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In order to obtain better corrosion resistance, the micro-arc oxidation (MAO) coating after adding (Y(NO₃)${}_3\cdot 6$H₂O was sealed by TiO₂ sol–gel method. Dynamic polarization curves and electrochemical impedance spectroscopy show that the prepared samples (Y(NO₃)${}_3\cdot 6$H₂O content was 1.5g/L) have good corrosion resistance ($1.2308\times 10^{-9}$A$\cdot {\mathrm{\text{cm}}}^{-2}$) in 3.5wt.% NaCl aqueous solution. Afterward, the samples with a concentration of 1.5 g/L of Y(NO₃)${}_3\cdot 6$H₂O were sealed by TiO₂ sol–gel. Through SEM, XRD and electrochemical test, it is found that the surface of the film is smooth, the phase is mainly in the form of Anatase-TiO₂ and has better corrosion resistance ($7.593\times 10^{-9}$A$\cdot {\mathrm{\text{cm}}}^{-2}$). This showed that sealing the doped 2024 aluminum alloy MAO film can significantly reduce the corrosion sensitivity and corrosion rate of the film.

The protective coatings on laser-welded joints of Zr–1Nb alloy were prepared by the micro-arc oxidation (MAO) process. The morphology, microstructure and phase composition of MAO coatings at different regions of Zr–1Nb alloy laser-welded joints were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion behaviors of the coatings. The MAO coatings at the fusion zone (FZ) and heat-affected zone (HAZ) were smooth with some small holes, while MAO coatings at base metal (BM) were relatively rough with large holes. The cross-sectional topography at FZ coatings was quite dense with less distribution of pores and cracks, which can effectively improve the protective properties of the coatings. HAZ coatings were divided into outer loose layer and inner dense layer. The BM coating layer possessing many holes was a typical sparse layer structure. The phase compositions of welded joints treated by MAO were diverse at different zones of the joints, but all of them consisted of the monoclinic ZrO₂ and tetragonal ZrO₂ phases. The coatings of FZ had the best corrosion resistance, compared with HAZ and BM. The results of EIS indicated that the thickness and defects of the inner compact layer played a key role in improving the corrosion resistance of the MAO coatings on the surface of the Zr–1Nb alloy laser-welded joints.

Micro-arc oxidation (MAO) is a novel technique for producing oxide films on metals like Al, Mg, Ti and Zr. A ceramic-like film containing Ca and P was deposited on Ti-6Al-4V substrate by MAO in an electrolytic solution using a pulsed power supply. XRD indicated that the film was a mixture of anatase, rutile and VO. SEM showed that the surface of the film was microporous with 1–2 μm pores. EDS gave the element distributions of the cross-section. The result indicated that elements like Ca and P could be incorporated into the film during MAO process. The microhardness test showed that the film had an average hardness of 540 HV_0.025.

Ceramic coatings were formed on a pure aluminum substrate in an aqueous solution of phosphate salt, using the pulsating micro-arc oxidation method. XRD indicated that the film composed of pure γ-Al₂O₃. SEM showed that the surface of the film was micro-porous with 1–5 μm pores. EDS gave the element distributions of the cross-section. The result indicated that elements from the electrolyte like P could be incorporated into the film during MAO process. The microhardness test showed that the film had an average hardness of 654 HV_0.025. The growth process of the Al₂O₃ ceramic-like film was discussed on the basis of the structure analysis.

Ti and its alloys are considered to be very promising in biological material field. But their biological activity and corrosion resistance in body fluid still need to be improved. As a novel technique for surface treatment, Micro-arc oxidation (MAO) is attracting interest from those who want to improve their implant properties through surface modification. This paper reviewed the application of MAO on biomedical Ti-alloys. The structure characteristic such as surface and cross-section micrographs, phase components, and element distribution were analyzed. The mechanical and biological properties of MAO films were further discussed on this basis. The effects of current density, voltage, and electrolyte on the structure and properties of the films were summarized.

Ceramic-like coatings were deposited on the substrate of the AZ91 alloy by micro-arc oxidation (MAO). Many methods like X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron diffraction spectroscopy (EDS) were used to analyze the structure and composition of the coating. XRD showed that the MAO coating was mainly composed of MgO. The results of SEM showed that the coating surface possessed a porous microstructure and the pore size was in the range of 1–3 μm. EDS indicated that the coating matrix was relatively dense and had a homogeneous thickness. The element silicon from the applied electrolyte could be absorbed into the coating in the MAO process. The hardness of the MgO coatings could reach 513 HV_0.025 that was about four times higher than that of the substrate. It is expected that the coatings have a great potential in the surface protection of magnesium alloys.

Micro-arc oxidation technique (MAO) treatment produces a layer of alumina film on the surface of the aluminum alloy. A hard and uniform tetrahedral amorphous carbon film (diamond-like carbon, DLC) was deposited on the top of the alumina layer of the 2024 aluminum alloy by a pulsed filtered catholic vacuum arc (FCVA) deposition system with a metal vapor vacuum arc (MEVVA) source. The morphology and tribological properties of the duplex DLC/Al₂O₃ coating were investigated by a scanning electron microscope (SEM) and a ball-on-disk sliding tester. These results suggested that the duplex DLC/Al₂O₃ coating had good adhesion and a low friction coefficient, which improved significantly the wear resistance of aluminum alloys.

Ceramic coatings have been synthesized on 6063 aluminum alloy by micro-arc oxidation (MAO) technique. The effects of current intensity on the chemical and mechanical properties of MAO coatings have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), micro-hardness test, adhesion measurement, and friction and wear tests. The fabricated samples consist mainly of α-Al₂O₃ and γ-Al₂O₃ phases. SEM results show that the surface of the prepared coatings is dense except some pores distributed over the coating surface. Mechanical measurements show that these kinds of samples have high hardness, excellent adhesion, and high friction resistance. The coating consists of a thin buffer layer at the metal surface followed by a relatively compact inner layer and finally a higher porous ceramic layer. Meanwhile, there is a nonuniform distribution of hardness for the samples in the plane parallel to the substrate.

Ceramic coatings have been synthesized on 6063 aluminum alloy by micro-arc oxidation (MAO) technique in the solution of Na₂B₄O₇ electrolyte with and without α-Al₂O₃ nanoadditive. Effects of α-Al₂O₃ nanoadditive on the phase composition, micro-structure, micro-hardness, adhesion and wear resistance of the prepared ceramic coatings have been investigated in this paper. The phase composition and microstructure of the MAO coatings were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDX) analyses, respectively. Micro-hardness, adhesion and tribological and wear tests were also performed. The results showed that the α-Al₂O₃ nanoadditive doped in the electrolyte had great influence on the structural and mechanical properties of the ceramic coatings.

Magnesium and its alloys are potential biodegradable implant materials due to their attractive biological properties. But the use of magnesium is still hampered by its poor corrosion resistance in physiological fluids. In this study, a HA-containing coating was fabricated by micro-arc oxidation (MAO). The active plasma species of micro-discharge was studied by optical emission spectroscopy (OES). The microstructure and composition were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion behavior and apatite-forming ability were studied by electrochemical tests and immersed samples in simulated body fluids (SBF). The results show that the microdischarge channel model is gas discharges and oxide layer discharges. The elements from the substrate and electrolyte take part in the formation of the coating. The MAO coating significantly improves the corrosion resistance of AZ31 magnesium alloy and enhances the apatite formation ability.

Micro-arc oxidation (MAO) coatings were prepared on closed-cell aluminum foams. The microstructure, elemental distribution and phase composition of the MAO coatings were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The corrosion resistances and compressive properties of the uncoated and coated aluminum foams were studied by electrochemical polarization test and mechanical test, respectively. The results show that the MAO coatings cover the surfaces of closed-cell aluminum foams. The average thickness of the MAO coatings is 7 μm. The MAO coatings are mainly composed of γ-Al₂O₃ phase. The corrosion resistances of the closed-cell aluminum foams are improved by MAO treatment. The as-received foams show a low corrosion potential (-1.36 V), on contrary, the MAO coated foams get an increase corrosion potential (-0.78 V). But the MAO coated foams show the marginal variation in compressive strengths when the thickness of the coatings could be negligible compared with the total thickness of the foams.

The ceramic coatings were prepared on 2A12 alloy by micro-arc oxidation in CH₃COONa–Na₂WO₄ electrolyte system with different concentration of KOH added. The effects of KOH in this electrolyte on micromorphology, phase compositions, adhesion and corrosion resistance of the coatings were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD), scratch test and electrochemistry workstation. The results show that KOH has a significant influence on the surface morphology, which can make the surface smoother. The adhesion of the coating becomes stronger with the increase of KOH in the electrolyte. The corrosion resistance of the coated specimen increases a lot compared with that of the substrate. And the lowest corrosion current density (I_corr) of the coating prepared in the electrolyte with KOH is about three orders of magnitude lower than that of the substrate.

Composite coating was prepared on AZ91D magnesium alloy with a new method which combined silica sol with micro-arc oxidation (MAO). The MAO coating was prepared on the basis of MAO solution, and then coated by sol–gel process. The composite coating was obtained after second MAO treatment. Scanning electron microscopy coupled with X-ray diffraction (XRD), energy spectrum analysis and electrochemical testing was applied to characterize the properties of MAO coating and composite coating. The experimental test results indicated that the Si element derived from SiO₂ gel particle embedded into the MAO coating by second MAO treatment. The surface of composite coating became dense and the holes were smaller with silica sol sealing process. The corrosion resistance of composite coating was improved than the MAO coating.

The grayish black film was prepared on AM50 magnesium alloy with a new method which combined chemical conversion with micro-arc oxidation (MAO). The optimum formula of chemical conversion was obtained by L₉(3⁴) orthogonal test. Meanwhile, the morphology, structure, composition and corrosion resistance of films were analyzed by scanning electron microscopy (SEM), energy spectrum analysis (energy dispersive X-ray spectroscopy (EDS)), X-ray diffraction (XRD), electrochemical tests and CuSO₄ drip experiment. The results indicated that Mo element was introduced into the MAO film by chemical conversion pretreatment. The surface of composite film was smooth and compact. The main phase composition of the composite film were SiO₂, Mo₉O${}_{26}$, MgSiO${}_{3,}$ Mg₂SiO₄ and Mo₉O${}_{26}$ was identified to be responsible for giving color to the film. The corrosion resistance of the grayish black film was improved obviously.

Due to the biocompatibility, mechanical strength and esthetics properties, titanium–porcelain prosthetic plays an important role in prosthodontics. However, weak bonding strength and considerable thickness of the porcelain restrict its application. Whether we can find a method to increase the bonding strength and reduce the thickness of the porcelain is an acute problem. In this study, ceramic coatings with similar color of nature teeth are fabricated on the surface of pure titanium by micro-arc oxidation (MAO). The colors, thickness and bonding strength of the coatings can be controlled by adding ferrous sulfate into the electrolytes. These new coatings can be used in titanium–porcelain prosthetic to substitute the opaque and dentin porcelain which can enhance the bonding strength and decrease the thickness of the porcelain compared with the conventional technique.

Biocompatibility is crucial for implants. In recent years, numerous researches were conducted aiming to modify titanium alloys, which are the most extensively used materials in orthopedic fields. The application of zirconia in the biomedical field has recently been explored. In this study, the biological ZrO₂ coating was synthesized on titaniumalloy (Ti6Al4V) substrates by a duplex-treatment technique combining magnetron sputtering with micro-arc oxidation (MAO) in order to further improve the corrosion resistance and biocompatibility of Ti6Al4V alloys. The microstructures and phase constituents of the coatings were characterized by scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), the surface wettability was evaluated by contact angle measurements. The results show that ZrO₂ coatings are porous with pore sizes less than 2$\mu$m and consist predominantly of the tetragonal ZrO₂ (t-ZrO${}_2)$ and cubic ZrO₂(c-ZrO${}_2)$ phase. Electrochemical tests indicate that the corrosion rate of Ti6Al4V substrates is appreciably reduced after surface treatment in the phosphate buffer saline (PBS). In addition, significantly improved cell adhesion and growth were observed from the ZrO₂/Zr surface. Therefore, the hybrid approach of magnetron sputtering and MAO provides a surface modification for Ti6Al4V to achieve acceptable corrosion resistance and biocompatibility.

Corrosion is generally an undesirable phenomenon in most engineering applications. The effects of micro-arc oxidation (MAO) coating on Al alloy on the galvanic corrosion performance have been studied. The surface micro-structures were characterized using scanning electron microscope (SEM) and metallographic microscope before and after corrosion. Electrochemical studies on the corrosion behaviors have been reported by using open circuit potential (OCP) and potentiodynamic polarization (PDP) measurements in a slurry solution. All measurements were consistent with each other and showed that the MAO coating can significantly reduce the corrosion rate of Al alloy. A model for elucidating corrosion mechanism and corrosion process of the MAO coating on Al alloy is proposed.

Micro-arc oxidation coatings were fabricated on 7E04 aluminum alloy substrates by micro-arc oxidation (MAO) in the electrolytes with the graphite addition varying from 0 to 8g/L (0, 2, 4, 6, 8g/L). The effect of graphite concentration on the surface morphologies, micro-hardness, thickness, phase composition and corrosion resistance of coatings was investigated. With the graphite powder concentration increasing, the oxidation voltage decreased gradually and the thickness of coatings firstly dropped down and then went up. It is found that the size of micro-pores and sintered discs declined with increase of graphite concentration. The XRD results indicated that MAO coatings mainly consisted of $\gamma$-Al₂O₃, $\theta$-Al₂O₃, SiO₂ and a little $\alpha$-Al₂O₃. The corrosion resistance of coatings was improved with the addition of graphite powder. The study reveals that the appropriate graphite powder in the electrolytes is essential to promote the performance of the coatings.

In this paper, retrogression and re-aging (RRA) heat treatment was applied to 7075 aluminum alloy in T6 condition at different times (30 and 90min) and temperatures (180^∘C and 240^∘C). While RRA heat treatments increase the corrosion resistance of the material, it does not harm its mechanical properties. On the other hand, the surface resistance of aluminum is low. Surface modifications are applied to overcome this deficiency. Among these, the micro-arc oxidation (MAO) method increases the corrosion resistance and attains excellent values in surface hardness. To better understand the RRA/MAO relationship, heat-treated (RRA) samples with four different parameters were coated with the MAO method. In this way, a ceramic oxide coating layer was created on the material surfaces. In order to determine the RRA parameter effect, the MAO process parameters are kept constant (anode voltage ($V_a$): 500V, cathode voltage ($V_k$): 300V, anode voltage open time ($t_a)$: 300$\mu$s, cathode voltage open time ($t_k$): 200$\mu$s, frequency: 160 coating with Hz, and process time: 20min). Surface properties (coating thickness, surface roughness, surface arc duct’s structure, etc.), phase analysis (X-ray diffraction (XRD)) and microstructures (coating cross-section studies: distance-dependent hardness, coating/backing material interface character, coating porosity ratio) were examined. XRD analysis showed that the main phase of the coatings is $\gamma$-Al₂O₃. A coating layer of around 125$\mu$m was achieved with the growth rate of 6$\mu$m/min. Surface roughness was between 5.5$\mu$m and 8$\mu$m for different RRA parameters. RRA/MAO relation with the characterization made was detailed, and predictions were made for the surface properties of the material (hardness, corrosion resistance, wear, etc.).

The growth process of micro-arc oxidation films on ZL109 cast Al-Si alloy was investigated and the relation between current density and arc voltage, arc time and the growth rate of the films was discussed. Additionally, the morphology, element distribution and phase components of the films of the ZL109 higher-silicon cast aluminium alloys were analyzed. Results indicated that in the oxidation process, arc voltage was a constant under the condition of varied current densities; moreover, when current density increased, arc time shortened, film thickness increased, and the growth rate of films first increased and then decreased. The oxidation film was mainly composed of mullite (3Al₂O₃ • 2SiO₂), α-Al₂O₃ and γ-Al₂O₃ phase, among which, the content of mullite was the highest.