## 2 Materials and Methods  
### Coating Production  
Fatigue and tensile specimens were prepared by cutting AZ91 Mg and AA6063 Al alloys whose chemical compositions are given in Table 1. The mechanical characteristics are shown in Table 2.  
Cr/CrN/TiCrN/TiCrCN multilayer nitride coatings (Figure 1) were coated by a CFUBMS system. A titanium target (99.95% pure) and a chromium target (99.95% pure), connected to a pulsed DC power supply, were used in the process of multilayer nitride coating. An ion cleaning procedure was initially performed on AZ91 and AA6063 materials at a negative voltage of 800 V for 40 min. After the above-mentioned procedure, a chromium interlayer was added to increase the adhesion between the base materials and the coating. At this stage, the base material's negative voltage was kept at 250 V, whereas a current of 2.5 A was applied to the chromium target for 10 min. Afterward, the current applied to the chromium target was set to 3 A and the CrN layer was proceeded to. During this 15 min stage, N\({}_{2}\) gas was supplied to ensure that the nitride phase was formed. The N\({}_{2}\) gas flow was kept constant at 7 sccm. Then, a current of 3 A was applied to the titanium targets for 15 min. For the TiCrN layer, a current of 3 A was applied to the titanium and chromium targets during the 15 min procedure. Finally, the TiCrCN layer was proceeded to. A current of 3 A was applied to the titanium and chromium targets, while a current of 1.5 A was applied to the carbon. The constant negative voltage applied to the base materials was 50 V. During the entire coating procedure, the operating pressure was kept at 2.5 \(\times\) 10\({}^{-3}\) torr, the frequency was kept at 100 kHz, the impulse duration was adjusted at 2 \(\upmu\)s, and the duty cycle was set at 20%.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Alloys** & **Al** & **Zn** & **Mn** & **Si** & **Cu** & **Fe** & **Cr** & **Others** & **Ti** & **Mg** \\ \hline AZ91 & 8.84 & 0.61 & 0.18 & 0.02 & 0.005 & - & - & - & Balance \\ AA6063 & Balance & 0.1 & 0.1 & 0.2–0.6 & 0.1 & 0.35 & 0.1 & 0.15 & 0.1 & 0.45–0.9 \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition of AZ91 and AA6063 alloys.  
\begin{table}
\begin{tabular}{c c c} \hline
**Mechanical Characteristics/Alloys** & **AZ91** & **AA6063 (T6)** \\ \hline Modulus of Elasticity (GPa) & 45 & 68 \\ \hline Tensile Strength (MPa) & 230 & 205–245 \\ \hline Yield Strength (MPa) & 150 & 170–210 \\ \hline Hardness (Brinell) & 63 & 75 \\ \hline Elongation (\%) & 3–7 & 12 \\ \hline Poisson ratio & 0.35 & 0.33 \\ \hline \end{tabular}
\end{table}
Table 2: Mechanical characteristics of AZ91 and AA6063 alloys.  
The films' structural properties and post-damage surface images were obtained using XRD, SEM, and EDS. Energy dispersive spectrometry (EDAX) determined the chemical composition of the Cr/CrN/TiCrN/TiCrCN multilayer nitride films. The SEM analyses of the morphology of fatigue-tensile fracture surfaces were conducted on an FEI QUANTA FEG 450 model scanning electron microscope. XRD measurements were made using a PANALYTICAL EMPYREAN model diffractometer equipped with CuK\(\alpha\) (\(\lambda\) = 1.5404) at a scanning range of 20-90\({}^{\circ}\) and a 2\({}^{\circ}\)/min scanning speed. A Mahr surface profilometer determined surface roughness (Ra) values.