Micro-arc Oxidation Coatings Research Articles

Effects of current density on the corrosion resistance of micro-arc oxidation coatings on Ti6Al4V welded joints

Effects of current density on the corrosion resistance of micro-arc oxidation coatings on Ti6Al4V welded joints

Tribological Properties of 7A04 Aluminum Alloy Enhanced by Ceramic Coating

The 7A04 Al alloy is a commonly used lightweight metal material; however, its low wear resistance limits its application. In this study, the wear resistance of this alloy was improved by preparing micro-arc oxidation (MAO) coatings, MAO/MoS2 composite coatings, and hard-anodized (HA) coatings on its surface. The friction and wear behaviors of these three coatings with diamond-like coated (DLC) rings under oil lubrication conditions were investigated using a ring–block friction tester. The wear rates of the coatings on the block surfaces were determined using laser confocal microscopy, and the wear trajectories of the coatings were examined using scanning electron microscopy. The results indicated that, among the three coatings, the MAO/MoS2 coating had the lowest coefficient of friction of 0.059, whereas the HA coating had the lowest wear rate of 1.47 × 10−6 mm/Nm. The MAO/MoS2 coatings exhibited excellent antifriction properties compared to the other coatings, whereas the HA coatings exhibited excellent anti-wear properties. The porous structure of the MAO coatings stored lubricant and replenished the lubrication film under oil lubrication. Meanwhile, the introduced MoS2 enhanced the densification of the coating and functioned as a solid lubricant. The HA coating exhibited good wear resistance owing to the dense structure of the amorphous-phase aluminum oxide. The mechanisms of abrasive and adhesive wear of the coatings under oil lubrication conditions and the optimization of the tribological properties by the solid–liquid synergistic lubrication effect were investigated. This study provides an effective method for the surface modification of Al alloys with potential applications in the aerospace and automotive industries.

Effects of ultrasonic treatment and current pulse frequency on properties of micro-arc oxidation coating on Mg alloy LAZ931

To enhance corrosion resistance of magnesium-lithium (Mg-Li) alloy LAZ931 in corrosive environments, a micro-arc oxidation (MAO) based surface treatment was carried out under a unipolar pulsed power supply. Effects of ultrasonic treatment and current pulse frequency on the performance of MAO coatings were investigated using scanning electron microscopy, electrochemical characterization, and friction wear tests. Results demonstrate that ultrasonic treatment greatly reduced the number density of micro-defects in the resulting MAO coatings and improved their overall performance. Corrosion and wear resistance of the MAO coating increased when current pulse frequency increased from 400 to 2000 Hz. Those findings are beneficial for improving the performance of MAO on Mg-Li alloys through optimizing the processing parameters.

Simultaneous improvement of wear and corrosion resistance of microarc oxidation coatings on ZK61 Mg alloy by doping with ZrO2 nanoparticles

Simultaneous improvement of wear and corrosion resistance of microarc oxidation coatings on ZK61 Mg alloy by doping with ZrO2 nanoparticles

Synergistic Effect of Polyurethane on Pore-Sealing and Lubrication of Microarc Oxidation Coating.

Microarc oxidation (MAO) is a widely used surface treatment technology. However, its processing inevitably leads to the presence of micropores, microcracks, and other defects in the MAO coating. These defects lead to suboptimal tribological performance and diminished corrosion resistance of the MAO coating. Thus, this study proposed a new pore-sealing method with environmental protection and high efficiency. Polyurethane reactive (PUR) composite coating was prepared on the surface of MAO coating. The sealing effect of the PUR sample was analyzed using scanning electron microscopy (SEM). The molecular structure and elemental changes of the coating were analyzed using X-ray photoelectron spectroscopy (XPS). The tribological properties were analyzed using a tribological testing machine, and the corrosion resistance was tested using an electrochemical workstation. The test results show that the lubricity and corrosion resistance of PUR sample are improved significantly compared with MAO coating. When the mass fraction of PUR is 3 wt %, the PUR sample shows the best lubrication performance, and the coefficient of friction decreases by 70.8%. Compared with the traditional sealing technology, this experimental method is simple and low-cost and has a wide application prospect, which promotes the development of MAO technology.

Influences of Sr(OH)2 on the Structure and Corrosion Resistance of Micro‐Arc Oxidation Coatings Formed on TC4 Titanium Alloy

ABSTRACTIn order to improve the comprehensive properties of TC4 titanium alloy, micro‐arc oxidation (MAO) coating is prepared on the surface of a TC4 substrate in a basic electrolyte with varying concentrations of Sr(OH)2. The surface morphology, phase composition, element distribution, and corrosion resistance of MAO coatings are analyzed by scanning electron microscopy, X‐ray diffraction, an energy‐dispersive spectrometer, the electrochemical test, and the erosion–corrosion test. The results show that the voltage of the MAO process increases and then decreases; the surface morphology and density of the coatings are greatly improved after doping Sr(OH)2. When the concentration of Sr(OH)2 is 0.6 g L−1, the thickness, hardness, and adhesion strength of the coating reach the maximum values of 37.04 μm, 836.2 HV, and 21.8 N, respectively. At this time, the wear resistance and corrosion resistance are the best, and the friction coefficient and the corrosion rate are the lowest: 0.3352 and 1.04 × 10−10 mm a−1, respectively.

Effect of adding Sr(OH)2 on the formation process of 2024 aluminum alloy micro-arc oxidation coatings

Micro-arc oxidation coatings with oxidation times of 15 S, 30 S, 60 S, 120 S, 240 S and 1800 S were prepared on 2024 aluminum alloy using silicate-phosphate electrolytes containing 0 g/L and 0.5 g/L Sr(OH)2, respectively. The effects of adding Sr(OH)2 on the oxidation voltage, surface morphology and phase composition of the 2024 aluminum alloy MAO coatings formation process were investigated using SEM, XRD and XPS. The results show that adding Sr(OH)2 increases and then decreases the oxidation voltage compared with that of the unadded Sr(OH)2, which has little effect on the morphology of the oxide coatings, the Sr2+ and PO43- in the solution are jointly involved in the formation of the MAO coatings. The main phase composition of the coating is γ-Al2O3, and the Sr element is uniformly distributed as SrCO3 in the coating.

Exploring the friction and wear characteristics of MAO and MoSe2 tailored micro-pocket interface across wide temperature range

For realizing friction reduction and wear resistance across a wide temperature range, this study proposes a novel interface structure combining micro-arc oxidation (MAO) coatings on titanium alloy with MoSe2 nanoparticles. Focus on investigating the tribological behavior of this structure across a wide temperature range (RT–500 °C) and its response to temperature. Through detailed characterization, the friction-reducing and lubricating mechanisms during wear were elucidated. The results demonstrate that this structure consistently exhibits friction reduction and lubrication capabilities, with low-shear MoSe2 particles and molybdenum oxides enhancing the micro-pocket structure. This research provides valuable insights for developing integrated self-lubricating interfaces and improving the high-temperature tribological performance of MAO coatings.

Tailored micro-arc oxidation coatings to match with parylene for enhanced tribological performance of Ta-12W alloys

To improve the tribological properties of tantalum-tungsten alloys, dual-layer composite coatings are designed using micro-arc oxidation (MAO) and chemical vapor deposition (CVD) techniques. By adjusting the concentrations of NaOH and NaF additives as well as the oxidation mode (constant current/voltage) of MAO process, the microstructure and properties of underlying MAO layers are tailored for better matching with the top parylene layer, aiming to develop composite coatings capable of service under heavy loads. A comprehensive study is conducted to evaluate the effect of these parameters on the growth, microstructure, bond strength, and mechanical properties of MAO coatings, along with their subsequent impact on the friction and wear behavior under various loads for composite coatings. The findings reveal that increasing the concentration of either NaOH or NaF significantly increases the coating thickness, surface porosity, crystallinity, and hardness of MAO coatings, by promoting the spark discharge activities. However, distinct interfacial growth behavior is observed due to anionic effects caused by OH− and F−. The oxidation mode plays a crucial role in determining the mechanical quality of coatings; the constant voltage mode facilitates dense coatings with high adhesion due to its self-repairing mechanism. Furthermore, wear tests confirm that the enhanced mechanical properties and open-porosity of the MAO coatings markedly improve the tribological performance of composite coatings. This improvement is attributed to the excellent load-bearing capacity provided by hard MAO layers, as well as the anchoring and storage capabilities of the textural porous surfaces. The optimized composite coating demonstrates long-lasting low friction even under heavy loads up to 80 N.

Effects of nanoceramic particle size and content on the wear resistance of micro-arc oxidation coatings for 2A12 aluminum alloy

The 2A12 aluminum alloy, renowned for its exceptional mechanical properties, encounters significant limitations in applications involving abrasive environments or frequent contact with other surfaces due to its inadequate wear resistance. This shortcoming substantially reduces the service life of components and escalate maintenance costs, highlighting the urgent need for advanced surface modification techniques. This study meticulously investigates the influence of nanoceramic particle size and content on the wear resistance of micro-arc oxidation (MAO) coatings, a surface modification technique renowned for forming dense, adherent ceramic layers on aluminum alloys. The systematic experimentation revealed that the incorporation of 150 nm α-Al2O3 nanoparticles into the MAO matrix achieves an optimal dispersion, leading to a substantial enhancement in wear resistance. The sample with a 5 g/L concentration of 150 nm α-Al2O3 nanoparticles doping in the MAO electrolyte demonstrated the lowest mass loss, underscoring its enhanced wear resistance. The enhanced microstructure, enriched with hard ceramic phases such as α-Al2O3 and mullite, plays a pivotal role in withstanding wear. Moreover, this study identified that an optimal balance of surface roughness and coating thickness is essential for achieving enhanced wear resistance. The findings of this research provide a strategy for the design of wear-resistant coatings tailored for high-performance applications across aerospace, automotive, and general engineering industries. By optimizing the particle size and concentration in MAO coatings, this study paves the way for the development of durable materials capable of thriving in demanding environments, thereby extending the service life and reducing maintenance requirements of components.

Enhancing Corrosion Resistance and Antibacterial Properties of ZK60 Magnesium Alloy Using Micro-Arc Oxidation Coating Containing Nano-Zinc Oxide

Enhancing Corrosion Resistance and Antibacterial Properties of ZK60 Magnesium Alloy Using Micro-Arc Oxidation Coating Containing Nano-Zinc Oxide

Influence of triangular texture composite MAO coating on the tribological properties of aluminum alloys

To compensate for the poor friction and wear performance of aluminum alloys, in this study, four triangular textures with different area occupancy rate (5.16 %, 9.17 %, 20.63 % and 28.07 %) were prepared on the surface of aluminum alloys by laser texture technology, subsequently micro-arc oxidation (MAO) coatings were prepared using MAO technique. The treated samples were characterised by X-ray diffraction, micro-morphological analysis, optical profiler and energy dispersive spectroscopy. The tribological performance under oil lubrication conditions was tested by using a pin-disc contact rotational friction wear tester. The results showed that the sharp corners at the edges of the texture caused current concentration, which resulted in more intense sparking and accelerated MAO growth, thus creating more pores and roughening the surface of the coating. In terms of tribological performance, the triangular texture with 9.17 % area occupancy rate showed the best tribological performance before MAO coating, compared with the substrate, the average friction coefficient decreased by 64.11 %; and after the MAO coating was applied, the triangular texture composite MAO coating with 9.17 % area occupancy rate still had the best tribological performance, and its average friction coefficient further decreased compared with that of the smooth MAO coating by 33.07 %. The analysis suggests that the texture can store abrasive particles to prevent the MAO coatings from being worn through, leading to coating failure, while the hard MAO coatings can resist contact stresses well enough to prevent the texture from being flattened, leading to the failure of the function of storing abrasive debris. Therefore, the integration between texture and MAO coating can greatly enhance the anti-wear properties of aluminum alloys.

A Study on Tailoring Silver Release from Micro‐Arc Oxidation Coating Fabricated on Titanium

This study is initiated with the aim of regulating the release of silver (Ag) as an antibacterial agent from the micro‐arc oxidation (MAO) coating. Herein, an external 5 wt% Tin(II) chloride (SnCl2) containing biodegradable polycaprolactone (PCL) layer is formed on the 0.8 wt% Ag‐incorporated MAO coating by the dip coating method. 5 wt% SnCl2 addition into PCL provides a steady release of Ag into concentrated simulated body fluid (1.5X SBF) from the underlying MAO coating at 37 °C. When the Ag release rate is taken into consideration, it is quantified as 0.0089 and 0.0586 ppm day−1 for PCL‐covered MAO and PCL‐free MAO coatings, respectively. It is finally concluded that the preliminary result of this study can be promising for minimizing the in vivo adverse effects of Ag+ ions arising from rapid release as well as maintaining antibacterial efficacy for prolonged periods, which is ideal for preventing the risk of postimplantation infections.

Influence of ultrasonic power modulation on the optimisation of aluminium alloy micro-arc oxidation coating properties

Conventional micro-arc oxidation (MAO) technology, due to the randomness and transient nature of arc discharge, produces coatings with numerous defects, poor abrasion resistance, and inadequate ability to effectively block the intrusion of corrosive media. In this paper, ultrasonic-assisted technology was used to study the effect of different ultrasonic power regulation on the performance optimization of MAO coating on 6061 aluminum alloy. The coating microstructure and elemental composition were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometer, and its hardness, wear resistance, corrosion resistance and other properties were tested. The results show that the prepared coating has the characteristics of high hardness, good wear resistance and excellent corrosion resistance under the assistance of ultrasonic power of 50 W. This is mainly attributed to the cavitation effect, mechanical effect and thermal effect generated by the corresponding ultrasonic power, which effectively enhances the surface quality, performance and service life of the coating. This study provides valuable insights into the interaction between ultrasound power and the resulting MAO coatings, and has important implications for optimizing ultrasound-assisted processes and improving coating properties.

Hysteresis Phenomena at Boiling in Liquid Film Flowing down the Tubes with Microarc Oxidation Coating

Hysteresis Phenomena at Boiling in Liquid Film Flowing down the Tubes with Microarc Oxidation Coating

Optimization of tribological properties and corrosion resistance of MAO coatings on LY12 aluminum alloy by co-doping with graphite particles and in situ formation of zinc phosphate

Micro-arc oxidation (MAO) coatings can enhance the wear and corrosion resistance of aluminum alloys, but the high coefficient of friction and surface pores can lead to failure in the field. In this study, zinc acetate and graphite particles are introduced to the electrolyte to form corrosion-resistant and lubricating MAO coatings on the LY12 aluminum alloy. Zinc phosphate nanocrystals formed in situ are uniformly distributed in the amorphous coating, while the extrinsic graphite is mainly located in the amorphous structure. Compared to the aluminum alloy substrate, the friction coefficient of the optimal MAO coating decreases to ∼0.2, and the wear rate decreases by nearly 10 times. Zinc phosphate nanocrystals generated in situ in the coatings release Zn2+ during corrosion attack to cover the vulnerable weak corrosion areas around the pores. The corrosion resistance is improved significantly, as manifested by the increase of the corrosion potential from −1.290 V to −0.811 V and the decrease of the corrosion current density from 3.172 × 10−6 A cm−2 to 4.320 × 10−10 A cm−2. The co-doped MAO coatings have better wear and corrosion resistance and are more suitable than the LY12 alloy in many engineering applications.

Growth mechanism and performance of MAO-AO composite coating obtained by two-stage process

Titanium alloys, with the high specific strength and low elastic modulus, are currently the research focus of many applications. Various surface treatments have been applied with the intention of improving their wear and corrosion resistance. Micro-arc oxidation (MAO) technology originated from anodic oxidation (AO) technology, which has the advantages of simple process, high efficiency and environmentally friendly electrolyte. However, due to the micro-arc discharge effect, MAO coatings inevitably show the characteristic of high porosity. In this study, post-treatment of MAO coatings using AO was proposed to obtain a composite coating with both high hardness and low roughness and porosity thereby compensating for the defects of MAO coatings. After preparation, each sample was analyzed and characterized. The results indicated that AO was main conducted in the discharge channel of MAO coating and an amorphous phase coating with the matrix oxide as main component was grown in situ. The composite coating showed a slight increase in thickness and significant reduction in roughness than MAO coating and with superior wear and corrosion resistance. In conclusion, this study provides new idea and data support for improving the performance of MAO coatings, which is expected to be widely used in the field of surface treatment.

Study of optical characteristics of microdischarges in the micro-arc oxidation process

Micro-arc oxide coatings of aluminum alloy AD31 samples were formed in an electrolyte containing 2 g/l NaOH and 9 g/l Na2SiO3 for 2, 4, 8, 16 min in the anode and anode-cathode modes. During the processing, the forming curve, the time dependences of the charge passed through the galvanic cell, and the optical parameters of the microdischarges were measured using the optical synchronization method developed by the authors with subsequent image recognition. The thickness of the coatings was measured using a point autofocus probe surface texture measuring instrument Mitaka PF-60, the surface morphology was studied using a VEGA3 scanning electron microscope using SBH surface topography. The elemental composition of the coatings was determined using a scanning electron microscope JSM-6610LV. The analysis of the elemental composition and morphology of the surface revealed that the inner layer of synthesized coatings consists of Al2O3, the outer layer consists of Al2O3·SiO2 mullite, and the ratio of phases Al2O3 and mullite changes with changes in current density and oxidation mode. The formation of mullite is due to the presence of Na2SiO3 in the electrolyte. It is shown that with increasing processing time and current density, the thickness, roughness and porosity of coatings increase. The interrelation of the optical parameters of microdischarges with the morphology of the surface and the elemental composition of the formed coatings is substantiated; it is shown that the ratio of illuminated and non-illuminated sections of the sample surface by microdischarges can be used as a rough estimate of the ratio of electron and ion currents corresponding to microplasma and electrochemical processes. A mathematical model describing the dependence of the coating thickness on the oxidation time based on Faraday's laws for electrolysis and the results of measuring the optical parameters of microdischarges and the electrical parameters of the MAO process without taking into account microplasma processes that do not lead to an increase in the thickness of coatings is proposed. The error of adequacy of the proposed model does not exceed ±10 %. The results of the study can be used in the development of a digital twin of the micro-arc oxidation process.

Research on micro-arc oxidation method based on passivation layer on non-valve metal low-carbon steel surface

In this study, a method for preparing micro-arc oxidation (MAO) coatings on the surface of non-valve metal low-carbon steel based on a passivation layer is proposed. Microstructure and elemental composition of the surface and cross-section were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive spectroscopy (EDS). Micro-scratch tests, tribological tests and electrochemical corrosion tests were conducted. Compared to MAO coatings, coatings obtained by pre-passivation treatment exhibit fewer surface micropores with smaller pore sizes, and both wear resistance and corrosion resistance are enhanced. In summary, the method for preparing MAO coatings based on the passivation layer on the surface of low-carbon steel provides feasible approach to improving the drawbacks of low-carbon steel.

Effect of electrolyte pH value on microstructure and corrosion resistance of black MAO coating on 6063 aluminum alloy

Black micro-arc oxidation (MAO) coatings were obtained on the surface of 6063 aluminum alloy by using the MAO technique in silicate electrolyte with colorant NH4VO3. The work aimed to study the effect of adjusting the electrolyte pH (pH10, pH11, pH12, and pH13) on the microstructure, mechanical properties, and corrosion resistance of black MAO coatings on 6063 aluminum alloy. The experimental results showed that the electrolyte pH had an important effect on the macroscopic morphology of the MAO coatings. As the electrolyte pH value increased, the process voltage of the sample decreased, the duration of the anodic oxidation stage and the spark discharge stage were shortened, while the duration of the micro arc oxidation stage was prolonged. Reasonable adjustment of the electrolyte pH could reduce the poriness and defects of the coatings, decrease the surface roughness of the coatings, and improve the quality and wear resistance of the coatings. The corrosion resistance of the black MAO coating obtained in the electrolyte with pH12 was superior and the corrosion current density was only 3.207×10−7 A/cm2. The immersion corrosion test further demonstrated the same results of corrosion resistance.