## 3 Results and Discussion
### Mechanical Properties of the Multi-Phases Coatings  
The change in the hardness and elastic modulus of C61900 Cu alloy with increasing indented depth is shown in Figure 4. The hardness of the unmodified C61900 Cu alloy substrate increased rapidly up to 6 GPa and then decreased gradually with the growth of depth, which was kept constant at 3 GPa. The elastic modulus of the C61900 Cu alloy shared the same trend until it reached about 125 GPa. The increase in the early hardness and modulus was attributed to the size effect and work hardening effect. For the multi-phase coatings, the hardness of the modified layer at 0-300 nm from the surface was lower than the C61900 Cu alloy for the F1 and F3 films, and the hardness of the nitrided F2 film was the highest at this stage, which was dependent on the particle size and the surface morphology, as indicated in Figure 3. The hardness and modulus for the nitrided F1 and F2 films followed the same changing trend. The maximum hardness value of nitrided F1 film and F2 film reached 5.5 GPa and 4 GPa, respectively. A critical indented depth of about 500 nm existed, where the hardness and modulus of the F1 film after nitriding were highest when the depth was over 500 nm. Figure 5 shows the morphology of the cross-sections and the EDS results after nitriding at 650 \({}^{\circ}\)C for 4 h. The thickness of the F1, F2, and F3 films after nitriding was 7 \(\upmu\)m, 6 \(\upmu\)m, and 3 \(\upmu\)m, respectively. The Ti content remained at a high level only in a range of about 650 nm from the surface for F1 and F2. Combined with the results of EDS and GIXRD, it was found that the multi-phase coatings consisted of a nitride layer as the outmost surface and a thicker intermediate layer of Cu-Ti and AlCu\({}_{2}\)Ti. In addition, a discontinuous Cu-rich layer appeared in the multi-phase coating of the nitrided F3 film. Due to the strong affinity between Ti and N, the Ti-N compounds formed on the surface after nitriding under high nitrogen content, so Cu accumulated in the subsurface. Figure 3: Surface morphology and the EDS results for the Cu-Ti film on C61900 Cu alloy after plasma nitriding at 650 \({}^{\circ}\)C for 4 h: (**a**) F1, (**b**) F2, (**c**) F3, (**d**) EDS result for point A in (**a**), (**e**) EDS result for region B in (**c**). The friction and wear properties of the C61900 Cu alloy and the modified surface were evaluated. The surface topography of the worn track and the friction coefficient of the C61900 Cu substrate against the WC ball showed serious surface wear with a deep worn track of 12.7 um (as displayed in our previous work [18]). As shown in Figure 6a, the friction coefficient of the C61900 Cu alloy was stable at 0.16. In comparison, the friction coefficient of the modified layer of nitrided F2 film and F3 film decreases to 0.1, while the friction coefficient of the nitrided F1 film slowly increased from 0.05 to 0.3. In addition, the wear rates of the F1, F2, and F3 films after plasma nitriding were 3.6 \(\times\) 10\({}^{-15}\) m\({}^{3}\)/Nm,  
Figure 4: Surface hardness and elastic modulus of the Cu substrate and three films after plasma nitriding at 650 \({}^{\circ}\)C for 4 h with increasing displacement: (**a**) hardness, (**b**) elastic modulus. Figure 5: Morphology of the cross-section (back-scattered electron image) and EDS results for the Cu-Ti films after plasma nitriding at 650 \({}^{\circ}\)C for 4 h: (**a**) and (**b**) for F1, (**c**) and (**d**) for F2, (**e**) and (**f**) for F3. \(1.0\times 10^{-15}\) m\({}^{3}\)/Nm, and \(3.8\times 10^{-14}\) m\({}^{3}\)/Nm, respectively, which were 98%, 99.5%, and 79% lower than the matrix, respectively, as shown in Figure 6b. The surface morphologies and cross-sectional profiles of worn tracks after nitriding at 650 \({}^{\circ}\)C for 4 h showed that peeling of the hard coating was induced during the wear test. The worn depth and width of F1 film reached 1.5 \(\upmu\)m and 200 \(\upmu\)m, respectively, as shown in Figure 7a,b.