In Situ Growth of High-Performance ZnAl-Layered Double Hydroxides/Al 2 O 3 Composite Coatings on Aluminum Alloy

: In this research, high-performance ZnAl-layered double hydroxides (ZnAl-LDHs) with different Zn 2+ concentrations were prepared on the surface of an anodized 1060 aluminum alloy using the in situ growth method, and the inﬂuence of Zn 2+ concentration on corrosion resistance and tribological behavior was further explored. The surface morphology, element distribution, phase composition


Introduction
Aluminum (Al) and its alloys are widely used in the aerospace, transportation, and marine industries due to their high strength ratio, good electrical and thermal conductivity, and excellent manufacturing characteristics [1][2][3]. The 1060 Al alloy is cost-effective and exhibits the advantages of light weight, high strength-to-weight ratio, excellent electrical conductivity, and good recyclability. However, some technical disadvantages remain unsolved. Al alloys possess a low hardness, a high coefficient of friction (COF) (ca. 0.4-0.8), and high wear rates (ca. 10 −4 -10 −5 mm 3 /m) [4,5]. Moreover, the uppermost thin protective oxide coating (ca. 1.5-10 nm) with high passivity on the surface of the Al alloy could be readily destroyed under the extreme chemical, physical, or mechanical conditions, and thus leads to increased cracks and corrosion [6,7]. To further expand the application field of Al alloys, it is of great significant to protect Al alloys from corrosion and wear damage, and further develop the coatings with both excellent corrosion resistance and tribological properties.
Anodic oxidation is a low-cost and simple operation that is beneficial to large-scale manufacturing [8][9][10], which has become a prevailing technique to obtain superior corrosion resistance and tribological properties upon the surface of diverse metal alloys such as magnesium (Mg), titanium, and aluminum [11][12][13]. The factors affecting the anodization hardness and interfacial bonding strength. The electrochemical tests were carried out using an electrochemical workstation composed of a three-electrode system, and the tribological experiments were conducted on a ball-on-disc rotating wear tester under dry wear. The anti-corrosion and tribological properties of the as-prepared composite coatings were discussed, and the effect of Zn 2+ concentration on the anti-corrosion property and tribological performance of ZnAl-LDHs/AAO composite coatings was analyzed. Based on the results of COF, wear loss, and worn surface studies, the lubrication mechanism for ZnAl-LDHs/AAO composite coatings was proposed, which could further optimize the lubricating behavior of two-dimensional layered materials.

Preparation of ZnAl-LDHs/AAO Composite Coatings
Commercial 1060 Al alloy (with nominal composition, wt%: Fe 0.243, Si 0.057, Cu 0.004, Ti 0.021, Zn ≤ 0.05, Mn ≤ 0.03, Mg ≤ 0.03%, and balance Al) was used as the substrate. Samples with dimensions of 40 mm × 30 mm × 0.5 mm were polished with water-resistant sandpaper of grade 400, 800, 1200, 1500, and 2000 grit successively, followed by cleaning in a mixed solution of alcohol and acetone (3:1 vol.) ultrasonically, rinsing in deionized water, and drying in ambient atmosphere naturally. All of the analytical grade chemicals were utilized without further purification. Deionized water was employed as a solvent.
The overall manufacturing process of preparing ZnAl-LDHs/AAO composite coatings on the surface of the Al alloy substrate is briefly illustrated in Figure 1. The obtained samples were put into alkali washing solution (NaOH (5 g/L), Na 2 CO 3 (20 g/L) and Na 3 PO 4 (20 g/L)) for 3 min and moved into acid washing solution (HNO 3 (40 g/L)) for polishing of 15 s. Then, all samples were washed with deionized water and dried. After the pre-treatment, the 1060 Al plates were anodized in phosphoric acid electrolyte (0.3 mol/L) for 1 h under current density of 0.01 A/cm 2 by using the direct current (DC) electrolytic treatment and a circulating water-cooling device (T = 20 • C). A certain amount of ZnSO 4 ·7H 2 O and (NH 4 ) 2 SO 4 were dissolved in 60 mL of deionized water to obtain solutions with different Zn 2+ concentrations (0.05 mol/L, 0.1 mol/L, 0.20 mol/L, and 0.30 mol/L) and their pH values were adjusted to 8.5 by adding ammonia water (1.2 mol/L). Subsequently, the AAO substrate was placed in the mixed solution using an autoclave at 90 • C for 24 h. There are two procedures of ionization equilibrium and coordination equilibrium in the formation of LDH. The aluminum source on the anodized aluminum surface continuously consumed the OHin solution, causing the ammonia to undergo ionization equilibrium in a positive direction. The occurrence of ionization equilibrium caused the concentration of NH 3 to become smaller, which promoted the dissociation of [Zn(NH 3 ) 6 ] 2+ , which in turn caused Zn 2+ to be released and the concentration of NH 3 to increase. At this time, the presence of Al 3+ , Zn 2+ , and OH − provided conditions for the growth of ZnAl-LDH films, which can be generated in situ on the AAO surface. After the in situ growth of LDH films on the surface of AAO, the substrates were taken out, rinsed with deionized water 6 times, and dried at 70 • C. Finally, the ZnAl-LDH/AAO composite coatings were successfully synthesized.

Characterization and Tests
To observe the cross-sectional profiles of ZnAl-LDHs/AAO composite coatings, specimens were completely wrapped using epoxy resin, left to set for 12 h, the cross-section of the specimens were then sandpapered until smooth. The surface morphologies and cross-sectional profiles of ZnAl-LDHs/AAO composite coatings were examined using SEM (FEI, Hillsboro, OR, USA) and the chemical composition was characterized using the energy dispersive spectrum (EDS). The crystal structure of the samples was confirmed using XRD (DX-2700B, Pingdingshan, China) over a 2θ range of 5-70 • and a scanning speed of 5 • min −1 . During the testing process, ZnAl-LDHs films were scraped from the surface of the AAO with a razor blade, and the powder was added to an appropriate amount of potassium bromide for grinding, and finally, the samples were prepared by KBr presses for infrared spectroscopy. The functional groups of the samples were acquired using spectroscopic analysis using the FT-IR (Spectrum 100, PerkinElmer, Waltham, MA, USA) in the range of 500 cm −1 to 4000 cm −1 at room temperature. The average hardness value (a mean of six measurements) was tested using a micro-hardness tester (HV-1000IS, Shanghai, China) with a diamond tetrapyramid indenter. The interfacial bonding force was examined by a pull-off tester (Ogdensburg, NY, USA), special glue was employed to bond the spindle and sample, and then stayed 12 h at room temperature.

Characterization and Tests
To observe the cross-sectional profiles of ZnAl-LDHs/AAO composite coatings, specimens were completely wrapped using epoxy resin, left to set for 12 h, the cross-section of the specimens were then sandpapered until smooth. The surface morphologies and crosssectional profiles of ZnAl-LDHs/AAO composite coatings were examined using SEM (FEI Hillsboro, OR, USA) and the chemical composition was characterized using the energy dispersive spectrum (EDS). The crystal structure of the samples was confirmed using XRD (DX-2700B, Pingdingshan, China) over a 2θ range of 5-70° and a scanning speed of 5° min −1 . During the testing process, ZnAl-LDHs films were scraped from the surface of the AAO with a razor blade, and the powder was added to an appropriate amount of potassium bromide for grinding, and finally, the samples were prepared by KBr presses for infrared spectroscopy. The functional groups of the samples were acquired using spectroscopic analysis using the FT-IR (Spectrum 100, PerkinElmer, Waltham, MA, USA) in the range of 500 cm −1 to 4000 cm −1 at room temperature. The average hardness value (a mean of six measurements) was tested using a micro-hardness tester (HV-1000IS, Shanghai China) with a diamond tetrapyramid indenter. The interfacial bonding force was examined by a pull-off tester (Ogdensburg, NY, USA), special glue was employed to bond the spindle and sample, and then stayed 12 h at room temperature.
The electrochemical characteristics of the samples were evaluated using an electrochemical workstation (CHI660B, Chenhua, Shanghai, China) after being soaked in 3.5 wt% NaCl solution for 72 h at room temperature with a typical three-electrode system. The samples were used as the working electrode, while a platinum sheet and a saturated calomel electrode (SCE) were the counter and the reference electrodes, respectively. The test frequency of electrochemical impedance spectroscopy (EIS) ranged from 1000 Hz down to 10 Hz, and the scanning voltage was 10 mV. The experiments were scanned from −0.5 V to +0.5 V with respect to the open circuit potential at a scan rate of 1 mV/s. The corrosion potential (Ecorr) and corrosion current density (icorr) were obtained from the potentiody- The electrochemical characteristics of the samples were evaluated using an electrochemical workstation (CHI660B, Chenhua, Shanghai, China) after being soaked in 3.5 wt% NaCl solution for 72 h at room temperature with a typical three-electrode system. The samples were used as the working electrode, while a platinum sheet and a saturated calomel electrode (SCE) were the counter and the reference electrodes, respectively. The test frequency of electrochemical impedance spectroscopy (EIS) ranged from 1000 Hz down to 10 Hz, and the scanning voltage was 10 mV. The experiments were scanned from −0.5 V to +0.5 V with respect to the open circuit potential at a scan rate of 1 mV/s. The corrosion potential (E corr ) and corrosion current density (i corr ) were obtained from the potentiodynamic polarization curves. The tribological experiment was conducted on a friction-abrasion testing machine (SFT-2M, Lanzhou, China), with a GCr15 steel ball (a diameter of 5 mm) as friction counterpart, a load of 1 N, a rotation speed of 200 RPM (revolutions per min), and a rotation radius of 2 mm. Subsequently, the worn surfaces of the samples were observed using SEM. The mass losses of samples were measured using an electronic analytical balance (EX225D, Ohaus, Parsippany, NJ, USA, with an accuracy of 0.1 mg and 0.01 mg readability); each group was measured five times to ensure the repeatability of the data and arithmetic means are used as the final experimental results.

Chemical and Phase Composition
The diffraction patterns of the ZnAl-LDHs/AAO composite coatings with different Zn 2+ concentrations are shown in Figure 2a. The (003), (006), and (012) reflection peaks appear at around 10 • , 19.9 • , and 34 • , indicating the successful synthesis of ZnAl-LDHs for all Zn 2+ concentrations, which is consistent with the results of the research by Shen et al. [38]. The appearance of the characteristic peak of Al 2 O 3 in all groups suggests that the composite coatings consist of ZnAl-LDHs and AAO. With the increase in Zn 2+ concentration in the solution, there is an obvious augment in the intensity of the LDH phases and a peak (003) narrowing, indicating more LDH platelets with high crystallinity contained in the deposited coatings. The phenomena may be due to the formation of compact LDH lamellas. Meanwhile, there is an obvious decrease in the intensity of the Al 2 O 3 phases, which can be attributed to the cover by the denser deposited LDH films. The characteristic diffraction peak of LDH films (003) undergoes a blue shift at the concentration of 0.2 mol/L and 0.3 mol/L. According to the Bragg reflection, the calculated value of basal interlayer spacing d (003) is 0.890 nm at the concentration of 0.05 mol/L and 0.10 mol/L. Therein, with the increase in Zn 2+ concentration, the basal interlayer distance increases to d (003) = 1.132 nm and d (003) = 1.168 nm at the concentrations of 0.20 mol/L and 0.30 mol/L, respectively. Here, the increased Zn 2+ concentration can both change the LDH's structural characteristics (morphologies and size) and the basal interlayer spacing between LDH layers. the increase in Zn 2+ concentration, the basal interlayer distance increases to d (003) = 1.1 nm and d (003) = 1.168 nm at the concentrations of 0.20 mol/L and 0.30 mol/L, respective Here, the increased Zn 2+ concentration can both change the LDH's structural character tics (morphologies and size) and the basal interlayer spacing between LDH layers. Figure 2b shows the FT-IR spectra of ZnAl-LDHs/AAO composite coatings with d ferent Zn 2+ concentration, where the peaks at 1138 cm −1 and 1624 cm −1 can be attributed the intercalated sulfate ions and the bending vibration of hydroxyl in interlayer wa molecules, respectively. The absorption peak at 3450 cm −1 can be assigned to the γ(Ostretching vibration and the interaction of interlayer water molecules between LDH plat The peaks that occur at the range of 617-837 cm −1 belong to M-OH characteristic abso tion peaks of metal-oxygen bonds between LDH layers. These values corresponded actly to the characteristic peaks of ZnAl-LDH [39]. Compared with the FT-IR spectra u der different Zn 2+ concentrations, the absorption peak of SO4 2− and the M-O absorpti peak of the interlayer metal-oxygen bonds are more obvious; this corresponds to the XR results. Hence, ZnAl-LDH films with complete functional groups and lamellae structu can be formed under different concentrations of Zn 2+ .   Figure 2b shows the FT-IR spectra of ZnAl-LDHs/AAO composite coatings with different Zn 2+ concentration, where the peaks at 1138 cm −1 and 1624 cm −1 can be attributed to the intercalated sulfate ions and the bending vibration of hydroxyl in interlayer water molecules, respectively. The absorption peak at 3450 cm −1 can be assigned to the γ(O-H) stretching vibration and the interaction of interlayer water molecules between LDH plates. The peaks that occur at the range of 617-837 cm −1 belong to M-OH characteristic absorption peaks of metal-oxygen bonds between LDH layers. These values corresponded exactly to the characteristic peaks of ZnAl-LDH [39]. Compared with the FT-IR spectra under different Zn 2+ concentrations, the absorption peak of SO 4 2− and the M-O absorption peak of the interlayer metal-oxygen bonds are more obvious; this corresponds to the XRD results. Hence, ZnAl-LDH films with complete functional groups and lamellae structure can be formed under different concentrations of Zn 2+ .

Surface and Cross-Sectional Morphology
The SEM images of ZnAl-LDH films at different Zn 2+ concentrations are shown in Figure 3. All the ZnAl-LDH nanoparticles possessed typical hexagonal symmetry structure with uniform size [40]. Apparently, the homogeneous ZnAl-LDH platelets are all perpendicular to the AAO substrate, which is consistent with the results of Wu [41]. With the increase in Zn 2+ concentration, it can be observed that the LDH nanosheets become more compact and the size of voids between them is reduced. Therein, the ZnAl-LDH nanosheets at the concentration of 0.3 mol/L possess a minimum size of about 3.75 µm and the resulting films are denser than those of other groups. In addition, the content ratio of the Al and Zn elements is about 1:12 (see Figure 3d). This is because that the AAO substrate is covered by the LDH films to a greater extent in this Zn 2+ concentration, which can also be verified by the covered "flower-like" cluster structure from Figure 3c.
different Zn concentrations are shown in Figure 4. Based on our earlier published works, the structure of AAO includes two parts (inner barrier layer and outer porous oxide layer), which also can be observed in the LDHs/AAO composite coatings [42]. It is clearly visible that the ZnAl-LDH films become thicker with the increased Zn 2+ concentration, among which the film thickness in the concentration of 0.3 mol/L is the largest (99.26 µm). Wang et al. prepared MgAl-LDHs/Al2O3 composite coatings on the surface of Mg alloy AZ31 and found that the thickness of composite coatings is enhanced with the increased concentration of Al2O3 nanoparticles [11]. When the Zn 2+ concentration ranges from 0.05 mol/L to 0.30 mol/L, the growth rate for the thickness of LDH films gradually becomes lower, indicating a limited improved effect of larger Zn 2+ concentration on the thickness growth of LDH films.  The cross-sectional photographs of the ZnAl-LDHs/AAO composite coatings with different Zn 2+ concentrations are shown in Figure 4. Based on our earlier published works, the structure of AAO includes two parts (inner barrier layer and outer porous oxide layer), which also can be observed in the LDHs/AAO composite coatings [42]. It is clearly visible that the ZnAl-LDH films become thicker with the increased Zn 2+ concentration, among which the film thickness in the concentration of 0.3 mol/L is the largest (99.26 µm). Wang et al. prepared MgAl-LDHs/Al 2 O 3 composite coatings on the surface of Mg alloy AZ31 and found that the thickness of composite coatings is enhanced with the increased concentration of Al 2 O 3 nanoparticles [11]. When the Zn 2+ concentration ranges from 0.05 mol/L to 0.30 mol/L, the growth rate for the thickness of LDH films gradually becomes lower, indicating a limited improved effect of larger Zn 2+ concentration on the thickness growth of LDH films. Figure 5a shows the hardness of AAO and ZnAl-LDHs/AAO composite coatings with different Zn 2+ concentrations, where the curve of ZnAl-LDH film thickness is added as a reference. It can be seen that the hardness of the composite coatings gradually decreases with the rise in Zn 2+ concentration. There is a minimum value in the concentration of 0.30 mol/L, and the corresponding value is 112 HV, which is about 50% of that of the AAO (230 HV). The results indicate that the structure of ZnAl-LDHs/AAO composite coatings is composed of the hard AAO and soft LDH films. However, the thickness of the ZnAl-LDH films at different Zn 2+ concentrations is inversely correlated with those of the hardness.

Hardness and Interfacial Bonding Strength
The interfacial bonding strength of samples was examined by a coating adhesion automatic scratch tester, and the average value (a mean of three measurements) was taken as the interfacial bonding strength. The interfacial bonding force between the ZnAl-LDH films and AAO matrix is shown in Figure 5b, it can be seen that the interfacial bonding forces of samples at different Zn 2+ concentrations are 7.20 MPa, 8.50 MPa, 8.75 MPa, and 8.85 MPa, respectively. The bonding force gradually enhances with the increase in Zn 2+ concentration, and reaches a maximum of 8.85 MPa at 0.30 mol/L. This may be due to the fact that the higher Zn 2+ concentration offers more divalent Zn 2+ for growth of LDH nanosheets and the further compact LDH films, which can be confirmed from the SEM images of ZnAl-LDH films in Figure 3. In addition, Zhang K et al. found that the bonding strength between micro-arc oxidation (MAO) coatings and 7N01Al alloy increases with the augment of NaAlO 2 concentration to a certain extent [43]. Here, the greater the power used for growth of the coating, the more beneficial it is for the metallurgical combination of the substrate and coatings, and the bonding strength will be enhanced accordingly. As a result, the less energy consumed by the electrolyte, the better the interfacial bonding strength. The energy employed in the electrolyte decreases with the reduction in its resistance, which is mainly connected to the concentration of conductive particles. Therefore, the higher the Zn 2+ concentration, the lower the corresponding resistance of the electrolyte will be, which will decrease the energy consumed by the electrolyte and the interfacial bonding strength will gradually enhance.  Figure 5a shows the hardness of AAO and ZnAl-LDHs/AAO composite coatings with different Zn 2+ concentrations, where the curve of ZnAl-LDH film thickness is added as a reference. It can be seen that the hardness of the composite coatings gradually decreases with the rise in Zn 2+ concentration. There is a minimum value in the concentration of 0.30 mol/L, and the corresponding value is 112 HV, which is about 50% of that of the AAO (230 HV). The results indicate that the structure of ZnAl-LDHs/AAO composite coatings is composed of the hard AAO and soft LDH films. However, the thickness of the ZnAl-LDH films at different Zn 2+ concentrations is inversely correlated with those of the hardness.

Hardness and Interfacial Bonding Strength
The interfacial bonding strength of samples was examined by a coating adhesion automatic scratch tester, and the average value (a mean of three measurements) was taken as the interfacial bonding strength. The interfacial bonding force between the ZnAl-LDH films and AAO matrix is shown in Figure 5b,   electrolyte will be, which will decrease the energy consumed by the electrolyte and the interfacial bonding strength will gradually enhance.

Corrosion Resistance of ZnAl-LDHs/AAO Composite Coatings
The potentiodynamic polarization curves are used to investigate the anti-corrosion behavior of different coated samples, as shown in Figure 6. The corrosion potential (Ecorr)

Corrosion Resistance of ZnAl-LDHs/AAO Composite Coatings
The potentiodynamic polarization curves are used to investigate the anti-corrosion behavior of different coated samples, as shown in Figure 6. The corrosion potential (E corr ) and corrosion current density (i corr ) derived from the curves are summarized in Table 1. Evidently, a higher corrosion potential or a lower corrosion current density represent a lower corrosion rate and a better corrosion resistance in a typical polarization curve. The AAO has the most negative E corr (−0.775 V) and the highest i corr (3.020 × 10 −7 A/cm 2 ) among all groups, which indicates that the AAO is easily corroded in chloride-containing solutions. All groups with ZnAl-LDHs/AAO composite coatings have the larger E corr values, meanwhile, their i corr values are two orders of magnitude lower than that of the bare AAO substrate. The reason may be that the ZnAl-LDH nanosheets passivate most of the active Al sites on the substrate surface to increase the potential [37]. Importantly, all coated groups appear to be very effective in protecting the AAO matrix from corrosion, the ZnAl-LDHs/AAO composite coating at the concentration of 0.30 mol/L has the highest E corr (−0.462) and the lowest i corr (2.435 × 10 −9 A/cm 2 ), indicating a superior corrosion resistance.
tings 2023, 13, x FOR PEER REVIEW LDH nanosheets gradually increased and the distribution of LDH more compact. Hence, LDH nanosheets with higher concentrations of role in sealing AAO film. Additionally, the thickness of the ZnAl-LD coatings became thicker, and the binding force became stronger with centration. And the effect of inhibiting the penetration behavior of cor hanced, which significantly improved the corrosion resistance of ZnA posite coatings.      Figure 7 shows the experimental data of the electrochemical impedance spectroscopy (EIS). The Nyquist curve (see Figure 7a) of ZnAl-LDHs/AAO composite coatings is composed of a capacitive arc, which evaluates the anti-corrosion behavior by the size of the capacitive arc radius. It can be observed that each coating has a larger capacitive arc radius and plays a role in blocking the corrosive medium as a resistive barrier layer during immersion. It is well-known that the low-frequency impedance modulus of Z (10 −2 -0 Hz) in the Bode plot (see Figure 7b) can reflect the corrosion resistance of the coatings [44]. Compared with other concentration conditions, the |Z| value of the low-frequency region of the LDHs/AAO composite coating in the concentration of 0.3 mol/L is significantly higher, which indicates that the composite coating prepared under this concentration has the best corrosion resistance. This result is consistent with the potentiodynamic polarization curve. To further confirm the corrosion resistance of ZnAl-LDHs/AAO composite coatings with different Zn 2+ concentrations, the physical model of ZnAl-LDHs/AAO composite coatings and the corresponding equivalent circuit are established, as shown in Figure 8. The fitting results are shown in Table 2. R sol represents the solution resistance, R ct is used to describe the interface transfer resistance, R ox represents the resistance between the LDH film and oxide film. It can be observed that R ct of ZnAl-LDHs/AAO composite coating at the concentration of 0.30 mol/L is the largest, which has the best corrosion resistance. ZnAl-LDHs/AAO composite coatings with good crystallinity were formed at different concentrations of Zn 2+ . With the continuous increase in Zn 2+ concentration, the distance between LDH nanosheets gradually increased and the distribution of LDH nanosheets became more compact. Hence, LDH nanosheets with higher concentrations of Zn 2+ played a better role in sealing AAO film. Additionally, the thickness of the ZnAl-LDHs/AAO composite coatings became thicker, and the binding force became stronger with increased Zn 2+ concentration. And the effect of inhibiting the penetration behavior of corrosive ions was enhanced, which significantly improved the corrosion resistance of ZnAl-LDHs/AAO composite coatings.

Groups
Ecorr    Figure 9a shows the relationship between the COF and sliding time under dry wear for all groups. It can be observed that the COF of AAO exhibit significant fluctuation at the initial stage of the test, and then gradually increase up to a relatively stable value (0.7) throughout the test. The larger COF can be attributed to the decreased effective contact area; the earlier published studies have proved that the actual contact area of surface affects the tribological properties significantly [45,46]. Likewise, bulges, cracks, and pores upon the surface of AAO reduced the effective contact area, which will generate severe abrasive wear, "edge crushing phenomena", along the edges of these defects and finally lead to more wear debris left on the surfaces [45,47]. The COF of samples with lower Zn 2+ concentrations (i.e., 0.05 mol/L and 0.10 mol/L) gradually become stable after the initial running-in stage, giving a relatively constant value about 0.2 and 0.35 until the end of the tests, respectively. At low Zn 2+ concentrations, it is easy for ZnAl-LDH nanosheets to peel away from the substrate surface caused by the lower interfacial bonding force (see Figure  5b), which resulted in the larger COF values. However, the sample with the Zn 2+ concentration of 0.20 mol/L has a long-term large COF of approximately 0.4 in the initial friction stage due to the reduced actual effective contact area resulted from relatively rough surface caused by the formation of a large number of "flower-like" LDH nanosheets (see Figure 3c), which results in the largest wear loss (1.1 mg) among all groups (see Figure 9b). In the case of the sample with a Zn 2+ concentration of 0.30 mol/L, a flat and low COF curve can be observed, which is reduced by 85.71% of that of AAO and also exhibits little fluctuation. The reason for this phenomenon is that the sample with the concentration of 0.30 mol/L, possessing a uniform and compact surface (see Figure 3d), has fewer collisions among micro-peaks during sliding process. The COF curves of samples with different Zn 2+ concentrations show larger distinction in terms of values, stability, and trend, indicating the significant effect for Zn 2+ concentration of ZnAl-LDHs on the COF. It was obvious that the COF curves for each group with ZnAl-LDHs/AAO composite coatings gradually fluctuated with the increase in degree of wear, but there was no evident transition point, implying that the LDH composite films still existed [48,49].   Figure 9a shows the relationship between the COF and sliding time under dry wear for all groups. It can be observed that the COF of AAO exhibit significant fluctuation at the initial stage of the test, and then gradually increase up to a relatively stable value (0.7) throughout the test. The larger COF can be attributed to the decreased effective contact area; the earlier published studies have proved that the actual contact area of surface affects the tribological properties significantly [45,46]. Likewise, bulges, cracks, and pores upon the surface of AAO reduced the effective contact area, which will generate severe abrasive wear, "edge crushing phenomena", along the edges of these defects and finally lead to more wear debris left on the surfaces [45,47]. The COF of samples with lower Zn 2+ concentrations (i.e., 0.05 mol/L and 0.10 mol/L) gradually become stable after the initial running-in stage, giving a relatively constant value about 0.2 and 0.35 until the end of the tests, respectively. At low Zn 2+ concentrations, it is easy for ZnAl-LDH nanosheets to peel away from the substrate surface caused by the lower interfacial bonding force (see Figure 5b), which resulted in the larger COF values. However, the sample with the Zn 2+ concentration of 0.20 mol/L has a long-term large COF of approximately 0.4 in the initial friction stage due to the reduced actual effective contact area resulted from relatively rough surface caused by the formation of a large number of "flower-like" LDH nanosheets (see Figure 3c), which results in the largest wear loss (1.1 mg) among all groups (see Figure 9b). In the case of the sample with a Zn 2+ concentration of 0.30 mol/L, a flat and low COF curve can be observed, which is reduced by 85.71% of that of AAO and also exhibits little fluctuation. The reason for this phenomenon is that the sample with the concentration of 0.30 mol/L, possessing a uniform and compact surface (see Figure 3d), has fewer collisions among micro-peaks during sliding process. The COF curves of samples with different Zn 2+ concentrations show larger distinction in terms of values, stability, and trend, indicating the significant effect for Zn 2+ concentration of ZnAl-LDHs on the COF. It was obvious that the COF curves for each group with ZnAl-LDHs/AAO composite coatings gradually fluctuated with the increase in degree of wear, but there was no evident transition point, implying that the LDH composite films still existed [48,49]. The wear losses of samples with different Zn 2+ concentrations under dry wear a shown in Figure 9b. It can be observed that the wear losses have a reduced trend with t increased Zn 2+ concentration, but there is a rise in the concentration of 0.20 mol/L, wh should be attributed to the long-term large COF caused by the rough peaks of surface the initial friction stage. High contact pressure due to the rough surface would crush p ticles and break the layered structure of ZnAl-LDHs, which also restricts its lubricati properties (see Figures 3 and 10). Therein, the wear loss of the sample with the Zn 2+ co centration of 0.30 mol/L is the lowest (0.20 mg), which is caused by the lower average CO and larger interfacial bonding strength. Additionally, the published literature also fou that the thickness and pore size of the AAO coatings has an effect on the tribological b havior to a certain degree [50,51]. As a result, the AAO parameters and LDH nanoshe may have a coupling impact on the friction and wear performance of ZnAl-LDHs/AA composite coatings.

Tribological Behavior of ZnAl-LDHs/AAO Composite Coatings
To further explain the effect of different Zn 2+ concentration on the tribological beha ior of ZnAl-LDHs/AAO composite coatings under dry wear, the SEM images and E results of the worn surfaces are acquired, as shown in Figure 10. The wear track widths samples with the Zn 2+ concentration of 0.05 mol/L, 0.10 mol/L, 0.20 mol/L, and 0.30 mo are 343.21 µm, 312.51 µm, 340.42 µm, and 296.77 µm, respectively. Therein, the wear sc width of the sample with the concentration of 0.30 mol/L is the minimum and the reduc width is nearly 23% of that of the sample with the concentration of 0.20 mol/L, showi excellent wear resistance. In addition, there were no evident streaks and debris generat on wear traces compared with other groups. This result is consistent with the wear loss of samples. The EDS results show that a stable friction-reducing LDH films on the surfa of the LDHs/AAO composite coatings after friction and wear tests, which further confirm the formation of the ZnAl-LDH film and its friction-reduction and wear resistance. summary, the lubricating mechanism of ZnAl-LDH nanosheets as lubricant additives that nanoparticles may fill pores, and the friction force is reduced by the relative slidi motion of uniform and smooth LDH layers, which further forms a protective film on t surfaces of friction pairs [39]. The wear losses of samples with different Zn 2+ concentrations under dry wear are shown in Figure 9b. It can be observed that the wear losses have a reduced trend with the increased Zn 2+ concentration, but there is a rise in the concentration of 0.20 mol/L, which should be attributed to the long-term large COF caused by the rough peaks of surface in the initial friction stage. High contact pressure due to the rough surface would crush particles and break the layered structure of ZnAl-LDHs, which also restricts its lubrication properties (see Figures 3 and 10). Therein, the wear loss of the sample with the Zn 2+ concentration of 0.30 mol/L is the lowest (0.20 mg), which is caused by the lower average COF and larger interfacial bonding strength. Additionally, the published literature also found that the thickness and pore size of the AAO coatings has an effect on the tribological behavior to a certain degree [50,51]. As a result, the AAO parameters and LDH nanosheets may have a coupling impact on the friction and wear performance of ZnAl-LDHs/AAO composite coatings.
To further explain the effect of different Zn 2+ concentration on the tribological behavior of ZnAl-LDHs/AAO composite coatings under dry wear, the SEM images and EDS results of the worn surfaces are acquired, as shown in Figure 10. The wear track widths of samples with the Zn 2+ concentration of 0.05 mol/L, 0.10 mol/L, 0.20 mol/L, and 0.30 mol/L are 343.21 µm, 312.51 µm, 340.42 µm, and 296.77 µm, respectively. Therein, the wear scar width of the sample with the concentration of 0.30 mol/L is the minimum and the reduced width is nearly 23% of that of the sample with the concentration of 0.20 mol/L, showing excellent wear resistance. In addition, there were no evident streaks and debris generated on wear traces compared with other groups. This result is consistent with the wear losses of samples. The EDS results show that a stable friction-reducing LDH films on the surface of the LDHs/AAO composite coatings after friction and wear tests, which further confirms the formation of the ZnAl-LDH film and its friction-reduction and wear resistance. In summary, the lubricating mechanism of ZnAl-LDH nanosheets as lubricant additives is that nanoparticles may fill pores, and the friction force is reduced by the relative sliding motion of uniform and smooth LDH layers, which further forms a protective film on the surfaces of friction pairs [39]. The friction and wear process can be divided into two stages: The first stage is the friction and wear of ZnAl-LDH films, which corresponds to the initial running-in period of the sliding, and the second stage is the friction and wear of ZnAl-LDHs/AAO composite coatings. The plastic deformation of matrix surface can be contributed to by transitory heat owing to dissipated energy caused by rough surface or asperity collisions, resulting in relatively higher resistance to sliding [52]. The ZnAl-LDH films play a role of lubrication in the process of friction due to its relatively weak van der Waals forces for easy shear between their laminates, which significantly reduces the COF for the AAO matrix. Moreover, wear loss and worn scars obtained an the effective reduction under the action of the LDHs with largest Zn 2+ concentration (0.30 mol/L) compared with the other groups. As shown in Figure 11, the tribological model of the ZnAl-LDHs/AAO composite coated sam- The friction and wear process can be divided into two stages: The first stage is the friction and wear of ZnAl-LDH films, which corresponds to the initial running-in period of the sliding, and the second stage is the friction and wear of ZnAl-LDHs/AAO composite coatings. The plastic deformation of matrix surface can be contributed to by transitory heat owing to dissipated energy caused by rough surface or asperity collisions, resulting in relatively higher resistance to sliding [52]. The ZnAl-LDH films play a role of lubrication in the process of friction due to its relatively weak van der Waals forces for easy shear between their laminates, which significantly reduces the COF for the AAO matrix. Moreover, wear loss and worn scars obtained an the effective reduction under the action of the LDHs with largest Zn 2+ concentration (0.30 mol/L) compared with the other groups. As shown in Figure 11, the tribological model of the ZnAl-LDHs/AAO composite coated samples is established. Smaller LDH nanosheets were easily embedded in the pores and defects of sliding surfaces, forming a thick tribofilm, which also smoothed the collision of asperities during actual friction process. The friction-reduction and anti-wear mechanism of ZnAl-LDHs/AAO composite coatings can be summarized as follows: The soft ZnAl-LDH films and hard AAO substrate act the bearing effect together, and the ZnAl-LDH films with composite flake structure are destroyed and deformed due to the action of load extrusion and frictional tangential force. Finally, the ZnAl-LDH lubricating films are formed between the friction pairs. Meanwhile, the LDH nanosheets stored in the cracks and holes of the AAO matrix will constantly move to the interface to replenish the lubricating films due to the action of load and tangential force friction. In addition, the LDH transfer films, namely, lubricating membrane, formation during the sliding friction process, are transferred from AAO to the surface of friction counterpart (ball), which changes the contact model and a lubricating interface is formed here. Therefore, the synergistic effect of the hard AAO matrix, ZnAl-LDH films, and LDH transfer films endows the ZnAl-LDHs/AAO composite coatings with excellent tribological properties. defects of sliding surfaces, forming a thick tribofilm, which also smoothed the collision of asperities during actual friction process. The friction-reduction and anti-wear mechanism of ZnAl-LDHs/AAO composite coatings can be summarized as follows: The soft ZnAl-LDH films and hard AAO substrate act the bearing effect together, and the ZnAl-LDH films with composite flake structure are destroyed and deformed due to the action of load extrusion and frictional tangential force. Finally, the ZnAl-LDH lubricating films are formed between the friction pairs. Meanwhile, the LDH nanosheets stored in the cracks and holes of the AAO matrix will constantly move to the interface to replenish the lubricating films due to the action of load and tangential force friction. In addition, the LDH transfer films, namely, lubricating membrane, formation during the sliding friction process, are transferred from AAO to the surface of friction counterpart (ball), which changes the contact model and a lubricating interface is formed here. Therefore, the synergistic effect of the hard AAO matrix, ZnAl-LDH films, and LDH transfer films endows the ZnAl-LDHs/AAO composite coatings with excellent tribological properties. Figure 11. Model of friction-reduction and wear resistance mechanism of ZnAl-LDHs/AAO composite coatings.

Conclusions
In this paper, ZnAl-LDHs/AAO composite coatings were synthesized by anodization and the in situ growth method on the surface of an Al alloy substrate at different Zn 2+ concentrations (0.05 mol/L, 0.10 mol/L, 0.20 mol/L, and 0.30 mol/L). The corrosion resistance and tribological properties of ZnAl-LDHs/AAO composite coatings were tested and analyzed. The effect of Zn 2+ concentration on the anti-corrosion property and tribological behavior of ZnAl-LDHs/AAO composite coatings was discussed. The key conclusions are summarized as follows: The prepared ZnAl-LDH nanosheets are all perpendicular to the AAO substrate, and the resulting ZnAl-LDHs/AAO composite coatings become uniform and compact with the increased Zn 2+ concentration, but some "flower-like" ZnAl-LDH platelets can be found on the surface with an Zn 2+ concentration of 0.2 mol/L. Meanwhile, the thickness and interfacial bonding strength of ZnAl-LDH films are both enhanced with the increase in Zn 2+ concentration.
All ZnAl-LDHs/AAO composite-coated samples have improved corrosion resistance, friction-reduction, and anti-wear properties. The sample with the highest Zn 2+ concentration possesses better corrosion resistance and acceptable tribological properties, especially 0.30 mol/L, which exhibits the best protected action for the Al alloy substrate.
The sliding pairs change from substrate/steel ball to ZnAl-LDH films/transferred films, and a lubricating interface formed here enhances the lubricity for the contact model.

Conclusions
In this paper, ZnAl-LDHs/AAO composite coatings were synthesized by anodization and the in situ growth method on the surface of an Al alloy substrate at different Zn 2+ concentrations (0.05 mol/L, 0.10 mol/L, 0.20 mol/L, and 0.30 mol/L). The corrosion resistance and tribological properties of ZnAl-LDHs/AAO composite coatings were tested and analyzed. The effect of Zn 2+ concentration on the anti-corrosion property and tribological behavior of ZnAl-LDHs/AAO composite coatings was discussed. The key conclusions are summarized as follows: The prepared ZnAl-LDH nanosheets are all perpendicular to the AAO substrate, and the resulting ZnAl-LDHs/AAO composite coatings become uniform and compact with the increased Zn 2+ concentration, but some "flower-like" ZnAl-LDH platelets can be found on the surface with an Zn 2+ concentration of 0.2 mol/L. Meanwhile, the thickness and interfacial bonding strength of ZnAl-LDH films are both enhanced with the increase in Zn 2+ concentration.
All ZnAl-LDHs/AAO composite-coated samples have improved corrosion resistance, friction-reduction, and anti-wear properties. The sample with the highest Zn 2+ concentration possesses better corrosion resistance and acceptable tribological properties, especially 0.30 mol/L, which exhibits the best protected action for the Al alloy substrate.
The sliding pairs change from substrate/steel ball to ZnAl-LDH films/transferred films, and a lubricating interface formed here enhances the lubricity for the contact model. Moreover, the increased bonding force between the ZnAl-LDHs and AAO substrate suppresses the excessive removal of ZnAl-LDH films, which provides good durability of lubrication. Data Availability Statement: The original codes and data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest:
The authors declare no potential conflict of interest with respect to the research, authorship, and/or publication of this article.