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The structure, phase composition, microhardness, and adhesive strength of a coating on an МЛ10 (ML10) magnesium alloy modified with Mg2SiO4 forsterite during microarc oxidation (MAO) have been studied. Sodium silicate additives were used as a forsterite precursor. A pattern has been established that reflects the minimum concentration of sodium silicate in the electrolyte composition, at which silicon compounds participate in the formation of a ceramic coating on magnesium and are introduced into it, forming complex compounds (Mg2SiO4). With the chosen MAO parameters and the basic composition of the electrolyte, the modification with silicon compounds begins with a concentration of sodium silicate in the electrolyte of 4 g/l. It has been established that at this concentration, a developed interface is formed, which increases the adhesive strength of the coating. In addition, the presence of forsterite in the coating leads to an increase in the thickness and microhardness of the coating.

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

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

Porous TiO2 ceramic coatings with high contents of Si and P were fabricated on a Ti6Al4V alloy using hybrid processing of laser surface melting (LSM) and microarc oxidation (MAO). The effect of LSM pretreatment on the growth behavior and mechanical properties of the MAO coating was studied. The results showed that the MAO coating on LSM pretreatment Ti6Al4V (LSM-MAO) had quite different surface morphologies, thickness, and compositions compared to the MAO coating on as-received titanium alloy. Grain refinement and dislocation in the LSM-treated Ti6Al4V alloy provided extra energy and more numbers of discharge channels during the MAO process. LSM–MAO coating exhibited a more uniform pore distribution, relatively small pores, and larger thickness compared to the MAO coating. The LSM-MAO coating had higher hardness and better wear resistance than the MAO coating. LSM is deduced to be an appropriate and efficient pretreatment method for achieving a high-quality MAO coating on titanium alloy. [ABSTRACT FROM AUTHOR]

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

Under the sodium aluminates' system, microarc oxidation treatment was conducted on the superhard aluminum alloy 7A04 for different times. The microstructure of microarc oxidation ceramic layer was investigated by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The influences of different oxidation times on the adhesion strength of ceramic layer and substrate, the morphology of surface and cross-section, the phase composition and the electrochemical properties were studied. The results indicated that the connection of the coating and substrate appears to be metallurgical bonding and dense ceramic layer, and the surface is in a 'volcanic vent' morphology, which is composed of -Al2O3 and little -Al2O3. The corrosion resistance of ceramic layer is improved significantly in contrast with that of the substrate. [ABSTRACT FROM AUTHOR]

In the present study, microarc oxidation (MAO) coatings were formed on ZL101A aluminum alloy in an electrolytic bath containing 3g/L KOH 2g/L Na2WO 4g/L KF. The morphology and wearing behavior were investigated. In both electrolytes, the additives were borates (Na2B4O718g/L) and silicates (Na2SiO3 18g/L), respectively. It was found that the coating formed in borates-containing electrolyte was of compact and smooth structure than that of the one formed in silicates-containing electrolyte at the optimum treatment time. It was found that all the coatings were composed of á-Al2O3 and ã-Al2O3. The microhardness and wear tests proved that the coating formed in borates-containing electrolyte was having better mechanical properties than those of the coating formed in silicates-containing electrolyte. [ABSTRACT FROM AUTHOR]

Microarc oxidation (MAO) coatings were prepared on 2024-T4 aluminum alloys using a pulsed bipolar power supply at different anode pulse-widths. After the MAO coatings were formed, the micropores and microcracks on the surface of the MAO coatings were filled with Fluorinated ethylene propylene (FEP) dispersion for preparing MAO self-lubricating composite coatings containing FEP. The effect of the anode pulse-width on the microstructure and wear resistance of the microarc oxidation coatings was investigated. The wear resistance of the microarc oxidation self-lubricating composite coatings was analyzed. The results revealed that the MAO self-lubricating composite coatings integrated the advantages of wear resistance of the MAO ceramic coatings and a low friction coefficient of FEP. Compared to the MAO coatings, the microarc oxidation self-lubricating composite coatings exhibited a lower friction coefficient and lower wear rates. [ABSTRACT FROM AUTHOR]

Microarc oxidation (MAO) composite coatings containing rutile TiO2 were produced on 2024 aluminum alloy in an electrolyte with nanorutile TiO2 particles. The microstructure and properties of the composite coatings were analyzed by SEM, EDS, laser confocal microscope, XRD, Vickers hardness tester, scratch test and friction test, respectively. The results showed that the composite coatings consisted of -Al2O3, -Al2O3, mullite and rutile TiO2. With increasing concentration of rutile TiO2 particles in the electrolyte, the compactness of the composite coatings was improved significantly. The abrasion performance of the microarc oxidation composite coatings containing rutile TiO2 was better than that of MAO coatings without rutile TiO2. [ABSTRACT FROM AUTHOR]

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

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

This study was initiated to compare the tribological performances of a 5754 quality aluminum alloy after hard anodic oxidation and microarc oxidation processes. The structural analyses of the coatings were performed using XRD and SEM techniques. The hardness of the coatings was determined using a Vickers micro-indentation tester. Tribological performances of the hard anodised and microarc oxidised samples were compared on a reciprocating wear tester under dry sliding conditions. The dry sliding wear tests showed that the wear resistance of the oxide coating generated by microarc oxidation is remarkably higher than that of the hard anodised alloy. [ABSTRACT FROM AUTHOR]

Ceramic coatings on friction stir welded (FSW) joints of AZ31B magnesium alloy were fabricated by microarc oxidation (MAO) method in silicate electrolyte. Microstructure, phase constituents, microhardness and electrochemical corrosion behaviors of bare and coated magnesium alloys at different zones of FSW joints for different oxidation time were investigated. The influence of microstructure at different zones on the growth of MAO coatings was analyzed. The results show that the MAO coatings on FSW joints are uniform, and they have almost the same morphology, phase constituents, hardness and corrosion resistance at base metal, stir zone and heat-affected zone. The properties of MAO coatings are independent on the microstructures of AZ31B alloy. In addition, the microstructures of magnesium alloy near the coating/alloy interface at different zones of FSW joint was not changed by microarc discharge process. [ABSTRACT FROM AUTHOR]

The experimental studies of the properties of the hardened surface layer, developed by the microarc oxidation method (MAO) on the surface of Al-Si ingots from AK12D alloy have been presented here. The effect of concentration of the electrolyte components on the properties of the MAO coating, such as microhardness, thickness, porosity have been studied. The corresponding regression equations to estimate the influence of the process parameters on the quality of the developed MAO-layer, have been set up. [ABSTRACT FROM AUTHOR]

Aluminium oxide ceramic composite coatings containing hexagonal BN (h-BN) powder were produced successfully by a two-step microarc oxidation process. The incorporation of h-BN powder was achieved by means of wiping an emulsion into the pretreated coatings between these two steps. The thermal shock resistance of the fabricated samples was tested via quenching method. The morphological characteristics and the distribution of elements were analysed by SEM, energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The microhardness of samples was determined by employing a Vickers indenter. The phase composition of composite ceramic coatings was investigated by X-ray diffraction. The results showed that the inclusion of h-BN powder has no influence on the growth and microhardness of alumina ceramic coatings. However, the modified coatings exhibit lower area and number of pores than conventional coatings. More significantly, the incorporation of a certain amount of h-BN powder improves the thermal shock resistance of aluminium oxide ceramic composite coatings even in a thick coating. [ABSTRACT FROM AUTHOR]

Abstract: Bioactive amorphous phase/TiO2 nanocrystal (APTN) composite coatings were fabricated by microarc oxidation (MAO) on Ti. The APTN coatings are composed of much amorphous phase with Si, Na, Ca, Ti and O elements and a few TiO2 nanocrystals. With increasing applied voltage, the micropore density of the APTN coating decreases and the micropore size of the APTN coating increases. The results indicate that less MC3T3-E1 cells attach on the APTN coatings as compared to Ti. However, the APTN coatings greatly enhance the cell proliferation ability and the activity of alkaline phosphatase. The amorphous phase and the concentrations of the released Ca and Si from the APTN coatings during cell culture have significant effects on the cell response. [Copyright &y& Elsevier]

Abstract: Ceramic coatings were obtained on AZ91D magnesium alloy by microarc oxidation under unipolar and bipolar modes in a silicate–aluminate dual electrolyte. The surface and cross-sectional morphology, coating thickness and phase composition of coatings were characterized by scanning electron microscopy (SEM), layer thickness meter and X-ray diffraction (XRD), respectively. The corrosion resistance of coatings in a 3.5% NaCl neutral solution was evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The results showed that the microarc oxidation (MAO) process in the unipolar mode was very similar to that in the bipolar mode. The average thickness of the coating obtained in the unipolar mode was thinner than that in the bipolar mode. Both coatings were mainly composed of MgO, Mg2SiO4 and MgAl2O4. Introduction of a cathodic component during the MAO process was helpful to obtain a uniform and compact coating. The electrochemical study indicated that the coating obtained in the bipolar mode was in better corrosion resistance due to its compact microstructure and thickness. [Copyright &y& Elsevier]

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]

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: 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: 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]

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: In this paper, thick and hard oxide coatings resistant to wear were produced on 2017A–T6 Al alloy by the microarc oxidation (MAO) technique in an alkali electrolyte consisting of different sodium silicate concentrations (0–8g/l). The coatings were characterized by means of optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and surface profilometry. Microhardness, scratch adhesion and pin-on-disk sliding wear tests were also performed to evaluate the tribological properties of the coatings. The influence of sodium silicate concentration on the structural and tribological properties of the MAO coatings was discussed. Results reveal that increasing sodium silicate concentration from 0 to 8g/l in the electrolyte caused an increase in the electrolyte conductivity (from 7.71 to 18.1mS/cm) and a decrease in positive final voltage (from 627 to 590V) in the MAO process. In response to the increase in sodium silicate concentration, the thickness, surface roughness (R a) and critical load (L c) corresponding to adhesive failure of the coatings were increased simultaneously from 74 to 144μm, and 4.4 to 6.58μm, and 127.76 to 198.54N, respectively. At the same time, the phase structure and composition of the coatings also varied by the participation of silicate ions in the reactions and their incorporation into the coating structure. Moreover, it was observed that the coating formed in the low sodium silicate concentration (4g/l) had higher surface hardness (2020HV) and improved wear resistance than the one (1800HV) formed in the high sodium silicate concentration (8g/l). The coatings produced in three different electrolytic solutions provided an excellent wear resistance and a load carrying capacity compared to the uncoated aluminum alloy. [ABSTRACT FROM AUTHOR]

Microarc oxidation (MAO) coatings were prepared on 2024 aluminum alloy in a Na2SiO3-KOH electrolyte with KMnO4 addition varying from 0 to 4 g/L. The microstructure and phases of the coatings were characterized by scanning electron microscopy (SEM) and X-ray diffractometry (XRD), respectively. The corrosion resistance of MAO coatings was evaluated by electrochemical potentiodynamic polarization in 5% (mass fraction) NaCl solution. The results show that when KMnO4 is added into base electrolyte, the growth speed of oxide coatings is increased obviously. The main phase of oxide coatings is Al2O3, and the contents of MnO2 and Mn2AlO4 phases are increased at the top of oxide coatings with increasing the concentration of KMnO4. The solute elements participate in forming the oxide coatings. When a proper concentration of KMnO4 (2.5 g/L) is added into the base solution, the micropores of the MAO coatings are small and compact, and the corrosion resistance of oxide coatings is increased largely. [ABSTRACT FROM AUTHOR]

Abstract: The corrosion-resistant ceramic coatings up to 80μm thick were fabricated on SiCP/AZ31 magnesium matrix composite by microarc oxidation (MAO) technique in Na3PO4 +KOH+NaF solution. The microstructure, composition and phase constituent of ceramic coatings were analyzed by SEM and XRD, and the electrochemical corrosion behaviour of coatings was evaluated by the electrochemical polarization method. The thicker coating is compact and displays a good adhesion to the composite substrate. The ceramic coatings consist of MgO, Mg2SiO4, MgF2, Mg3(PO4)2, furthermore, a few residual SiC phases were also found in the coatings by means of SEM observation and EDX analysis. Most of SiC reinforced particles in the oxidized composite substrate have transformed into the oxides under microarc discharge sintering, but a few residual SiC reinforcements in the MAO coatings have not disrupted the continuity of coatings. So the corrosion resistance of the SiCP/AZ31 composite is greatly improved by MAO surface treatment, however, the corrosion resistance of coated composite also depends on the coating thickness. [Copyright &y& Elsevier]

Abstract: TiO2-based coatings containing P (T–P) were prepared on Ti6Al4V by microarc oxidation (MAO) with applied voltages of 200–400V in an electrolyte containing (NaPO3)6 and NaOH. The surfaces of the T–P coatings became rough and the thickness increased with increasing the applied voltage. Above 200V, anatase was found on the surface, and rutile was observed at 400V. With increasing the coating thickness, the O and P concentrations increase; while Ti and Al concentrations decrease. Ti, O and P elements display a uniform distribution character around the micropores on the surface of the T–P coating formed at 300V. However, the inner of the micropores exhibits a high Ti concentration and low O and P concentrations due to the graded distributions of Ti, O and P elements in the T–P coating. The apatite-forming ability of the T–P coating formed at 300V was evaluated by immersing in a simulated body fluid (SBF) for 28 and 56 days. The results indicate that biomimetic apatite was formed on the surface of the T–P coating after immersion in SBF for 56 days. And the further cell experiment indicates that the T–P coating can provide surface suitable for the MG63 cell proliferation. [Copyright &y& Elsevier]

Abstract: Chemical- and heat treatment was performed to modify the surface of the microarc oxidized TiO2-based (TOB) film containing P to produce nano-scale compounds containing Na, Ti and O elements. In the TOB film, anatase and rutile nanocrystals were randomly distributed in P-doped matrix. On the surface of the chemically treated TOB (C-TOB) film, amorphous titanium oxide containing Na shows nano-scale ribbonlike morphology. Na, Ti and O show uniform distribution in the outer layer of the C-TOB film along surface depth. Chemical treatment did not alter the surface roughness of the TOB film obviously; however, it improved its hydrophilic property. Heat treatment has no influence on the chemical states of Ti, Na and O, as well as wetting ability, elemental composition and atomic concentration in the outer layer of the C-TOB film. However, the phase compositions and surface morphology of the C-TOB film after heat treatment are dependent on the heat treatment temperature. [Copyright &y& Elsevier]

Abstract: Three functional coatings (namely Al-C, Si-P-Al and P-F-Al coating) were fabricated by microarc oxidation method on Ti6Al4V alloy in different aqueous solutions. The microstructure, phase and chemical composition of coatings were investigated using scanning electron microscope, X-ray diffraction and energy dispersive spectroscopy. The interface adhesion failure mode of the coating is revealed by shear, tensile and thermal shock methods. The coatings exhibit high adhesion strength by the quantitative shearing test, registering as 110, 70, and 40MPa for Al-C, Si-P-Al and P-F-Al coating, respectively. The tensile test of the coated samples shows that microarc oxidation treatment does not significantly deteriorate mechanical properties of substrate titanium alloy. The observations of the coating failure after subjected to the identical tensile elongation of 3.0% are well in agreement with those results of the shear test. The thermal cycle test indicates that all the coatings have good anti-thermal shocking properties. [Copyright &y& Elsevier]

Abstract: The microstructure, composition and corrosion performance of oxide coatings formed on AM60B alloy using microarc oxidation techniques at different waveforms of applied current densities were investigated within this study. It is found that the use of optimizing current density waveforms, i.e. decaying freely current density in the later stage and stepped decreasing current density, significantly improved the microstructure of oxide coatings compared with the constant current density mode, which are connected with changes in behaviors of spark discharges on the surface in oxidation process. The optimal waveform of current density is showed to be decaying freely current density in the later stage, which results in sealing the originally formed large micropores. The optimisation of the microstructure results in a significant improvement of the corrosion resistance of oxide coating. [Copyright &y& Elsevier]

Abstract: The ceramic oxide films with a thickness of 120 μm were prepared on γ-TiAl intermetallic compound by alternating-current microarc oxidation in silicate electrolyte. Isothermal oxidation of TiAl alloy coated with the ceramic oxide film was carried out in air at 1100 °C. Result showed that the dense layers in the oxide film were mainly composed of Al2TiO5 and TiO2 rutile phase, while the loose layer contained a large amount of amorphous SiO2 besides Al2TiO5 and TiO2 rutile phase. Thermal shock tests revealed a good adhesion between the films and TiAl substrate, and the oxidation resistance of the TiAl alloy was improved. [Copyright &y& Elsevier]

Abstract: Ceramic coatings were formed on Ti6Al4V alloy surface by microarc oxidation (MAO) in a Na2SiO3 system solution. Unlubricated, smear oil and oil bath lubricated fretting tests were performed on MAO coatings against 52100 steel on a fretting wear tester. Microstructural investigation of the worn surfaces was performed and the wear mechanisms were studied. The results show that the coatings are mainly composed of rutile and a small amount of anatase TiO2, both in nano grain structure. Friction coefficient of microarc oxidation coatings under oil bath lubrication was significantly reduced, favorable stable at 0.15, which indicates that the coatings with oil lubricated lowered the shear and adhesive stresses between contact surfaces, consequently alleviating the possibility of initiation and propagation of cracks in the inner layer of the coating or titanium alloy substrate. [Copyright &y& Elsevier]

Abstract: Ceramic coatings were formed on the surface of Ti6Al4V alloy by microarc oxidation (MAO) in a Na2SiO3 system solution using a pulsed bipolar power supply. The microstructure, phase and elemental composition of the ceramic coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The distribution of mechanical properties across the coating and its adhesion strength were determined by a nanoindentation and a shear test respectively. The tribological properties were evaluated using a pin-on-disk wear tester. The results show that the coatings are mainly composed of rutile and a small amount of anatase TiO2, with amorphous compounds from electrolyte components incorporated. The hardness and elastic modulus are about 8.5 GPa and 87.4 GPa for the compact region of the coating, and about 4 GPa and 150 GPa for the substrate. Similar profiles of hardness and elastic modulus along the coating of about 45 μm thickness is found, and both values in the compact region remain mainly constant before finally declining to below those of the substrate in the outer loose region. The adhesion strength is about 70 MPa. The friction coefficient of the coating against steel is as low as 0.2–0.3 at <1 N load and at <2500 sliding cycles. However, the oxidation wear mechanism occurring at the later stage of wear tests leads to a gradual increase of friction coefficient. [Copyright &y& Elsevier]

Abstract: The coatings mainly composed of nanostructured TiO2 were deposited on Ti6Al4V alloy by microarc oxidation (MAO). The duplex coatings of microarc oxidation combined with spraying graphite process were fabricated for the antifriction purpose. The tribological properties of unpolished, polished and duplex coating against steel under dry friction conditions were examined. It is found that antifriction property of the polished microarc oxidation coating is superior to that of the unpolished one. The improvement is attributed to the low surface roughness and the nanocrystalline structure of coatings. The duplex coating exhibits best antifriction property, registering a lower and steady friction coefficient of ≈0.12 than that of the polished microarc oxidation coating sliding in the similar condition. The good tribological property is attributed to the specially designed duplex structure, the coating adhering strongly to the substrate and serving as the load-supporting underlayer and the graphite layer on top of it working as solid lubricant. [Copyright &y& Elsevier]

Abstract: Microarc oxidation coatings were fabricated on Ti6Al4V alloy in NaAlO2 containing solution. Microstructure and adhesion properties of the coatings were evaluated. For antifriction purpose, a novel duplex coating of microarc oxidation combined with spraying graphite process was developed. The results show that the microarc oxidation coating with 20μm thickness is compact and uniform in the inner layer, while porous in the surface layer. The coating has high adhesion strength of about 110MPa to the substrate. The novel duplex coating exhibits good antifriction property, registering friction coefficient of ≈0.12, which is 5 times lower than that of the microarc oxidation coating sliding in the similar condition. The good tribological property is attributed to the specially designed duplex structure, the coating adhering strongly to the substrate and serving as the underlying loading layer and the graphite layer on top of it working as solid lubricant. [Copyright &y& Elsevier]

Abstract: Ceramic coatings were formed on a Ti6Al4V alloy in a Na2SiO3–KOH–(NaPO3)6 aqueous solution, using the pulsating microarc oxidation method. A constant voltage (500V) was applied to reveal the effect of the positive-pulse duty cycle on the growth rate, microstructure and phase composition of the coating. To obtain the optimum process, a constant current density (60mAcm-2) combined with a stepped adjusting of the positive-pulse duty cycle was also proposed. The coatings are mainly composed of anatase and rutile (TiO2). As the duty cycle increases, rutile increases and becomes the predominant phase. The coating formed at a duty cycle of 8% indicates an optimum combination of structure and thickness. When applying a constant current density, the growth rate of coatings is significantly increased compared with the application of a constant voltage. Further, using the stepped adjusting of the duty cycle, the microstructure of the coatings is improved obviously. [Copyright &y& Elsevier]

Ceramic coatings were fabricated on a Ti6Al4V alloy surface by microarc oxidation (MAO) in Na2 SiO3 – (NaPO3)6 aqueous solutions with and without NaAlO2 additive using an AC power supply. The effect of NaAlO2 on microstructure, composition, and homogeneity of ceramic coatings were characterised using SEM, XRD, and EPMA. The antifriction property of the coatings with optimised microstructure sliding against SAE 52 100 steel ball was investigated on a pin-on-disc friction and wear tester. The results show that the addition of NaAlO2 into Na2 SiO3 – (NaPO3)6 solution assists the formation of more dense, uniform, and thicker coatings and increases rutile TiO2 content in the coatings. The optimised coating sliding against the steel has a friction coefficient as low as 0.2 – 0.3 at an applied load of 0.5 N and sliding cycle below 2500, which is much smaller than that of uncoated Ti6Al4V against the same counterpart. The transferring of material from the softer steel ball onto the coating surface is the main wear event, while the microarc oxidation coating is characterised by slight abrasive wear and adhesive wear. [ABSTRACT FROM AUTHOR]

Ceramic coatings were fabricated on Ti6Al4V alloy surface by microarc oxidation (MAO) in Na2SiO3–(NaPO3)6–NaAlO2 solution using an AC power supply. Microstructure and phase composition of coating were characterized by SEM and XRD, respectively. The antifriction property of the coating with and without solid lubricant sliding against SAE 52100 steel ball was investigated on a pin-on-disk friction and wear tester. The results show that the microarc oxidation coating is relatively dense and uniform, mainly composed of rutile and anatase. The coating sliding against the steel has friction coefficient as low as 0.2–0.3 at an applied load of 0.5 N and sliding cycle below 2500, which is much smaller than that of uncoated Ti6Al4V against the same counterpart. The transferring of materials from the softer steel ball onto the coating surface is the main wear event, while the microarc oxidation coating is characterized by slight abrasive wear and adhesive wear. Introducing solid graphite lubricant into the porous surface of microarc oxidation coating significantly improves the long-term antifriction property (registering friction efficient of 0.15 in the long-term sliding) under a similar sliding condition. This improvement is attributed to the reduction of materials that are transferred from the softer steel ball onto the coating surface. [ABSTRACT FROM AUTHOR]

Ceramic coatings were fabricated by microarc oxidation in galvanostatic regime on Ti6Al4V alloy in (NaPO3)6–NaF–NaAlO2 solution. The growth, microstructure and phase composition of coatings were investigated using scanning electron microscope and X-ray diffraction. With increasing treatment duration, coating growth varies from rapidness to tardiness accompanied by gradually roughening in appearance. Meanwhile, phase transformation of anatase to rutile occurs. The crystalline AlPO4 is involved in the coatings via high-temperature thermolysis of hydrated aluminium polyphosphates in the nearby discharging channels. The stepped current regime enables coating structure to be controllable. The mechanical properties distribution across the coating thickness and the adhesion strength were determined by nanoindentation and shear test, respectively. A similar evolution profile of hardness and elastic modulus across the coating thickness is found: remaining high values (5.5 and 69.1 GPa) in the compact region before finally declining to low values (5.1 and 65.6 GPa) in the looser region. The adhesion strength of substrate/coating interface is about 40 MPa. [Copyright &y& Elsevier]

In order to optimize the production regime for effective coatings of required thickness on Ti6Al4V surface, a comparative study was made on the kinetics of ceramic coatings growth, and was associated with the evolution of microstructure and phase composition during microarc oxidation (MAO) under constant voltage and constant current density regime, respectively, using an Ac pulse power supply in Na2SiO3–KOH–(NaPO3)6 solution. The thickness of the resulting coatings was measured using a coating thickness gauge based on eddy principle. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were employed to characterize the microstructure and phase composition of the coatings. The results show that as treatment time increases, the coating grows in an exponential function accompanied by a gradually decaying current density for constant voltage control mode (U=500 V), For constant density control mode (J=60 mA/cm2), voltage increases rapidly up to a relatively stable value and the coating grows linearly; compared with the constant voltage mode, the coating grows faster. Phase composition and microstructure evolution during MAO process are almost independent of control mode. With increasing treatment time, the predominant phase composition varies from anatase to rutile, which indicates that phase transformation of anatase into rutile occurs in the duration of oxidizing process. Meanwhile, the size of micropores existing on the coating surface increases and thus the surface becomes much coarser. [Copyright &y& Elsevier]

Abstract: We prepared various porous structures on magnesium substrates by microarc oxidation (MAO) pretreatment in different solutions, and then controllably deposited electroless nickel (EN) layers on them under identical plating conditions. The results indicate that the microstructures, electrical and electrochemical properties of EN layers are highly dependent on the porous surface. Thin layer including tiny nickel granule is deposited on NaF-solution-pretreated substrates, but thick layer, consisting of typical nickel nodule, is formed on substrates oxidized in Na2SiO3 solution, while excellent electrical conductivity and corrosion resistance are obtained. Whereas, the reticular-structured layers with wormlike nickel as well as high thickness and resistivity are area-selectively deposited on the substrate pretreated in Na3PO4 solution. [Copyright &y& Elsevier]