MAO Coatings Research Articles

Interactions Effect Among the Electrolytes on MAO Coatings of AZ91D Mg Alloy

Interactions Effect Among the Electrolytes on MAO Coatings of AZ91D Mg Alloy

Influence of the interelectrode distance and the ratio of the cathode and anode areas on the formation rate and properties of MAO coatings

Influence of the interelectrode distance and the ratio of the cathode and anode areas on the formation rate and properties of MAO coatings

Identification of regularities of formation of the phase-structural state and properties of coatings obtained by micro-arc oxidation of high-strength V95 alloy

The influence of electrolyte composition and electrolysis modes on the surface morphology, growth kinetics, phase-structural state, and mechanical properties of MAO coatings on the V95 aluminum alloy (with Zn as the main alloying element) was studied. The possibility of forming a continuous MAO coating on the V95 alloy with a base layer thickness of more than 100 μm and a surface roughness of less than 5 μm was found. The study of the growth kinetics of MAO coatings on the V95 alloy showed that the highest growth rate of the base coating layer (about 0.83 μm/min) occurs in the 1 g/L KOH+6 g/L Na2SiO3 electrolyte. Although an increase in the relative content of the silicate component (Na2SiO3) provides a fairly high growth rate of the coating, the growth rate of the base layer remains rather low (0.38–0.40 µm/min). This makes the oxidation process in these electrolytes less technological.The study of the phase-structural state of the base coating layer showed that it has a crystalline structure, in which the main phase is aluminum oxide γ–Al2O3 (90–97 %). Crystallites of α-Al2O3 and mullite (3Al2O3∙2SiO2) are formed as the second phase. With an increase in process time in electrolytes with the highest relative content of the alkaline component (1 g/L KOH+6 g/L Na2SiO3), the relative content of the hardest α-Al2O3 phase increases (up to 5 %). The hardness of such coatings with an oxidation time of 180 minutes reaches 14,000 MPa. Thus, the study has shown the advisability of using MAO treatment for the V95 alloy, since it can significantly increase the surface hardness and thereby guarantee its high wear resistance. The combination of high hardness with a relatively high coating growth rate makes it possible to recommend oxidation in the 1 g/L KOH and 6 g/L Na2SiO3 electrolyte as the most technologically advanced for improving the mechanical characteristics of the surface of products made of V95 alloy

Phase composition and properties of mao coatings on titanium alloys

Problem. MAO technology is well developed for aluminum alloys. As for titanium alloys, the MAO technology of surface hardening for them requires further research. Goal. In order to improve the mechanical characteristics of titanium alloys it is necessary to establish patterns of influence of regimes and electrolyte composition during MAO process on the phase composition, microhardness and tribological characteristics of coatings obtained by microarc oxidation of VT3-1 and PT-3V titanium alloys. Methodology. X-ray structural analysis (DRON-3) in radiation Кα-Cu, microhardness measurement (PMT-3) with 100 gr load, measurement of coating thickness (vortex thickness gauge BT-10NTs), tribological tests (SMC-2) was used. Results. The MAO treatment of the alloys was carried out in an alkaline electrolyte (KOH) with the addition of sodium aluminate (NaAlO2), sodium hexametophosphate (NaPO3)6 and water glass (Na2SiO3, ρ ≈ 1.4 g/cm3). A metallographic study of the cross section of titanium alloys with MAO coating made it possible to establish that the coating has a two-layer structure consisting of a technological (porous) and main (dense, base) layers. The results of X-ray diffraction analysis showed that the main phases of the coating are the phases TiO2 – rutile, TiO2 – anatase, as well as compounds Al2TiO5 – aluminum titanate. The specific volume of the formed oxides exceeds the volume of the titanium base on which it is formed. So, for TiO2 Voх/Vmet. ≈ 1.8; for Al2TiO5 – 4.4; for mullite – 13.9. This factor contributes to the formation of continuous coatings. It was established that in order to achieve the highest hardness, the presence of the NaAlO2 component in the electrolyte is necessary. This made it possible to achieve a hardness of 11000 MPa of MAO coating on VT3-1 alloy using an electrolyte of 1.75 g/L KOH + 1 g/L Na2SiO3 + 2 g/L NaAlO2. For the PT-3V alloy, the use of an electrolyte of a similar composition can achieve a hardness of 7500 MPa. It is shown that the use of MAO processing of titanium alloys allows to reduce the dry friction coefficient in the VT3-1 – SCh20 system to 0.1. Originality. The structure, phase composition and properties are determined by electrolysis conditions and coating thickness. Practical value. The results obtained are a recommendation for surface hardening of PT-3V and VT3-1 titanium alloys for their further use in friction elements.

Possibilities of phase-structural engineering and properties of microarc oxide coatings on the AMг3 alloy

Purposeof the work: to establish the regularities of the alkali-silicate electrolyte composition influence and the conditions of electrolysis during microarc oxidation of the AMг3 aluminum alloy on the kinetics of the oxide coating formation, its structure and the conditions for the formation of the α-Al2O3 phase. Results. The possibility of forming high-density MAO coatings on the AMг3 aluminium alloy during electrolysis in alkali-silicate electrolytes has been determined. The regularities of the kinetics of MAO coatings growth at different electrolyte composition and oxidation time were revealed. It was found that the phase composition of MAO coatings consists of the following phases − α-Al2O3, γ-Al2O3, and mullite (3Al2O3•2SiO2), the ratio of which changes with a change in the amount of sodium silicate (Na2SiO3). It was found that addition of an inorganic soluble salt dopant to the electrolyte, which contains oxygen (K2Cr2O7), leads to a qualitative change in the phase composition − an increase in the amount of α-Al2O3 phase in the coating composition and an increase in hardness. In this case, it becomes possible to obtain MAO coatings of small thickness (up to 90 μm) with a high content of the α-Al2O3 phase (up to 40 %), which cannot be carried out in an alkali-silicate electrolyte. Analysis of the effect of high-temperature annealing of MAO coatings on the shape of the diffraction spectrum made it possible to revealthe appearance of tetragonality in the crystal lattice of the γ-А12О3 phase. The structural state with a tetragonally distorted lattice is a stage of γ-А12О3 → α-А12О3 transformation. Scientific novelty. It was found that when using an alkaline-silicate electrolyte for electrolysis, the addition of liquid glass (Na2SiO3) leads to an increase in the growth rate of the coating, but at the same time it stimulates the formation of mullite (3Al2O3 × 2SiO2) as a phase component. An increase in the alkaline (KOH) component leads to a decrease in the growth rate (to 0,6…0,7 μm/min) and stimulates the formation of the γ-Al2O3 phase. The formation of the α-Al2O3 (corundum) phase is stimulated with a long process duration, when the thickness of the dielectric layer and the breakdown power increase. An increase in the amount of α-Al2O3 leads to an increase in the hardness of the coatings. Isothermal annealing at temperatures exceeding 1 000 °C stimulates the γ-Al2O3 → α-Al2O3 polymorphic transformation. The initial stage of such a transformation is the appearance of tetragonality in the crystal lattice of the γ-Al2O3 phase. Practical value. This study allows us to propose an additive to the alkali-silicate electrolyte in the form of a K2Cr2O7 salt, which increases the rate of formation of MAO coatings to 1,4 μm/min and, at the same time, qualitatively and quantitatively changes the phase composition of the coatings, increasing its hardness.

Lithium-modified MAO coating enhances corrosion resistance and osteogenic differentiation for pure magnesium

In this study, the MAO coating with and without Li addition was fabricated on pure Mg to enhance corrosion resistance and osteogenic differentiation for pure Mg. The SEM, XRD and XPS were used to analyze the microstructure and chemical composition of the MAO coatings. The immersion test were conducted to evaluate the degradation behavior of the MAO coated Mg in term of pH and hydrogen evolution. MTT and quantitative PCR (qPCR) analysis were performed to assess osteogenic differentiation of the MAO coatings. The results showed that the addition of Li could decrease the number of micropores and cracks on the MAO coating. Accordingly, the Li-MAO coating with less micropores and cracks was found to exist better corrosion resistance compared with Li free MAO coating. More importantly, the in vitro results of MTT and quantitative PCR (qPCR) analysis suggested that Li addition on MAO could promote the osteogenic-related genes expression with respect to Li free MAO coating, indicating a better osteogenic ability of Li-MAO coating.

The effect of electrolysis conditions during microarc oxidation on the phase-structural state, hardness and corrosion resistance of magnesium alloys

By the method of microarc oxidation for different types of electrolytes (which include KOH, Na2SiO3, Н2О2, NaOH, NaAlO2, Na5P3O10, NaF) and electrolysis conditions, multifunctional ceramic coatings on a magnesium alloy were obtained. The phase composition of the coating includes magnesium oxide (MgO), spinel MgAl2O4, Mg2SiO4 and Мg3(РО4)2 compounds. The phase composition of the coatings is determined by the composition of the electrolyte. The obtained MAO coatings provide high hardness, which is 1500 to 7300 MPa, as well as high corrosion resistance. The results obtained make it possible to recommend MAO coatings on magnesium alloys both as an external (functional) layer and for the formation of an underlayer for the subsequent application of protective coatings (varnishes, polymers, polytetrafluoroethylene, in particular).

Zn- or Cu-containing CaP-Based Coatings Formed by Micro-Arc Oxidation on Titanium and Ti-40Nb Alloy: Part II-Wettability and Biological Performance.

This work describes the wettability and biological performance of Zn- and Cu-containing CaP-based coatings prepared by micro-arc oxidation on pure titanium (Ti) and novel Ti-40Nb alloy. Good hydrophilic properties of all the coatings were demonstrated by the low contact angles with liquids, not exceeding 45°. An increase in the applied voltage led to an increase of the coating roughness and porosity, thereby reducing the contact angles to 6° with water and to 17° with glycerol. The free surface energy of 75 ± 3 mJ/m2 for all the coatings were determined. Polar component was calculated as the main component of surface energy, caused by the presence of strong polar PO43− and OH− bonds. In vitro studies showed that low Cu and Zn amounts (~0.4 at.%) in the coatings promoted high motility of human adipose-derived multipotent mesenchymal stromal cells (hAMMSC) on the implant/cell interface and subsequent cell ability to differentiate into osteoblasts. In vivo study demonstrated 100% ectopic bone formation only on the surface of the CaP coating on Ti. The Zn- and Cu-containing CaP coatings on both substrates and the CaP coating on the Ti-40Nb alloy slightly decreased the incidence of ectopic osteogenesis down to 67%. The MAO coatings showed antibacterial efficacy against Staphylococcus aureus and can be arranged as follows: Zn-CaP/Ti > Cu-CaP/TiNb, Zn-CaP/TiNb > Cu-CaP/Ti.

Corrosion-fatigue behavior of micro-arc oxidation coated 6061-T6 Al alloy

The corrosion-fatigue response of Al alloys becomes an important consideration for qualifying them for aerospace and other structural applications, the MAO coatings with three different thicknesses namely 10 µm, 50 µm and 90 µm were provided on peened (SP + MAO) and un-peened (Plain MAO) substrates. Further, the corrosion (potentiodynamic polarization) tests in 3.5 wt% NaCl solution and corrosion-fatigue tests at 5-different maximum alternating stress levels were evaluated for each material condition. In addition, the surface roughness, surface and cross-sectional coating morphologies in as deposited and corroded conditions; fractured morphology and corrosion-fatigue crack propagation features were evaluated; correlated with the corresponding corrosion rates and corrosion-fatigue life. The results obtained highlight the role of corrosion medium in significant degradation of corrosion-fatigue life as compared to the plain-fatigue life of both bare substrate and MAO coated conditions. A new parameter namely the normalized corrosion-fatigue life index (NCFI) provides a quicker insight and possible benchmarking of the coating’s ability to withstand the damaging action of simultaneous corrosion and fatigue. A 50 µm thick SP + MAO coating was identified as a candidate choice to withstand diverse degradation modes namely the simple corrosion, plain-fatigue and corrosion-fatigue.

STRUCTURE AND PROPERTIES OF HETEROGENEOUS CATALYST ON AN OXIDE CARRIER BASED ON ALUMINIUM

In this study, the MAO technology was optimized in order to obtain a high development of coatings and their good adhesion to the substrate for the creation of a highly efficient heterogeneous catalyst on the oxide carrier of the Pt/Al2O3 system. Oxide coatings of different phase composition, surface structure and roughness, with a thickness of 20 –100 μm were obtained by MAO methods on D16 alloy and A97 commercial aluminum. It is shown that the structure and morphology of the surface of the coatings is determined by the conditions of electrolysis, which allows their optimization to improve efficiency. It is determined that the surface coating layer consists of the following phases: γ-Al2O3, α-Al2O3 and 3 Al2O3 • 2SiO2 (mullite). On the D16 alloy there was a predominant formation of γ-Al2O3 and α-Al2O3 phases. Increasing the duration of microarc oxidation increases the content in the coating of the α-Al2O3 phase. It is established that the use of MAO technology allows to change the morphology of the surface and its roughness in a wide range. The formation of two types of surface inhomogeneity in the planar growth region of coatings is revealed: with a relatively small size of such formations (up to 50 μm) and with the formations of the second type, the sizes of which in the planar growth region of MAO coatings reach 300 μm.It is determined that the purification coefficient of nitrogen oxide in Pt/Al2O3 catalysts is temperature dependent. At relatively low (150°C) and high (300°C) temperatures the highest value of α is achieved in systems based on MAO coatings with binary (γ-Al2O3 and α-Al2O3) phase composition. In the temperature range of the most effective cleaning (200–250°C) the greatest influence on their efficiency, apparently, has a surface morphology, namely, the formation of large (up to 300 μm) formations of surface inhomogeneity in the growth plane. In this case, the purification coefficient (for the reduction of nitrogen oxides) α reaches 92 %.The results of the study can be used in the development and creation of a new type of catalysts for the Pt/Al2O3 system.

Layer by layer assembled chitosan (TiO2)-heparin composite coatings on MAO-coated Mg alloys

Magnesium based alloys are ideal biodegradable materials owing to their appropriate mechanical properties and good biocompatibility. However, the poor corrosion resistance of Mg alloy limits its wide application. We fabricated self-sealed antibacterial MAO coatings and layer by layer assembled (LBL) composite coatings on Mg-3Zn-0.5Sr alloy. Three layers of chitosan and 1, 3, 6 cycles of LBL films containing TiO2 were alternately immobilized on MAO coating. The effects of heparin and layer numbers on corrosion resistance, wettability and hemocompatibility of composite coatings were explored. The results showed that 3 cycles of LBL film possessed uniform, dense and adhesive morphology, excellent apparent anticoagulant ability. It could effectively seal the pores in MAO coating and prevent the samples from corroding too fast.

Corrosion chemical mechanism in aggressive liquid of MAO coatings based on OT-4 titanium alloys

Microarc oxidation of alloys is widely used to obtain multifunctional coatings with unique properties. One of the topical issues in the application of the microarc oxidation is to increase the corrosion resistance of titanium alloys. In this article, the corrosion resistance of various microarc oxidation coatings of a titanium alloy in chemically aggressive liquids is determined. Comparative analysis of microarc oxidation coatings chemical resistance of the titanium alloy OT-4 showed a high coating resistance to the corrosive liquid exposure. The highest corrosion coefficient was determined when the alloy was exposed to concentrated sulphuric and hydrochloric acids. Virtually no corrosion of microarc oxidation coatings was observed in dilute acids (H2SO4 and HCl) and NaOH alkali solution.

Effects of graphene on the microstructure and properties of MAO coatings formed on AA7050

Abstract Ceramic coatings were prepared on the surface of 7050 highstrength aluminum alloy using micro-arc oxidation in an aluminate electrolyte with added graphene. To analyze the surface morphology, roughness, phase composition, and corrosion resistance, scanning electron microscopy, X-ray diffraction, X-ray photoelectron, and electrochemical measurements were used, respectively. The addition of 9 g · L-1 of graphene to the electrolyte decreased the micro-pore size of the composite coatings and improved the density. In addition, with the addition of graphene, the roughness was the lowest, and the corrosion resistance was significantly improved.

Thickness and corrosion resistance optimization of Micro-ARC oxidation coating on Mg-Al-Li-Zn alloy using Taguchi approach

Micro arc oxidation method has been developed in the field of surface protection of magnesium alloys and considered as a simple, highly effective, commercial and environmentally friendly method in industry. MAO coatings are fabricated on novel Mg-Al-Li-Zn alloy to improve the anti-corrosion performance of surface by using friendly alkaline electrolytes under a high electrical potential. The Taguchi method and optimal analysis are used to identify the effects of the three factors including current density, processing time and electrical frequency on coating’s characteristics. The results have shown that the main factor that affects coating thickness and corrosion resistance of coating is the processing time. The results obtained by optimal conditions are consistent with prediction values of Taguchi analysis. The thickness of the coating can help to improve the long-term corrosion protection of a MAO coating in corrosive environments.

Effects of graphene on the microstructure and properties of MAO coatings formed on AA7050

Abstract Ceramic coatings were prepared on the surface of 7050 highstrength aluminum alloy using micro-arc oxidation in an aluminate electrolyte with added graphene. To analyze the surface morphology, roughness, phase composition, and corrosion resistance, scanning electron microscopy, X-ray diffraction, X-ray photoelectron, and electrochemical measurements were used, respectively. The addition of 9 g · L-1 of graphene to the electrolyte decreased the micro-pore size of the composite coatings and improved the density. In addition, with the addition of graphene, the roughness was the lowest, and the corrosion resistance was significantly improved.

Investigating local corrosion behavior and mechanism of MAO coated 7075 aluminum alloy

In order to study the effect of defects on the corrosion behavior of MAO coatings, global electrochemical method including potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) was used to measure the corrosion resistance of MAO coatings. Local electrochemical measurements including local impedance spectroscopy (LEIS) and scanning vibrating electrode technique (SVET) were used to study the local corrosion mechanism. The confocal microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to observe the microstructure and phases of the samples. Results of global electrochemical measurement demonstrate that the scratches cause the reduction of the impedance and the increase of the corrosion current density. LEIS results show the impedance value distribution of the coating with the scratch. With the changes over time, the minimum impedance value of the scratch decreases first and then increases from the third hour. SVET results show the corrosion current density distribution of the scratched coating. The maximum corrosion current density at the scratch increases first and then decreases. The results of local electrochemical measurements show that the corrosion rate at the scratch decreases when the reaction lasted 3 h. Confocal microscopy found that the scratch is expanding with increasing time, and the depth and cross-sectional area of the coating is 5 times deeper and 10 times larger than that of 7075Al with scratch at the first 6 h. The 6-day immersion test proved that the scratch coating still has the protective effect on the substrate. The main corrosion product is Al(OH)3. A physical model reflecting the mechanism of the scratch propagation and the formation of the corrosion product film is proposed.

Phase composition of coatings on the D16 alloy during micro-arc oxdation in alkaline and silicate electrolytes

Goal. The influence of electrolysis conditions at different electrolyte compositions on the phase formation of coatings obtained by micro-arc oxidation (MDO) on an aluminum alloy D16 was studied. Method. For electrolysis, two types of electrolytes were used: alkaline electrolyte (solution (KOH) in distilled water), silicate electrolyte (with different percentages of Na2SiO3 component). Research results. It was found that the phase composition of the MAO coatings obtained in an alkaline (KOH) electrolyte mainly consists of γ - Al2O3 phases and, to a much lesser extent, the α-Al2O3 phases. An increase in the KOH concentration leads to a shift in the γ – Al2O3 → α – Al2O3 polymorphic reaction toward the formation of the hardest α-Al2O3 phase (corundum). The formation of the preferred orientation of the growth of crystallites of γ – Al2O3 and α – Al2O3 phases during their formation in an alkaline electrolyte was not detected. Scientific novelty. A significant influence on the mechanism and processes of coating formation is made by the addition of liquid glass (Na2SiO3) in the electrolyte. In this case, the growth rate of the coating increases significantly, but the size of the ordering regions decreases from crystalline to X-ray amorphous. The phase composition of the MAO coatings, when they are formed in a silicate electrolyte, varies from a mixture of the γ - Al2O3 phase and mullite (3Al2O3 • 2SiO2) with a low content of liquid glass (10 g/l Na2SiO3) to the formation of only the X-ray amorphous phase with a high content of liquid glass in the electrolyte (50 g/l Na2SiO3). Practical significance. It was concluded that the use of an alkaline or silicate electrolyte with different percentages allows a wide variation of both the phase composition and structural state (α- Al2O3 and γ- Al2O3 phases, mullite (3Al2O3 • 2SiO2) or X-ray amorphous state) and the kinetics of growth the coating itself.

Preparation and Corrosion Behavior in Marine Environment of MAO Coatings on Magnesium Alloy.

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.

Aggregation of MAO Coatings and Crack Formation in the Microgrooves on an Aluminum Surface

Aggregation of MAO Coatings and Crack Formation in the Microgrooves on an Aluminum Surface

Tribocorrosion behavior of Ca–P MAO coatings on Ti6Al4V alloy at various applied voltages

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