Micro-arc Oxidation Process Research Articles

Effect of ultrasound on the physicochemical, mechanical and adhesive properties of micro-arc oxidized coatings on Ti13Nb13Zr bio-alloy

Implant surgeries are increasingly challenging due to their rising number. Achieving the desired biomaterial surface properties to ensure a strong bond with human tissue is a significant issue. This study investigates the influence of ultrasound (US) during the micro-arc oxidation (MAO) process on Ti13Zr13Nb bio-alloy, an area not previously explored, to enhance titanium alloy coatings’ properties for biomedical applications. Porous calcium-phosphate-based coatings were successfully deposited on Ti13Zr13Nb using MAO and ultrasound micro-arc oxidation (UMAO). Various properties such as morphology, chemical composition, topography, wettability, surface free energy, thickness, adhesion to the substrate, as well as mechanical and corrosion characteristics were thoroughly analyzed. Cytocompatibility was assessed using human osteoblasts. Using US during the MAO process increased coating roughness (up to ~ 17%), core height (up to 22%), isotropy (up to 17%), thickness (up to ~ 46%), and hardness (up to ~ 18%), depending on MAO parameters and US mode. Optimal coating performance was achieved at 136 mA, 600 s, and a sinusoidal US setting, resulting in the highest isotropy (~ 79%) and rutile quantity (2.6%), the lowest elastic modulus (~ 57 GPa), and the contact angle of ~ 70°, all of which could have contributed to enhancing osteoblast viability in vitro. This study, for the first time, underscores the importance of using the US during the MAO in tailoring the Ti13Zr13Nb for specific biomedical applications.

Study on the coating-formation process of micro arc oxidation coating on AZ31B magnesium alloy with Sc(NO3)3 addition

To investigate the effect of the addition of Sc(NO3)3 on the coating-formation process of AZ31B magnesium alloy MAO coating, microarc oxidation was conducted in a silicate-phosphate electrolyte containing 0.5 g/L Sc(NO3)3 for 15 s, 30 s, 60 s, 120 s, 240 s, and 1800 s. The oxidation voltage, surface morphology, and phase composition of the coatings were characterized using SEM, XRD, and XPS. The results indicated that Sc(NO3)3 hydrolytic complexation forms Sc(OH)2+ which leads to more uniform discharge micropores on the MAO coating surface, increasing the oxidation voltage. Compared to the MAO sample without Sc(NO3)3, the coating surface becomes denser and smoother. The MAO coating phase composition is MgO and Mg2SiO4, with Sc present as Sc2O3 in the coating.

Microstructure, MAO performance, interfacial characteristics and corrosion behavior of FSW joint of Al–Mg-Sc alloy

The microstructure evolution, micro-arc oxidation (MAO) performance, and corrosion behavior of Al–Mg-Sc alloy friction stir welded (FSW) joint were investigated. The microstructure observations indicated that compared with the base metal (BM), the stirring zone (SZ) and thermo-mechanical affected zone (TMAZ) showed notable grain defects and an enormous number of high angle grain boundaries (HAGBs). Additionally, a more conspicuous presence of non-uniform recrystallized grains and HAGBs was noticed in the thickness direction within the SZ. FSW process induced the precipitation of Al3Mg2 (β phase) at grain boundaries in the heat-affected zone (HAZ) of joint. The microstructural changes and precipitation induced by the FSW process influenced the electrical conductivity, resulting in the differences in micro-arc discharge in various zones of the FSW joint in MAO process, which in turn affected the thickness, porosity, and corrosion resistance of MAO ceramic film. HRTEM observation suggested that a transition layer composed of nanocrystalline and amorphous Al2O3 with average thickness of 2–4 nm was found at the film/substrate interface. Electrochemical tests suggested that a heterogeneous structure in various regions of FSW joint resulted in varying susceptibility to localized corrosion. HAZ/TMAZ had the worst anti-corrosion performance. After MAO treatment, the anti-corrosion performance of SZ and HAZ/TMAZ in FSW joint was significantly improved, especially SZ.

SLM Magnesium Alloy Micro-Arc Oxidation Coating

In this study, we utilized Selective Laser Melting (SLM) technology to fabricate a magnesium alloy, and subsequently subject it to micro-arc oxidation treatment. We analyzed and compared the microstructure, elemental distribution, wetting angle, and corrosion resistance of the SLM magnesium alloy both before and after the micro-arc oxidation process. The findings indicate that the SLM magnesium alloy exhibits surface porosity defects ranging from 2% to 3.2%, which significantly influence the morphology and functionality of the resulting film layer formed during the micro-arc oxidation process. These defects manifest as pores on the surface, leading to an uneven distribution of micropores with varying sizes across the layer. The surface roughness of the 3D-printed magnesium alloy exhibits a high roughness value of 180 nanometers. The phosphorus (P) content is lower within the film layer compared to the surface, suggesting that the Mg3(PO4)2 phase predominantly resides on the surface, whereas the interior is primarily composed of MgO. The micro-arc oxidation process enhances the hydrophilicity and corrosion resistance of the SLM magnesium alloy, thereby potentially endowing it with bioactivity. Additionally, the increased surface roughness post-treatment promotes cell proliferation. However, certain inherent defects present in the SLM magnesium alloy samples negatively impact the improvement of their corrosion resistance.

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.

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.

Preparation and Wear Resistance of Porous TC4 Titanium Alloy Micro‐arc Oxidation Coating Produced by Selective Laser Melting

The porous TC4 titanium alloy prepared by selective laser melting (SLM) is modified by micro‐arc oxidation (MAO) technology. By introducing active elements such as calcium (Ca) and phosphorus (P) into the electrolyte, a wear‐resistant active coating is constructed on the surface of porous TC4. The effects of MAO process parameters on the surface morphology and wear resistance of the coating are discussed. The results show that the thickness and adhesion of the coating increase with the increase of the first‐stage oxidation time, and decrease with the increase of the second‐stage oxidation time. In all processes, the MAO‐2 coating has better surface morphology and performance, with a median pore size of 0.51 μm and a binding force of 18 N. XPS analysis confirms that the coating is mainly composed of TiO2, Ca2P2O7, and Ca(PO3)2 phases. The study of tribological behavior shows that the wear form of the substrate is abrasive wear, while the wear form of the coating is adhesive wear. The friction coefficient of the coating is significantly lower than that of the substrate, and the wear resistance is better. This study provides an effective way to optimize the surface properties of porous TC4 titanium alloy.

Influence of titanium carbide particles on the characteristics of microarc oxidation layer on Ti6Al4V alloy

The Microarc Oxidation (MAO) layer on titanium alloy was mainly composed of TiO2, and there were some defects, such as holes and cracks, which made the performance of the MAO layer not ideal. To enhance the properties of the MAO layer, titanium carbide (TiC) particles were added to the electrolyte of a phosphate–silicate system as an additive. Consequently, the MAO layers containing the TiC phase on Ti6Al4V alloy were produced. The MAO process, composition, microstructure, and hardness of the MAO layer were comprehensively analyzed. Their frictional performance was assessed under reciprocating friction conditions without lubrication. The findings suggested that added TiC particles in the electrolyte played a significant role in creating the MAO layer, enhancing its thickness. The electrolyte without TiC particles produced an MAO layer primarily composed of TiO2 in two different mineral forms (rutile and anatase). Adding TiC particles resulted in the presence of TiC within the MAO layer, thereby facilitating the formation of a reinforced oxide layer. This addition also led to an improvement in the densification of the layer and a reduction in porosity. Notably, corrosion resistance testing indicated that incorporating 6 g l−1 TiC into the electrolyte resulted in superior performance compared with that obtained from the base electrolyte alone by achieving 1.4 times higher corrosion resistance. Moreover, a hardness value of 690 HV for the MAO layer was attained at a content level of 9 g l−1 TiC, demonstrating a significant 65% enhancement compared to the base oxide layer. This finding also demonstrated significantly enhanced friction property with a wear-volume reduction to 0.81 mm3. The findings on the relationship between the preparation of the MAO layer and its structure and properties can provide valuable guidance for designing and preparing the MAO layer.

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.

Enhanced bioactivity and mechanical properties of silicon-infused titanium oxide coatings formed by micro-arc oxidation on selective laser melted Ti13Nb13Zr alloy

The titanium scaffolds and surface-porous implants are manufactured by fast and cheap additive methods such as selective laser melting (SLM). The obtained irregularity and relative biological inertness of the titanium need proper surface modification. This research is aimed at forming bioactive multicomponent oxide coatings by micro-arc oxidation (MAO) at different process parameters on the selective laser-melted Ti13Zr13Nb alloy. The MAO process was carried out in a base solution containing 0.15 mol/L Ca and 0.1 mol/L GP, with different amounts of metasilicate added under two different applied voltages. Scanning electron microscopy, atomic force microscopy, X-ray electron diffraction spectroscopy, nanomechanical and nano scratch tests, water contact angle measurements, phosphate deposition observed in long-term immersion tests, and biological LDH and MTT assays were performed. The results showed that the coating microstructure, morphology, and properties were significantly dependent on process parameters. In particular, the metasilicate addition and its rising content in the electrolyte resulted in the higher development of the surface followed by better viability of osteoblasts at no necrosis, with no negative change in the other parameters, usually close to those obtained on silicon-free coatings. The obtained results prove that the addition of silicon to the electrolyte at the partial expense of calcium can be favorable for long-term titanium implants made by selective laser melting.

Recent Patents of Micro-arc Oxidation Technology

Abstract: Micro-arc oxidation (MAO) is a new surface treatment technology that can improve wear resistance, corrosion resistance, high voltage insulation, and other metal properties. Therefore, this technology is widely used in automobile manufacturing, aerospace, ship anti-corrosion, medical equipment, sewage treatment, and other fields. This paper reviews the representative patents of micro-arc oxidation technology at home and abroad. The micro-arc oxidation processing device, the process method of micro-arc oxidation processing, the power supply and electrical parameters of micro-arc oxidation, the preparation of electrolytes, and the methods of anticorrosion and protection of metal surface are analyzed. The development trend of micro-arc oxidation technology in the future is discussed. Micro-arc oxidation is a simple, green, and environment- friendly surface treatment technology, and there will be more patents in the field of micro- arc oxidation in the future.

Enhancing Tribological Performance of Micro-Arc Oxidation Coatings on 6061 Aluminum Alloy with h-BN Incorporation

Micro-arc oxidation (MAO) coatings of aluminum alloy have great potential applications due to their high hardness and wear resistance. However, the micro-pores and defects formed in the discharge channels during the MAO process limit its application in the corrosion field. This study delves into the impact of h-BN nanoparticles into MAO coatings on their structure, corrosion resistance, phase composition, and tribological properties. The results show that the incorporation of h-BN particles reduces the porosity and surface roughness of the coating while enhancing its hardness and wear resistance. The best corrosion resistance is obtained at a concentration of 2 g/L h-BN. An analysis of worn surface morphology, corrosion resistance, and friction coefficient change was conducted to evaluate the performance of this coating. This method provides a new approach to enhance the surface hardness and wear resistance of aluminum alloys, which is significant for expanding the application of aluminum alloys in corrosion environments.

Influence of Current Duty Cycle and Voltage of Micro-Arc Oxidation on the Microstructure and Composition of Calcium Phosphate Coating

The micro-arc oxidation (MAO) technique was employed to produce calcium phosphate coatings on titanium surfaces using an electrolyte composed of hydroxyapatite and calcium carbonate in an aqueous solution of orthophosphoric acid. The coatings’ morphology and composition were regulated by adjusting electrical parameters, specifically the duty cycle and voltage. This study examined the effects of the duty cycle and voltage during the MAO process on the microstructure and composition of calcium phosphate coatings on VT1–0 titanium substrates. Scanning electron microscopy (SEM) was utilized to analyze the microstructure and thickness of the coatings, while X-ray diffraction (XRD) was employed to determine their phase composition. The findings reveal that the surface morphology of the calcium phosphate coatings transitions from a porous, sponge-like structure to flower-like formations as the duty cycle and voltage increase. A linear increase in the voltage within the applied duty cycles led to a rise in the size of the forming particles of amorphous/crystalline structures containing phases of monetite (CaPO3(OH)), monocalcium phosphate monohydrate (Ca(H2PO4)2·H2O), and calcium pyrophosphate (γ–Ca2P2O7).

Influence of OH− on the initiation and growth mechanism of pore structure during microarc oxidation of Ti-6Al-4 V alloy

The presence of pores in a microarc oxidation (MAO) coating is an inherent characteristic that results from the expulsion and rapid solidification of molten material during the MAO process. The pore size in the MAO coating on the titanium alloy strongly affects the biocompatibility, but how to control it is unclear. The present study utilizes the advantages of NaOH to modulate pore size and plasma discharge, thereby effectively regulating the pore structure of the coating. The FIB-lift-out method was employed to directly extract the cross-sectional samples from the coating, enabling a detailed investigation of the pore microstructure using transmission electron microscopy (TEM). The structure surrounding the discharge channels exhibit layered zoning characteristics, with an amorphous layer near the discharge channel and a distinct crystal structure further away from it. The addition of NaOH enables the regulation of electrochemical and energy release processes, while also facilitating efficient discharge and spark generation to effectively disrupt the oxide layer and establish a plasma discharge channel.

Recent Research on Pore Sealing

Abstract: The article underscores the pivotal significance of sealing membrane pores in augmenting material performance and delves into the foundational principles governing membrane pore sealing. It thoroughly scrutinizes the evolution of membrane pore sealing technology, with a particular emphasis on anodization and micro-arc oxidation processes. Methods for sealing membrane pores are classified into hydration, organic, inorganic techniques, and other approaches to foster comprehension. Furthermore, the article deliberates on novel techniques for sealing membrane coatings, assesses their merits and demerits, and proposes enhancement strategies. These endeavors are geared towards propelling the advancement of membrane pore sealing technology, thereby enhancing efficiency, sustainability, and multifunctionality.

Preparation and properties of micro-arc oxidation ceramic coatings using machine vision technology

In this paper, a machine vision system is combined with non-immersion micro-arc oxidation (MAO) technology, using color as a visual feature, and automatically determining the MAO process through a self-developed color image segmentation program for HSV channel. The robotic arm accurately regulates the machining position and duration. Dense ceramic oxide coatings of similar quality to the artificially prepared coatings were successfully prepared on aluminum (Al) surface. The surface morphology and elemental composition of the samples were characterized using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The performance of the coatings was tested through thickness tests, hardness tests, and roughness tests. The tribological properties were tested by friction experiments, and its corrosion resistance was tested by electrochemical corrosion tests. The results confirm the effectiveness of a machine vision system for MAO process control, which is capable of preparing high-quality coatings comparable to manual operations. This not only solves the need for precise film formation in different application scenarios, but also provides a more efficient, controllable and innovative solution for the MAO process.

Enhanced Surface Properties of TiO2-Based Coatings via Stevia-Assisted Spark Suppression: Insights from Density Functional Theory Calculations

This study investigates the enhancement of surface properties in TiO2-based coatings on the Ti-6Al-4V alloy through micro-arc oxidation (MAO), employing stevia sugar as a novel additive. By incorporating stevia sugar into acetate–glycerophosphate–tetraethoxysilane solutions used in MAO treatment, the porous morphology of TiO2-based oxide layers is regulated. The incorporation of stevia moderates plasma discharge intensity, facilitating the formation of a uniform silicon-rich structure characterized by reduced porosity and pore size. This effect is attributed to the interaction between stevia and tetraethyl orthosilicate (TEOS), which modifies the TEOS hydrolysis process, thereby enhancing structural uniformity and stability while concurrently reducing plasma discharge intensity. Additionally, theoretical calculations offer a valuable understanding of the reactivity and interactions of stevia, TEOS, and their complex during the MAO process, laying the groundwork for further research and optimization in this promising field.

The growth behavior and performance of microarc oxidation coating on AZ91/Ti composite: Influence of Ti-reinforcement phase and electrolyte

Magnesium matrix composites with both high strength and ductility have been achieved by introducing pure Ti particles. However, the properties of the surfaces of the composites need to be improved by surface technology, such as micro-arc oxidation (MAO). In this study, we investigated the influence of the Ti-reinforcement phase on coating growth and evolution by subjecting both AZ91 alloy and AZ91/Ti composite to MAO treatment using silicate-based and phosphate-based electrolytes. Results revealed that the Ti-reinforcement phase influenced the MAO process, altering discharge behavior, and leading to a decreased cell voltage. The vigorous discharge of the Ti-reinforcement phase induced the formation of coating discharge channels, concurrently dissolving and oxidizing Ti-reinforcement to produce a composite ceramic coating with TiO2. The MAO coating on the AZ91/Ti composite exhibited a dark blue macromorphology and distinctive local micromorphological anomalies. In silicate electrolyte, a “volcano-like” localized morphology centered on the discharge channel emerged. In contrast, treatment in phosphate-based electrolyte resulted in a coating morphology similar to typical porous ceramic coatings, with visible radial discharge micropores at the reinforcement phase location. Compared to the AZ91 alloy, the coating on the AZ91/Ti composite exhibited lower thickness and higher porosity. MAO treatment reduced the self-corrosion current density of the AZ91/Ti surface by two orders of magnitude. The silicate coating demonstrated better corrosion resistance than the phosphate coating, attributed to its lower porosity. The formation mechanism of MAO coatings on AZ91/Ti composites in phosphate-based and silicate-based electrolytes was proposed.

Microstructure and properties of a MAO/PA/MoS2 composite coating formed on 6063 aluminum alloy by micro arc oxidation

Different self-sealing and lubricating composite coatings were produced on 6063 Al alloy using micro arc oxidation (MAO) with MoS2 and/or phytic acid (PA) incorporated into the base electrolyte. The tribological properties and electrochemical corrosion behavior of the coatings were detailed investigated in this paper. The results showed that the coefficient of frictional (CoF) of MAO/MoS2 coating decreased 29.6 % compared to that of the pure MAO coating due to the excellent self-lubricating of MoS2. Meanwhile, the CoF of MAO/PA/MoS2 coating reduced 39.5 % attributed to a thicker and more compact coating obtained by the addition of PA as an intermediate chelating agent besides the self-lubricating effect of MoS2 particles. The pores were sealed by MoS2 during MAO process due to the electrodeposition effect which resulted in a positive shifting of the corrosion potential. However, the improvement of corrosion resistance for the MAO/MoS2 coating was limited as the intrinsic defects in MoS2 lattice preferred to form micro galvanic cell. The corrosion current density of the MAO/PA/MoS2 coating was three orders of magnitude lower than that of pure MAO coating. The thin PA layer produced at the substrate/coating interface of the MAO/PA/MoS2 coating can act as a barrier to suppress the galvanic corrosion, inducing the best corrosion resistance of the coating.

Effect of Ti3AlC2 particles on microstructure and tribological properties of micro-arc oxidation layer on TC4 alloy

TiO2 metal oxide layers rich in Ti3AlC2 phase was successfully prepared on TC4 alloy by micro-arc oxidation (PECC or ASD) process. Analysis of the material phases contained in the PECC layer by X-ray diffractometry; scanning electron microscopy and energy spectroscopy were used to analyses the pores and elemental distribution on the surface of the specimens; the cross-sectional thickness of PECC layers were measured by metallographic microscope; confocal microscopy was used to measure the surface roughness of the sample; High temperature wear resistance of PECC layers with different Ti3AlC2 concentration was compared by tribological wear testing. The results show: 1) The incorporation of Ti3AlC2 particles plays an obvious catalytic role in the growth of PECC layer and the thickness is up to 74.74 μm; 2) When the concentration of Ti3AlC2 gradually increases, the PECC layers fluctuates obviously, and the deposition sealing effect is remarkable; 3) When the concentration is 4-6 g/L, the alloy has the best wear resistance, volume wear is only 1.98×10-4 mm3 /N·m. Experiments have shown that surface modification of the alloy using the PECC process can further broaden the application of the alloy in the field of anti-wear