Micro-arc Oxidation Coatings Research Articles

Dual self-healing effects of salicylate intercalated MgAlY-LDHs film in-situ grown on the micro-arc oxidation coating on AZ31 alloys

Micro-arc oxidation (MAO) coatings only provide the passive protection of common Mg alloys (AZ31), especially causing severe localized corrosion under accidental damage. Herein, a novel MgAlY-Layered double hydroxides (LDHs) film loaded with salicylate is successfully prepared on flash-MAO coated AZ31 alloys, by the one-step hydrothermal and intercalation methods. The in-situ grown LDHs platelets seal the porous morphology, significantly improving the corrosion performance. Moreover, self-healing measurements (SVET) and characterizations (EDS, XRD, and FT-IR) demonstrate that the endowed active corrosion protection is attributed to the corrosion inhibition of anion-exchange reaction, and dual self-healing effects of yttrium oxide and Y(III) - salicylate complexes.

Preparation of HA-containing coating by one-step MAO on titanium alloys through synergistic effect of calcium gluconate and calcium glycerophosphate

In a base solution composed of calcium gluconate (CaGlu2) and magnesium gluconate (MgGlu2), the influences of calcium glycerophosphate (Ca-GP), sodium phytate (Na12Phy), phosphoric acid (H3PO4) and constant voltage on the amounts of calcium (Ca) and magnesium (Mg) elements in micro-arc oxidation (MAO) coatings were systematically investigated using an orthogonal experiment. The results show that anodic coatings with the Ca/P molar ratio of 1.35 and containing high bioactive hydroxyapatite (HA) phase are successfully fabricated using one-step MAO treatment by adjusting processing parameters. Ca ions enter into MAO coatings by both electromigration and diffusion. H3PO4 is an effective film-forming agent and the increased concentration significantly decreases both the Ca and Mg amounts. Our results show that compared with Ca-GP, CaGlu2 can effectively improve the Ca amount of anodic coatings. Sole Ca-GP or CaGlu2 cannot significantly improve the Ca amount, while Ca-GP and CaGlu2 can synergistically improve the Ca amount in MAO coatings.

Study on Growth Mechanism and Characteristics of Zirconium Alloy Micro-Arc Oxidation Film

Ceramic coatings on R60705 zirconium alloy were prepared on the surface by the micro-arc oxidation (MAO) technique in electrolytes containing Na2SiO3, NaOH, and Na2EDTA. The growth behavior of the MAO ceramic coatings at different stages, including growth rate, microstructure, and phase composition, was investigated using the method of direct observation of the boundary area. The results showed that the growth of the MAO coatings on R60705 zirconium alloy occurred in both inward and outward directions. At an oxidation time of 5 min, the thickness of the oxidation layer increased fastest, reaching 103.43 μm, with a growth rate of 0.345 μm/s. After 5 min, the growth rate decreased and tended to level off around 15 min, with a thickness and growth rate of 162.7 μm and 0.181 μm/s, respectively. The total thickness of the coatings continuously increased throughout the process, with the outward growth thickness always higher than the inward growth thickness. The composition of the zirconium alloy micro-arc oxidation coatings mainly consisted of monoclinic zirconia (m-ZrO2), tetragonal zirconia (t-ZrO2), and a small amount of SiO2. The main elements in the coatings were Zr, O, and Si. The corrosion resistance of the zirconium alloy micro-arc oxidation coatings increased first and then decreased with increasing oxidation time, with a corrosion current density of 8.876 × 10−9 A·cm−2 at 15 min, indicating the best corrosion resistance.

Influence of applied stress on the corrosion resistance of micro-arc oxide coatings on AZ31B magnesium alloy

ABSTRACT A micro-arc oxidised (MAO) protective coating was formed onto AZ31B magnesium alloy samples which were then loaded into ‘C' rings to apply compressive and tensile stresses. The influence of these applied stresses on the protective ability of the coating in simulated body fluid was assessed by surface characterisation and electrochemical testing. It was found that the undulating surface and pore structure of the MAO ceramic coatings was affected by the stress during immersion with a greater number of closed pores under compression and more open pores, with additional cracks, under tension. During the earlier stages of immersion, compressive stresses enhanced the corrosion resistance whilst tensile stresses accelerated corrosion. However, at later stages of immersion both compressive and tensile stress caused enhanced corrosion with tensile stresses consistently worse than compressive stresses.

Enhanced long-term corrosion protection of 2A14 aluminum alloy: Hybrid effect of micro-arc oxidation coating and cerium based conversion treatment

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.

Highly corrosion-resistant ZIF-8-integrated micro-arc oxidation coating on Mg alloy

The long-term corrosion resistance of conventional micro-arc oxidation (MAO) coatings on magnesium alloy is limited by the inherent micro-scale pores. To address this issue, zeolitic imidazolate framework-8 (ZIF-8) and zinc gluconate (ZnG) loaded ZIF-8 (ZnG@ZIF-8) were synthesized and incorporated into the MAO coating. The electrochemical and immersion tests showed that the MAO composite coating doped with ZIF-8 and ZnG@ZIF-8 ((ZIF-8 + ZnG@ZIF-8)/MAO) displayed optimal corrosion resistance. The corrosion current density (jcorr) of the pristine (ZIF-8 + ZnG@ZIF-8)/MAO composite coating (3.38 nA cm−2) is 63.3 times lower than that of the blank MAO coating (214 nA cm−2), indicating the significant enhancement in corrosion protection of the nanoparticle to the coatings. Even after 32 days of immersion in a 3.5 wt.% NaCl solution, the jcorr of the former was as low as 87.3 nA cm−2, much lower than that of the blank MAO coating (1710 nA cm−2). These findings provide new insights into the design of corrosion-resistant MAO composite on magnesium alloy surfaces via the incorporation of inhibitor-loaded ZIF-8 nanocontainers.

In situ Micro‐Arc Oxidation‐Induced Growth of Black Ceramic ZrO2 and CuO Coatings on Magnesium Alloys and their Electrochemical Analysis

AbstractIn this paper, a new environmentally friendly micro‐arc oxidation (MAO) colorant coating was proposed, which greatly improved the corrosion resistance of magnesium alloy. Herein, the black MAO coatings with uniform color and superior performance were successfully prepared on AZ91D alloy to expand the application of magnesium alloy. It was found that the color of the MAO coatings changes from white (80.76) to black (19.92) with the increased concentration of cupric pyrophosphate, attributed to the formation of copper compounds (CuO) in the outer layer of MAO coatings. In addition, electrochemical corrosion tests were also carried out in 3.5 wt% NaCl solution. And it was found that the corrosion potential and current were −1.32 V and 1.06×10−6 A/cm2, respectively, which increased by 0.15 V and decreased by 3.16×10−6 A/cm2 compared to those without cupric pyrophosphate. In conclusion, black MAO coatings produced at a cupric pyrophosphate concentration of 6 g/L, an oxidation time of 17 min, and a termination voltage of 375 V had an excellent performance and superior corrosion resistance. Therefore, it was provided a new idea for the research of surface in‐situ treatment technology with the composite of zirconia salt electrolyte and cupric pyrophosphate.

Recent Advances on Biocompatible coating on Magnesium alloys by Micro Arc Oxidation Technique

Magnesium alloys are suitable biological material because of its favourable mechanical qualities, high biocompatibility, and biodegradability. However, it has poor corrosion resistance and has rapid dissolution in the corrosive environment which will weakens its mechanical characteristics. The surface characteristics of magnesium alloy must thus be changed using a suitable surface modification technology, such as micro arc oxidation (MAO). This article examines recent developments and advancements in biodegradable surface coatings applied to magnesium alloys. It was observed there are four steps of MAO process, the formation of a thinner and denser barrier, commencement of oxides in bare Ca-Mg matrix following the presence of sparks; the horizontal expansion of the oxide layer, and finally thickening of MAO coating. It was observed that characteristics of MAO coating can changed by varying electrical parameters like duty cycle, current density, type of power output, frequency, and processing time. It was noticed that when all other factors are held constant, duty cycle, processing time, and frequency primarily effect the coating's porosity, number of cracks and thickness, which in turn influences how well the coating performs. DC, AC, pulsed bipolar, and pulsed unipolar, are the four categories into which the current regimes are classified. It was found that, unipolar current mode MAO coatings found to be rough, highly porous, and vulnerable to microcracks due to stronger spark discharge. MAO coating produced in a bipolar current type of mode have larger pores but are more uniform in thickness and compact. It was noticed that the in-vitro cell assays showed cells L929 on the Ca-P coated Mg alloy to have considerably good adhesion, a high growth rate, and strong proliferation (p 0.05). In other words, the cytocompatibility was greatly enhanced by the Ca-P coating. It was discovered that the Ca-P coated Mg alloy improved cell responsiveness and encouraged early bone formation at the implant/bone interface by both conventional pathological examination and immunohistochemistry investigation. The Ca-P coating was found to be an effective method for raising the surface bioactivity of Mg alloy. It was also observed that the calcium phosphate coating deposited by MAO process improve surface biomineralization which is the main mechanism behind bioactivity. Functional groups that are present on surface engage electrostatically through calcium and phosphate ions from solutions to start the biomineralization process. Calcium phosphates have excellent biocompatibility and are quite comparable to the mineral makeup of bone. The current study aims to investigate the bioactivity of calcium phosphate coatings and the characteristics of magnesium and its alloys.

Effect of zinc or copper doping on corrosion resistance and anti-oxidative stress of strontium-based micro-arc oxidation coatings on titanium

In osteoporosis, the osteogenic microenvironment maintains a high level of oxidative stress (OS) due to the overaccumulation of reactive oxygen species (ROS). It not only significantly destroys the bioactivity of titanium (Ti) implants, but also enhances Ti implant corrosion, which has a significant impact on the early osteointegration of Ti implants. In this study, using microarc oxidation (MAO) technology, we constructed strontium/zinc (Sr/Zn)- and strontium/copper (Sr/Cu)-doped MAO coatings on the surface of the Ti implant and studied their corrosion resistance and osteoinductive properties under normal and OS conditions. Apart from their chemical compositions, their physical and chemical properties (roughness, hydrophilicity, morphology, and crystal structure) were similar. In the anti-corrosion test, the MAO-Sr/Zn groups (especially MAO-Sr/Zn3) exhibited perfect performance in both normal and OS microenvironments, whereas MAO-Sr/Cu showed poor corrosion resistance. In terms of biological behavior, based on the perfect osteogenesis and anti-OS properties of Sr and Zn, MAO-Sr/Zn3 exhibited the best performance in terms of cell viability, mineralization, and anti-OS ability, followed by MAO-Sr/Cu2, due to a suitable proportion of Sr/Cu. Therefore the excellent comprehensive performance of MAO-Sr/Zn3 showed that it has broad potential for implant repair in patients with osteoporosis.

Effects of soft sparking on micro/nano structure and bioactive components of microarc oxidation coatings on selective laser melted Ti6Al4V alloy

We used the soft sparking mode to fabricate microarc oxidation coatings containing micro/nano-scale pores and bioactive Ca/P components on Ti6Al4V alloy samples formed by selective laser melting. The effect of soft sparking on the microstructure and corrosion resistance of the coatings and the codeposition mechanism of the electrolyte anions in the soft sparking layer were investigated using different negative/positive charge ratios. The results showed that the negative charge significantly decreased the intensity of microarc discharge, resuling in the soft sparking phenomenon, which promotes the formation of a thin amorphous TiO2 layer at the barrier layer/substrate interface. In turn, this interface facilitated the electron tunneling and aggregation of the electrolyte anions, which leads to soft sparking at voltages significantly lower than the breakdown voltage of the anodic oxide film on the selective laser melted Ti alloys. Soft sparking promoted the codeposition of the electrolyte anions, including PO43− and Ca2+ ions, in the form of bulk amorphous phases in the coating formed by soft sparking. The soft sparking layer contained amorphous phases, which were corroded more readily than that the outer ceramic layer. However, the submicro/nano-scale pores in the soft sparking layer improved its corrosion resistance. With an increase in negative/positive charge ratio, the effect of soft sparking on the surface morphology of the MAO coating weakened, and the amounts of Ca and P codeposited in the soft sparking layer increased. Finally, the corrosion resistance of the coating was found to first increase and then decrease, reaching the maximum at the 1.1 ratio.

Investigation of the Characteristics of MAO Coatings Formed on Ti6Al4V Titanium Alloy in Electrolytes with Graphene Oxide Additives

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.

Improved Mechanical Property and Corrosion and Wear Resistance of High-Voltage Aluminium Wires by Micro-Arc Oxidation Coating

New surface treatments are required to improve the resistance of high-voltage aluminium conductors to corrosion and wear. This paper describes the preparation of ceramic coatings on the surface of aluminium wires by the micro-arc oxidation (MAO) technology. The surface morphology, mechanical and tribological properties, electrical resistivity, wear and corrosion resistance of the MAO-treated aluminium samples are examined and compared not only with the untreated samples but also between the use of different current densities in the MAO process. The MAO treatment can increase the tensile strength of the wire and mitigate the wire elongation at excessive high temperature. In addition, the increased surface hardness and the overlapping micro-porous distribution of the MAO coating improve the resistance of the treated sample to wear and corrosion respectively, protecting the aluminium substrate from the joint effects of wear and corrosion. Furthermore, the MAO coating has little impact on the electrical resistivity of aluminium conductors and can mitigate the resistivity change due to corrosive damage. The MAO technology providing a potential idea for the surface treatment of aluminium conductors is expected to extend the service life and alleviate maintenance needs of aluminium conductors operating in harsh environments.

Preparation and antibacterial properties of ZnSr-doped micro-arc oxidation coatings on titanium

Titanium and titanium alloys are biologically inert materials. The lack of rapid osseointegration and resistance to bacterial infection remain unsolved problems for these materials. To overcome these problems, zinc (Zn) and strontium (Sr) at different concentrations were doped into the surface coating of implants by one-step micro-arc oxidation (MAO) technology. The microstructure, elemental content, chemical composition and corrosion resistance of the coating were studied. The antibacterial activity and biocompatibility of the coating were evaluated by antibacterial tests and proliferation tests of bone marrow mesenchymal stem cells (BMSCs). The results show that the coating is mainly composed of the rutile phase and anatase phase. The content of zinc in the coating decreases with increasing strontium concentration. The addition of zinc and strontium in the electrolyte does not significantly change the microstructure and physical and chemical properties of the coating. The ZnSr-TiO2 coating shows good antibacterial activity against Staphylococcus aureus and Porphyromonas gingivalis, which can effectively promote the proliferation and differentiation of bone marrow mesenchymal stem cells (BMSCs), and the cells on the coating with a higher strontium content exhibit greater proliferation.

Incorporation of Al2O3 and ZrO2 ceramics to AZ31 magnesium alloys composite coating using micro-arc oxidation method

In this research, a composite coating with Al2O3 and ZrO2 particles have been applied on AZ31 magnesium alloy by micro-arc oxidation (MAO) technique. The alkaline electrolyte included a constant based composition and different composition of the Al2O3 and ZrO2 additives. Microstructure observations reveal that the surface pores of composite coating reduced during addition of ZrO2 and Al2O3 ceramic particles. The hardness of coating increased from about 380 for non-added to 620 MPa for Al2O3+ZrO2 added coating and wear rate reduced about 8 times. Wettability of the coating increased by incorporation of Al2O3 and/or ZrO2 particles while, Al2O3 is more effective than ZrO2. Addition of the ceramic particles enhanced the hydrophilicity properties of surface in wettability test and a contact angle of 43° was obtained for coating including Al2O3+ZrO2. The antibacterial properties of MAO coatings showed that S. aureus bacterium is more sensitive to the zirconia and alumina particle than S. typhimurium bacterium after 24 h of incubation.

Influences of Sc(NO3)3 on characteristics of micro-arc oxidation coatings

ABSTRACT Micro-arc oxidation (MAO) coatings were fabricated on 2195 aluminium-lithium alloy substrate, using a silicate and phosphate electrolyte with variable scandium nitrate (Sc(NO3)3) concentrations. The properties of the coatings were analysed by scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical workstation. The comprehensive analysis demonstrates that adding proper amount of Sc(NO3)3 increased the oxidation voltage, which transformed the worm shape discharge micropores into round-hole shape, and the MAO coatings presented a double-layer structure. The primary phase composition of the coatings was γ-Al2O3, α-Al2O3, Sc2O3 and Al3Sc. The optimum concentration of Sc(NO3)3 was 0.6 g L−1. The hardness reached a maximum value of (1284 ± 43.5) HV, which was about 2.5 times higher than that without adding. The self-corrosion current density reduced from 138.00 × 10− 7 A cm−2 (substrate) to 3.65 × 10− 7 A cm−2, which was approximately two orders of magnitude lower, indicating that the corrosion resistance was improved.

Influence of different modes of microarc oxidation of titanium on the electrochemical properties and surface morphology of the obtained coatings

ABSTRACT The influence of Microarc Oxidation (MAO) modes of titanium substrate in electrolytes of the original composition is investigated. Used electrolytes no contain sodium silicate but includes nickel sulphate, sodium aluminate and alkali. The morphology and composition of the obtained coatings were studied by electron microscopy. The electrochemical behaviour was studied using polarisation curves and electrochemical impedance spectroscopy (EIS). Proposed Equivalent circuits are adequately corresponded to experimental EIS data with no more 6% error. The dependence between the elements of the Equivalent circuits and the morphological characteristics depending on the processing is considered. It is shown that MAO coatings on titanium have high corrosion resistance in an aggressive sulphuric acid. The electrochemical properties of coatings undergo irreversible changes in the direction of deterioration after anodic polarisation. It has been established that nickel doping of MAO coatings on titanium does not allow them to serve as anode materials.

In-situ formation of Ag nanoparticles in the MAO coating during the processing of cp-Ti

Silver nanoparticle (Ag-NP) containing antibacterial micro-arc oxidation (MAO) coatings have already been synthesized over titanium-based materials via the MAO process employed in silver acetate (AgC2H3O2) containing electrolyte. However, the way of incorporation and in-situ formation of Ag-NPs within the MAO coating have not been documented yet. Present work was initiated to reveal the mechanism of Ag-NP formation within the MAO coatings. Thus, the structure of the MAO coating fabricated on commercial purity titanium in the AgC2H3O2-containing electrolyte was investigated by electron microscopy techniques. To this end, the cross-sectional high-resolution electron microscopy studies were carried out on lamella cut out with the focused ion beam technique, and these investigations were backed by X-ray photoelectron spectroscopy measurements of chemical composition on the surface of the MAO coating. These studies revealed that Ag is dispersed in the form of nanoparticles throughout the coating and that a higher density was confirmed closer to the micro-pores.

Influence of content of silicon nitride nanoparticles into micro-arc oxidation coating of titanium on bactericidal capability and osteoblastic differentiation

Bone implants with osteogenic and bactericidal capability are particularly promising for orthopedic applications. Herein, micro-arc oxidation (MAO) coatings containing silicon nitride (Si3N4) on titanium (Ti) surface (MSN) were fabricated by dispersing different content of Si3N4 nanoparticles in the electrolyte. Compared with pure Ti and MAO coating, incorporation of Si3N4 nanoparticles into the MAO coating caused significant change in surface characters (e.g., nano-topography, hydrophilicity and surface energy) of MSN, and the change depended on the content of Si3N4 nanoparticles. MSN with the changed surface characters exhibited remarkable improvement of cell response (e.g., multiplication and osteoblastic differentiation), which significantly enhanced with the increase of Si3N4 content. Furthermore, MSN displayed excellent bactericidal ability, which improved with the increase of Si3N4 content. As a result, the enhancements of cellular response and bactericidal capability were ascribed to the increase of Si3N4 content in the coating, and MSN with high content of Si3N4 possessed outstanding osteogenic and anti-bacterial effects. The study demonstrated that incorporation of Si3N4 nanoparticles into the MAO coating led to novel implantable materials of MSN, which exhibited excellent cytocompatibility, osteogenic bioactivity, bactericidal ability, suggesting that MSN possessed a promising potential for applications in orthopedics.

Undoped and diamond-doped MAO coatings prepared on Ti6Al4V: Microstructure, wear, corrosion, and biocompatibility properties

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.

Effects of K2TiF6 and Electrolyte Temperatures on Energy Consumption and Properties of MAO Coatings on 6063 Aluminum Alloy

To decrease energy consumption and improve the performance of micro-arc oxidation (MAO) films on 6063 Al alloy, a policy of K2TiF6 additive and electrolyte temperature control was adapted. The specific energy consumption relied on the K2TiF6 additive and more particularly on the electrolyte temperatures. Scanning electron microscopy demonstrates that electrolytes with 5 g/L K2TiF6 can effectively seal the surface pores and increase the thickness of the compact inner layer. Spectral analysis shows that the surface oxide coating consists of γ-Al2O3 phase. Following 336 h of the total immersion process, the impedance modulus of the oxidation film, prepared at 25 °C (Ti5-25), remained 1.08 × 106 Ω·cm2. Moreover, Ti5-25 has the best performance/energy-consumption ratio with a compact inner layer (2.5 ± 0.3 μm). This research found that the time of the big arc stage increased with the temperature, resulting in producing more internal defects in the film. In this work, we employ a dual-track strategy of additive and temperature providing an avenue to reduce the energy consumption of MAO on alloys.