The nominal chemical composition of 2024 Al alloy used in this study is given in Table 1. The duplex coating was prepared on 2024 Al alloy substrate with two procedures (deposition/nitriding). The process of depositing Ti film was conducted by a closed field unbalanced magnetron sputtering ion plating system (Teer, UDP450/4) with high quality Ti target (99.999%). The base pressure of the chamber is 4 × 10^-3 Pa. Prior to the deposition, substrates were etched by Ar plasma sputtering at a bias of -400 V for 40 min. A bias of -70 V was employed on substrates and the power of Ti target was switched to 1.8 kW for depositing process holding for 4 h. Plasma nitriding (PN) was performed in a nitriding unit (LDMC-30, 30 kW). The chamber was evacuated to below 5 Pa by a rotary pump and then heated up to 430 ° C by plasma atmosphere generated by a gas mixture (40% N₂-60% H₂). The nitriding process was held at a constant pressure of 226 Pa for 8 h. Then, the specimens were cooled down in a nitrogen gas flow.

Table 1

Table 1

Table 1 Chemical composition of 2024 Al alloy (wt%) Si Fe Cu Mn Mg Cr Zn Ti Al 0.22 0.40 3.81 0.40 1.40 0.05 0.14 0.05 Bal.

Table 1 Chemical composition of 2024 Al alloy (wt%)

The phase structures of the surface duplex coating were analyzed by X-ray diffraction (type, Philips X’ Pert diffractometer) using CuKα radiation (λ = 0.15406 nm). A scanning electron microscope (SEM, FEI QUANTI 200F) equipped with energy dispersive X-ray spectrometer (EDS) was used to evaluate the surface and cross-sectional morphologies and chemical composition analysis of the surface duplex coatings. The surface hardness of untreated and coated specimen was measured using a Vickers microhardness tester (HV-1000) at the indentation load 10 g for 15 s. Pin-on-disk wear tests were used to examine wear resistance of the coated and uncoated 2024 Al alloy specimens with a normal load of 2 N, under dry sliding condition at room temperature and ambient humidity. During wear tests, the cylindrical pins (20 mm in diameter and 5 mm in height) of the 2024 Al alloy (before and after plasma nitriding) were rotated against a stationary hardened chromium steel (HRC 64) ball of 5 mm in diameter at a speed of 200 r/min (0.1 m/s) for 1200 s. The tested specimens were weighted by an electronic balance with accuracy up to 0.1 mg before and after the wear test to calculate mass loss. The specific wear rate (W_N) was calculated by the expression: W_N = Δ M/(L × n), where Δ M (g) is the weight loss, L (N) is the contact load and n (r) is the total number of turns. The worn surfaces and wear debris of tested specimens were characterized by scanning electron microscopy (SEM).

The XRD patterns of Ti coated 2024 Al alloy before and after plasma nitriding are illustrated in Fig. 1. As can be seen from Fig. 1(a), Ti film with hexagonal structure was obtained by magnetron sputtering method, which was consistent with previous studies^{[21] and [22]}. Obviously, nitride (identified as TiN_0.3, JSCPDS No. 41-1352) was formed on the surface of Ti coated Al alloy after plasma nitriding. According to Ti-N phase diagram^[23], nitrogen-rich solid solutions and compounds containing low nitrogen could occur below 700 ° C while TiN and Ti₂N usually form at higher temperature. In the present study, with low nitriding temperature (430 ° C), conventional nitrides were difficult to form. TiN_0.3 crystallized in a hexagonal structure with the space group P63/mmc, which was similar to that of α -Ti. Thus, the compound TiN_0.3 could be identified as α -Ti (N) solution. As can be seen from Fig. 1(b), intermetallic phase Al₁₈Ti₂Mg₃ and Al₃Ti were also formed in the modified layer of Al alloy.

Fig. 1

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	Fig. 1 XRD patterns of Ti coated specimen: (a) before PN treatment, (b) after PN treatment at 430 ° C for 8 h.

The surface morphologies of Ti coated Al alloy before and after plasma nitriding are illustrated in Fig. 2. It can be seen from Fig. 2(a), a dense Ti film with facet microstructure was successfully deposited on the surface of 2024 Al alloy. The size of the Ti particles was in the range of about 250 nm. After plasma nitriding, Ti particles changed to much finer particles with the size of 50 nm, resulting in much more dense coating on the surface of 2024 Al alloy (Fig. 2(b)). Some particles grew into clusters with the size of about 450 nm. In order to understand the composition change of Ti film after plasma nitriding, the EDS analysis was carried out. Fig. 2(c) shows the EDS spectra on the surface of nitrided specimen. The presence of high nitrogen content indicated that Ti film was successfully nitrided. The nitrided particles were identified as TiN_0.3 by previous XRD analysis.

Fig. 2

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	Fig. 2 SEM images showing surface morphologies of Ti coated specimen: (a) before PN treatment, (b) after PN treatment at 430 ° C for 8 h, (c) EDS spectra on the surface of nitrided specimen

In order to understand the changes that occurred between the Ti film and 2024 Al alloy substrate, the cross-section microstructure observation was conducted. Fig. 3(a) gives the cross-section morphologies of Ti coated 2024 Al alloy before and after plasma nitriding. The deposited Ti film was uniform with thickness of about 3.33 μ m. No interface reaction was observed during the deposition process. After plasma nitriding, three layers with distinct interface between each other formed on the surface of 2024 Al alloy (Fig. 3(b)). The corresponding elements depth profiles across the transverse section of the modified layer displayed in Fig. 3(c) show that two kinds of reaction occurred during plasma nitriding Ti coated 2024 Al alloy, i.e. (a) reaction between N atoms and Ti film to produce nitrides; (b) reaction diffusion between Ti film and 2024 Al alloy substrate to produce intermetallic layer. According to the XRD results and elements depth profiles, the duplex coatings comprising three sublayers identified as the outmost TiN_0.3 layer in white color, the middle Al₃Ti layer and the inside Al₁₈Ti₂Mg₃ layer. The approximate thickness of the three sublayers was 1.5, 1.1 and 5.5 μ m, respectively. Thus, the duplex coating with total thickness of about 8.1 μ m was successfully fabricated on the surface of 2024 Al alloy.

Fig. 3

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	Fig. 3 Cross-section micrographs (backscatter SEM image) of Ti coated specimen: (a) before PN treatment, (b) after PN treatment at 430 ° C for 8 h, (c) elements depth profiles of specimen after PN treatment.

The dry sliding wear behaviors of the untreated and plasma nitrided specimen were evaluated by using a ball-on-disc tribometer. As can be seen from Fig. 4, the wear rate of the plasma treated specimen has a significant decrease as compared to the untreated one. As is known to all, the wear resistance was related to the surface hardness of materials. Materials with higher hardness usually exhibited better wear resistance. The surface hardness of the plasma nitrided specimen is more than 450 HV_0.01, which is about five times harder than that of the untreated one. Thus, the nitrided specimen was expected to possess better wear resistance.

Fig. 4

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	Fig. 4 Surface hardness and wear rate (WN) of the untreated and the treated specimens

The friction coefficients vs sliding time curves of the plasma nitrided and untreated specimens are shown in Fig. 5. The stable friction coefficients of the tested specimens were found to be similar (0.52 for the untreated one, 0.5 for the nitrided one). However, the sliding curves of the untreated samples exhibited larger fluctuation than the nitrided one. This large fluctuation in the sliding curves resulted from the lower surface hardness of the substrate.

Fig. 5

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	Fig. 5 Frictional coefficient curves of the untreated and the nitrided specimens with a GCr15 ball under 2 N.

Fig. 6 shows the SEM micrographs of the worn surfaces of the untreated 2024 Al alloy generated through testing at a normal load 2 N. The wear track of the untreated sample was illustrated in the pattern of grooves and rides extending parallel to each other in the sliding direction (Fig. 6(a)). The grooves and ridges were caused by plastic deformation and micro-ploughing of the soft alloy by the hard steel counterface, accompanied by the removal of material^[24]. Material smearing and dark colored compact surface layers were also visible in the wear track. EDS analysis was carried out to understand the changes that occurred on the sliding surface of the untreated Al alloy during wear. As can be seen from the EDS spectrum in Fig. 6(c), the dark colored area (marked by A) contained a high content of oxygen, indicating the occurrence of oxidation reaction. The oxidation caused by the local high temperature due to compact contact between the hard steel counterface and the soft Al alloy. The EDS spectrum in Fig. 6(d) revealed that the chemical composition of area B was close to that of the 2024 Al alloy. It can be concluded that the wear mechanism of the 2024 Al alloy under the present test condition exhibited the typical characteristic of adhesive wear, which was in accordance to the extensive reports for soft and ductile materials such as copper and gold^[25].

Fig. 6

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	Fig. 6 SEM images showing worn surface morphologies of the untreated specimen: (a) low magnification, (b) high magnification, (c) EDS spectrum of the area A, (d) EDS spectrum of the area B

Fig. 7 shows the SEM micrographs of the worn surfaces of the nitrided 2024 Al alloy. Obviously, the wear track width for the nitrided specimen (Fig. 7(a)) was much narrower than that of the untreated one. The ploughed grooves inside the wear tracks and formation of wear debris were evidence of abrasive wear caused by the hard particles ploughing into the pin. A significant decrease of craters, finer grooves, less smearing were observed on the worn surface of the nitrided specimens (Fig. 7(a)), resulting in less material removing from the tested surface. As can be seen from the high magnification micrograph of the wear track of the nitrided sample (Fig. 7(b)), fine wear debris dispersed in the craters (area A) and cracks were also visible. The EDS spectrum in Fig. 7(c) revealed that the wear debris in area A contained large amounts of nitrogen and titanium. These debrises came from the spalling of the outside nitride layer (TiN_0.3). The hard nitride particles acted as abrasive medium, resulting in the ploughing grooves. The EDS spectrum in Fig. 7(d) showed that oxidation occurred in area B. Moreover, the large amount of Al in area B indicated that the modified layer was worn through under the present test condition.

Fig. 7

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	Fig. 7 SEM images showing worn surface morphologies of the nitrided specimen: (a) low magnification, (b) high magnification, (c) EDS spectrum of the area A, (d) EDS spectrum of the area B

The wear debris of the untreated specimen displayed metallic shining with white color while the nitrided one exhibited black color, which was readily identifiable during the test by the naked eye. Fig. 8 gives the wear debris morphologies for the tested specimens. Obviously, the wear debris of the nitrided specimen has a much smaller size than that of the untreated one, in agreement with the wear rate in Fig. 4. The wear debrises of the untreated specimen were large flake-like particles (about 200 μ m, in Fig. 8(a)). The stratified morphology of the wear debris was the evidence of an adhesion mechanism for the untreated alloy, which is in agreement with previous studies^{[26] and [27]}. Two types of debris morphology for the nitrided specimen were observed in Fig. 8(b), i.e. the fine particles of 1.5-4.0 μ m in diameter and large particles with a plate-like morphology (about 50-180 μ m in diameter). The distinct shape and size of wear debris of the test specimens originated from their different hardness. Since the soft Al alloy has good plastic deformation ability, it was inclined to machine by hard steel counterface. While the hexagonal compound TiN_0.3 and intermetallics (Al₃Ti and Al₁₈Ti₂Mg₃) exhibited higher hardness and brittle tendency. Cracks could occur in the outside hard layer enduring cyclic load during wear test. As a consequence, the fine nitride particles (Fig. 2) peeled from the surface layer to form abrasive particles.

Fig. 8

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	Fig. 8 SEM micrographs of wear debris morphologies: (a) the untreated specimen, (b) the nitrided specimen