## 2 Materials and Methods
### Mechanical Tests  
The coated piston ring samples underwent hardness, roughness and abrasion tests to determine the wear resistance of the coating and possible usage area in industry. A reciprocating tribometer abrasion test device was used to scratch the surface using a 100 Cr6 steel ball (\(\O 5\)) moving across the coated surfaces under dry-sliding circumstances, as shown in Figure 2. With a standard load of 20 N and a stroke of 4.5 mm, the scratcher moved at a linear sliding speed of 2400 mm/min, subtending to a distance of 280 m. The temperature and relative humidity ranges for the tribological analyses of the steel specimens were 20-25 \({}^{\circ}\)C and 55-60%, respectively.  
The electronic balance (Shimadzu ATX224) was used to correctly weigh each sample before and after the friction test. The coefficient of friction (COF) was automatically calculated by the system considering the normal load and friction force between the pairs. To determine the wear resistance limits of the surface coatings, a constant high load was applied under dry-sliding conditions (20 N, Hertzian contact stress of approximately  
\begin{table}
\begin{tabular}{c c c} \hline
**Analysis** & **Device** & **Technical Specs** \\ \hline \multirow{2}{*}{Laser-cutting} & \multirow{2}{*}{\
\begin{tabular}{c} Senfeng \\ (Jinan City, China) \\ \end{tabular} } &
\begin{tabular}{c} Laser power: 1.5–3 kW \\ Positioning accuracy: \(\pm\)0.05 mm \\ Max. speed: 80 m/min \\ \end{tabular} \\ \hline \multirow{2}{*}{SEM and EDX} & FEI Quanta 650 & Resolution: 1.2 nm @ 30 kV \\  & (Hillsboro, OR, USA) & Magnification: 6–10\({}^{6}\times\) \\ \hline \multirow{2}{*}{XRD} & Rigaku Miniflex II & X-ray source: Cu anode \\  & (Tokyo, Japan) & (\(\lambda\) Cu K\(\alpha\) = 1.5418 Å), \\  & & Measurement range (20) = 2–145\({}^{\circ}\) \\ \hline \multirow{2}{*}{Raman} & Renishaw In Via Qontor & Wavelength range: 200–2200 nm \\  & (Wotton-under-Edge, UK) & Lasers: 229–1064 nm \\  & & Stability: \(\pm\)0.01 cm\({}^{-1}\) \\ \hline \multirow{2}{*}{AFM} & Park Systems NX10 & Field of view: 480 \(\times\) 360 \(\upmu\)m \\  & (Suwon, Republic of Korea) & (10\(\times\) objective lens) \\ \cline{1-1}  & & CCD: 5 MP \\ \hline \multirow{2}{*}{Profilometer} & Filmetrics Profilm 3D & Thickness range: 50–10 mm \\  & (Quezon City, Philippines) & Camera: 2592 \(\times\) 1944 (5 MP) \\ \hline \end{tabular}
\end{table}
Table 2: Technical specifications of the analysis devices.  
Figure 2: Abrasion test bench.  
1.7 GPa) [31]. The mass loss values of the samples obtained from the abrasion tests were transformed into wear rates using Equation (1):  
\[\phi=\frac{A*\frac{\updownarrow}{F*s}} \tag{1}\]  
\(\O\): wear rate (mm\({}^{3}\)/Nm)  
\(A\): trace area (mm\({}^{2}\))  
\(\ell\): amount of reciprocation (mm)  
\(F\): normal load (N)  
\(s\): stroke (m)  
The tribological tests were performed on the region where the main plasma bombardment (perpendicular plasma from 10 mm distance) was applied. A portable roughness measuring device (MarSurf M300) with a capacity of up to 350 \(\upmu\)m was utilized to capture the macro roughness data, and AFM imaging was employed to assess the R\({}_{\text{a}}\) of the samples within a scanning area of 5 \(\upmu\)m \(\times\) 5 \(\upmu\)m. The surface hardness of all the specimens were measured using a Bruker Universal Mechanical Tester (UMT) with a Vickers indenter tip, applying a 100 g load for a duration of 15 s.