coat-ing deposition} & \multirow{4}{*}{\(3.2\times 10^{-3}\)} & \multirow{4}{*}{250} & 4 h step time \\ \cline{3-3}  & & & Ar flow rate: 250 sccm \\ \cline{3-3}  & & & \(-\)65 V substrate bias \\ \cline{3-3}  & & & Graphite targets (UBMS mode, 5 KW) \\ \cline{3-3}  & & & Mo-WC target (HIPIMS mode, 2 KW for C2, 4 KW for C4 and 6 KW for C6) \\ \hline \hline \end{tabular}
\end{table}