J.Cent.South Univ.(2025) 32: 867－881
1 Introduction
Copper, while widely used as an electronic material, suffers from drawbacks such as low strength, ease of oxidation, and easy corrosion in the marine environment [1], which have become significant factors constraining the development of copper-based equipment. To enhance the performance of copper-based materials, common approaches involve creating coating or plating to the surface or incorporating reinforcing phases to prepare composite materials [2, 3]. A protective layer with self-healing [4] capability will improve the wear resistance, oxidation resistance and corrosion resistance of the copper-based materials, and consequently the robustness in various applications will be significant enhanced. Currently, a straightforward method to prepare self-healing layers is to embed healing agents in the coatings. Such coatings include microcapsules, microtube-type [5, 6], porous materials [7], and microcontainer-like structures [8, 9].
TiO2 nanotubes (TiO2 NTs) exhibit advantages such as dielectric effects [10], photoelectric conversion [11], ion storage [12], strong resistance to chemical corrosion [13], high biocompatibility [14] and the capability of cargo loading [15]. These features make them ideal nanocontainers for corrosion resistance. The Ni/Cr co-doped TiO2 NTs synthesized by SHABAN et al [16] exhibited higher photocatalytic efficiency and stability compared with the non-doped and Ni-doped ones. LUO et al [17] prepared a structurally optimized TiO2 NT thin layer on Ti6Al4V bone screws. The formation of an interface layer between the TiO2 nanotubes and the Ti6Al4V substrate enhanced the bond between them, thereby improving the resistance to fretting corrosion. Therefore, TiO2 NTs, with a large specific surface area, high porosity, and uniformity [18], can overcome the poor dispersity of conventional microcapsules and serve as an enhanced version of microcontainers. After suitable modifications such as cargo-loading or doping, they will exhibit enhanced anti-corrosive, antibacterial, photocatalytic and photoelectrochemical properties. Regarding the TiO2 NTs methods [19], previous reports have elucidated the fundamental distinction between one-step and two-step anodization methods for titanium. In the two-step anodization process, the stable ion current and current at the bottom of the NTs are ensured, creating conditions conductive to the successful preparation of more ordered TiO2 NTs.
The “MAX” phase [20], with a chemical formula of Mn+1AXn, is a transition metal-based ternary carbide or nitride featured by hexagonal crystal structure. It combines the thermal and electrical conductivity, processability of metallic materials, and excellent properties of ceramics such as high strength, oxidation resistance and corrosion resistance. It has found extensive applications in aerospace, nuclear engineering, and the national defense industry, garnering widespread attention [21]. TORRES et al [22] employed heat treatment on a magnetron-sputtered Ti-Al-C multilayer system, repeating the sputtering of pure Ti/Al/C elemental single layers in a cyclic manner 22 times. The Ti2AlC MAX phase was successfully prepared at temperatures below 850 ℃, while the Ti3AlC2 MAX phase was obtained at temperatures higher than 850 ℃. The mechanical properties, including hardness and elastic modulus, of both MAX phases were evaluated through nanoindentation testing. LI et al [23] deposited Ti50Al50 (at.%) alloy target on a 304 stainless steel substrate at room temperature using filtered cathodic vacuum arc (FCVA) and subsequently annealed at 800 ℃ for 1 h. A smooth and dense Ti3AlC2 coating was prepared by varying the C2H2 flow rate. Experimental results indicate that with an increase in C2H2 flow rate from 9 to 12 mL/min, the coating not only enhances its hardness and elastic modulus but also improves overall corrosion resistance. Given the excellent performance of MAX phases, we have chosen MAX phase materials as the reinforcing cap layer for TiO2 NTs, thereby achieving the goal of a three-dimensional protective coating.
In this work, the PEUMS-PVD technique [24, 25] was employed to sputter a Ti layer film on the Cu substrate surface, and a two-step anodization method was used to induce the growth of TiO2 NTs microcontainers with adjustable and highly ordered pore structures. Doping of nano-sized Al particles as repair agents was realized. During the oxidation process, Al generates a dense Al2O3 film, preventing further oxidation of the substrate material. Al2O3 itself has a high modulus and good adhesion with the substrate, achieving the purpose of crack healing and restoring material strength, resulting in a microcontainer-type self-healing protective layer. Further deposition of Ti-Al-C synthetic targets, followed by high-temperature annealing, promoted the nucleation of Ti3AlC2-MAX phase, forming a framework-reinforced covering layer. Simultaneously, a gradual change in composition created mechanical interlocking, enhancing the bond between TiO2 NTs and the covering layer. PEUMS-PVD technology [26] can produce films with good density and adhesion. Additionally, adjusting factors such as sputtering power, substrate temperature, and substrate bias can improve the quality of the film coating, making it a promising coating preparation method. The use of nano-sized Al particles as dopants in TiO2 NTs microcontainers provides a new approach to self-healable anti-corrosion coatings.
2 Experimental
2.1 Materials
Copper sheets were purchased from Yangzhou Xiangwei Machinery Co., Ltd. (Jiangsu, China). High-purity Ti target, high-purity Al target, and Ti-Al-C alloy target were all purchased from Zhongnuo New Material (Beijing) Technology Co., Ltd. Ammonium fluoride (NH4F) was obtained from Da Mao Chemical Reagent Co., Ltd. (Tianjin, China). Ethylene glycol ((CH2OH)2), hydrogen peroxide (H2O2), and nitric acid (HNO3) were purchased from China National Pharmaceutical Group Corporation Chemical Reagent Co., Ltd. (Shanghai, China). All chemical reagents were used as received without further purification.
2.2 Preparation of TiO2 NTs on surface of copper substrate
TiO2 NTs were prepared in two steps using a combination of PEUMS-PVD and anodization. The schematic diagram of the PEUMS-PVD apparatus is shown in Figure 1. The deposition process was carried out in a high-purity Ar atmosphere using RF from a Ti target, with Cu serving as the substrate material. The Cu substrate was processed into circular discs with dimensions of Φ15 mm×2 mm, mechanically polished, placed on a rotating stage, and subjected to vacuum evacuation to less than 8×10-4 Pa. The Cu surface was then subjected to 60 s of ion source cleaning using the PEUMS-PVD equipment to remove any oxides and contaminants that may have adhered to the substrate surface. The rotating stage was set in motion, and the temperature was raised to 200 ℃ and maintained until the sputtering process concluded. The bias voltage was set at 100 V, and the deposition duration was 1.5 h. After sputtering, chemical polishing of the Cu-based Ti surface was conducted for 400 s using a polishing solution (0.17 wt% NH4F, 2.28 vol% H2O2, 2.28 vol% HNO3, and 95.27 vol% H2O), followed by repeated rinsing with water and anhydrous ethanol and drying with nitrogen gas. TiO2 NTs were prepared on the Cu-based Ti surface using a two-step anodization process. In the first step, the voltage was maintained at 70 V for 150 s, followed by rinsing with water and anhydrous ethanol, and drying with nitrogen gas. The second step involved maintaining the voltage at 2 V for 1 h, followed by removal and rinsing. In addition, the experimental Cu-based Ti for anodization served as the working electrode, with a Pt plate as the counter electrode. The electrolyte consisted of a 94.50 vol% ethanol solution, 0.50 wt% NH4F, and 5.00 vol% H2O.
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2.3 Doping of nano-sized Al and preparation of Ti3AlC2 covering layer
The doping of nano-sized Al particles was achieved through DC sputtering of high-purity Al target using PEUMS-PVD in a high-purity Ar atmosphere. The specific process parameters were as follows: a power of 400 W and a deposition time of 2 min. The deposition experiment of the Ti3AlC2 covering layer was carried out immediately after the doping of nano-sized Al particles. A Ti-Al-C alloy target with an atomic ratio of 3:1:2 was used, and RF sputtering was conducted in a high-purity Ar atmosphere. To investigate the effect of temperature on the structure and performance of the Ti3AlC2 covering layer, the deposition temperature during Ti-Al-C sputtering was set at 700, 750, and 800 ℃, with a deposition time of 1 h. The preparation process of the Cu-based Ti/Al-doped TiO2 NTs/Ti3AlC2 self-healing three-dimensional coating is illustrated in Figure 2.
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2.4 Characterization
TiO2 NTs and three-dimensional coating samples were observed for morphology using a scanning electron microscope (SEM, ZEISS GeminiSEM 300, Germany) equipped with an EDS detector. The surface morphology and roughness of the coating were observed using an atomic force microscope (AFM, Bruker Dimension Icon, Germany). The structure of the coating was determined using Raman spectroscopy (InVia, Renishaw, UK) with excitation by an Ar ion laser with a wavelength of 532 nm, covering a wavelength range of 100-1800 cm-1. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA) was used to characterize the chemical states of the coating. The acquired XPS spectra were fitted and analyzed using Avantage.
Additionally, an electrochemical workstation (VersaSTAT 3F: Mode-500, USA) was employed to evaluate the corrosion resistance of the samples through electrochemical impedance spectroscopy (EIS) and polarization curves (Tafel). The tests were conducted using a conventional three-electrode system in a 3.5 wt% NaCl solution simulating marine corrosion. The working electrode (WE) was a three-dimensional protective layer, a 2 cm ×2 cm platinum sheet was the counter electrode (CE), and a saturated calomel electrode (SCE) was used for the reference electrode (RE). The specimens were first subjected to open circuit potential (OCP) test in the 3.5 wt% NaCl solution before the AC impedance test, and the EIS test was performed after the voltage of the OCP was kept stable below 5 mV fluctuation. The scanning frequency range of the EIS test was 0.01 Hz-10 kHz, and the amplitude of the applied sinusoidal interference signal was 10 mV. The scanning direction was from high frequency to low frequency, and a shielding box was used to shield the interference from other signals in the surrounding area during the test, and the results were obtained by using the ZSimpWin software to fit the experimental data to the original experimental data. The EIS test was completed, followed by the Tafel test, in which the initial and final potentials were set to 250 mV above and below the open circuit potential (OCP), the test scan rate was 0.1666 mV/s, the step size was 3 s, and the scan points were 1001. The initial analysis of the coating immersion was conducted using electrochemical noise (EN), and the fast Fourier transform (FFT) was employed to transform the time domain into the frequency domain to calculate the power spectral density (PSD) and infer the different corrosion types and processes occurring after coating immersion.
3 Results and discussion
3.1 Deposition thickness and structure of Ti3AlC2
To ensure the successful growth of TiO2 NTs on the Cu substrate surface and control the deposition time, step testing was conducted to measure the film thickness after 1 h of deposition. As shown in Figure 3(a), at the same power, the thickness of the film produced by RF process is approximately 72% of that produced by DC process, and under the same process, the higher the power, the thicker the film layer. The structure of the Ti3AlC2 coating was obtained through the Raman spectra of Ti3AlC2 coatings at different annealing temperatures, as shown in Figure 3(b). The peak at 268 cm-1 in the Raman spectrum at 800 ℃ is associated with the vibration modes of Ti3AlC2 [27], indicating the presence of the Ti3AlC2 phase in the coating. As the temperature increases from 700 to 800 ℃, the overall intensity of this peak in the Raman spectrum shows a trend of increasing from none to higher, indicating an increase in the content and crystallinity of Ti3AlC2 with the rising temperature. In addition, the peaks at 420 and 605 cm-1 are standard Raman peaks for TiC. No significant signals were recorded beyond the wavenumber range of 1200-1600 cm-1, which is associated with amorphous or disordered carbon materials. This indicates that the carbon layer deposited by magnetron sputtering grows in an amorphous state [21].
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XPS was further employed to characterize the bonding structure of the as-deposited Ti3AlC2 coating. Figure 3(c) shows the XPS spectra of Ti in coatings prepared at different annealing temperatures along with the corresponding peak fitting. Peaks located at 458.5 and 464.3 eV were attributed to the Ti—O bond of oxygenated titanium compounds [28] in the as-annealed coating at 800 ℃. The presence of the Ti—O bond is speculated to result from the reaction of titanium with residual oxygen in the chamber during the coating preparation process. Peaks at 454.7 and 460.6 eV correspond to Ti 2p1/2 and Ti 2p3/2 [29] in titanium carbide compounds, respectively. The presence of Ti—C bonds confirms the formation of Ti3AlC2 in the coating, which is consistent with the Raman results in Figure 3(b). The Ti 2p spectra and corresponding peak fitting for the coatings at 700 and 750 ℃, combined with Raman spectra, confirm the presence of Ti—C bonds. Figure 3(d) shows the Al 2p spectrum and the corresponding peak fitting. There is a main peak in the Al 2p spectrum, and the fitting results indicate the presence of two peaks. The high-intensity peak located near 74.2 eV is typically associated with the Al—O bond [28] in Al2O3. This is because the deposited surface Al undergoes oxidation to form Al2O3. The low-intensity peak at 72.8 eV is believed to be associated with the Al—Ti or Al—Al bonds in the Ti3AlC2 phase [27]. Furthermore, there is no significant change in the Al 2p spectrum of the deposited coating with variations in annealing temperature. The fitting results of the C 1s spectrum of the Ti3AlC2 coating are shown in Figure 3(e). The peak centered at 281.6 eV in the C 1s spectrum of the coating annealed at 800 ℃ is attributed to the C—Ti bond in the Ti3AlC2 phase [30]. Peaks at 284.8, 286.7 and 288.9 eV are associated with C—C, C—O and C—O bonds [31, 32], respectively. The presence of these peaks may be attributed to impurities generated during the phase formation process. Of course, the C 1s spectra of the coatings annealed at 700, 750 and 800 ℃ are essentially the same. The appearance of the C—Ti bond in the C 1s spectrum further confirms the formation of the Ti3AlC2 phase in the coating.
3.2 Morphology of TiO2 NTs and Ti3AlC2
By using a two-step anodization method and varying the working voltage and anodization time in the first step, TiO2 NTs with adjustable pore size and ordered growth were prepared. To verify this, SEM was used to observe the growth of TiO2 NTs on the surface of the Cu substrate. Figure 4(a) depicts the surface and cross-sectional morphology of TiO2 NTs, where it is observed that at an anodization voltage of 70 V, TiO2 NTs have a uniform pore size ranging from 90 to 100 nm. In the upper right image of the cross-section, TiO2 NTs have a height of 500-550 nm and exhibit an ordered arrangement. Figure 4(b) presents SEM observation of the surface morphology of the coating after deposition and annealing. It is observed that TiO2 NTs are completely sealed in the coating after deposition and annealing, with a needle-like crystal structure on the surface, arranged densely without cracks. Combining the upper right image for a larger-scale surface morphology, the overall surface of the coating is relatively flat and smooth, indicating good film-forming performance after magnetron sputtering and high-temperature annealing.
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Figures 4(c) and (d) reveal the 2D and 3D morphology of the deposited coating after annealing using AFM. The coatings after deposition and annealing at different temperatures exhibit a completely dense and smooth structure. Subsequently, the surface roughness of the coatings was digitally assessed. Part of the data are listed in the supplementary material. As shown in Figure 4(d), the Ra value remains relatively unchanged at 800 ℃, maintaining a flat profile. Therefore, the surface morphology of the deposited coating is not dependent on annealing processes but plays a crucial role in the magnetron sputtering deposition process. However, the appearance of “needle-like” and “flat” growth morphologies of the coating at different annealing temperatures suggests a potential correlation with the annealing temperature. In conclusion, the coating exhibits good smoothness and low roughness at both macro and micro-nano scales.
3.3 Electrochemical performance
Corrosion resistance capability of the Ti3AlC2 three-dimensional coating was evaluated using electrochemical impedance spectroscopy (EIS). Figures 5(a)-(c) represent the results of the EIS tests in a 3.5 wt% NaCl solution. In general, the larger the radius of the Nyquist plot, the better the corrosion resistance performance of the material. It is observed from Figure 5(c) that the coatings at different annealing temperatures all exhibit incomplete semicircles. Particularly, with the increase in temperature, the curvature of the semicircle in the Ti3AlC2 three-dimensional coating becomes larger, indicating greater resistance to corrosion [33]. When the phase angle appears over a wide frequency range with the phase staying close to -90°, it indicates the presence of a capacitive passivation film [34], suggesting that the corrosion resistance of the Ti3AlC2 three-dimensional coating is optimal at 800 ℃. The modulus of the low-frequency impedance at 0.01 Hz in the Bode plot can also be used to assess the material’s corrosion resistance [35]. From Figure 5(a), it can be observed that with the increase in annealing temperature, the impedance value at the lowest frequency in the Bode plot increases, similarly indicating that the corrosion resistance of the Ti3AlC2 three-dimensional coating is optimal at 800 ℃.
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Figure 5(d) presents the Tafel curves for the Cu substrate and Ti3AlC2 three-dimensional coatings at different annealing temperatures in a simulated marine environment with a 3.5 wt% NaCl solution. According to the extrapolation method, the Tafel curve was processed, and the slope of the anode region and the cathode region of the curve was fitted by VersaStudio software, and the abscissa at the intersection point corresponded to the corrosion current density (Jcorr) and the ordinate corresponded to the corrosion potential (Ecorr). The anode and cathode Tafel constants are determined by applying the Butler Volmer equation. The fitting is based on Levenberg-Marquardt’s nonlinear least squares fitting method, which provides a numerical solution to the problem of minimizing nonlinear functions in parameter space. Based on the Stern and Geary relationship, the polarization resistance (Rp) can be evaluated. In general, the lower the corrosion current density, the better the corrosion resistance of the material [38, 39]. Additionally, a higher corrosion potential corresponds to better corrosion resistance, with the corrosion current density playing a dominant role. The corrosion current density, corrosion potential, and yearly corrosion rate of the coating at different temperatures are presented in Table 1. As the temperature increases, the corrosion resistance of the coating improves, indicating that the high-temperature annealing is helpful for enhancing the substrate’s corrosion resistance. Furthermore, at an annealing temperature of 800 ℃, the corrosion current density is the lowest, reaching 4.5643×10-8 A/cm2, and the annual corrosion rate was 1.0957×10-5 mm/a. This is attributed to the reduction of the impurity phase in the coating and the increase of the Ti3AlC2 phase, which favors the enhancement of the corrosion resistance of the coating. This observation aligns with the results obtained from the EIS tests.
| Sample | Ecorr/mV | Jcorr/(A·cm-2) | Cathodic slope, βb/(mV·decade-1) | Anodic slope, βa/(mV·decade-1) | Corrosion rate/ (mm·a-1) | Polarization resistance, RP/(Ω·cm-2) |
|---|---|---|---|---|---|---|
| Substrate | -278.8 | 1.509×10-6 | 43.7 | 132.7 | 3.6474×10-4 | 294.3 |
| 700 ℃ | -375.7 | 1.044×10-6 | 93.3 | 97.4 | 2.507×10-4 | 720.4 |
| 750 ℃ | -317.3 | 3.815×10-7 | 37.3 | 87.0 | 9.1592×10-5 | 459.1 |
| 800 ℃ | -176.8 | 4.564×10-8 | 66.4 | 18.7 | 1.0957×10-5 | 1005.2 |
By intentionally scratching the surface of the Ti3AlC2 three-dimensional coating prepared under 800 ℃ conditions, its self-healing performance is evaluated and compared with the Cu substrate. The results of the electrochemical impedance tests are presented in Figures 5(e, f, g). The capacitive arc of the scratched coating is larger compared to the Cu substrate. This is because after scratching, the Al particles in TiO2 NTs of the coating come into contact with the air, forming Al2O3 by reacting with oxygen. This creates an effective barrier to inhibit the penetration of corrosive media. In the Tafel performance test shown in Figure 5(h), the scratched coating exhibits a lower corrosion current density and a more positive corrosion voltage compared to the Cu substrate. This indicates that the 800 ℃ Ti3AlC2 three-dimensional coating maintains strong corrosion resistance even after being scratched, providing protective effects for the Cu substrate. Therefore, we chose the R(C(R(CR))) equivalent circuit model to fit the EIS test results of the coating, and the fitting circuit diagram is shown in Figure 5(i), and the results obtained are shown in Table 2. Here, CPEc represents the capacitance at the coating-electrolyte interface, CPEdl represents the double-layer capacitance at the substrate-electrolyte interface, Rs represents the solution resistance, Rp represents the pore resistance, and Rct represents the charge transfer resistance [36]. According to reports [37], the higher the pore resistance, the greater the charge transfer resistance, indicating better corrosion resistance of the material. With the increase in annealing temperature, the resistance value of Rct also increases, thereby indicating that the Ti3AlC2 three-dimensional coating at 800 ℃ has the best corrosion resistance.
| Sample | Rs/(Ω·cm-2) | Rp/(Ω·cm-2) | Rct/(Ω·cm-2) | CPEc/(Ω-1·cm-2·sn) | CPEdl/(Ω-1·cm-2·sn) |
|---|---|---|---|---|---|
| Substrate | 13.61 | 2.326×102 | 4.487×103 | 9.576×10-1 | 5.289×10-1 |
| 700 ℃ | 10.48 | 8.322×103 | 1.480×104 | 8.201×10-1 | 9.990×10-1 |
| 750 ℃ | 13.55 | 6.215×103 | 4.009×104 | 8.268×10-1 | 9.709×10-1 |
| 800 ℃ | 10.56 | 2.763×103 | 7.879×104 | 8.260×10-1 | 2.644×10-5 |
The self-healing effect of the Ti3AlC2 three-dimensional protective layer prepared at 800 ℃ was evaluated by scratching the surface of the Ti3AlC2 three-dimensional protective layer prepared at 800 ℃. After the protective layer was cut, traces of O and Al elements were observed from the EDS image (Figure 6, shown by arrows), and it was judged that the TiO2 nanotubes were successfully doped with Al particles. The purpose of doping Al particles as a repair agent is that after the coating is damaged, the Al particles in the TiO2 nanotubes are released into the solution or briefly exposed to air, and the oxidation of the Al particles can form a dense Al2O3 dense layer, which prevents the matrix material from being further oxidized, and then forms a barrier that effectively inhibits the penetration of the corrosive medium. The presence of O and Al elements was observed at the scratches of EDS images (Figure 6), indicating that the surface protective layer was repaired through the self-healing ability of Al particles, to protect the Cu substrate. At the same time, when the subsequent Al2O3 dense layer is damaged, the remaining Al particles in the TiO2 nanotubes will continue to release and oxidize to form the Al2O3 dense layer, thereby achieving the effect of corrosion inhibition. The doping of nano-Al makes a layer of protection effect for the protection of Cu matrix by three-dimensional protective layer, which has a protective effect on possible repeated destruction, thereby prolonging the service life of copper-based electronic materials.
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3.4 Corrosion resistance evaluation during the initial immersion of the coating
The Ti3AlC2 three-dimensional coating, annealed at 800 ℃, was immersed in a 3.5 wt% NaCl solution for 2, 6, 12 and 24 h. The post-immersion coating was subjected to electrochemical performance testing, enabling a quantitative assessment of the early-stage corrosion resistance of the Ti3AlC2 three-dimensional coating exposed to a marine environment [40, 41]. Figures 7(a)-(c) depict the results of EIS tests for both unimmersed and immersed conditions. From the EIS data, it can be observed that the corrosion resistance of the coating slightly decreases after immersion compared to the unimmersed state. However, the overall corrosion resistance of the coating remains relatively stable after immersion. Conversely, in Figure 7(d), the Tafel test results show that the corrosion current density of the coating slightly increases, and the corrosion voltage becomes more positive after 2 h of immersion in the 3.5 wt% NaCl solution. However, the corrosion resistance performance remains relatively stable. After 6 h of immersion in the 3.5 wt% NaCl solution, the corrosion resistance of the coating slightly improves, suggesting a potential association with the formation of a passivation film on the coating. However, after 12 and 24 h of immersion in the 3.5 wt% NaCl solution, the change in the corrosion resistance of the coating is relatively small compared to the unimmersed state. This indicates that the coating exhibits stable data in the early immersion test period, demonstrating excellent corrosion resistance. Table 3 presents the calculated data from the Tafel electrochemical tests, providing a visual representation of the outstanding corrosion resistance of the coating in the early immersion period.
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| Sample | Ecorr/mV | Jcorr/(A·cm-2) | Corrosionrate/(mm·a-1) |
|---|---|---|---|
| 0 h | -176.8 | 4.5643×10-8 | 1.0957×10-5 |
| 2 h | -153.9 | 8.6937×10-8 | 2.2077×10-5 |
| 6 h | -109.2 | 7.7722×10-8 | 1.8659×10-5 |
| 12 h | -129.0 | 8.2992×10-8 | 1.9924×10-5 |
| 24 h | -106.1 | 1.1103×10-7 | 2.6657×10-5 |
Figures 7 (e)-(h) depict the electrochemical noise spectra of the 800 ℃ Ti3AlC2 three-dimensional coating corroded for different durations in the 3.5 wt% NaCl solution. The electrochemical noise was used to investigate the local corrosion of the coating in the 3.5 wt% NaCl solution, determining the damage caused by the corrosion medium to the coating [41-44]. When the coating is corroded in Figure 7(e) for 2 h, the fluctuation frequencies of potential noise and current noise show an overall inverse trend, with potential noise slowly decreasing and current noise slowly increasing. The noise data in the spectra exhibit an approximately symmetric distribution, indicating that, at this stage, the 800 ℃ Ti3AlC2 three-dimensional coating is in the dissolution phase. The corrosion is mainly characterized by uniform corrosion, suggesting that the coating provides effective protection for the Cu substrate. In Figures 7(f) and (g) at 6 h and 12 h, it is observed that the noise data still exhibit an approximately symmetric distribution, indicating that this stage is still in the dissolution phase. However, in the late stage of the current noise in Figures 7(f) and (g), transient peaks appear (as can be observed in the enlarged region in Figure 7(g)). This is due to the insufficient density of the coating, with small pores appearing in the shallow layer of the coating. As a result, the corrosive medium begins to erode it, but the self-healing property of the coating quickly hinders further corrosion by the corrosive medium. In Figure 7(h), after the current noise and voltage noise undergo a decline, transient peaks with rapid increases and decreases are observed. The stabilized values of the potential noise after the decline are relatively small compared to those measured in the early corroded coating. At this point, the corrosive medium easily penetrates the coating. The Al particles doped in TiO2 NTs within the coating quickly impede further corrosion by the corrosive medium, reducing the corrosion rate of Cl-. This leads to a state where some regions of the coating are undergoing both corrosion and the formation of a passive film simultaneously [45]. At this stage, it is hypothesized that the corrosion process will involve both uniform corrosion and localized corrosion, with pitting corrosion being predominant in the localized corrosion. Of course, the noise results correspond to the relatively stable state of the coating observed in the early immersion period in the electrochemical impedance spectroscopy (EIS) and Tafel polarization curve results.
Figure 7(i) displays the power spectral density (PSD) graphs for the 800 ℃ Ti3AlC2 three-dimensional coating immersed in the 3.5 wt% NaCl solution for different durations (2, 6, 12 and 24 h) [46-48]. The PSD curves exhibit three characteristic parameters: white noise level (W), the slope (k) of the high-frequency linear portion, and the cutoff frequency (_Xml/alternativeImage/74542E74-4DF8-4736-9B5F-080990E05251-M001c.jpg)
3.5 Coating protection mechanism
The protective mechanism of the Ti3AlC2 three-dimensional coating is illustrated in Figure 8. Initially, during the immersion in the 3.5 wt% NaCl solution, the majority of the undamaged coating effectively prevents the penetration of water molecules or corrosive agents. However, with the prolonged immersion time, certain regions of the coating will inevitably undergo erosion, forming cracks or holes that accelerate the infiltration of the corrosive medium. TiO2 NTs, serving as microcontainers in the intermediate layer of the three-dimensional coating, release Al particles stored in their tubes when the outermost surface of the three-dimensional coating is damaged. These particles react with O2, H2O and Cl- to generate Al2O3 and Al2O3·3H2O. Al2O3 itself has a high modulus and good bonding with the substrate, achieving the purpose of healing crack defects. This process inhibits further corrosion of the coating, demonstrating a proactive self-repair capability [49]. Therefore, the Ti3AlC2 three-dimensional coating exhibits excellent protective effects on the Cu substrate.
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4 Conclusions
In this study, we developed a method to prepare a self-healing three-dimensional coating of Ti/Al-doped TiO2 NTs /Ti3AlC2 on the surface of a Cu substrate by combining PEUMS-PVD with anodizing, aiming to achieve long-term corrosion resistance. The corrosion resistance performance in a 3.5 wt% NaCl solution was evaluated using electrochemical methods, scratch testing, and initial immersion tests. The specific conclusions are as follows:
1) Uniformly sized, ordered TiO2 NTs microcontainers were successfully prepared on the surface of the Cu substrate. A Ti3AlC2 three-dimensional coating was fabricated at different annealing temperatures, achieving the goal of a corrosion-resistant coating.
2) Three-dimensional coatings at different annealing temperatures exhibited uniform surface morphology. With increasing temperature, the corrosion resistance of the coating became stronger, attributed to a reduction in the outermost layer’s heterogeneous phases and an increase in Ti3AlC2 phases. The Tafel anodic slope is higher than the cathodic for the substrate and the coating at 750 ℃, and the anodic reaction is the slowest. For the coating produced at 800 ℃, the cathodic slope is higher than the anodic, and the cathodic reaction is the slowest. The Ti3AlC2 three-dimensional coating annealed at 800 ℃ exhibited the highest corrosion resistance by EIS and Tafel tests. Artificial scratch experiments validated the protective effect of Al particle-doped TiO2 NTs within the three-dimensional coating, forming a high modulus protective layer to achieve a self-repairing effect.
3) Tafel and EIS tests on the coated samples after immersion indicated that the coating still possessed robust corrosion resistance, providing protection to the Cu substrate. We preliminarily explored the corrosion mechanism by the time-domain and frequency-domain results of electrochemical noise in the early stages of coating immersion, with uniform corrosion being the primary mode and a minor occurrence of localized corrosion.
The protective layer is a coating with both metallic and ceramic properties, and as a protective layer for copper-based electronic materials, it is superior to the organic protective layer in some fields. On the other hand, Al-doped TiO2 nanotubes are used as the slow-release agent of the protective layer, which makes up for the corrosion of the matrix after the damage of the protective layer of a single inorganic material, and realizes multi-effect protection. In addition, the strategy is highly novel, and the current coating technology and anodizing are emerging in the industrial side, and the process is economical when the industry is put into high-volume production. It takes a certain transition time from experimental conditions to industrial applications, although it cannot be quickly applied to the industrial market at this stage, it is believed that soon, industrial modernization will be more advanced and perfect, and the technology has huge room to play.
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WANG Guo-qing, WANG Ning, HU Yi-teng, WANG Jing, GAO Chuan-hui, WANG Jie, XUE Jun-jie, YAN Ke-xin. Corrosion resistance of self-healing three-dimensional Ti/Al-doped TiO2 nanotubes Ti3AlC2 coating deposited by magnetron sputtered on copper [J]. Journal of Central South University, 2025, 32(3): 867-881. DOI: https://doi.org/10.1007/s11771-025-5902-x.
王国庆,王宁,胡艺腾等.自修复三维Ti/Al掺杂TiO2纳米管Ti3AlC2镀膜溅射在铜上的耐腐蚀性[J].中南大学学报（英文版）,2025,32(3):867-881.