## 2 Materials and Methods  
A \(\phi\)20 mm C61900 aluminum bronze bar was sliced into \(\phi\)20 \(\times\) 4.5 mm sections for the surface modification process by magnetron sputtering and subsequent thermos plasma nitriding. The chemical composition of the C61900 alloy is listed in Table 1, which shows that it is composed of copper, aluminum, iron, etc. The obtained substrates were first treated for solid solution by water quenching after heating at 930 \({}^{\circ}\)C for 2 h. The Cu-Ti gradient films of different compositions were firstly fabricated with the UDP450 closed field unbalanced magnetron sputtering equipment, and the processing parameters are listed in Tables 2 and 3. The subsequent thermos plasma nitriding of the obtained films was undertaken to generate a multi-phase coating in a homemade pulsed plasma multicomponent infiltration furnace (LDMC-30 kW), with a nitrogen and hydrogen gas flow rate ratio of 0.30.1 L/min to generate a high nitrogen phase. Detailed information on the magnetron sputtering and plasma nitriding processes can be found in previous studies [31; 32]. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Al** & **C** & **Si** & **Pb** & **P** & **S** & **Fe** & **Mn** & **Sn** & **Zn** & **Cu** \\ \hline
8.95 & 0.01 & 0.06 & 0.02 & 0.05 & 0.06 & 3.87 & 0.35 & 0.09 & 0.08 & Bal. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The chemical composition of C61900 Cu alloy (wt.%). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Total Ar** & **Substrate Bias** & **Sputtering Time** & **Substrate-to-Target** & **Workpiece Rotation** & **Vacuum Chamber** \\
**Pressure (Torr)** & **(V)** & **(h)** & **Distance (mm)** & **Speed (r/min)** & **Temperature (\({}^{\circ}\)C)** \\ \hline \(3.7\times 10^{-2}\) & \(-70\) & 3 & 120 & 5 & 20\(-\)150 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The fixed parameters for the magnetron sputtering process. The phase structures of the gradient Cu-Ti films and the multi-phase coating were characterized by the D/max-rB rotary anode X-ray diffractometer, and the CuK\(\alpha\) target was applied at a voltage of 40 kV and current of 30 mA, with a scanning range from 20\({}^{\circ}\)-100\({}^{\circ}\) and scanning speed of 5\({}^{\circ}\)/min. The XRD patterns were compared with the standard PDF (ASTM) card to complete the phase structure calibration. The phase structures of the multi-phase coating were also analyzed by grazing incidence X-ray diffraction at an incident angle of 1\({}^{\circ}\). The surface morphology and cross-sectional morphology of the pre-fabricated coatings were examined by a scanning electron microscope (SEM, SUPRA 55 SAPPHIRE), equipped with an energy dispersive spectrometer (EDS). The hardness and elastic modulus of the multi-phase coatings were measured by a nano-indenter, where the continuous stiffness measurement (CSM) mode was applied at continuous indentation depths. The scratching test was then performed on the MFT-4000 multifunctional material surface performance tester to evaluate the adhesion of the coating. The device was continuously loaded by a diamond scriber in automatic mode at a loading speed of 100 N/min and maximum load of 100 N, where the sensor recorded the acoustic emission signals and the friction force signals. When the film layer peeled off, the critical load of the binding force between the coating and the substrate could be obtained. In addition, the pin-on-disk wear performance was measured against a WC ball of 5 mm in diameter. The test was carried out under dry friction conditions with a positive pressure of 4 N, a rotating speed of 200 r/min and a test time of 1800 s. A Wyko NT8000 white light interferometer was used to measure the cross-sectional profiles of the worn tracks, by which the volume wear rate was calculated [31]. For the first principle calculation, the K point and kinetic energy cut-off were optimized to make the energy difference between the two settings less than 0.01 eV.