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The work addresses the issues of controllability of the microarc-oxidation process. This problem is mostly due to the difficulty of analytically characterizing the heterogeneous physical–chemical processes, which are nonlinear and nonstationary, that take place during microarc oxidation. To tackle this issue, a digital twin of the process is being developed, within which an analytical model is proposed to describe the behavior of the equivalent electrical circuit of the galvanic cell during coating deposition. The proposed analytical model of the process is nonlinear and nonstationary, which is attributed to the abrupt decrease in the active resistance of the coating during dielectric breakdowns. Based on experimentally obtained current and voltage oscillograms and the proposed analytical model, parametric identification of the electrical parameters of oxide coatings is performed using the matrix-operator method on the orthonormal Walsh basis. The matrix-operator method is selected because of its applicability to solving problems with both linear and nonlinear equations, as well as stationary and nonstationary parameters and variables, along with the relative simplicity of the mathematical and algorithmic implementation of computations. The calculations result in weight functions of the electrical model of the microarc-oxidation process, reflecting the relationship between the voltage and current in the galvanic cell (coating conductivity). The mean values of the weight functions (average conductivity) decrease during coating deposition, which confirms the adequacy of the proposed model and enables its use for implementing real-time procedures for monitoring coating properties. The scientific novelty of the work lies in the possibility of the real-time control of microarc oxidation through parametric identification of the electrical parameters of coatings using the matrix-operator method. [ABSTRACT FROM AUTHOR]

This work proposed a strategy of indirectly inducing uniform microarc discharge by controlling the content and distribution of β-Mg17Al12 phase in AZ91D Mg alloy. Two kinds of nano-particles (ZrO2 and TiO2) were designed to be added into the substrate of Mg alloy by friction stir processing (FSP). Then, Mg alloy sample designed with different precipitated morphology of β-Mg17Al12 phase was treated by microarc oxidation (MAO) in Na3PO4/Na2SiO3 electrolyte. The characteristics and performance of the MAO coating was analyzed using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), contact angle meter, and potentiodynamic polarization. It was found that the coarse α-Mg grains in extruded AZ91D Mg alloy were refined by FSP, and the β-Mg17Al12 phase with reticular structure was broken and dispersed. The nano-ZrO2 particles were pinned at the grain boundary by FSP, which refined the α-Mg grain and promoted the precipitation of β-Mg17Al12 phase in grains. It effectively inhibited the "cascade" phenomenon of microarcs, which induced the uniform distribution of discharge pores. The MAO coating on Zr-FSP sample had good wettability and corrosion resistance. However, TiO2 particles were hardly detected in the coating on Ti-FSP sample. [ABSTRACT FROM AUTHOR]

The repair and reconstruction of large bone defects after bone tumor resection is still a great clinical challenge. At present, orthopedic implant reconstruction is the mainstream treatment for repairing bone defects. However, according to clinical feedback, local tumor recurrence and nonunion of bone graft are common reasons leading to the failure of bone defect repair and reconstruction after bone tumor resection, which seriously threaten the physical and mental health of patients. On this basis, here the self-developed low modulus Ti-12Mo-10Zr alloy (TMZ) was chosen as substrate material. To improve its biological activity and osteointegration, calcium, oxygen, and phosphorus co-doped microporous coating was prepared on TMZ alloy by microarc oxidation (MAO). Then, black phosphorus (BP) nanosheets were incorporated onto MAO treated TMZ alloy to obtain multifunctional composites. The obtained BP-MAO-TMZ implant exhibited excellent photothermal effects and effective ablation of osteosarcoma cancer cells under the irradiation of 808 nm near infrared laser, while no photothermal or therapeutic effects were observed for TMZ alloy. Meanwhile, the structure/component bionic coating obtained after MAO treatment as well as the P-driven in situ biomineralization performance after incorporation of BP nanosheets endowed BP-MAO-TMZ implant with synergistic promoting effect on MC3T3-E1 osteoblasts' activity, proliferation and differentiation ability. This study is expected to provide effective clinical solutions for problems of difficult bone regeneration and tumor recurrence after tumor resection in patients with bone tumors and to solve a series of medical problems such as poor prognosis and poor postoperative quality of patients life with malignant bone tumors. [ABSTRACT FROM AUTHOR]

For the first time, a promising method for protecting the surface of metals and alloys from external high-temperature effects is proposed and substantiated experimentally by creating a functionally graded coating with a finishing ceramic layer and a precursor intermetallic layer. The technology of gas dynamic cold spraying of a mechanical mixture of powders based on aluminum valve metal with heat treatment is developed. [ABSTRACT FROM AUTHOR]

It is known that the methods of electrolytic oxidation, including traditional anodizing and microarc oxidation, implemented according to classical schemes in electrolytic baths with mechanical, pneumatic, and magnetic mixers, cannot provide high-quality homogeneous oxide coatings on the inner surfaces of products made from valve metal alloys. In this paper, the principal possibility of obtaining high-quality uniform coatings on the extended internal surfaces of products, including the surfaces of deep holes, when an electrolyte solution is passed through them, is revealed. It has been experimentally proved that, based on the proposed approach, it is possible to process products with deep holes, achieving a ratio of both minimum and maximum values of the thickness and the breakdown voltage of coatings on the internal and external surfaces in a range of 0.85–0.93. A possibility of forming coatings exclusively on the internal surfaces of products without changing the state of their external surfaces, moreover, without using expensive electrolytic baths—inside internal cavities of products themselves, is shown. A good correlation between the values of the thickness and of the breakdown voltage of coatings has been established. As a result, a possibility of indirectly determine the coatings thickness based on the results of their breakdown voltage measuring has been substantiated. [ABSTRACT FROM AUTHOR]

The influence of the parameters of thermal, electrochemical, and microarc oxidation on the laws of formation of oxide barrier coatings on a VT23 titanium alloy (Ti–Al–V–Mo–Cr–Fe system) is studied. The resistance of the formed coatings to the subsequent vacuum annealing at 800°C for 1 h is examined. The coating thickness is shown to increase with the oxidation temperature or voltage. The coatings produced by thermal and microarc oxidation are most resistant to heating in vacuum, but those produced by microarc oxidation contain pores. The anodic oxide coating is completely dissolved upon vacuum annealing. [ABSTRACT FROM AUTHOR]

A MA8 magnesium alloy substrate was pre-impregnated in aqueous solutions of cerium(III) nitrate and glycerol before microarc oxidation, which was used to form oxide ceramic coatings from a silicate–alkaline electrolyte containing fluoride ions. The corrosion-protection properties of these coatings on the pre-impregnated alloy surface were assessed using potentiodynamic polarization curves. Electrochemical parameters calculated from the potentiodynamic polarization curves showed that the presence of the oxide ceramic coating on the surface of MA8 magnesium alloy significantly reduces the corrosion current density and increases the polarization resistance. It was determined that the pre-impregnation of the MA8 magnesium alloy substrate in aqueous solutions of cerium(III) nitrate and glycerol affects the mechanism of the corrosion damage. [ABSTRACT FROM AUTHOR]

Questions of raising the efficiency and controllability of the process of depositing protective microarc oxide coatings with specified properties on products made of light metals and alloys (aluminum, magnesium, titanium) are examined. An intelligent information-measurement system is developed for measuring the parameters of microarc oxide coatings which contains unique hardware, program, and information support. For the hardware support of the system a technological current source is created that specifies the technical regimes, strength and shape of the current (a sinusoidal or pulsed signal is possible) which to a great extent determines the parameters of the formed coatings. Original circuit solutions in the area of pulsed power electronics, along with modern electronic components, are used for developing the source. The technological current source has a high efficiency and provides a complete range of regulation of the parameters of the energy effect on a test sample where a coating is being formed. The developed intelligent informationmeasurement system is intended for high current measurements of the process parameters and properties of the resulting coatings; it can be used for automatic monitoring of microarc oxidation, as well as for scientific studies. Intelligent application and the knowledge bank of the system ensure intelligent choice of the optimum technological parameters of microarc processing, along with automatically controlled synthesis of microarc oxide coatings with specified properties. [ABSTRACT FROM AUTHOR]

This article provides the results of corrosion resistance tests of nanostructured oxide-ceramic coatings produced by microarc oxidation on aluminum alloys in a boric acid electrolyte. The coatings were formed on a microarc oxidization plant used in anode-cathode mode. Accelerated corrosion tests were conducted by alternating the immersion of the specimens in sodium chloride solution with their drying in the air. At the microarc oxidation of aluminum alloys in a boric acid electrolyte, the corrosion levels directly depend on microarc oxidation mode and chemical composition of oxidated alloys. At a density of 10–20 A/dm2, continuous corrosion is observed, whereas at a current density of 20–30 A/dm2 staining is observed, which is caused by the fact that corrosive medium penetrates through the pores and produces poultice corrosion. The oxide ceramic coatings on D16 alloys exhibit a weaker resistance to corrosion than on AMg2 and AD1 alloys. It has been established that the corrosion values of aluminum alloys with nanostructured oxide-ceramic coatings are severalfold as low as in uncoated alloys. [ABSTRACT FROM AUTHOR]

The effect of the duration of the microarc-oxidation process on the thickness, roughness, and optical characteristics (solar absorbance αs and emissivity ε) of white and black thermal-control coatings formed on an aluminum alloy AMg6 is studied experimentally. It is found that αs decreases with an increase in the thickness of the coatings with increasing microarc-oxidation-treatment duration. For the white coatings, a decrease in their roughness is accompanied by an increase in ε, and vice versa. For the black coatings, the main role is played by the emissivity factor of the coating, which is determined by the content of vanadium oxide in it. A comparative analysis of the optical characteristics of thermal-control coatings obtained using the microarc-oxidation process on various aluminum alloys show that it is better to form white coatings of the solar-reflector class on aluminum alloy AMg3 and black coatings of the true-absorber class on an aluminum alloy AMg6 with a microarc-oxidation (MAO) treatment duration of 25 min. The obtained experimental data can serve as a framework for the development of a basic technology for the formation of thermal-control coatings on products made of aluminum alloys. [ABSTRACT FROM AUTHOR]

Microarc Oxidation (MAO) technology can prepare a ceramic-like oxide coating. It is an effective technique to increase the wear and corrosion resistance of magnesium alloys. The MAO coating has many defects such as holes and cracks. As well, the coating on magnesium alloys typically consists mainly of MgO, which absorbs water or dissolves in a humid or acidic environment. Therefore, the MAO coatings are not adequate for the long-term protection of magnesium alloys. MAO coatings that are composed of ZrO2, or which have ZrO2 with a compact microstructure, can improve the protection of magnesium alloys. This paper critically summarizes the progress of MAO coatings containing ZrO2 on magnesium alloys from four aspects: electrolyte compositions, electrical parameters, the two-step MAO process, and post-treatment of MAO coating. The future research directions for MAO coatings containing ZrO2 on magnesium alloys are discussed. [ABSTRACT FROM AUTHOR]

Microarc oxidation (MAO) is an effective surface treatment method for Ti alloys to allow their application in extreme environments. Here, binary electrolytes consisting of different amounts of sodium phosphate and sodium silicate were designed for MAO. The surface morphology, composition, and properties of MAO coatings on Ti−6Al−4V alloy treated in 0.10 mol/L electrolyte were investigated to reveal the effect of PO43− and SiO32− on the growth kinetics of the MAO coatings, using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and potentiodynamic polarization. The results showed that PO43− is beneficial for generating microarcs and forming pores within the coating, resulting in a thick but porous coating. SiO32− facilitates the blocking of pores in the outer deposition layer and impedes the generation of microarcs, resulting in a thin dense coating. The thickness, density, phases content, and polarization resistance of the MAO coatings are primarily affected by the intensity of microarcs for low SiO32− contents, and by the number of microarcs when the SiO32− content is sufficiently high. The thickness of MAO coatings obtained in P/Si electrolytes shows a piecewise linear increase with increasing process time during the three stages of microarc discharge. SiO32− is beneficial to the density increase of the coating formed in the previous stage of microarcs discharge, but slows down the growth of the coating formed in the next stage. [ABSTRACT FROM AUTHOR]

To improve the corrosion resistance of 7075 aluminum alloy microarc oxidation (MAO) coating, graphene of 3 g/L was adsorbed and deposited in the coating. Then, the microarc oxidation–sol–gel (MAO-SG) composite coating was obtained by sealing the micropores of the coating with TiO2 sol–gel. Electrochemical tests of 7075 aluminum alloy matrix, undoped sample, doped graphene sample and sealed sample were carried out in 3.5wt.% NaCl solution. The results show that compared with the blank sample, the self-corrosion potential of the sealed sample is increased by 98.1%, and the self-corrosion current density is reduced by 4 orders of magnitude. After immersion corrosion for 720 h, the corrosion rate of MAO-SG composite coating is 3.4 × 10–5 mm/a, and its corrosion resistance is still better than that of single MAO coating. This is due to the strong conductivity of graphene and the blocking effect of TiO2 sol–gel. [ABSTRACT FROM AUTHOR]

The effects of high- and low-frequency coupled square wave pulses and concentration of TiC on TiC/Al2O3 coatings were determined through microarc oxidation on 6063 aluminum alloy. Scanning electron microscopy results showed that the "stack" of micropores and the thickness increased with the increase in TiC concentration and high-frequency pulse amplitude. X-ray diffraction and x-ray photoelectron spectroscopy results showed that the composite coatings primarily consisted of γ-Al2O3, α-Al2O3, TiC, and Al2SiO5. The TiC content increased, the surface hardness of the coatings improved from 781.5 to 1321.4 Hv, and the corrosion current density decreased from 1.497 × 10−6 to 8.979 × 10−10 A cm−2. The friction coefficient decreased from 0.52 to 0.41 at the TiC content of 3 g/L and high-frequency pulse amplitude of 30 V. The TiC content and high-frequency pulse amplitude improved the corrosion and friction performance of the TiC/Al2O3 coatings. [ABSTRACT FROM AUTHOR]

Heat transfer and critical heat flux on flat rectangular (16 × 24 mm) heat-releasing surfaces (HS) made of aluminum alloy D16 and modified by microarc oxidation (MAO) were investigated. Two types of coatings made using different electrolytes were studied: KOH and phosphates. Coatings had good adhesion to the HS and high dielectric strength. The thickness of coatings was 20 and 7 µm, respectively. The studies were carried out using freon R21 under conditions of natural convection with horizontal orientation of the HS. For comparison, measurements were carried out on identical test sections without a modifying coating: the first section was polished; the second one was ground with a roughness of 20 µm. It was shown that the heat transfer intensity on the modified HS was not lower than that on the ground HS with a roughness of 20 µm, and it was almost twice as high as that on the polished HS. The values of the critical heat flux on the studied surfaces were almost the same. The use of these types of coatings provides electrical insulation of the heat-releasing element without compromising the efficiency of heat removal. [ABSTRACT FROM AUTHOR]

The the main results of development of thermodynamic physicochemical simulation of microarc oxidation on an aluminum alloy in an atypical aqueous borate electrolyte of predefined composition under conditions of varying temperature are discussed. In the scope of this study, the appropriate methods of solution and thermodynamic databases have been developed for efficient implementation of the simulation system. The roles of each component of the aqueous borate electrolyte as well as of aluminum substrate have been substantiated, and the influence of state variables (composition and temperature) on physicochemical transformations has been determined for the conditions of microarc oxidation in borate solutions. The results define the basis for application of the obtained regularities and subsequent optimization of microarc oxidation aimed at production of coatings with preset operation properties. [ABSTRACT FROM AUTHOR]

A method for modification of calcium-phosphate coatings, formed by microarc oxidation, with Fe–Cu nanocomposite bicomponent particles is proposed. The effect of modification of calcium-phosphate coatings, formed on titanium, with Fe–Cu nanoparticles on their morphological, structural and mechanical characteristics is evaluated. It is found out that the highest adhesion strength (33 MPa) is exhibited by the coatings with a thickness of 42–50 μm, which were formed at the microarc oxidation voltage of 200 V in the electrolyte containing Fe–Cu nanoparticles. These coatings are characterized by a roughness (Ra) of 3.3 μm, a surface porosity of 21% and a higher content of calcium compared to that in the initial unmodified coating. [ABSTRACT FROM AUTHOR]

The wear resistance of sheet magnesium alloy AZ31 (Mg – 2.90% Al – 0.98% Zn – 0.13% Mn) with microarc oxidation coatings after shot peening is determined. The microstructure of the alloy is studied by scanning electron microscopy. The effect of the shot peening on formation of cracks and pores in the alloy after microarc oxidation and on the roughness of the surface is determined in order to predict the wear resistance of the alloy under dry friction. [ABSTRACT FROM AUTHOR]

The article considers the technological aspects of temperature estimation in an aluminum specimen with the possible formation of an aluminum-containing coating on its surface using the combined technology of cold gas dynamic deposition and microarc oxidation. The experimental results for the temperature flow distribution over the thickness of the metal surface under the cold gas dynamic deposition are presented. The temperature of the heat flow during spraying was varied from 200 to 600°C. It was found that the operating temperature of the aluminum substrate with a thickness of 8 mm at the standard thermal deposition modes is not more than 120°C, which is one of the advantages of the used technology over the known methods of plasma and magnetron sputtering. The temperature effect of the microarc oxidation on the surface of a specimen depending on various technological modes is estimated. It was found that the surface temperature of the specimen oxidized in an alkaline electrolyte with liquid glass additives at current densities of 20–30 A/dm2 is not more than 90°C, which shows the absence of thermal transformations. [ABSTRACT FROM AUTHOR]

The results of studying the morphology, porous structure, and corrosion and diffusion properties of boehmite-containing calcium phosphate coatings formed by microarc oxidation on titanium substrates are presented. Boehmite nanoparticles obtained as a result of the AlN hydrolysis were deposited on the surface of microarc coatings with a developed relief. With an increase in the duration of the preliminary ultrasonic treatment (UST) of the AlN suspension in the interval of 20–60 min, the size of agglomerates of boehmite nanoparticles on the coating surface is decreased from 200 to 40 μm. The surface porosity of the modified coatings and the area occupied by boehmite particles are decreased from 36 to 33% and from 27 to 10%, respectively. Agglomerates with boehmite nanoparticles after 60 min of UST of an AlN powder suspension are distributed in the coating more uniformly than after 20 min of treatment. When the duration of UST of the initial suspensions is increased, the effective diffusion coefficients of the model biological fluid in porous coatings is decreased from 7.98 × 10–11 to 7.25 × 10–11 m2/s. Modification of calcium phosphate coatings with boehmite nanoparticles by varying the duration of UST of AlN powder increases the corrosion resistance of the surface layers. [ABSTRACT FROM AUTHOR]

A thickness distribution model of a coating formed under microarc oxidation of aluminum and its alloys is considered for two extreme cases: a point cathode and a cathode parallel to the sample surface. In the case of the point cathode, the change in the thickness of the formed aluminum coatings was determined. It was experimentally established that the best results for the uniform distribution of the coating thickness in the process of microarc oxidation are provided by the condition of plane-parallel installation of electrodes (cathodes) relative to the plane of the oxidized sample. The main technological factors determining the operational properties of coatings formed in this way are considered. [ABSTRACT FROM AUTHOR]

The characteristics of the oxide layer deposited by microarc oxidation (MAO) on Al – Si alloys in a silicate-alkaline electrolyte are studied. The thermal conductivity of the layer at 50 – 120°C is determined using a special facility by the method of stationary heat flow. The thickness, the porosity, the elemental composition, the content of the amorphous phase, and the size of the crystallites in the oxide layer are determined. The results obtained are analyzed and the conditions for the lowest thermal conductivity of the oxide layer are determined. [ABSTRACT FROM AUTHOR]

This work presents the results of studies of structural and physicomechanical characteristics of oxide-ceramic coatings obtained on aluminum by microarc oxidation in an alkaline silicate electrolyte with various contents of silicate ions using alternating and direct currents. [ABSTRACT FROM AUTHOR]

Three types of surface treatments were performed on AZ80 magnesium (Mg) alloy by laser shock peening (LSP), microarc oxidation (MAO), and laser shock peening followed by microarc oxidation (LSP/MAO). The stress corrosion resistance of treated and untreated specimens was carried out using slow strain rate tensile testing at a strain rate of 5 × 10−7/s in simulated body fluid. The influence of microstructure on the stress corrosion behavior was analyzed by x-ray diffraction, transmission electron microscopy, scanning electron microscopy, and electrochemical measurement. The results showed that the LSP/MAO specimen has a higher stress corrosion resistance compared to the AZ80 Mg alloy substrate, the LSP-treated specimen, and the MAO-treated specimen. The factors improving the stress corrosion resistance of the LSP/MAO specimen are its nanocrystalline material, compressive residual stress, and favorable (0002) basal texture of its reconstructed layer. [ABSTRACT FROM AUTHOR]

There has been a constant requirement from the clinic to develop biomedical titanium (Ti) implants with high osteogenic ability. In this study, we clarified a novel mechanism of how MAO (microarc oxidation) coating of Ti implants facilitates osteogenic differentiation of human bone marrow mesenchymal stem cells (hB-MSCs) by activating ERK1/2-miR-1827-Osterix signaling pathway in vitro. MAO surface of titanium implant was more favorable to promote osteogenic differentiation than SLA and AOS coating. Besides, titanium implants regulated hB-MSCs osteogenesis through the p38 MAPK pathway and ERK1/2 might be the most efficient target. Furthermore, MAO coating induced osteogenic differentiation though ERK1/2-miR-1827 pathway. Finally, we verified miR-1827 regulated osteogenic differentiation partially through Osterix. Our study reveals novel insights that MAO surface of titanium implant is a prior choice for biomedical trial and for its use in periprosthetic osteolysis (PIO) treatment in an evidence-based rationale. [ABSTRACT FROM AUTHOR]

The paper deals with one of the most promising, intensively developing methods of depositing functional oxide coatings onto aluminum and titanium alloys, namely microarc oxidation (MAO) performed in the pulsed current mode. MAO layers can be obtained due to both oxidation of the substrate material and thermochemical transformations of electrolyte components and their subsequent melting on the part surface. It is shown how the black coating appears on the surface of aluminum and titanium alloys due to the MAO process. The structure, phase and elemental compositions of the resulting coatings are studied in this paper. [ABSTRACT FROM AUTHOR]

The effect of nickel, cobalt, and iron sulfates' additives in the alkali-silicate electrolyte for microarc oxidation on the element composition and electrochemical behavior of MAO-coatings formed on titanium has been studied. It has been established that these additives were incorporated into an oxide layer. The increase of the content of these components in the electrolyte resulted in the decrease of their content (along with silicon) in the coating and the increase of the titanium content. This dependence can be related to the MAO process mechanism specifics and, first of all, to changes in temperature–time characteristics of microarc discharges. [ABSTRACT FROM AUTHOR]

The possibility of the application of a mobile unit with several travelling electrodes to form oxide coatings via the microarc oxidation on various surfaces of large-scale articles made of titanium alloys has been demonstrated. The application of several electrodes has promoted improving the treatment performance and forming more homogeneous oxide layer on the treated surface. [ABSTRACT FROM AUTHOR]

The problem of increasing the antifriction properties of oxide-ceramic coatings obtained by microarc oxidation on the surface of aluminum alloys is considered. This problem was solved by the impregnation of oxide-ceramic coatings with solutions of polymers (oligomers), which, upon further heat treatment and polymerization, give the oxide-ceramic coatings antifriction properties. Laboratory tests of the tribotechnical properties of the obtained polymer–ceramic coatings were carried out. The areas of possible practical application of the obtained polymer-ceramic coatings are considered. [ABSTRACT FROM AUTHOR]

The dielectric properties of coatings on AK6 alloy formed with a microarc oxidation method in two electric modes in alkaline-silicate electrolytes are estimated. It is shown that both modes, a galvanostatic mode and an arbitrarily falling power mode in alternating current circuits, make it possible to obtain coatings with a thickness of 30–60 μm, with sufficiently high electrophysical parameters: bulk specific resistance ρv = 3–9 × 109 Ω m and dielectric strength E = 9–14 V/μm. It is established that higher values of ρv and E in the both modes can be achieved in the solutions of 1 g/L KOH + 6 g/L of liquid glass (LG) and 12 g/L of LG. In terms of absolute value, the parameters of the coatings formed in the mode of arbitrarily falling power exceed the same characteristics of the oxide layers formed under conditions of galvanostatic mode by a factor of 1.5–2.5. [ABSTRACT FROM AUTHOR]

The paper presents experimental data on the determination of the tribological properties of commercially pure titanium with different microstructures, with and without a coating. As a result of the performed experiments, it was established that the integral value of the friction coefficient, as well as its adhesive component, are structurally sensitive parameters. It is noted that the ultrafine-grained structure obtained as a result of severe plastic deformation contributes to a reduction in the friction coefficient, as well as to an increase in the load-bearing capacity of the triboconjugation. [ABSTRACT FROM AUTHOR]

Microarc oxidation (MAO), an electrochemical process that employs aqueous electrolytes, is adopted to impart corrosion resistance to magnesium alloys. To study the influence of the inhomogeneity in the microstructure on the characteristics of the coating and therefore on the properties, the weld of a ZM21 alloy was considered. The surfaces of the weld metal and the base metal were treated using MAO with silicate and phosphate-based electrolytes. Corrosion behavior of the coated and uncoated base metal and weld metal in 0.1M NaCl was compared using potentiodynamic polarization tests. The results showed that the inhomogeneity in the microstructure of the substrate altered the nature of the coatings and their corrosion resistance was not the same for the electrolytes used in MAO. While for the silicate-based electrolyte, the coated weld metal showed better corrosion resistance than the base metal; a reversal in the corrosion property was noticed with phosphate-based electrolyte. [ABSTRACT FROM AUTHOR]

A self-sealing bioceramic coating was prepared on AZ80 magnesium alloy by one-step microarc oxidation (MAO) in an alkaline silicate solution in the presence of HA nanoparticles and K2TiF6. The influence of oxidation time on the microstructure and corrosion resistance was investigated. The surface and cross-sectional morphologies of the coatings were characterized by scanning electron microscopy, while their phase structure and chemical composition were obtained by x-ray diffraction analysis and energy-dispersive spectroscopy, respectively. The corrosion behavior of the coatings was evaluated by electrochemical measurements in a simulated body fluid. The results show that the self-sealing bioceramic coatings are mainly composed of MgO, MgF2, Ti3O5, Mg2SiO4, and HA phases, which effectively improve the corrosion resistance of the AZ80 magnesium alloy. The added K2TiF6 plays a key role in producing self-sealing coating, while the HA nanoparticles improve the bioactivity of the coating. Thus, the self-sealing bioceramic coating obtained by the one-step MAO process may develop into a suitable surface treatment technology for bone implants in clinical application. [ABSTRACT FROM AUTHOR]

The formation of oxide coatings on D16 aluminum alloy by microarc oxidation (MAO) treatment in the anodal-cathodal mode with discharge interruption in the basic silicate-alkali electrolyte (composed of three solutions) and in the mode without charge interruption in a silicate-alkali electrolyte of similar composition is studied. We find that the mechanism of formation of oxide ceramic coatings depends on the mode of MAO treatment. Addition of sodium tetraborate to the electrolyte has an impact on the properties of oxide ceramic coatings fabricated using the mentioned treatment modes. We demonstrate that MAO treatment in the anodal-cathodal mode with discharge interruption gives rise to a higher content of high-temperature aluminum oxide phase (α-Al2O3) in coatings and enables the formation of thin (up to 25 μm) oxide ceramic coatings that retain all unique properties of this type of coatings. [ABSTRACT FROM AUTHOR]

A computational approach to the determination and evaluation of internal stresses arising in functional coatings on the parts of machines during operation at high temperatures is explained. The method for calculation of temperature on the coating–metal base adhesion boundary is suggested in this study. The possibility of controlling acceptable mechanical stresses in the coating depending on its thickness and operating temperature, as well as the chemical and phase composition of the material of base and coating, is explained. [ABSTRACT FROM AUTHOR]

FGUP TsNII KM "Prometey" have proposed a new approach to creating porous functional ceramic coatings on the surface of steel by means of low-temperature heterophase transport combined with microarc oxidation (MAO). The adhesion of the coating to metal is at least 60 MPa. An intermediate layer of the coating contains nickel-aluminum intermetallide compounds and imparts mechanical strength during service at temperatures above 750°C. It is shown for the example of nickel that catalytic active substances with promoters can be deposited on the surface of porous coatings from solutions of their salts or hydroxides followed by decomposition as a result of heat-treatment; metal can also be deposited into the pores of a MAO coating. [ABSTRACT FROM AUTHOR]

In this work, microarc oxidation (MAO) technology was used to form oxide ceramic coating on the surface of aluminum alloy. The combined additives nano-TiO2 and nano-ZnO were added into the silicate electrolyte, and the effect of the compound nano-additive on microstructure and properties of MAO coating was investigated. The results show that compared with those of the nano-additive-free coating formed on aluminum alloy, the thickness, hardness, abrasion resistance and corrosion resistance of the nano-additive-containing coating are obviously improved. The surface of coating with nano-additive becomes smooth, dense, and there are less porosities and microcracks. Moreover, the content of crystal phase α-Al2O3 and γ-Al2O3 increases visibly on the nano-additive-containing MAO coatings, and new phases Al3Ti and Zn0.6Ti0.4 are detected in the coatings, which are mainly contributed to the excellent corrosion resistance and abrasion resistance of the film. When the contents of nano-TiO2 and nano-ZnO are, respectively, 4 and 2 g·L−1, the film has better comprehensive performance, the thickness and hardness of the film could reach 52 μm and HV 692, respectively. [ABSTRACT FROM AUTHOR]

Microarc oxidized coatings on Ti6Al4V alloy are produced in a basic Na2SiO3-(NaPO3)6-NaAlO2 electrolyte with different concentrations of sodium tungstate. The effects of sodium tungstate on the characteristics of the coating are investigated through current-time response, microstructure, composition, hardness, bonding strength and antifriction measurements. The coating is mainly composed of Al2TiO5, TiO2, Al2O3 and amorphous substances regardless of the concentration of sodium tungstate. The sodium tungstate is transformed into trace amounts of WO3, which significantly promotes the growth of the coatings. The bonding strength and antifriction performance of the coatings are enhanced with the concentration of sodium tungstate less than 4 g/L, whereas excessive sodium tungstate results in a coarse outer layer with more microdefects, which deteriorates the wear resistance of the coating. Based on the results of scratch and wear test with energy-dispersive x-ray spectroscopy of the worn surface and debris, the optimal concentration of sodium tungstate doped into the electrolyte is identified, and this concentration yields superior tribological property. Excellent wear resistance is achieved by combining a high bonding strength and hardness with a less defective outer layer. [ABSTRACT FROM AUTHOR]

Formation of vanadium-containing oxide ceramic coatings in the stagewise microarc oxidation process on the surface of the D16 aluminum alloy is studied. Conclusions as to the mechanism of their growth are made on the basis of analysis of the change in the element composition and surface morphology of the coatings. The prospects of using such coatings as catalytically active systems are shown. [ABSTRACT FROM AUTHOR]

The influence of nickel sulfate additives to acid and basic electrolytes for microarc oxidation on the structure, composition, and properties of coatings formed on VT1-0 titanium were studied to produce poorly soluble titanium anodes. We established the possibility of incorporating nickel (nickel oxide) into the composition of coatings. X-ray diffraction analysis showed that the coatings contained nickel oxide β-NiO with a cubic lattice, TiO2 in the form of rutile, and SiO2 in the form of high-temperature β-cristobalite. The maximum thickness and nickel content in the surface layer and the minimum values of both the anodic dissolution currents and the electrical strength were obtained for the coatings formed in silicate-alkali (3 g/L KOH + 4.5 g/L Na2SiO3) electrolyte with the addition of 1 g/L of nickel sulfate NiSO4. [ABSTRACT FROM AUTHOR]

The review is devoted to microarc (or plasma-electrolytic) oxidation (MAO) of carbon-graphite materials with the purpose of protecting their surfaces from oxidation, primarily, high-temperature oxidation. It is found that the formation of protective coatings on graphite under certain conditions occurs in accordance with trends similar to the formation of oxide layers on valve metals. The phenomenological model of the mechanism of formation of MAO coatings on graphite is given. In particular, the results of our own studies are described, as well as the data on the achieved level of characteristics of obtained MAO coatings are given. [ABSTRACT FROM AUTHOR]

The morphology and the structure of zinc- or copper-containing calcium phosphate coatings deposited by the microarc oxidation (MAO) method at different voltages on pure titanium and low-modulus Ti-40 wt % Nb alloy substrates are investigated. The morphology of the MAO coatings on both substrates is composed of sphere-shaped structural elements 8-42 μm in size and pores 1-15 μm in size. When the process voltage is increased from 200 to 300 V, these structural elements grow and partially destruct. It is ascertained that the MAO voltage increase leads to the linear increase in the surface porosity of the coatings from 14 to 24%. It is established that the effective diffusion coefficients of the model biological fluid in porous coatings vary from 0.85 × 10-10 to 9.0 × 10-10 m2/s. Under the increase in the structure element size, the effective diffusion coefficient of the model biological fluid increases in the MAO coatings deposited on a titanium substrate and decreases in those deposited on Ti-40 wt % Nb alloy. The observed difference is related to the increase in the crystalline phase fraction. [ABSTRACT FROM AUTHOR]

A promising duplex coating was prepared by microarc oxidation (MAO) and enameling processes onto polished Ti6Al4V alloy. The TiO ceramic coating deposited by MAO was characterized and then combined with an enameling treatment in order to improve the tribological and corrosion resistance of Ti6Al4V alloy. The morphology, phase composition, and hardness of MAO and MAO/enameling-coated Ti6Al4V alloy were evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Vickers microhardness tester, respectively. The tribological performance was investigated using a ball-on-disk tribometer. The corrosion resistance was studied using immersion tests and potentiodynamic polarization. Wear tests show that the enamel coating on the MAO-coated surface causes a reduction in the friction coefficient. Immersion tests demonstrate that the duplex coating is more effective in improving the corrosion resistance of Ti6Al4V than the MAO coating especially at high temperature (80 °C). Potentiodynamic polarization curves reveal that the corrosion potential of the duplex coating increases by about 250 mV and the corrosion current density is slightly lower than that of the MAO coating. The duplex coating is superior to the stand-alone MAO coating in improving the tribological and corrosion behavior of Ti6Al4V. [ABSTRACT FROM AUTHOR]

Ceramic-like coatings with a thickness of up to 40 μm are formed on aluminum composites without additives and with copper additives (1 and 4.5%) in a silicate-alkaline electrolyte by microarc oxidation. The composites are prepared by powder metallurgy (cold pressing and sintering in forevacuum). An increase in the copper concentration in the composites to 4.5% leads to the retardation of anode voltage growth on the initial stage of oxidation corresponding to the formation of a barrier layer. The coatings are studied by scanning electron microscopy, X-ray microanalysis, X-ray photoelectron spectroscopy, and X-ray diffraction. The morphology of their surface corresponds to the morphology of the surface of coatings on compact aluminum alloys. According to X-ray photoelectron spectroscopy, a thin 1-μm layer forms on the surface. It consists predominantly of electrolyte components. X-ray diffraction analysis shows that the coatings mainly consist of γ-Al2O3 oxide as well as the η-Al2O3 phase, the peaks of which are broadened. This broadening is characteristic of the amorphous component and may be due to the presence of nanocrystalline regions in the coating structure. In the coatings on the composite Al + 4.5% Cu, mullite Al2SiO5 and copper oxide CuO are also found. The excess aluminum content may be associated with residual unoxidized aluminum inclusions in the structure of the coatings. [ABSTRACT FROM AUTHOR]

A model of the growth of a calcium phosphate coating on a zirconium substrate with respect to the formed oxide sublayer and formation of phases is proposed. The influence of the parameters of the model on the growth of the coating and the evolution of the stresses in the oxide layer and in the coating is investigated. It is shown that the content of the substances formed in the coating increases with the increasing constants of the reaction rates, increasing diffusion coefficients, or with a decreasing voltage of the process. It is shown that the stresses in the vicinity of the interface (the substrateoxide layer) are higher when there is an oxide layer than in its absence. [ABSTRACT FROM AUTHOR]

In this work, the morphology, phase composition, and corrosion properties of microarc oxidized (MAO) gas tungsten arc (GTA) weldments of AZ31 alloy were investigated. Autogenous gas tungsten arc welds were made as full penetration bead-on-plate welding under the alternating-current mode. A uniform oxide layer was developed on the surface of the specimens with MAO treatment in silicate-based alkaline electrolytes for different oxidation times. The corrosion behavior of the samples was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy. The oxide film improved the corrosion resistance substantially compared to the uncoated specimens. The sample coated for 10 min exhibited better corrosion properties. The corrosion resistance of the coatings was concluded to strongly depend on the morphology, whereas the phase composition and thickness were concluded to only slightly affect the corrosion resistance. [ABSTRACT FROM AUTHOR]

The properties of the hardened surface layers formed by microarc oxidation on Al-Si alloy billet are investigated. The microhardness, thickness, and porosity of the surfaces formed depend on the alloy structure. [ABSTRACT FROM AUTHOR]

The composite coatings containing HBN were prepared on 2024 aluminum alloy by microarc oxidation in the electrolyte with nano-HBN particles. The microstructure, surface roughness, phase composition, hardness, adhesion strength and wear resistance of composite coatings were analyzed by SEM, EDS, laser confocal microscope, XRD, Vickers hardness tester, scratch test and ball-on-disc abrasive tests. The results revealed that composite coatings were mainly composed of γ-AlO, α-AlO, mullite and HBN. With increasing the content of HBN particles in the electrolyte, the size and number of the pores on the surface of composite coatings decreased significantly. Compared to the MAO coatings without HBN, the composite coatings exhibited better wear resistance, as demonstrated by the lower friction coefficient and the lower wear rate. [ABSTRACT FROM AUTHOR]

Using prototype human synovial fluid, we have synthesized a calcium phosphate nanopowder consisting of two crystalline phases (whitlockite and carbonate-containing hydroxyapatite). The nanopowder was used to produce calcium phosphate (monetite) coatings on titanium substrates by microarc oxidation. It has been shown that, with increasing microarc oxidation voltage (from 200 to 300 V), the mass, thickness, roughness parameter, and Ca : P ratio of the coatings increase linearly. Increasing the voltage to 300 V and the calcium phosphate coating growth time to 10 min allows a single-phase coating up to 100 μm in thickness, consisting of monetite nanocrystals, to be produced on a metallic surface and its adhesion strength to be improved. [ABSTRACT FROM AUTHOR]