## 3 Results and Discussion
### Mechanical Properties of the Multi-Phases Coatings
For the nitrided F2 film, only slight scratch marks remained on the modified layer, with a depth and width of only 0.7 \(\upmu\)m and 150 \(\upmu\)m, respectively. However, the depth and width of the abrasive groove for the nitrided F3 film reached 4.3 \(\upmu\)m and 400 \(\upmu\)m, respectively. The EDS results for the A-F region in Figure 7 are listed in Table 5. The high O content indicates that oxidation occurred during the wear process. For the nitrided F1 film and F2 film, the N atoms in region B and region D reached 15.9 at.% and 6.7 at.%, respectively, which indicates that the nitrides formed, and this can effectively improve the wear resistance. The acoustic signal curves and friction force curves of the multi-phase coatings on the C61900 Cu alloy are shown in Figure 8. The friction force increased slowly as the load grew to be about 18 N, and no change in the acoustic signal was detected. During the scratching test of the modified surfaces, the load continuously increased. When the normal load increased to the critical value, not only did a sudden change in the friction force occur, but also a sharp increase in the acoustic signal could be detected due to the fracture or separation of the multi-phase coatings. Thus, the critical load of failure could be obtained, which was dependent on the coating thickness, the residual stress, and the mechanical properties of the substrate (e.g., hardness), as well as the test parameters (e.g., ram radius and sliding speed) [33,34]. With the further increase in the load to 45 N, a spike in the acoustic signal was induced at the critical load, indicating that the brittle coating was broken, as shown in Figure 8a. For the nitrided F2 film, the critical load for the multi-phase coating was 38 N, and the substrate was gradually exposed at a distance of about 1 mm from the starting point, as shown in Figure 8b,c. Similarly, for the nitrided F3 film, the  
Figure 6: The friction coefficient and wear rate of C61900 Cu alloy and the modified surface: (**a**) friction coefficient, (**b**) wear rate. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Elements** & **C** & **N** & **O** & **Al** & **Ti** & **Fe** & **Cu** & **W** \\ \hline A & 5.97 & 4.63 & 54.41 & 0.75 & 28.8 & 3.49 & 0.86 & 1.08 \\ B & 9.44 & 15.9 & 31.38 & 1.44 & 38.05 & 2.51 & 1.28 & - \\ C & 9.29 & 2.63 & 51.69 & 0.86 & 14.19 & 19.73 & 0.63 & 0.99 \\ D & 7.44 & 6.7 & 49.53 & 1.75 & 20.97 & 11.63 & 1.67 & 0.31 \\ E & 11.44 & - & 34.65 & 11.01 & 6.12 & 2.04 & 34.74 & - \\ F & 12.81 & - & 26.52 & 12.68 & 10.54 & 2.04 & 35.41 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Semi-quantitative EDS results for areas A–E in Figure 7 (at.%). friction force gradually increased with the load to about 17 N, and no change in the acoustic signal appeared until the load increased to the critical load of about 44 N. The friction force curve also fluctuated at the intensity peak of the acoustic signal, as shown in Figure 8d. It is well known that the wear resistance is closely related to the surface hardness and elastic modulus. In the present work, the wear resistances of the three types of coatings were all improved, although the measured surface hardness of the modified coatings was even lower than that of the Cu substrate at the initial indentation depth (see Figure 4a). The measured average hardness by the indentation could not reflect the wear resistance appropriately. In addition, the scratching force as well as the acoustic emission signal always showed some fluctuation. Therefore, it is of great importance to study the mechanical properties of the possible formed phases by first principle calculations to provide basic knowledge on the obtained surface properties of the multi-phase coatings. Figure 7: The surface morphology and cross-sectional profile of the worn tracks after plasma nitriding at 650 \({}^{\circ}\)C for 4 h: **(a,b)** for F1 film, **(c,d)** for F2 film, **(e,f)** for F3 film.