## 1 Introduction  
The C61900 aluminum bronze alloy has been widely applied in the electronics, electrical, automotive, aviation, and astronautics fields. However, the deficient wear resistance of Cu alloys makes surface modification necessary [1, 2, 3, 4], especially for Cu alloy devices used in harsh environments. Plasma nitriding of titanium alloys has been investigated to fabricate a titanium nitride layer for its excellent wear resistance and high hardness [5, 6, 7]. However, the disparity between the hard coating and the soft Cu substrate results in low adhesion [2, 5]. To solve this problem, a gradient layer is proposed to achieve a higher interface strength by metallurgical bonding in thermal diffusion [8]. Specifically, an intermediate Cu-Ti intermetallic layer is regarded as the most appropriate candidate to be the transition medium between the titanium nitride layer and the Cu substrate [9, 10, 11], where the formation of a thin TiN and Cu\({}_{3}\)Ti\({}_{3}\)O layer contributes to the improvement of wear resistance by plasma nitriding [12]. In addition, the multi-phase coating containing nanocrystalline or micro grains, such as TiN/TiAl and TiN/TiCN, also enhances ductility [13, 14, 15]. [PERSON] et al. investigated the plasma-assisted nitriding process on Ti-6 Al-4V alloy, and they found that the main phases of the nitrided layer were \(\delta\)-TiN, \(\epsilon\)-Ti\({}_{2}\)N and the solid solution of N atoms dissolved in \(\alpha\)-Ti [16]. In our previous studies [17, 18], it was reported that the different Cu-Ti intermetallics of varying mechanical properties might form during the thermal diffusion process [19, 20], and their formation was affected by the addition of N atoms [21; 22; 23; 24; 25]. From the Ti-N phase diagram, it can be seen that \(\delta\)-TiN (Fm3m), \(\delta^{\prime}\)-Ti\({}_{2}\)N (41/amd), \(\varepsilon\)-Ti\({}_{2}\)N (P42/mmm), \(\alpha\)-Ti, and \(\beta\)-Ti of nitrogen-containing solid solutions all exist in this system, where the solubility of N atoms in \(\alpha\)-Ti is 23 at.% [26]. To be mentioned, the interaction between Al and Cu, Al and Ti during the thermos plasma nitriding process must be considered due to the high content of the Al atom in the C61900 Cu alloy (8.95 wt.%). The projection in the liquid phase and the isothermal section of the Al-Cu-Ti ternary phase diagram at 500 \({}^{\circ}\)C shows that the AlCu\({}_{2}\)Ti phase can form in a variable composition interval [27]. The formation mechanism of AlCu\({}_{2}\)Ti in plasma nitriding of Cu-Ti films on C61900 Cu alloy is attributed to the elemental Al diffusion process in the C61900 Cu alloy and the Ti atoms in the film layer. However, the above mentioned researches mainly focused on the experimental results of the microstructure and the optimal mechanical properties in the series of experiments. The design and control of the phase compositions based on the first principle study have been rarely explored. Actually, both the phase stability and the mechanical properties of the Ti-N compounds and the Cu-Ti intermetallics have been investigated [28; 29; 30], but there are still few reports on the Ti-Cu-N and Ti-Al-Cu ternary systems that industry demands.  
In the present study, surface modification of C61900 Cu alloy by thermos plasma nitriding of the prefabricated Cu-Ti film was undertaken, and the wear resistance was evaluated experimentally. Combining the calculation by the first principle method and thermodynamic analysis, the microstructure formation and phase composition of the Cu-Ti-N and Al-Cu-Ti systems were investigated to reveal the mechanism of the improved wear resistance, where the formation potential, phase stability, and mechanical properties of the Ti-N nitride compounds and Al-Cu-Ti compounds and the essence of the improved wear resistance of the multi-phases coating were explored. The conclusions could provide useful instructions for the surface modification of C61900 alloys by the combination of magnetron sputtering and plasma nitriding processes.