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• Electroless Ni plating under pressure impregnation enhances the corrosion property. • The defects in the MAO coating are well filled with Ni under pressure impregnation. • Ionization of metal atoms and electron transport are obviously hindered. The corrosion protection ability of the microarc oxidation (MAO) coating cannot be maximized due to its inherent defective structure, and further hole sealing treatment has been a prevailing strategy for enhancing the corrosion resistance. Herein, the electroless Ni plating under pressure impregnation was demonstrated to be significantly effective to remedy the defects of the porous MAO coating, the voids and cracks in the MAO coating were filled with Ni well, resulting in the blockage of the invasive route of the corrosive Cl− ions, and the ionization of the metal matrix was unable to occur, the MAO coating with ultrahigh electrical resistivity hardly transferred the electron from the Ni layer to substrate, hence, the corrosion property of the MAO coating was obviously enhanced. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mg alloy AZ91D was treated by PEO in alkaline silicate electrolytes with various concentrations of fluoride ions. [•] Fluoride ions have pronounced catalytic activity on the formation of oxide layer. [•] Fluoride-containing amorphous phase is located close to the metal–oxide interface. [•] The best corrosion protection is achieved when an electrolyte contains high concentrations of silicate and moderate concentrations of fluoride. [Copyright &y& Elsevier]

It is shown that heating of an internal layer of microarc coating up to temperatures at which insignificant rates of heat removal from microdischarges that possess a temperature of no less than 1200°C are ensured leads to realization of a sufficient time interval for the occurrence of almost 100% transformation of low-temperature aluminum-oxide modifications into high-temperature ones. Insufficient heating of a number of areas of the coating to these temperatures is the main reason for formation of a composite microarc coating that contains both low- and high-temperature aluminum-oxide modifications in its internal layer. [ABSTRACT FROM AUTHOR]

Abstract: Magnesium has attracted much attention as a class of biodegradable metallic biomaterials. In this study, a silicate electrolyte-based micro-arc oxidation (MAO) treatment was adopted to prepare forsterite-containing MAO coatings on a ZK60 magnesium alloy in order to decrease the degradation rate and increase the biological property of the alloy. Four anodization voltages were chosen to prepare the MAO coatings. The cell experiment showed a cytotoxicity of grade 0 for the MAO-coated alloy to L929 cells and the hemolytic ratio was dramatically decreased for the MAO-coated alloy compared with the bare one. The corrosion resistance and the degradation behavior of the MAO-coated ZK60 alloy were studied using drop tests, electrochemical measurements and immersion tests. The results indicate that the MAO coating could effectively decrease the initial degradation rate of the alloy. The corrosion resistance of MAO coating was increased with the elevation of the preparation voltage. A degradation model for ZK60 alloy with a forsterite-containing MAO coating was proposed. Based on the model, the MAO-coated alloy experiences destruction and restoration simultaneously, and the coating fails in a peeling-off mode. [Copyright &y& Elsevier]

Highlights: [•] SiC particles inhibit sparking discharge and microarc oxidation coating growth. [•] SiC particles undergo intense oxidation before sparking discharge occurs. [•] Effect mechanism of SiC particle on microarc oxidation process is revealed. [•] SiC particles degrade the corrosion resistance of microarc oxidation coating. [Copyright &y& Elsevier]

Abstract: Porous titanium was prepared by pressureless sintering of titanium beads with diameters of 100, 200, 400 and 600μm. The results indicated that the mechanical properties of porous titanium changed significantly with different bead diameters. Plastic deformations such as necking phenomenon and dimple structure were observed on the fracture surface of porous titanium sintered by beads with diameter of 100μm. However, it was difficult to find this phenomenon on the porous titanium with a titanium bead diameter of 600μm. The microarc oxidized coatings were deposited on its surface to improve the bioactivity of porous titanium. Furthermore, the cefazolin sodium/chitosan composited films were fabricated on the microarc oxidized coatings for overcoming the inflammation due to implantation, showing good slow-release ability by addition of chitosan. And the release kinetic process of cefazolin sodium in composited films could be possibly fitted by a polynomial model. [Copyright &y& Elsevier]

Objective This study aimed to evaluate the effects of the microarc oxidation surface on cell morphology and cytoskeleton. Methods Pure titanium with a diameter of 15 mm and a thickness of 1 mm was divided into four groups: grooved surfaces (group G); sandblasted surfaces (group SB); grooved microarc oxidation surfaces (group GMAO); and sandblasted microarc oxidation surfaces (group SBMAO). Osteoblast cells were cultured in each group. The morphology and proliferation of the cells on the titanium surface were observed by scanning electron microscope (SEM). The cytoskeleton was evaluated by laser scanning confocal microscope (LSCM). Results The osteoblasts were inoculated after 12 h; these osteoblasts then spread along the surface of the titanium plate. In GMAO and SBMAO groups, the osteoblasts converged in the hole. The actin fibers in each group were clearly visible. In particular, the actin fibers in GMAO and SBMAO groups were arranged in parallel and formed bundles that extended into the holes. Conclusion After microarc oxidation, the titanium surface significantly affected the morphology and cytoskeleton of osteoblasts. [ABSTRACT FROM AUTHOR]

In this study Fe-Si binary alloy containing 8 at. % Si and pure iron were pack boronised at 900°C, 1000°C and 1100°C for 3 h using Ekabor II powder. The microstructure, chemical composition, phase contents and microhardness of the formed boride coatings were characterized by means of XRD, SEM-EDS and Vickers Microhardness measurements. The saw-tooth morphology was observed on both substrates, it tended coarsen with boriding temperature and especially with the addition of Si to the substrate. Silicon did not dissolve in the boride layer but accumulated between boride layer and Fe-8Si substrate and formed a Si rich transition zone with an average hardness of 500 HV. The average hardness value of the boride layers on both substrates were approximately 2200 HV. The boride layer thickness increased near-linearly with boronizing temperature for both materials. The presence of silicon in the substrate reduced the thickness of boride layer compared to the boride layer thickness on pure iron. The amount of boron rich FeB phase compared to Fe2B phase was higher on the boride layer of Fe-Si alloy than of pure iron. [ABSTRACT FROM AUTHOR]

Various oxide films have been grown on AZ31 magnesium alloy by microarc oxidation in alkaline phosphate solution with addition of different compounds (Na2SiO3, NaAlO2). Because of the anodic oxide porosity, arising from the sparking treatment, organo-functional silane coatings have been applied to seal pores and cracks and achieve a more effective protective system. The surface and cross-section morphologies of the anodic films were analysed by scanning electron microscopy. The phase composition was determined by X-ray diffraction. Bare and anodised AZ31 specimens were post-treated with silanes, namely N-phenil-3-aminopropyltrimethoxysilane and Bis (trimethoxysilylethyl) benzene. The corrosion resistance of anodic oxides and oxide-silane composite coatings was evaluated in dilute Harrison solution by means of single-cycle anodic polarisation. The results showed that the addition of different salts to the alkaline phosphate solution affects the surface morphology and the phase composition of the coatings and, as a consequence, the corrosion resistance behaviour. Both silane post-treatments provide further corrosion protection to samples anodised in aluminate containing solution. [ABSTRACT FROM AUTHOR]

Abstract: Micro-arc oxidation (MAO) films on AZ31 magnesium alloy were modified by high-intensity pulsed ion beam (HIPIB) irradiation with ion energy of 300keV at 200A/cm2 with up to 10 shots. Scanning electron microscopy, X-ray diffractometry and micro-hardness testers were used to characterize the surface properties of the irradiated MAO films. The thickness of remelted layer increased and then decreased, and the maximal value of 10μm was obtained at 200A/cm2 with 5 shots. The phase structure of the ablated surface still consisted of Mg2SiO4 and MgO, which are the same as that of the original ones. Surface roughness of the ablated surface increased and then decreased with the increase of shot number. The surface roughness for the original MAO film is about 2.10μm, it decreased to 1.18μm with 1 shot irradiation and then increased to 4.13μm with irradiation shots up to 10. Correspondingly, the surface energy of the ablated surface augmented, resulting in the tedious decrease of static contact angle from 145.9° for original film to 49.7° for the film with 10 shots. The ablation modification enhanced the continuity and compaction of the MAO films on AZ31 magnesium alloy. [Copyright &y& Elsevier]

Based on the experimental data, the mechanism of the influence of the cathodic component of the ac current on the growth kinetics and the limiting thickness of the microarc coatings formed on the surface of aluminum and magnesium alloys is suggested. It is shown that, during microarc oxidation (MAO), the cathode component of the ac current either increases (in the case of the Al alloy) or decreases (in the case of the Mg alloy) the number of effective microcharge discharges due to an increase in the electrolyte pH in the through pores of the coating and the elevation of the temperature of the MAO process. [ABSTRACT FROM AUTHOR]

The method of microarc oxidation of materials is described and its advantages over the anodization conventionally used to obtain protective coatings on the surface of light aluminum and constructional alloys are shown; the main notions of the mechanism of this process in the understanding of the authors of this article are presented. [ABSTRACT FROM AUTHOR]

The mechanism of the transition of the electrical breakdown to a thermal one upon MAO was confirmed in a polarized metal-oxide-electrolyte system. The inclusion of reactance in the circuit makes it possible to create coatings with controlled morphology and defectiveness. [ABSTRACT FROM AUTHOR]

Abstract: Microarc oxidation (MAO) coatings were prepared on AZ91D magnesium alloy in a phosphate electrolyte with and without K2TiF6, respectively. The morphology, composition and phase structure of the MAO coatings were investigated. Corrosion resistances and photocatalytic properties of the MAO coatings were evaluated using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) in 3.5wt.% NaCl solution and photocatalytic decomposition of methylthionine blue (MB) solution in incandescent light. Gray coatings formed in the phosphate electrolyte were mainly composed of MgO and amorphous phase. Compared with the phosphate electrolyte, black coatings with fewer and smaller micropores were formed in the electrolyte with K2TiF6. Moreover, Titanium in the form of Mg2TiO4 and TiO2 was detected in the black coatings. The results of corrosion tests show that the MAO coatings formed in the phosphate electrolyte with and without K2TiF6 both improved significantly the corrosion resistance of the magnesium alloy. Photocatalytic measurements indicated that the MAO coatings formed in the electrolyte with K2TiF6 exhibited photocatalytic behaviors. [Copyright &y& Elsevier]

Abstract: A thin protective film on Zr–1Nb alloy was fabricated by AC microarc oxidation (MAO) in phosphate electrolyte. The morphology, microstructure and phase constituents of MAO films under different discharge voltages were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. Their surface roughness and microhardness were measured. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to evaluate corrosion behaviors of films. The equivalent circuits of EIS plots for bare Zr–1Nb alloy and MAO films were proposed. It was found that all of films at different discharge voltages consisted of the monoclinic ZrO2 and tetragonal ZrO2 phases, but the content of monoclinic ZrO2 was much higher than that of tetragonal ZrO2 phase. Microhardness of MAO films is over 570HV, which is higher than that of Zr–1Nb alloy substrate with 245HV. The film fabricated in higher voltage shows lower corrosion current density and higher impedance, which displays better corrosion resistance. The EIS results identify that the inner compact layer of MAO films plays a key role in improving the corrosion resistance of Zr–1Nb alloy. [Copyright &y& Elsevier]

Ceramic coatings with thickness of 27 µm were fabricated on Mg-7Li alloy in Na2SiO3-C6H18O24P6 solution by microarc oxidation (MAO). The morphology and phase composition of MAO coatings were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion behavior of the bare and MAO coated Mg-7Li alloy was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that the MAO coatings were composed of MgO, Li2O, and Mg2SiO4, and there existed some micropores on the coating surface with a diameter of 3-20 µm. The corrosion potential ( Ecorr) and corrosion current density ( Icorr) of the MAO coated alloy were about −1.4761 V and 7.204 × 10−7 A/cm2, respectively. The Ecorr of the MAO coated alloy increased by 109.6 mV and its Icorr decreased by three orders compared with that of the bare Mg-7Li alloy. The EIS plots indicated that the impedance of the MAO coated alloy was 15 times higher than that of the bare alloy. The fitting parameters showed that the resistance of the MAO coatings was far greater than that of the bare alloy. The dense intermediate layer and the transition layer of the MAO coatings acted as a barrier to hinder the proceeding of solution permeation, remarkably improving the corrosion resistance of the Mg-7Li alloy. [ABSTRACT FROM AUTHOR]

Abstract: Ti and N co-doped diamond-like carbon ((Ti:N)-DLC) film was deposited on the MAO coated substrate using a hybrid beam deposition system, which consists of a DC magnetron sputtering of Ti target and a linear ion source (LIS) with C2H2 and N2 precursor gas. The microstructure and properties of the duplex (Ti:N)-DLC/MAO coating were investigated. Results indicate that the (Ti:N)-DLC top film with TiN crystalline phase was formed. Ti and N co-doping resulted in the increasing I D/I G ratio. The significant improvement in the wear and corrosion resistance of duplex (Ti:N)-DLC/MAO coating was mainly attributed to the increased binding strength, lubrication characteristics and chemical inertness of (Ti:N)-DLC top film. The superior low-friction and anti-corrosion properties of duplex (Ti:N)-DLC/MAO coating make it a good candidate as protective coating on magnesium alloy. [Copyright &y& Elsevier]

Abstract: This paper addresses the long-term corrosion behavior of microarc oxidation coated Mg alloys immersed in simulated body fluid for 28days. The coatings on AZ31 Mg alloys were produced in the electrolyte of sodium phosphate (Na3PO4) at the concentration of 20g/L, 30g/L and 40g/L, respectively. Scanning electron microscope (SEM) and optical micrograph were used to observe the microstructure of the samples before and after corrosion. The composition of the MAO coating and corrosion products were determined by X-Ray Diffraction (XRD). Corrosion product identification showed that hydroxyapatite (HA) was formed on the surface of the corroded samples. The ratio of Ca/P in HA determined by the X-ray Fluorescence (XRF) technique showed that HA is an acceptable biocompatible implant material. The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were employed to characterize the corrosion rate and the electrochemical impedance. The corrosion resistance of the coated Mg alloys can be enhanced by optimizing the electrolyte concentrations for fabricating samples, and is enhanced after immersing the coated samples in simulated body fluid for more than 14days. The enhanced corrosion resistance after long-term immersion is attributed to a corrosion product layer formed on the sample surface. The inhibition mechanism of the corrosion process is discussed and presented with an animation. [Copyright &y& Elsevier]

Abstract: We report a strategy to prepare biocompatible metasilicate-based ceramic coatings by microarc oxidation (MAO) on AZ91D magnesium alloy in sodium silicate solution with Ca(H2PO4)2 additive. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used to assess the phase composition and microstructure of the coatings. The corrosion behavior of the coated samples was evaluated by electrochemical and immersion tests in simulated body fluid (SBF). The results show that the corrosion resistance of AZ91D alloy is significantly improved due to the formation of dense coatings with biocompatible CaSiO3, CaMgSiO4 and Mg2SiO4 phases. Increasing Ca(H2PO4)2 content in the based sodium silicate leads to better corrosion resistance of AZ91D alloy. The results suggested a promising method for surface modification of degradable magnesium alloy as biomedical applications. [Copyright &y& Elsevier]

Oxide films were produced on AZ31 magnesium alloy by microarc oxidation (MAO) in an alkaline phosphate solution at different concentrations of NaAlO2. Surface and cross-section morphologies were assessed by scanning electron microscopy and phase composition by X-ray diffraction. The corrosion resistance was evaluated in a diluted Harrison solution using anodic polarisation tests. The results show that the properties of the oxides are affected by the NaAlO2 concentration. The thickness of the coatings increases with aluminate concentration from 5 μm in aluminate free electrolyte to 12 μm in electrolyte containing 60 g dm-3 after anodising (5 min; 135 V). The coating consists of two layers: an outer layer with high concentration of aluminium and an inner magnesium rich layer. Increased corrosion resistance was found in the samples anodised in a solution containing 30 g dm-3 NaAlO2. [ABSTRACT FROM AUTHOR]

Abstract: Plasma electrolytic oxidation (PEO) of aluminum alloy 5052 in alkaline-silicate electrolytes having different SiO2/Na2O ratios (silicate indexes) was studied. For all the electrolytes 20–90μm thick technological layer was obtained; composition, structure and properties of the oxidized layer were studied. For each sample, the oxidized layer consists of a denser internal and looser external sublayer. While for “n =1 electrolytes” the oxidized layer is mainly formed by several kinds of alumina, the principal constituent of the oxidized layer for “n =3 electrolytes” is mullite. Measurements of microhardness evidenced that it is apparently not influenced by the kind of silicate (n =1 or n =3) and by its concentration in the electrolyte. Electrolytes with silicate index n =3 ensure better corrosion protection than those with n =1. Corrosion protection parameters are significantly better for all PEO oxidized samples than for the untreated Al5052 alloy. [Copyright &y& Elsevier]

Abstract: Microarc oxidation coatings were fabricated on AZ31 magnesium alloy in the electrolyte of sodium phosphate. Potentiodynamic polarization and electrochemical impedance spectroscopy tests were employed to investigate the electrochemical corrosion behavior. The corroded surface was characterized by an optical microscope and X-ray diffraction. The influence of chloride ion concentration on the corrosion resistance of microarc oxidation coated AZ31 alloy is discussed. The corrosion current density enlarged ratio and the charge transfer resistance reduced ratio indicated that the extent of the corrosion damage of microarc oxidation coated AZ31 alloy is much higher when chloride ion concentration is greater than 5%. A corrosion mechanism related to the effect of chloride ion concentrations on the corrosion behavior is proposed. [Copyright &y& Elsevier]

Abstract: The electrochemical corrosion behavior of microarc oxidation (MAO) coatings produced at various oxidation times on AZ31 Mg alloys was studied in a simulated body fluid (SBF). The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to characterize the corrosion behavior. The influences of the MAO time on the microstructure and corrosion properties are discussed. The initial porosity of the MAO coating was evaluated by the potentiodynamic polarization method. Post-corrosion phase identification showed that hydroxyapatite (HA) was formed on the surface of the samples. The ratio of Ca/P in HA was determined by the X-ray Fluorescence (XRF) technique. A physical model of the corrosion process with equivalent circuits at different corrosion stages is proposed. [Copyright &y& Elsevier]

Abstract: Black and gray microarc oxidation (MAO) coatings were prepared in a phosphate electrolyte with and without K2TiF6 on 2A70 aluminum alloy, respectively. Voltage–time curves were recorded during the MAO process. The effects of K2TiF6 on the morphology, composition, abrasive resistance and corrosion resistance of MAO coatings were investigated. The results showed that the MAO coating produced in the electrolyte with K2TiF6 was thicker, and more uniform than that produced in the electrolyte without K2TiF6. Ti was detected in the surface of the MAO coating formed in the electrolyte with K2TiF6. The results of abrasive resistance and corrosion resistance showed that the MAO coating formed in the electrolyte with K2TiF6 exhibited better abrasive resistance and corrosion resistance. [Copyright &y& Elsevier]

Abstract: Ceramic coatings were fabricated on ZK60 magnesium alloy substrate by microarc oxidation (MAO) in Na2SiO3–KOH base electrolyte with four kinds of additives (i.e. KF, NH4HF2, C3H8O3 and H2O2). The effects of these additives on microstructure and property of coatings were investigated. The surface morphology, phase composition and corrosion resistance of the ceramic coatings were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and simulation body fluid (SBF) immersion test respectively. It is found that different additives can change the spark discharge phenomenon during microarc oxidation. It is proved that both potassium fluoride (KF) and ammonium bifluoride (NH4HF2) promote discharge and accelerate reaction while the introduce of glycerol (C3H8O3) leads to the refining of sparks and reduction of thermal effects. Results also demonstrate that the introduce of hydrogen peroxide (H2O2) contributes to the increase of coating surface roughness and enlargement of surface micropore size. XRD results indicate that the ceramic coatings are mainly composed of Mg2SiO4, MgSiO3 and SiO2. The introduce of H2O2 hinders the reaction between SiO2 and MgO and creates favorable conditions for the formation of the MgO phase. The ceramic coatings formed in base electrolyte containing 7g/L NH4HF2 and 5mL/L C3H8O3 exhibit the highest corrosion resistance. [Copyright &y& Elsevier]

Microarc oxidation (MAO) treatment produces a thick coating on the 15SiCp/2024 aluminum matrix composite. After pretreatment of ion implantation, a thin diamond-like carbon film (DLC) was deposited on the top of polished coating by a pulsed filtered cathodic vacuum arc (FCVA) deposition system with a metal vapor vacuum arc (MEVVA) source. The morphology and tribological properties of the duplex /DLC multiplayer coating were investigated by Raman spectroscopy, scanning electron microscopy (SEM) and SRV ball-on-disk friction tester. It is found that the duplex /DLC coating had good adhesion and a low friction coefficient of less than 0.07. As compared to a single or DLC coating, the duplex /DLC coating on aluminum matrix composite exhibited a better wear resistance against ball under dry sliding, because the coating as an intermediate layer improved load support for the top DLC coating on 15SiCp/2024 composite substrate, meanwhile the top DLC coating displayed low friction coefficient. [ABSTRACT FROM AUTHOR]

Failure of fixation caused by loosening of pedicle screws in osteoporosis is a problem in spinal surgery. We compared the in vivo fixation strength between pedicle screws treated with microarc oxidation (MAO) and untreated screws in an osteoporotic model of ovariectomized sheep. The MAO treated and untreated screws were placed in lumbar vertebral bodies. After 3 months of implantation, biomechanical tests, micro-CT analysis, and histological observations were conducted to examine the performance of the two groups. At time 0, no significant difference was found between the two groups in biomechanical tests ( p > 0.05); 3 months later, higher pull-out strength and load with less displacement were detected in the MAO-treated group ( p < 0.05). Micro-CT analysis showed that the tissue mineral density, bone volume fraction, trabecular thickness, and trabecular number in the MAO-treated group were all higher than those in untreated group ( p < 0.05), and trabecular spacing was smaller ( p < 0.05). Histologically, the bone-implant interface in the MAO-treated group was better than that in untreated group ( p < 0.05). In conclusion, pedicle screws with a bioactive surface treated by MAO can improve screw fixation strength in osteoporotic spines in sheep. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 30:1296-1303, 2012 [ABSTRACT FROM AUTHOR]

With the help of the PIXE and RBS methods, the elemental composition of a ceramic coating on Zr alloy, surface layers of heat-treated TiNi alloy, and Ar implanted pyrolytic graphite is investigated. It is shown that the PIXE and RBS methods are complementary and their combined use can provide additional information on the elemental composition of the surface layers. It was determined that the microarc oxidation of Zr alloy in an electrolyte with addition of ultrafine YO powder leads to introduction of Y in the substrate material. An effect of the surface segregation of Ti was found in TiNi alloy after the heat treatment. [ABSTRACT FROM AUTHOR]

Abstract: This paper addresses the effect of pulse frequency on the corrosion behavior of microarc oxidation (MAO) coatings on AZ31 Mg alloys in simulated body fluid (SBF). The MAO coatings were deposited by a pulsed DC mode at four different pulse frequencies of 300Hz, 500Hz, 1000Hz and 3000Hz with a constant pulse ratio. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were used for corrosion rate and electrochemical impedance evaluation. The corroded surfaces were examined by X-ray diffraction (XRD), X-ray fluorescence (XRF) and optical microscopy. All the results exhibited that the corrosion resistance of MAO coating produced at 3000Hz is superior among the four frequencies used. The XRD spectra showed that the corrosion products contain hydroxyapatite, brucite and quintinite. A model for corrosion mechanism and corrosion process of the MAO coating on AZ31 Mg alloy in the SBF is proposed. [Copyright &y& Elsevier]

Ceramic coatings have been successfully prepared on biocompatible AZ31 magnesium alloy substrates using microarc oxidation (MAO) technique. Residual stresses attributed to the MgO constituent of the coatings at different oxidation voltages have been evaluated by x-ray diffraction using the sin ψ method. It is found that tensile residual stresses were present in the coatings, and they decreased from 1418 to 545 MPa as the oxidation voltages increased from 250 to 350 V. Correlations between the residual stresses and microstructural morphology have been discussed. The residual stress characteristics are attributed to the microcracks and the new phase formation during the MAO process. [ABSTRACT FROM AUTHOR]

Abstract: Al2O3/ZrO2 composite coating was prepared on 2A70 aluminum alloy by microarc oxidation (MAO) in a phosphate electrolyte containing K2ZrF6. The effects of K2ZrF6 in the electrolyte on the coating thickness, surface morphology, elemental composition and abrasive resistance were investigated. The results indicated that the MAO coating thickness increase due to the addition of K2ZrF6 in the electrolyte. In the electrolyte containing 6.0g/L K2ZrF6, the coating was the most uniform and compact. The MAO coating formed in the electrolyte without K2ZrF6 was mainly composed of γ-Al2O3 and a little α-Al2O3. Zirconium in the form of t-ZrO2 was detected in the MAO coatings formed in the electrolyte containing K2ZrF6, and the Zr content increases with increasing concentration of K2ZrF6. A γ-Al2O3 and t-ZrO2 nanocomposite coating was formed in the electrolyte containing 6.0g/L K2ZrF6, and which exhibited the highest abrasive resistance. [Copyright &y& Elsevier]

Abstract: Wormhole-shaped TiO2/Ti photoelectrodes were prepared by microarc oxidation method in H2SO4 solution. The morphology and structure were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The photocatalytic (PC) activity of TiO2/Ti photoelectrodes was evaluated by the degradation rates of alachlor in aqueous solution. All results showed that their porous increased with the anodization voltage, and the crystal structure transferred from anatase to rutile TiO2. The photoelectrodes, which were anodized 10 min at 160 V in 0.5 M H2SO4 solution, were composed of mixed crystal structures of anatase and rutile TiO2 and have optimal PC performance. Alachlor was almost completely removed by TiO2/Ti photoelectrodes after 120 min under the illumination of 150W xenon light. [Copyright &y& Elsevier]

The ceramic coatings were prepared on open-cell aluminum foams by microarc oxidation (MAO) treatment in an alkaline-silicon electrolyte. The morphology, microstructure, elemental distribution, and phase composition of the MAO coatings were investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction, respectively. The corrosion behaviors of the coated and uncoated foams were evaluated by electrochemical polarization measurement. The results show that the MAO coatings cover the surface of open-cell aluminum foams. The coatings were composed of an external porous layer and an internal dense layer. The main phase of the MAO coating phase is γ-AlO. The coated aluminum foams exhibit more positive corrosion potential and lower corrosion current density compared with the uncoated aluminum foams. [ABSTRACT FROM AUTHOR]

Abstract: Microarc oxidation (MAO) coatings on 5052 aluminum alloy are prepared in silicate–hypophosphite electrolytes with sodium tungstate. The effects of sodium tungstate concentrations and current density on the surface morphology, phase composition and properties of the coatings are investigated. With the addition of sodium tungstate in the electrolyte and increase of current density, the final voltage at the microarc discharge process increases. The results also show that the MAO coatings are composed mainly of α-Al2O3 and γ-Al2O3 and the proportion of α-Al2O3 and γ-Al2O3, pore size, surface roughness as well as thickness of the coatings strongly depend on the sodium tungstate concentration and current density. Thus, the hardness, friction coefficient and corrosion resistance of the coatings are significantly influenced by the magnitude of the current density and sodium tungstate concentration. These oxide films on aluminum were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thickness gage, and polarization curves, respectively. [Copyright &y& Elsevier]

Abstract: A ceramic coating with thickness of 20–70μm was grown on titanium plate in Al(OH)3 colloidal solution by microarc oxidation (MAO) in constant current mode. It is found that the as-grown coating consists of rutile TiO2 phase together with a thin layer of Ai2TiO5 phase near the surface of the coating. After removing the Ai2TiO5 layer by polishing, a single phase of rutile TiO2 coating is achieved, which is different from results reported previously, where the coating is usually composed of two phases of rutile and anatase TiO2. It is suggested that the formation of the coating with single phase of rutile TiO2 is related to the existence of Al(OH)3 in the solution. The growth rate of the coating increases with increasing current density in the range of 17–23A/dm2, but it increases little in the range of 23–30A/dm2. The rutile TiO2 coating looks compactness and solidity in the coating grown in the density range of 17–23A/dm2 but looseness and insubstantiality in the range of 23–30A/dm2. The hardness and elastic modulus of the rutile TiO2 decreases with the density increasing. The mechanism of formation of the coating is discussed in the present work. [Copyright &y& Elsevier]

Titanium (Ti) and its alloys are used extensively in implants due to their excellent biocompatibility and mechanical properties. However, Ti-based implant materials have specific complications associated with their applications, such as the loosening of implanted host interface owing to unsatisfactory cell adhesion and the susceptibility of the implants to bacterial infections. Hence, a surface that displays selective biointeractivity, i.e., enhancing beneficial host cell responses but inhibiting pathogenic microbial adhesion, would be highly desirable. This study aims to confer long-lasting antibacterial properties and good biocompatibility on Ti via the microarc oxidation technique. The biocompatibility of the Ti surface was evaluated by cytotoxicity test, and the bacteriostasis rate was evaluated by antibacterial efficacy. The results showed that the implant surface might be nontoxic to cell and its long-lasting antibacterial properties could be significantly improved. These results indicate that such microarc oxidation coatings are expected to have good potential in transcutaneous implant applications. [ABSTRACT FROM AUTHOR]

In this study, the surface of pure zirconium was coated by microarc oxidation method under constant electrical parameters and the same electrolytic composition for the duration of 5, 15, 30, 45, 60, 90, 120 minutes. The phase content, surface morphology and microhardness of the formed coatings were characterized by X-ray diffraction, scanning electron microscopy, profilometry and microhardness measurements. The coating was porous in nature and contains significant amount of microcracks. The coating is formed by the crystalline phases of monoclinic (m- ZrO2) and tetragonal (t-ZrO2) zirconium oxide and duration of the process did not affect the phase content. The existence of the silicon in addition to zirconium and oxygen in the coatings was evident. The thickness of the coating increased linearly from 5 μm to 135 μm with the increase of the coating duration. The surface roughness was raised from 0.5 μm to 6.4 μm with the coating duration. The average coating microhardness was measured as 600 HV compared to the pure zirconium substrate's microhardness of 200 HV and there was no systematical change with the coating duration. [ABSTRACT FROM AUTHOR]

Abstract: Microarc oxidation (MAO) films on AZ31 magnesium alloy were treated by high-intensity pulsed ion beam (HIPIB) irradiation with ion energy of 300keV at ion current density of 200A/cm2. A remelted layer of a few micrometers was produced on irradiated MAO films. The corrosion resistance of MAO films was characterized by potentiodynamic electrochemical test in 3.5% NaCl solution. The anodic polarization behavior of MAO samples exhibited a transition from the active anodic dissolution for the original one to the passivation–pitting breakdown for the modified films. The passivation–pitting breakdown voltage of modified films increased with multi-shot irradiation, i.e. from a value of −1420mV(SCE) at 1 shot to −800mV(SCE) at 5 shots, and the corresponding passivation current density decreased by two orders of magnitude. The irradiated MAO films have a higher corrosion potential than the original one, reaching a maximal value of −1350mV(SCE) at 5 shots from the original −1580mV(SCE). The electrochemical impedance spectrum (EIS) of modified MAO films was measured with varying the immersion time in 3.5% NaCl solution. The Nyquist impedance plots from modified films may show only a capacitive loop at the immersion time of 5h, and the inductive loop occur at longer immersion time, whereas original films presenting both capacitive and inductive loops at all the immersion time. Bode plots of MAO films before and after irradiation were obviously distinguished in shape at low frequency range, especially at short immersion time. The evolution of Nyquist and Bode plots with immersion time was discussed in association with the processes of electrolyte penetration into the MAO structure and resultant reaction at the film-substrate interface. It is concluded that the improvement in the continuity and compaction of MAO films accounts for the enhanced corrosion resistance of the films irradiated by HIPIB. [Copyright &y& Elsevier]

Magnesium and its alloys have been used in many industries, but they are reactive and require protection against aggressive environments. In this study, a compact and relatively pore-free oxide coating was formed on AZ91D magnesium alloy to improve its corrosion resistance by means of the microarc oxidation (MAO) process via the addition of CeCl as an additive into the MAO electrolyte. Morphologies and compositions of the coatings were studied with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. It was found that with the addition of CeCl into the MAO electrolyte, the obtained coating showed more uniform morphology compared with the coating produced in the CeCl free electrolyte. EDS analysis confirmed the presence of cerium in the coating formed in the electrolyte with CeCl. Polarization tests results showed that the corrosion current density of the coating formed in the CeCl containing electrolyte was less than that of the coating formed in the electrolyte without CeCl. Furthermore, electrochemical impedance spectroscopy (EIS) tests indicated that the coating formed in the electrolyte with the addition of CeCl improved the corrosion resistance of the substrate significantly. [ABSTRACT FROM AUTHOR]

Abstract: Ceramic coatings with high emission were fabricated on Ti6Al4V alloy by microarc oxidation (MAO) with additive FeSO4 into the electrolyte. The microstructure, chemical composition and chemical state of the coatings were determined by SEM, XRD, EDS and XPS, respectively. The bonding strength between the coating and substrate was studied by tensile strength test, together with the thermal shock resistance of the coating. The results showed that Fe content in the coating layer significantly affect its thermal emissivity. The relative content of Fe in the coatings surface increased at first and then decreased with increasing the concentration of FeSO4 in electrolytes, so does the emissivity of the coatings. The bonding strength became weaker with increasing the concentration of FeSO4. In addition, the coating remains stable over 40 cycles of thermal shocking. The coating formed at 3g/L FeSO4 demonstrates the highest an average spectral emissivity value around of 0.87, and bonding strength higher than 33MPa. [Copyright &y& Elsevier]

Abstract: The formation, cell response and apatite-forming ability of novel TiO2-based coatings containing Si, Ca and Na (SCN) on titanium formed by microarc oxidation (MAO) were investigated. The oxidation time has obvious effect on the surface morphology and SCN elements concentrations of the MAO coatings. The current results indicated that the MAO coatings containing SCN possess good apatite-forming ability, which is highly dependent on the applied voltage. And high applied voltage could facilitate for improving the apatite-forming ability of the MAO coatings due to the introduction of a large quantity of SCN elements. It was firstly observed that the induced biomimetic apatite could grow into the pores of MAO coatings, which could facilitate for the fixation of apatite layer. The A-type slightly substituted carbonated-HA and B-type slightly substituted carbonated-HA with HPO4 2− group were obtained after immersion of the MAO coatings in a simulated body fluid. As expected, the MAO coatings show good biocompatibility according to the cells proliferation. [Copyright &y& Elsevier]

Abstract: To form bioactive microarc oxidized (MAO) coatings on titanium, the exploration for introducing various elements into the TiO2-based MAO coatings has been continually noticed. In this work, novel MAO coatings containing Si, Ca and Na (SCN) elements were prepared on titanium. The elemental composition, mechanical properties, corrosion resistance and apatite-forming ability of TiO2-based MAO coatings containing SCN were investigated. The surface hardness, elastic modulus, elastic recovery and corrosion resistance of the MAO coatings containing SCN were improved obviously by increasing the applied voltages. And the effect of applied voltage on the wetting ability of MAO coatings containing SCN is not significant. The current results indicated that the MAO coating containing SCN possesses good apatite-forming ability. The apatite-bonding structure is highly dependent on the chemical reactivity of the materials surface in simulated body fluid. [Copyright &y& Elsevier]

Abstract: The effects of a titania sol in an alkaline phosphate electrolyte on the thickness, surface morphology, composition and abrasive resistance of a microarc oxidation (MAO) coating on 2A70 aluminum alloy were investigated. The voltage–time curve was used to characterize MAO process. The coating thickness increased with increasing the concentration of the titania sol in the electrolyte, accompanied with the increase in surface nonuniformity and size of micropores in the coating surface. The MAO coating formed in the titania sol free electrolyte was mainly composed of γ-Al2O3 and a little α-Al2O3. Titanium in the form of Al2TiO5, Ti2O3 and TiO was detected in the MAO coatings obtained in the electrolyte containing titania sol, and the content increases with increasing the concentration of titania sol. The MAO coating formed in the electrolyte containing 6.0vol.% titania sol exhibited the highest microhardness and abrasion resistance. [Copyright &y& Elsevier]

Abstract: The Mg–Ca alloy system has been proposed as a potential new kind of degradable biomaterial with possible application within bone. Here microarc oxidation (MAO) coatings were fabricated on top of a Mg–Ca alloy using different applied voltages and the effect of applied voltage on the surface morphology and phase constitution, hydrogen evolution, pH variation in the immersion solution and in vitro biocompatibility of the MAO coating on the Mg–Ca alloy were extensively studied. It was found that the thickness and pore size of the MAO coating increased with the increasing applied voltage, whereas some micro-pores could be seen inside the 400V treated MAO coating. The 360V treated MAO coating gave the best long-term corrosion resistance during a 50days immersion test. All the MAO coatings could promote MG63 cell adhesion, proliferation and differentiation in comparison with the uncoated Mg–Ca alloy sample, due to significantly reduced Mg ion release and pH value variations in the culture medium. After 5days culture well-spread and elongated MG63 cells could be seen on the surface of the 360V and 400V MAO coatings, in contrast to no cells on the uncoated Mg–Ca alloy sample. In summary, MAO showed beneficial effects on the corrosion resistance of, and thus improved cell adhesion to, the Mg–Ca alloy, and should be a good surface modification method for other biomedical magnesium alloys. [Copyright &y& Elsevier]

Abstract: Microarc oxidized (MAO) coatings containing Si, Ca and Na elements (SCN) were prepared on titanium. The morphology, phase composition and elemental concentrations of the MAO coating surfaces highly depended on the applied voltages. The MAO coatings containing SCN show an atomic concentration gradient along the coating depth. The chemical states of Ti2+, Ti4+, Ca2+, Si2+ and O2− were observed in all the MAO coatings. The novel cefazolin sodium/chitosan composite films were deposited on the MAO coatings for application in therapy of the inflammation resulting from implanting. The increases of depositing times and cefazolin sodium concentration could increase the accumulative release concentration of cefazolin sodium, and could improve the sustaining-release ability of drug film. Also, the addition of chitosan could improve the sustaining-release ability of drug film, indicated that chitosan is a promising delivery carrier for cefazolin sodium. [Copyright &y& Elsevier]

Abstract: Sphene/titania composite coatings were fabricated on titanium by a hybrid technique of microarc oxidation (MAO) and heat treatment. The high-applied voltages promote the formation of sphene in the MAO coatings after heat-treatment. Heat treatment could change the surface morphology of the MAO coatings such as roughness, macropores size and density and the thickness of the MAO coatings. Increasing the heat-treatment temperature decreased the atomic concentration ratios of Ti/Si and Ti/Ca of the MAO coatings. The chemical states of Ti4+, Ca2+, Si2+ and O2− were observed on all the coatings. Additionally, Ti2+ was detected in the MAO and heat-treated MAO coatings at 600 and 700°C. The heat-treatment has obvious effect on the chemical states of Si, Ti and O elements due to the formation of sphene and oxidation of TiO phase of the MAO coating, but did not affect that of Ca. In the heat-treated MAO coatings at 800°C (MAO-H8), the titanium surface shows a MAO top layer and oxidized interior layer. A concentration gradient in components in the MAO layer of the MAO-H8 coating was formed. [ABSTRACT FROM AUTHOR]

Abstract: The mechanical and corrosion resistance of hydrophilic sphene/titania composite coatings on titanium formed by a hybid technique of microarc oxidation (MAO) and heat-treatment were investigated. The results indicated that the heat-treatment could improve the hardness, elastic modulus, elastic recovery and corrosion resistance of the MAO coatings, and reinforce the interface bonding between MAO coatings and titanium. A cefazolin sodium/chitosan drug film was prepared on the coating surfaces. The drug load procedures such as the addition of chitosan obviously increased the sustained-release ability of drug films. In addition, the increase of cefazolin sodium concentration could increase the accumulative release concentration of cefazolin sodium. The sustaining-release ability of drug films deposited on the MAO and heat-treated MAO coatings is similar. In the interior of drug film, the physical and chemical bonding reactions such as Coulombic interactions, van der Waals force and H-bonding etc. could be produced, through the chemical group interactions such as –OH and –NH2 groups of chitosan with –Czolin sodium. [ABSTRACT FROM AUTHOR]

Abstract: The process of microarc oxidation was studied as a pre-treatment for adhesive bonding of magnesium alloys. Porous microarc oxidation films on AZ31B magnesium alloy were prepared in a sodium silicate electrolyte by a high-frequency bipolar pulsing mode. The influences of duty cycle on film porosity and lap-shear strength of the bonding joints were studied. The higher duty cycle increased the film porosity and slightly decreased the thickness of the oxide film. The microarc oxidation film consisted mainly of MgO and MgSiO3. The increase of duty cycle resulted in the increase of MgO in the oxide film. As the duty cycle increased, the lap-shear strength increased. The highest lap-shear strength 24.5MPa was obtained under the duty cycle of 40%. The reason was attributed to the larger porosity and enhanced mechanical interlocking effect. [ABSTRACT FROM AUTHOR]