## 3 Results and Discussion  
### Microstructure and Phase Compositions of the Multi-Phase Coatings  
The compositions of the fabricated Cu-Ti films on the surface of the C61900 Cu alloy measured by EDS are listed in Table 4. The Cu-Ti film with Ti/Cu atomic ratios was close to 7:1, 7:4, and 1:2 for F1, F2, and F3. Figure 1 shows XRD patterns of the Cu-Ti films obtained on the C61900 Cu alloy. The composed phases differed depending on the Ti/Cu atomic ratio. Specifically, the crystalline phases of the F1 film were mainly \(\beta\)-Ti phase, Ti\({}_{2}\)Cu phase, and a trace of \(\alpha\)-Ti phase, as shown in Figure 1a. The intense peak at 2\(\theta\) = 38.2\({}^{\circ}\) was from \(\beta\)-Ti rather than \(\alpha\)-Ti, where the formation of \(\beta\)-Ti phase was promoted by the Cu substrate for the same body-centered cubic structure. In comparison, the main phases of the F2 film were Ti\({}_{2}\)Cu and a small amount of CuTi phase, while the F3 film contained no crystalline Cu-Ti phase, as shown in Figure 1b,c, respectively. The surface morphology of the Cu-Ti  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Process** & **Ti Target Power (W)** & **Ti Target Current (A)** & **Cu Target Power (W)** & **Cu Target Current (A)** \\ \hline F1 & 2500 & 7.00 & 100 & 0.33 \\ F2 & 2000 & 5.69 & 300 & 0.84 \\ F3 & 1000 & 3.02 & 450 & 1.18 \\ \hline \end{tabular}
\end{table}
Table 3: The parameters for the fabricated films with different Cu, Ti contents.  
films also changed, as shown in Figure 1d-f, where the F1 film was smooth with uniform particles, and the F2 and F3 films were much finer.  
The three Cu-Ti films prepared on the C61900 Cu alloy were then treated by plasma nitriding at 650 \({}^{\circ}\)C for 4 h. Except for the Cu-Ti intermetallics and Ti-N compounds, an AlCu\({}_{2}\)Ti phase also formed after plasma nitriding, as shown in Figure 2a. The grazing incidence XRD of the Cu-Ti films after nitriding at 650 \({}^{\circ}\)C indicated that the TiN and TiN\({}_{0.3}\) phases existed in the outmost layer, and the diffraction peak of the Cu\({}_{3}\)Ti and AlTi phases in the F2 film coated surface was enhanced simultaneously after nitriding at 650 \({}^{\circ}\)C for 4 h, as shown in Figure 2b.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{**Coating Process**} & \multicolumn{2}{c}{**Film Composition**} & \multicolumn{2}{c}{**Film Thickness**} \\ \cline{3-5}  & & **Cu (at.\%)** & **Ti (at.\%)** & **(\(\upmu\)m)** \\ \hline \multirow{3}{*}{C61900} & F1 & 12 & 88 & 3.5 \\  & F2 & 36 & 64 & 3.5 \\ \cline{1-1}  & F3 & 65 & 35 & 2.9 \\ \hline \hline \end{tabular}
\end{table}
Table 4: The composition and thickness of the Cu-Ti films fabricated by magnetron sputtering on C61900 Cu alloys.  
Figure 1: XRD results and SEM images of the Cu-Ti films on C61900 Cu alloy: (**a,d**) for F1, (**b,e**) for F2, (**c,f**) for F3.  
Figure 2: (**a**) XRD results of the Cu-Ti films after plasma nitriding at 650 \({}^{\circ}\)C for 4 h, (**b**) GLXRD results of the Cu-Ti films after plasma nitriding at 650 \({}^{\circ}\)C for 4 h.  
After nitriding at 650 \({}^{\circ}\)C for 4 h, the surface morphology of the Cu-Ti films changed, as shown in Figure 3. The discontinuous white Fe-rich phase formed on the surface, which was attributed to the sputtering of the Fe element in the hollow cathode net, as indicated by the EDS results for the selected area. For F2 and F3 films, the size of the Fe-rich phase particles became larger (about 2-3 \(\upmu\)m). In addition, no Cu element for F1 film after nitriding was detected, and the Cu content on the surface of F3 film decreased.