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The structural and phase transformation of microarc oxidation (MAO) coatings on 6061 aluminum alloy at 300∘C/3 MPa steam were evaluated. After 10 days steam corrosion, the weight loss of coated alloys with 15 μ m and 75 μ m thickness was only about 1/10 and 1/50 of weight gain of bare alloy, respectively. The γ -Al2O3 and α -Al2O3 phases in the MAO coatings were stable at 300∘C/3 MPa steam, but many fine needle-like or brick-like boehmite crystalline particles also appeared on coating surface. The boehmite had a large influence on the permittivity of coatings at low frequency. The MAO treatment on 6061 aluminum alloy improved its corrosion resistance at high-temperature and high-pressure steam. [ABSTRACT FROM AUTHOR]

Micro/nano-structured coatings with antibacterial function were prepared by microarc oxidation (MAO) treatment on Ti6Al4V alloy in a silicate/phosphate electrolyte with a NaF additive. The microstructure, phase composition, and corrosion resistance of the coatings modified by adding NaF (0.15–0.5 M) were examined using scanning/transmission electron microscopy, energy dispersive spectroscopy, atomic force microscopy, X-ray diffraction, and potentiodynamic polarization. The results showed that the incorporation of F ion reduced the threshold voltage for electrons avalanche on the surface film of the Ti alloy, and increased the intensity and lifetime of discharge. MAO coatings with 100–500 nm nano-pores and 1–20 μ m micro-pores were formed by the modification of the NaF additive. The F ions promoted microarc discharge as well as phase transformation from the metastable anatase to the stable rutile phase. The F ions also promoted the generation of penetration cracks and bubbles in the coating. The surface roughness, phase content, and thickness of the coating were enhanced by the NaF additive. However, the corrosion resistance of the coating first increased and then decreased with the increasing F ion concentration, reaching a maximum when the NaF content was 0.25 M. [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) 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]

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]

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]

Black and gray microarc oxidation coatings have been obtained on 2A70 Aluminum alloy in phosphate electrolyte with and without titania sol, respectively. The growth process of the microarc oxidation coating in the electrolyte with titania sol was investigated. The coating was characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray. The coating thickness was measured by eddy current thickness meter. The results show that the titania sol increase the growth rate of microarc oxidation coating. In both cases the composition of coatings contain Al and O, and are mainly composed of γ-Al2O3 and AlPO4. Compared with the gray coating, large amount of Ti is found on the surface of black coating. The titania sol added in the electrolyte results in Ti in the coating, in the form of TiO and Al2TiO5. [ABSTRACT FROM AUTHOR]

The wear resistances of AZ91D magnesium alloy with and without microarc oxidation (MAO) coating and Ti6Al4V alloy in artificial saliva were investigated at room temperature. The wear resistance of AZ91D magnesium alloy in artificial saliva was significantly improved after microarc oxidation treatment. The volume loss of untreated AZ91D magnesium alloy sample was 20.95 times of that of AZ91D magnesium alloy with MAO coating, and that of Ti6Al4V was 5.42 times of that of MAO. Furthermore, the wear resistance of AZ91D magnesium alloy was lower than that of Ti6Al4V alloy in artificial saliva. The wear mechanisms of AZ91D magnesium and Ti6Al4V were discussed. It was found that the wear mechanism of the MAO was associated with abrasion and microfracture. There were two dominative wear mechanisms for AZ91D alloy and Ti6Al4V alloy under the loading conditions used in the experiment, namely, micro-machining wear and deformation-induced wear. [ABSTRACT FROM AUTHOR]

The thick ceramic coatings were prepared on 6061 aluminum alloys by microarc oxidation in a silicate electrolyte. The morphology and phase constituent of the coatings with different thickness were studied by scanning electron microscope, and X-ray diffraction. Scratch and thermal shock tests were employed to evaluate the adhesion between the coating and alloy substrate. The maximum microhardness and its position away from alloy/coating interface increases with increasing the coating thickness. The critical scratch force for the coating break was about 55 N in the 50 μm thick coating. Thermal shock resistance of the coatings depend on their thickness and thinner coating has better thermal shock resistance. The coated alloy with 50 μm thick coating can be subjected to 50 times thermal cycles of heating up to 500°C followed by quenching into water. [ABSTRACT FROM AUTHOR]

The effects of the electrical bias conditions on oxide film during titanium MAO (microarc oxidation) were investigated. Since DC voltage showed very low stabilized currents but produced oxide films with poor quality, the magnitude of applied voltages, duty cycles, and frequencies were modulated to control the quality of the oxide film. Pulsed voltage showed increased stabilized currents but produced better oxide films. The change in the stabilized currents was attributed to the change of the resistances of the film and local electrolyte adjacent to the anode. Analysis of the film thickness and the resistances of the film calculated from the experiments led to conclusion that the resistance of the electrolyte adjacent to the anode was increased if the negative ions attracted towards the anode were not reacted as fast as the positive ions at the cathode. The increase in resistance was attributed to the excess negative ions repelling other negative ions. In order to relax the negative ions, it appeared that a pulsed voltage source was essential. However, higher off-duty cycle was not the best choice to produce good oxide films indicating that the electrical bias condition depended on the experimental configuration. [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]