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Multiple advantages, such as good formability, high specific strength, excellent thermal conductivity, and high corrosion resistance, enable aluminum alloy wide application in various fields; however, low surface hardness and poor wear resistance limit its further development. In this study, three surface modification coatings were successfully prepared on the surface of 7A04 aluminum alloy by microarc oxidation (MAO) and a combination of hard anodizing treatment (HA) and physical vapor deposition (PVD), named MAO, HA+W+DLC, and HA+Ti+ta-C, respectively. The microstructure, hardness, and tribological properties of the three coatings and the 7A04 aluminum alloy substrate were studied. The results show that the surface quality and hardness of the coated samples were higher than those of the 7A04 aluminum alloy and that the HA+Ti+ta-C coating possessed the highest hardness of 34.23 GPa. Moreover, the wear resistance of the two multilayer coatings was significantly improved during the ring-block wear tests under oil lubrication, exhibiting a wear rate of 1.51 × 10−7 mm3/N·m for HA+W+DLC and 1.36 × 10−7 mm3/N·m for HA+Ti+ta-C. [ABSTRACT FROM AUTHOR]

Magnesium alloys have excellent properties such as high specific strength, rigidity, good damping, electromagnetic shielding, castability, and machinability. However, corrosion, creep, and fatigue resistance are generally the properties that need to be improved. In this study, a hybrid coating was formed on AZ31 magnesium alloy with different surface roughness at different frequencies. In the hybrid coating, an oxide coating was first grown on the surface by the micro-arc oxidation method, and then, the epoxy coating was grown on this oxide coating by the dipping method. The characterization of the coatings was performed by XRD, SEM, and EDS analysis. Furthermore, the relationship between fatigue life and surface roughness was also discussed in this study. The fatigue life and strength of the coated samples were determined as a result of the tests performed on the fatigue device. It was concluded that the fatigue life and strength of hybrid coatings were much better compared to uncoated and single-layer coated samples. [ABSTRACT FROM AUTHOR]

A biomimetic coating was prepared on β-titanium alloy surface using micro-arc oxidation to enhance its bone regeneration property in diabetic environment. The research probed into the complex interactions between crystalline phase, chemical composition, and surface morphology in modulating osteoblast behavior. The resultant coating displayed an open, porous structure, characterized by surface porosities ranging from 19.3 % to 39.8 % and a roughness distribution spanning 22.9 to 37.9 nm. XRD and XPS analyses confirmed a hydroxyapatite crystal structure within the coatings. Elemental quantification revealed Ca/P ratios from 1.49 to 2.44, aligning closely with the physiological bone ratio of 1.67. Under the synergistic effects of hyperglycemia and advanced glycation end-products (AGES), Ca and P ion release from the MAO coating was expedited, thereby inducing osteogenic differentiation of bone marrow stem cells. In addition, the Ca2+ released from the coating would complex with AGES, reducing its inhibitory effect on cell growth and thus promoting cell proliferation. Cellular experiments showed that higher coating porosity and its beneficial ions release facilitated the growth environment and promoted cell proliferation and differentiation. This study utilized coating design and corrosion phenomena to provide a theoretical basis for titanium alloys in clinical applications.

Coatings with a thickness from ~40 to ~50 µm on Ti6Al4V titanium alloy were formed by micro-arc oxidation in a silicate-hypophosphite electrolyte with additions of graphene oxide. Micro-arc oxidation treatment was carried out in the anode–cathode mode (50 Hz) with a ratio of anode to cathode currents of 1:1, a total density of 20 A/dm2, and a treatment duration of 30 min. The effect of the graphene oxide concentration in electrolytes on the thickness, roughness, hardness, surface morphology, structure, composition of micro-arc oxidation coatings, and its electrochemical corrosion behavior in 3.5% NaCl solution was studied. The input of graphene oxide additives into the base silicate hypophosphite electrolyte led to an increase in the hardness of micro-arc oxidation coatings. Electrochemical polarization studies and impedance data showed that the best characteristics in terms of corrosion-protective ability among coatings formed in electrolytes with graphene oxide additives were those formed in the electrolyte with a graphene oxide concentration of 0.1 g·L−l. A further increase in the graphene oxide concentration in the electrolyte did not improve the protective properties of micro-arc oxidation coatings. [ABSTRACT FROM AUTHOR]

In this study, AA7075 and AA6082 alloy bearing materials were coated with Micro Arc Oxidation (MAO) method by keeping the electrolyte composition, electric current, coating period, and coating thickness fixed. Tests were conducted on AA7075 and AA6082 material bearings coated and non-coated with Statically Loaded Journal Bearing Test Apparatus (SLJBTA) with 5, 10, 50, 100, 250, 500, 750, and 1000 rpm shaft speeds and under the static bearing loads of 150, 250 and 500 N with the lubricant temperature of 40 °C. Mineral-based oil was used in the lubrication of the test bearings. The friction coefficients (µ) occurring in the bearing were measured. Scanning Electron Microscopy (SEM) was used in the surface analyses of the bearing samples, and the material analyses were conducted with x-ray diffraction (XRD). It was determined from the experiments that while µ values were higher in both MAO-coated samples than the non-coated samples under the boundary lubrication conditions in low revolutions, the µ values of the coated samples decreased in high revolutions. Also, µ values increased in non-coated samples. In the aluminum alloys (AA6082 and AA7075), while MAO coating caused the µ values to increase under boundary lubrication conditions, it caused the µ values to decrease under liquid film lubrication conditions. [ABSTRACT FROM AUTHOR]

The pitting corrosion behavior of AZ91 alloy before and after micro-oxidation treatment in 3.5% NaCl solution was investigated by cyclic potentiodynamic polarization (CPDP) and optical and SEM observations of corroded surfaces at different polarization potentials. The CPDP results show that both the alloy and the MAO-coated alloy suffer from pitting corrosion and it is difficult for pits to stop growth once the pits initiates. It is revealed that the air-formed MgO film on AZ91 alloy, the MAO coating along with corrosion products (mainly Mg(OH)2) formed during CPDP can significantly influence the kinetics of the redox reactions of Mg, and further influence the propagation behavior of pitting corrosion. The optical and SEM images show that the corrosion products on AZ91 alloy are dense and protective but on the MAO-coated corrosion products are very loose. Such observations support the analyses of CPDP results that pits on AZ91 alloy spread to the width whereas pits on MAO-coated alloy propagate to the depth. Overall, pitting corrosion on the MAO-coated alloy can be very severe once the coating ruptures and post-treatments are necessary to provide a promising corrosion protection for the Mg alloys.

Micro-arc oxidation is an effective method to enhance surface wear resistance. This paper comparatively studies the fretting and sliding wear properties of micro-arc oxidation-treated 7075 aluminum alloy at different temperatures. Research indicates that both the untreated and MAO-coated samples exhibit peak friction coefficient values due to adhesion at elevated temperatures. At both room temperature and 150 °C, the fretting wear rate of the MAO-coated sample is lower than that of the sliding wear rate, which can be attributed to distinct surface damage mechanisms and third body behavior under different wear modes. Adhesive wear is more prominent in MAO-coated samples during high-temperature fretting compared to room temperature, while abrasive wear and spalling are more prevalent during high-temperature sliding. [ABSTRACT FROM AUTHOR]

Coatings with a thickness from 27 to 62 µm on electron beam melted Ti-6Al-4V have been formed by micro-arc oxidation (MAO) in a silicate-hypophosphite electrolyte. MAO tests in the anode-cathode mode (50 Hz) with an anode-to-cathode current ratio of 1:1 and sum current densities 10 and 20 A/dm2 were carried out. The duration of the MAO treatment was 30 and 60 min. The effect of the processing parameters on the structural properties of the MAO treated coatings was studied. The current density and treatment time significantly affect the coating thickness and surface roughness. The values of these characteristics increase as the current density increases. The effect of thermal cycling tests on surface morphology, thickness and roughness, and elemental and phase composition of MAO coatings was analyzed. After 50 cycles of thermal cycling from +200 °C to −50 °C, no cracking or delamination of coatings was observed. Coatings formed in 30 min at a current density of 20 A/dm2 turned out to be the best in terms of such indicators as surface morphology, thickness, and roughness. [ABSTRACT FROM AUTHOR]

• Both AZ91 alloy and MAO-coated alloy are susceptible to pitting corrosion. • Pits on AZ91 alloy spread to width and they propagate to depth on MAO-coated alloy. • The MAO-coated alloy corrodes severely after coating ruptures. The pitting corrosion behavior of AZ91 alloy before and after micro-oxidation treatment in 3.5% NaCl solution was investigated by cyclic potentiodynamic polarization (CPDP) and optical and SEM observations of corroded surfaces at different polarization potentials. The CPDP results show that both the alloy and the MAO-coated alloy suffer from pitting corrosion and it is difficult for pits to stop growth once the pits initiates. It is revealed that the air-formed MgO film on AZ91 alloy, the MAO coating along with corrosion products (mainly Mg(OH) 2) formed during CPDP can significantly influence the kinetics of the redox reactions of Mg, and further influence the propagation behavior of pitting corrosion. The optical and SEM images show that the corrosion products on AZ91 alloy are dense and protective but on the MAO-coated corrosion products are very loose. Such observations support the analyses of CPDP results that pits on AZ91 alloy spread to the width whereas pits on MAO-coated alloy propagate to the depth. Overall, pitting corrosion on the MAO-coated alloy can be very severe once the coating ruptures and post-treatments are necessary to provide a promising corrosion protection for the Mg alloys. [ABSTRACT FROM AUTHOR]

Coatings with a thickness from ~40 to ~50 µm on Ti6Al4V titanium alloy were formed by micro-arc oxidation in a silicate-hypophosphite electrolyte with additions of graphene oxide. Micro-arc oxidation treatment was carried out in the anode–cathode mode (50 Hz) with a ratio of anode to cathode currents of 1:1, a total density of 20 A/dm2, and a treatment duration of 30 min. The effect of the graphene oxide concentration in electrolytes on the thickness, roughness, hardness, surface morphology, structure, composition of micro-arc oxidation coatings, and its electrochemical corrosion behavior in 3.5% NaCl solution was studied. The input of graphene oxide additives into the base silicate hypophosphite electrolyte led to an increase in the hardness of micro-arc oxidation coatings. Electrochemical polarization studies and impedance data showed that the best characteristics in terms of corrosion-protective ability among coatings formed in electrolytes with graphene oxide additives were those formed in the electrolyte with a graphene oxide concentration of 0.1 g·L−l. A further increase in the graphene oxide concentration in the electrolyte did not improve the protective properties of micro-arc oxidation coatings.

Mg-Zn-Ca metal glass shows good promising applications in biomaterials. MAO coating and Ca-P coating was applied on Mg69Zn27Ca4 metal glass in order to improve the alloy biodegradation resistance and biocompatibility. Microstructure observation, electrochemical experiment, immersion experiment and cell cytotoxicity experiment (MG63 cells) were carried out. The result revealed that Ca-P coating exhibited better degradation resistance than MAO coating due to its relatively thick and uniform structure. The cell cytotoxicity showed both coatings showed better bioactivity than the uncoated Mg69Zn27Ca4 metal glass. Moreover, Ca-P-coated samples had better bioactivity than those of MAO coated. Therefore, Ca-P coating improved not only the degradation resistance but also the bioactivity of Mg69Zn27Ca4 metal glass. Ca-P coating has a good promising application on Mg69Zn27Ca4 metal glass. [ABSTRACT FROM AUTHOR]

A bio‐ceramic coating was prepared on the surface of ZK60 magnesium alloys by micro‐arc oxidation (MAO) method. The substrate (BM) and coated (MAO) specimens were pre‐corroded in a simulated body fluid (SBF) for 12 h. Strain‐controlled and stress‐controlled loading modes were used to conduct fatigue tests for the specimens with non‐corroded and pre‐corroded, and the cyclic deformation behavior of different specimens was studied. The fatigue life of pre‐corroded MAO specimen is higher than that of pre‐corroded BM specimen. The mechanism of cyclic deformation under different loading conditions is related to twinning and slip. For the same specimens, the higher the absolute value of ratcheting strain, the lower the fatigue life. A modified total strain energy model is proposed, and the precision of life prediction is higher than that of traditional fatigue model. Highlights: A bio‐ceramic MAO coating was prepared on the surface of ZK60 magnesium alloys.Fatigue tests were carried out by strain‐controlled and stress‐controlled loading.Ratcheting behavior of non‐corroded and pre‐corroded specimens was studied.A modified total strain energy model was used to predict the fatigue life. [ABSTRACT FROM AUTHOR]

This paper investigates the tribological effects of composite oxide coatings grown on ZA-27 surfaces under dry sliding condition. Coating processes were conducted using three different electrolytic solutions through micro-arc oxidation (MAO) method. The base electrolytic solution without any particles as additives was labeled as MAO-Coated, whereas electrolytic solution labeled as Coated-A contained graphite particles and Coated-B contained hexagonal boron nitride (h-BN) particles incorporated into base solution. The morphology of each coated samples were analyzed using a Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD). The friction and wear tests were conducted using a pin-on-disc tester in order to evaluate the tribological behavior. Results indicated the friction coefficients of both Coated-A and Coated-B samples were lower when compared to that of uncoated ZA-27 alloy. Further comparative studies showed Coated-A samples containing graphite particles resulted in the lowest friction coefficient. [ABSTRACT FROM AUTHOR]

[Display omitted] • A composition-structure bionic MAO coating was prepared on β-titanium alloy surface. • Ca and P ion release patterns of MAO coating in diabetic environment were studied. • The bionic MAO coating possessed excellent biocompatibility in diabetic environment. • Ca and P ion release of coating promote osteogenic property in diabetic environment. A biomimetic coating was prepared on β-titanium alloy surface using micro-arc oxidation to enhance its bone regeneration property in diabetic environment. The research probed into the complex interactions between crystalline phase, chemical composition, and surface morphology in modulating osteoblast behavior. The resultant coating displayed an open, porous structure, characterized by surface porosities ranging from 19.3 % to 39.8 % and a roughness distribution spanning 22.9 to 37.9 nm. XRD and XPS analyses confirmed a hydroxyapatite crystal structure within the coatings. Elemental quantification revealed Ca/P ratios from 1.49 to 2.44, aligning closely with the physiological bone ratio of 1.67. Under the synergistic effects of hyperglycemia and advanced glycation end-products (AGES), Ca and P ion release from the MAO coating was expedited, thereby inducing osteogenic differentiation of bone marrow stem cells. In addition, the Ca2+ released from the coating would complex with AGES, reducing its inhibitory effect on cell growth and thus promoting cell proliferation. Cellular experiments showed that higher coating porosity and its beneficial ions release facilitated the growth environment and promoted cell proliferation and differentiation. This study utilized coating design and corrosion phenomena to provide a theoretical basis for titanium alloys in clinical applications. [ABSTRACT FROM AUTHOR]

Coatings with a thickness from 27 to 62 µm on electron beam melted Ti-6Al-4V have been formed by micro-arc oxidation (MAO) in a silicate-hypophosphite electrolyte. MAO tests in the anode-cathode mode (50 Hz) with an anode-to-cathode current ratio of 1:1 and sum current densities 10 and 20 A/dm2 were carried out. The duration of the MAO treatment was 30 and 60 min. The effect of the processing parameters on the structural properties of the MAO treated coatings was studied. The current density and treatment time significantly affect the coating thickness and surface roughness. The values of these characteristics increase as the current density increases. The effect of thermal cycling tests on surface morphology, thickness and roughness, and elemental and phase composition of MAO coatings was analyzed. After 50 cycles of thermal cycling from +200 °C to −50 °C, no cracking or delamination of coatings was observed. Coatings formed in 30 min at a current density of 20 A/dm2 turned out to be the best in terms of such indicators as surface morphology, thickness, and roughness.

Titanium and its alloys find extensive application within the realms of dentistry and orthopedics due to their remarkable chemical stability. Nevertheless, they are still susceptible to corrosion in harsh environments. Micro-arc oxidation (MAO) emerges as a potent methodology for enhancing the corrosion resistance of titanium and its alloys, achieved through the generation of a ceramic stratum atop the material's surface. The corrosion resistance of MAO coatings in clinical applications is closely related to their microstructure. This manuscript systematically explores the effect of annealing temperature on the phase composition and microstructure of MAO coatings. Furthermore, the evolution of the surface properties and the crystal structure of coatings is meticulously discussed and illustrated. The results show that the corrosion resistance of coatings exhibits an ascending trajectory followed by a subsequent decline, coinciding with the augmentation of the annealing temperature. This behavior can be attributed to the structural evolution of coatings caused by the gradual increase of the annealing temperature. At the optimum temperature (400 °C), the annealing treatment leads to the crystallization of the coating as well as to a decrease in the surface porosity, resulting in the formation of a compact and continuous multilayer microstructure. The decrease in porosity can be attributed to the diffusion-reaction of oxygen from the air with titanium from the discharge channel (titanium oxide), resulting in the formation of TiO 2 particles that effectively fill the pores. The decrease in corrosion resistance of the coatings after annealing at 800 °C is due to the phase transition of TiO 2 which leads to the expansion of cracks inherent in the oxide layer to form A-H cracks. Therefore, this study shows that the microstructure of MAO coatings can be adjusted by post annealing treatment (400 °C) to obtain a desirable oxide layer with appropriate porosity, crystallinity and chemical composition, thus improving its corrosion resistance. [Display omitted] • The surface of the MAO layer with different annealing temperatures exhibits distinct micro-morphologies. • After annealing, the Ti in the MAO layer grows along the discharge channel and forms TiO2 particles. • The annealing treatment enlarges the microdefects inherent in the MAO layer. [ABSTRACT FROM AUTHOR]

To enhance the ablation resistance of Ti6Al4V alloy, a SiC-modified nanocomposite ceramic coating was fabricated by a novel one-step liquid plasma-assisted particle deposition sintering (LPDS) method. A microarc oxidation (MAO) ceramic coating was prepared on the surface of titanium alloy as a comparison. The micromorphologies and formation mechanisms of two ceramic coatings were investigated with the assistance of scanning electron microscopy (SEM), X-ray diffractometer (XRD), and transmission electron microscopy (TEM). The thickness of the SiC nanocomposite coating (about 39 μm) was significantly increased by 129% compared to the MAO coating (about 17 μm). Additionally, ablation tests of Ti6Al4V alloy, MAO coating, and SiC nanocomposite coating were conducted at 1000 °C using a butane gas lance. Then, the variation of microstructure, phases, elemental composition, and thickness before and after ablation was also comparatively analyzed. The results indicate that the SiC nanocomposite coating exhibits the smallest thickening (29 μm) after ablation at 1000 °C for 360 s, which is 39% and 46% compared with the Ti6Al4V substrate and MAO coating, respectively. The nanocomposite coating is intact and cracks-free after the ablation test. SiO 2 , formed during the ablation of SiC nanocomposite coatings, blocks oxygen diffusion into the substrate, which plays a protective role in the coating. Therefore, the SiC nanocomposite coating by a novel LPDS technique exhibits superior ablation resistance to enhance the thermal protection performance of titanium alloy substrate. • A novel SiC nanocomposite coating by LPDS technique is prepared on the Ti6Al4V alloy. • No peeling occurred in the SiC nanocomposite coating after ablation for 360 s at 1000 °C. • The LPDS coating mainly contains SiC and SiO 2 to absorb the stress effect caused by thermal shock. • SiO 2 formed during the ablation of SiC nanocomposite coatings blocks oxygen diffusion into the substrate effectively. [ABSTRACT FROM AUTHOR]

• For the Mg-0.06Cu alloy, the MAO coating on the as-solution condition shows much better corrosion resistance than that on the as-cast condition. • The MAO coating on the Mg-0.06Cu alloy not only inhibits the appearance of high pH value in chloride containing solution but also shows an excellent antibacterial ability. • Solution treatment combined with MAO coating shows a good potential for the Mg-0.06Cu alloy for clinical applications. For a Mg-0.06Cu alloy, the solution treatment significantly decreased the corrosion rate compared with the as-cast condition due to the decrease of the amount of cathodic Mg 2 Cu phase particles. The micro-arc oxidation (MAO) produced a coating layer that separated the substrate from the solution so that the MAO treated condition had a corrosion rate that was 1/76 of that of the as-cast condition. Antibacterial test revealed that MAO coating on as-cast and solution treated conditions possessed excellent antibacterial ability, which might be attributed to the porous structure of the coating and the presence of a trace amount of copper. [ABSTRACT FROM AUTHOR]

• Cu was introduced into the micro-arc oxidation coating successfully. • Cu enhanced the corrosion resistance of the MAO coating. • Cu + MAO coating could effectively inhibit the bacterial proliferation. Copper (Cu) has a good antibacterial effect and micro-arc oxidation (MAO) coating has good corrosion resistance for magnesium (Mg) alloys. If they are combined together, the coated Mg alloy is expected to have both good corrosion resistance and antibacterial effect. In this work, the degradation, antibacterial property and cytotoxicity of a Cu-containing MAO coating on an extruded Mg-2Zn-1Gd-0.5Zr alloy were systematically studied. The results revealed that the addition of Cu could further improve the degradation resistance of MAO coated alloy. After two weeks immersion, the corrosion rate of Cu + MAO coated alloy was 0.16 mm/y, lower than those of both MAO coated and uncoated alloy. The release of Cu2+ from Cu + MAO coated alloy inhibited the bacterial proliferation. After 12 h co-culture, the antibacterial rate reached 96%. Cytotoxicity test (MG63 cell) showed that Cu + MAO coated alloy had good biocompatibility. The Cu + MAO coating has great potential for application on Mg alloys due to its good corrosion resistance, antibacterial property and good biocompatibility. [ABSTRACT FROM AUTHOR]

Micro-arc oxidation (MAO) coating with different oxidation times was prepared on aluminum alloy to improve the impact wear resistance of aluminum alloy. Surface morphology and impact wear behavior of the coating with different oxidation times were investigated. Results showed that the MAO coating formed on aluminum alloy had a porous and island structure comprising α-Al 2 O 3 and γ-Al 2 O 3. Coating thickness increased with increasing oxidation time. The coating also had a lower impact force and higher energy absorption ratio than the substrate but had a lower wear volume, which decreased with increasing oxidation time. These results indicated that MAO coating can improve the impact wear resistance of aluminum alloy. Further study revealed that the wear mechanism of aluminum alloy was oxidation wear and contact fatigue spalling, whereas that of MAO coating was contact fatigue wear. • The characterization and impact wear behavior of the MAO coating with different oxidation times was investigated. • The wear mechanism of aluminum alloy and MAO coating was discussed. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • LBL coating formed MgO, Mg(OH) 2 , and calcium phosphate ceramics on the surface. • Combination of LBL and BMP-2 coating enhanced the corrosion resistance of Mg alloy. • Levels of cell differentiation increased with the concentration of BMP-2 coating. • The thickest and most uniform new bone layer was formed in the ALB50 group. Abstract To control the bio-absorption rate of Mg alloy and improve its biocompatibility, a BMP-2 of various concentrations (20, 50, and 100 ng/mL) was immobilized in the carrier layer formed by micro-arc oxidation + hydrothermal treatment on the surface of Mg alloy. Their morphological and chemical properties were evaluated by FE-SEM with EDS and XRD. The electrochemical corrosion behavior was examined, and the cytotoxicity was assessed by WST and ALP assay. After in vivo test during 2 and 4 weeks in the rat tibia, histological observation was performed. In the carrier layer, calcium phosphate ceramic and oxide layers (MgO and Mg(OH) 2) were formed. The layers enhanced the corrosion resistance. It was maintained even after loading the BMP-2. In the BMP-2-loaded group, the proliferation and differentiation of the osteoblast cells were promoted by the continuous release of the BMP-2. After implantation for 2 weeks, all BMP-2-loaded groups showed better biodegradation rate and osseointegration than the control group. After 4 weeks, the group with 50 ng/mL of BMP-2 exhibited the slowest biodegradation rate of all the BMP-2-loaded groups. This effected on the enhancement of new bone formation and the stable bone growth. Therefore, for the short term of implantation, this double coating method could be applied onto biodegradable Mg metal as an orthopedics and dental implant. For the long term of implantation over 4 weeks, to treat Mg implant with 50 ng/mL of BMP-2 was determined to be optimal. [ABSTRACT FROM AUTHOR]

The microstructure of cold extruded 7075 aluminum alloy after annealed at various temperatures was investigated by EBSD and TEM. The results show that the grain size, dislocation density and Schmid factors vary significantly by annealing temperature. The alloy annealed at 300 °C owns a more compact and uniform coating with better corrosion resistance compared with those of other temperatures due to the combination of fine grain size and suitable electrical conductivity. The corrosion resistance of the bare alloy decreased with an increase of annealing temperature results from increased grain size, rate of high angle grain boundaries (HAGBs) and Schmid factors. • The microstructure of 7075 aluminum alloy after annealed at various temperatures was investigated by EBSD and TEM. • The sample annealed at 300 °C owns a most compact and uniform MAO coating due to grain size and electrical conductivity. • The corrosion resistance of bare alloy decreases with an increase of annealing temperature. • The coating of the sample annealed at 300 °C showed a best corrosion resistance. [ABSTRACT FROM AUTHOR]

The aim of this work was to explore the effect of microscopic surface topographies of the Ti-6Al-4 V alloy on the coating growth behaviour during the initial stages of micro-arc oxidation (MAO). Firstly, MAO treatments were carried out on the surfaces of Ti-6Al-4 V alloys with different surface morphologies for 5, 10 and 15 min respectively. Second, the effects of substrate topographies on the roughness, porosity, thickness, and microstructure of the coatings were studied. Then, the corrosion resistance of the MAO-treated titanium alloys was assessed by electrochemical analysis. Finally, the formation mechanisms and morphological results of the oxide layer were discussed and illustrated graphically. The results showed that the surface morphologies of the substrate affected the micro-arc discharge behaviour during the process of oxidation. Notably, rough areas on the substrate surface facilitated the breakdown of the passivation film, which was evidenced by the early appearance of plasma discharges in these areas. However, surface morphology with uneven distribution of pores observed in the oxide layer formed by the rough substrate. The coatings grown on the grooves were more porous than those grown in other areas, due to the non-uniformity of the micro-discharge on the rough substrate surface. • The appearance of micro-discharges on the rougher sample occurred earlier than that of the flat sample. • The surface morphology of coating is uneven due to uneven micro-discharge on the substrate surface. • The porous structure of the grooved area is determined by the discharge characteristics of the substrate surface morphology. [ABSTRACT FROM AUTHOR]

To improve the corrosion performance of magnesium alloys in the marine environment, the MAO, MAO−Cu2CO3(OH)2·H2O and MAO−Cu2P2O7 ceramic coatings were deposited on AZ91D magnesium alloys in basic electrolyte and the discoloration mechanism of the Cu-doped MAO coatings and the corrosion behavior of the three MAO coatings in the artificial seawater solution were investigated by SEM, EDS and XPS. The results indicated that the formation and discoloration mechanism of the brown MAO ceramic coatings were attributable to the formation of Cu2O in the coatings. Though the three MAO coatings had a certain protective effect against the corrosion of AZ91D substrate in the artificial seawater, the distinctive stratification phenomenon was found on the MAO−Cu2P2O7 coated sample and the corrosion model of the MAO−Cu2P2O7 coatings in the immersion experiment was established. Therefore, the brown Cu-doped MAO coatings were speculated to significantly reduce the risk of the magnesium parts in marine environments.

Abstract Coating is one of the effective methods to improve the corrosion resistance of the biodegradable Mg alloys. In this work, comparative study on the effects of micro-arc oxidation (MAO) coating, Sr P coating and Ca P coating on the biodegradable and wear properties of Mg-2Zn-1Gd-0.5Zr alloy were investigated. The results showed that the degradation deposition contained Gd on the surfaces of both MAO coating and Sr P coating, which stabilized the coating, thereby increasing the degradation resistance of the alloy. The wear test showed that MAO coating and Ca P coating exhibited good wear resistance. For a 5 N loading, the sliding distances of MAO coating, Sr P coating and Ca P coating were 2.24 m, 0.29 m and 9.00 m respectively, before the coatings were completely destroyed. This investigation revealed that the MAO coating exhibits the best degradation resistance and the Ca P coating exhibits the best wear resistance. Highlights • (1) The degradation products contained Gd could increase the degradation resistance of the MAO and Sr P coated alloys. • Gd 2 O 3 in MAO coating could stabilize the coating and delay the degradation process. • The rod-like deposition could act as a barrier to improve the wear resistance of the Ca P coated alloy. [ABSTRACT FROM AUTHOR]

The novel cerium based conversion (CeC) treatment was performed on 2A14-T3511 aluminum alloy micro-arc oxidation (MAO) coatings with thickness of 15 μm and 30 μm, respectively. The sealing effect of CeC treatment on MAO coatings and the chemical composition of the sealant were characterized. The influence of coating structure on the deposition of cerium compounds was systematically discussed. Additionally, long-term corrosion behavior was investigated using electrochemical methods. The results showed that CeC treatment significantly improved the long-term corrosion protection of MAO coatings. This is attributed to the physical sealing and chemical corrosion inhibition of the Ce based deposits. [Display omitted] • The CeC treatment reduces the porosity and surface roughness of the MAO coating. • The deposition mechanism of Ce compounds is related to the structure of MAO coatings. • The CeC treatment improves the long-term corrosion protection of MAO coatings. • Ce based deposits provide physical sealing and chemical corrosion inhibition effects. [ABSTRACT FROM AUTHOR]

In order to improve the wear resistance and biocompatibility, undoped and diamond-doped micro-arc oxidation (MAO) coatings are prepared on Ti6Al4V in this paper by using electrolyte without/with diamond nanoparticles. The microstructure, wear resistance, corrosion resistance, and cytocompatibility performance of the coatings are studied and compared. The results show that undoped and diamond-doped MAO coatings have similar surface morphologies and thicknesses. With the increase of the MAO time, the surface morphology of the two types of coatings translates from the micro-sized traditional volcano pores into Micro/Nano-pores composite structure. The diamond-doped MAO coatings have higher wear resistance than the undoped MAO coatings and the original Ti6Al4V substrate. All the MAO coatings have higher corrosion resistance than the Ti6Al4V substrate. However, the diamond doping has no obvious influence on the corrosion resistance. In vitro assays indicate that both the two kinds of MAO coatings have a better ability to enhance the adhesion, spreading, proliferation, and osteogenic differentiation of MC3T3-E1 osteoblasts compared with the original Ti6Al4V substrate. Considering the high wear resistance, the diamond-doped MAO coatings are more promising for Ti6Al4V alloy. • Diamond nanoparticles are successfully doped in MAO coatings prepared on Ti6Al4V. • The coatings consist of anatase and rutile TiO 2 , and are doped with diamond and graphite. • The diamond doped MAO coatings have higher wear resistance than undoped MAO coatings. • The doping of the diamond nanoparticles maintains the corrosion resistance of the MAO coating. • The adhesion, proliferation, and differentiation of osteoblasts are visibly enhanced. [ABSTRACT FROM AUTHOR]

Earle's Balance Salt Solution (EBSS) provides an alternative to the conventional simulated body fluids (c-SBF) and has been shown to better simulate the corrosion conditions in vivo. In this work, a series of tests were conducted to explore the corrosion performance of MAO-coated AZ31 samples in EBSS vs. c-SBF. Samples were produced by varying MAO process parameters and then immersed up to 21 days in both EBSS and c-SBF. The corrosion rates were evaluated by the electrochemical impedance spectroscopy and potentiodynamic scanning. Scanning electron microscope (SEM) was used to compare the progression of microcracks across the surface of the coatings. The evaluation of cross-sectional thickness showed an increase in MAO coating thickness with the process voltage. MAO samples with a thicker coating generally have higher impedance and lower current density at the initial immersion time point of 0.5 h. Samples in EBSS showed higher initial impedance and lower current density values as compared to c-SBF counterparts for all process groups. Samples in EBSS demonstrated a much slower corrosion rate than c-SBF samples because of the decreased chloride content and CO 2 buffering mechanism of the EBSS. [ABSTRACT FROM AUTHOR]

• Orange peel oil (OPO) was blended with gasoline in the ratio of 10:90, 20:80, 30:70. • The piston crown was coated using a micro-arc oxidation process. • In-cylinder combustion flame was visualized using combustion chamber endoscopy. • BTE of OPO10 blend with coated piston was comparable to gasoline without coated piston. • Coated piston reduced the HC and CO emissions of OPO10 blend by 3.1% and 3.6% The scope of this work investigates the feasibility of using orange peel oil (OPO)–gasoline blends in a spark-ignition engine for the first time. Initially, fuel characterization of OPO is performed, and the fuel properties are reported. Subsequently, engine experiments were performed in a spark-ignition engine at a constant speed with different engine loads. Coating studies were performed by coating the engine piston based on the micro-arc oxidation technique with a view of improving the engine performances and emissions. Then, the in-cylinder combustion images were visualized using AVL visioscope software to justify and explain the experimental engine findings. The experimental engine results reveal that OPO10 (10% OPO + 90% gasoline) showed slightly lesser brake thermal efficiency and nitrogen oxide and higher hydrocarbon and carbon monoxide emissions when compared to gasoline. With the increase in the fraction of OPO in the blend, the combustion is delayed, and the maximum net heat release is reduced due to unfavourable physical properties of OPO, such as increased boiling range and viscosity. With coating, the limitations with the physical properties of OPO are overcome as the prevention in heat losses favoured the combustion. The BTE of OPO10 with coated piston is comparable to that of gasoline without coated piston, whereas HC and CO emissions were lowered by 3.1% and 3.6%, respectively. In-cylinder combustion images showed that the diffusion flames are predominant for higher blends of OPO. With coating, the premixed combustion is improved in that OPO10 showed less diffusion fraction compared to gasoline. Overall, the fundamental flame investigation study justifies the experimental engine findings of the OPO fuelled SI engine. [ABSTRACT FROM AUTHOR]

The effects of ageing treatment on corrosion behavior of 7075 aluminum alloy coated by MAO have been investigated. The results suggest that precipitates (GPI, GP II zones, η and η') formed after ageing play a significant role on the thickness, porosity and corrosion resistance of MAO coating. After MAO coating is deteriorated during corrosion test, TEM observations show that coarse and discontinuously distributed precipitates at grain boundary contribute to the electrochemical corrosion resistance. HAADF-STEM images and EDS maps exhibit that the Mg segregation at grain boundary decreases with increasing the ageing time which reduces the stress corrosion cracking (SCC) susceptibility. • There are two ageing hardening peaks appearing at 36 h and 128 h respectively during long time ageing. • Ageing treatment will affect the MAO coating properties as the electrical conductivity of specimens changed by ageing. • Thicker and less porous MAO coating combined with less Mg segregation at grain boundary shows great SCC resistance. [ABSTRACT FROM AUTHOR]

Bioactive composite coatings containing hydroxyapatite on magnesium alloy AZ91D are prepared successfully by microarc oxidation (MAO) coupled with sol-gel treatment. The morphologies of the composite coatings formed by MAO treatment at 0.5 A · cm−2current density coupled with sol-gel treatment appear to be much denser and smoother than the coatings produced at 1.0 A · cm−2. The Ca/P ratio (1.70) of the composite coatings produced at 0.5 A · cm−2is slightly higher than the ratio of the sol prepared (1.67), due to thesubstitutegroups in apatite lattice according to the Fourier transform infrared spectroscopy test. Moreover, electrochemical tests illustrate that the composite coatings (0.5 A · cm−2) have higher open-circuit potential, corrosion potential, and corrosion resistance than the composite coatings at 1.0 A · cm−2. Immersion tests have been performed showing an enhanced bioactivity and apatite-forming ability. Consequently, combining the MAO and sol-gel treatments provides a promising candidate for further investigation. [ABSTRACT FROM AUTHOR]

• The corrosion failure progress and mechanism of MAO coating has been studied. • The surface roughness affects the bonding strength of the MAO coating significantly. • The high surface roughness could dramatically improve the corrosion resistance of MAO coating. In this study, micro-arc oxidation (MAO) coatings were formed on ZM5 Mg alloy substrate with different surfaces roughness. Therefore, the influence of surface roughness on the corrosion failure progress and mechanism of the MAO coated ZM5 Mg alloy has been investigated. The microstructure, bonding strength, and corrosion resistance of the MAO coatings were studied by SEM, LSCM, bonding strength measurement, and electrochemical test. The results showed that the surface roughness affects the porosity and bonding strength of the coatings. While it had little effect on the thickness and chemical composition of the MAO coating. Due to the high surface porosity, the coating of the grooved sample had poor corrosion resistance in the initial immersion. But, after a long-time immersion, the grooved sample was still protected by the coating, while the smooth sample showed severe damage. The strong bonding strength between the coating and groove sample was the key factor for enhancing the corrosion resistance of MAO coated ZM5 Mg alloy. [ABSTRACT FROM AUTHOR]

Abstract: The poor corrosion resistance of magnesium alloys is a dominant problem that limits their clinical application. In order to solve this challenge, micro-arc oxidation (MAO) was used to fabricate a porous coating on magnesium alloys and then electrochemical deposition (ED) was done to fabricate rod-like nano-hydroxyapatite (RNHA) on MAO coating. The cross-section morphology of the composite coatings and its corresponding energy dispersion spectroscopy (EDS) surficial scanning map of calcium revealed that HA rods were successfully deposited into the pores. The three dimensional morphology and scanning electron microscopy (SEM) image of the composite coatings showed that the distribution of the HA rods was dense and uniform. Atomic force microscope (AFM) observation of the composite coatings showed that the diameters of HA rods varied from 95nm to 116nm and the root mean square roughness (RMS) of the composite coatings was about 42nm, which were favorable for cellular survival. The bonding strength between the HA film and MAO coating increased to 12.3MPa, almost two times higher than that of the direct electrochemical deposition coating (6.3MPa). Compared with that of the substrate, the corrosion potential of Mg–Zn–Ca alloy with composite coatings increased by 161mV and its corrosion current density decreased from 3.36×10−4 A/cm2 to 2.40×10−7 A/cm2 which was due to the enhancement of bonding strength and the deposition of RNHA in the MAO pores. Immersion tests were carried out at 36.5±0.5°C in simulated body fluid (SBF). It was found that RNHA can induce the rapid precipitation of calcium orthophosphates in comparison with conventional HA coatings. Thus magnesium alloy coated with the composite coatings is a promising candidate as biodegradable bone implants. [Copyright &y& Elsevier]

Abstract: Protective composite coatings were obtained on a magnesium alloy by micro-arc oxidation (MAO) and sol–gel technique. The coatings consisted of a MAO layer and a sol–gel layer. The microstructure and composition of the MAO coating and the composite coatings were analyzed by scanning electron microscopy (SEM) and energy dispersive X-rays (EDX). Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and total immersion tests were used to evaluate the corrosion behavior of these coatings in a 3.5wt.% NaCl solution. The results show that the sol–gel layer provides corrosion protection by physically sealing the pores in the MAO coating and acting as a barrier. The composite coatings can suppress the corrosion process by preventing the corrosive ions from transferring or diffusing to the magnesium alloy substrate. This enhances the corrosion resistance of the magnesium alloy AZ91D significantly. [Copyright &y& Elsevier]

In this paper, the micro arc oxidation (MAO) method was utilized to prepare the ceramic coatings on Mg-5Y-7Gd-1Nd-0.5Zr substrate. Effects of different reaction time and NH 4 VO 3 concentration on the morphology, composition, absorptance, emissivity, corrosion resistance and adhesion strength of coatings were investigated. These results show that the as-prepared coatings were mainly composed of Mg, Si, P, V, O and Na elements, and all of them existed as amorphous state. With the increases of reaction time and NH 4 VO 3 concentration could improve the thickness, roughness and reduce the lightness ( L value), and so the absorptance and emissivity of prepared coatings increased. Meanwhile, the corrosion resistance of coatings was much better than that of Mg alloy, exhibiting an increasing tendency as the reaction time and NH 4 VO 3 concentration. However, too more NH 4 VO 3 might cause many pores and micro-cracks generated on the surface of coatings, which slightly reduced the corrosion resistance. Although scratch tests denote that the adhesion strength of coatings decreased with the increase of thickness, T4/N4 with the maximum thickness still had good adhesion to the substrate and remained stable after thermal shock experiments. In general, T4/N4 possessed the optimal comprehensive performances, which would expand the application range of Mg-5Y-7Gd-1Nd-0.5Zr in aerospace and optics. • The thermal control coating was got on Mg-5Y-7Gd-1Nd-0.5Zr Alloy by MAO process. • The coating's color mainly depends on the amount and chemical state of V element. • The optimal coating possesses 0.952 of absorptance and 0.941 of emissivity. • Too more pores and micro-cracks slightly reduce the corrosion resistance. [ABSTRACT FROM AUTHOR]

• Graphene oxide/ZrO 2 coating on N36 Zr alloy was prepared and analyzed. • Corrosion and fretting wear behavior of graphene oxide/ZrO 2 coating was investigated. • The mechanism on improving corrosion and fretting wear behavior was investigated. ZrO 2 coatings combined with graphene oxide (GO) was prepared on N36 zirconium (Zr) alloy by micro-arc oxidation (MAO) technique. MAO coating shows a typical volcano-like structures with pores and cracks. The MAO coating with GO is composed with m-ZrO 2 and t-ZrO 2. The electrochemical test indicates that MAO coating provides good corrosion protection to Zr alloy, and corrosion resistance enhance significant with GO concentration increasing. The increasing in corrosion resistance is mainly due to that GO particles in the coating act as a barrier to the diffusion pathway of electrolyte and restrain aggressive electrolyte accessing to the substrate. Fretting wear test indicate that MAO coating with GO exhibits a lower coefficient of friction (COF) and wear volume compared with that of Zr alloy substrate and MAO coating without GO. Optimal concentration of GO in MAO coating is 0.05 g/L. Low COF and wear volume is attributed to the lubrication function of GO particle, and the lubrication of GO particle is caused by low shear stress of different GO layer. Wear mechanism of MAO coating with GO is mainly adhesive wear. Good corrosion and wear resistance prompt the MAO coating of Zr alloy becoming accident tolerant candidate cladding material used for reactor in nuclear power plant. [ABSTRACT FROM AUTHOR]

To improve the corrosion performance of magnesium alloys in the marine environment, the MAO, MAO–Cu2CO3(OH)2·H2O and MAO–Cu2P2O7 ceramic coatings were deposited on AZ91D magnesium alloys in basic electrolyte and the discoloration mechanism of the Cu-doped MAO coatings and the corrosion behavior of the three MAO coatings in the artificial seawater solution were investigated by SEM, EDS and XPS. The results indicated that the formation and discoloration mechanism of the brown MAO ceramic coatings were attributable to the formation of Cu2O in the coatings. Though the three MAO coatings had a certain protective effect against the corrosion of AZ91D substrate in the artificial seawater, the distinctive stratification phenomenon was found on the MAO–Cu2P2O7 coated sample and the corrosion model of the MAO–Cu2P2O7 coatings in the immersion experiment was established. Therefore, the brown Cu-doped MAO coatings were speculated to significantly reduce the risk of the magnesium parts in marine environments. [ABSTRACT FROM AUTHOR]