Characteristics Of Microarc Oxidation Coatings Research Articles

Influences of Sc(NO3)3 on characteristics of micro-arc oxidation coatings

ABSTRACT Micro-arc oxidation (MAO) coatings were fabricated on 2195 aluminium-lithium alloy substrate, using a silicate and phosphate electrolyte with variable scandium nitrate (Sc(NO3)3) concentrations. The properties of the coatings were analysed by scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical workstation. The comprehensive analysis demonstrates that adding proper amount of Sc(NO3)3 increased the oxidation voltage, which transformed the worm shape discharge micropores into round-hole shape, and the MAO coatings presented a double-layer structure. The primary phase composition of the coatings was γ-Al2O3, α-Al2O3, Sc2O3 and Al3Sc. The optimum concentration of Sc(NO3)3 was 0.6 g L−1. The hardness reached a maximum value of (1284 ± 43.5) HV, which was about 2.5 times higher than that without adding. The self-corrosion current density reduced from 138.00 × 10− 7 A cm−2 (substrate) to 3.65 × 10− 7 A cm−2, which was approximately two orders of magnitude lower, indicating that the corrosion resistance was improved.

Effects of the Power Supply Mode and Loading Parameters on the Characteristics of Micro-Arc Oxidation Coatings on Magnesium Alloy

Micro-arc Oxidation (MAO) is a technology for non-ferrous metal surface treatment through growth ceramic coating in situ. To determine the influence of the power supply mode and the loading parameters on the film forming of magnesium alloy micro-arc oxidation processing, the different power supply modes of pulsed direct current DC, pulsed bipolar current (BC) and the pulsed with a discharge loop current (DLC) was used with MAO technology on the AZ91D magnesium alloy. The power load parameters were optimized. The average energy consumption was calculated. Results showed that the role of the negative voltage in the bipolar pulse power supply is to restrain the large arc tendency. Under the pulse power supply with a discharge loop, the current and energy consumption decreases with the increase of the discharge resistance at the same pulse parameters. The big arc phenomenon can be effectively avoided and the impact of load capacitance could be effectively avoided by using the pulse power supply with a discharge loop. Moreover, the processing of the micro-arc oxidation is stable, the arc point is uniform, the surface of the film is smooth, the hole is uniform and the coating is dense, and the film efficiency is improved effectively.

Effect of CoSO4 on the characteristics of micro-arc oxidation coatings

ABSTRACTOxidation coatings were prepared on the surface of AZ31B magnesium alloy by micro-arc oxidation in the electrolytes with CoSO4 addition varying from 0 to 0.8 g/L. The effects of different concentrations of CoSO4 on the properties and microstructure of the coating were analysed by Scanning Electron Microscope (SEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD) and Electrochemical Workstation. The results revealed that the addition of different concentrations of CoSO4 caused the voltage variation, which resulted in the changes of microstructure, phase composition and element distribution of coatings. Furthermore, it led to the improvement of coating properties. It was found that the oxidation voltage increased firstly and then decreased. Because of CoSO4 addition, the morphology of the coating was changed. XRD patterns and XPS confirmed that Co3O4, MgO and Mg2SiO4 were generated. The addition of CoSO4 could effectively improve the hardness, corrosion resistance and thermal-shock resistance.

Effect of MgO micro-powder on the characteristics of micro-arc oxidation coatings

ABSTRACTMgO micro-powder of different concentrations from 0 to 8 g L−1 was added in the electrolyte. Effects of MgO micro-powder concentration on properties of micro-arc oxidation (MAO) coating were analysed by scanning electron microscopy, energy dispersive spectrometer, X-ray diffraction, electrochemical workstation, thermal shock tester equipment, etc. The result revealed that the concentration of MgO has evident effects on the properties of the MAO coating. When MgO micro-powder increased, the oxide voltage, the coating thickness and hardness increased first and then decreased. In this case, the diameter of the hole on the coating surface also increased and more linear holes appeared. The phases mainly consist of γ-Al2O3, α-Al2O3, MgAl2O4 and Mg2SiO4. As a result, the corrosion resistance and the adhesion of the MAO coating increase first and then decrease mainly related to the surface morphologies and phase of the coating. The coating has good thermal shock resistance. The performance of micro-arc oxidation coating with MgO micro-powder was improved.

Effect of MgO MicroParticles on Characteristics of Microarc Oxidation Coatings Fabricated on Pure Titanium

Effect of MgO MicroParticles on Characteristics of Microarc Oxidation Coatings Fabricated on Pure Titanium

Effect of Al2O3 Particle Concentration on the Characteristics of Microarc Oxidation Coatings Formed on Pure Titanium

Effect of Al2O3 Particle Concentration on the Characteristics of Microarc Oxidation Coatings Formed on Pure Titanium

Characteristics of microarc oxidation coatings deposited on magnesium using alkaline and acidic electrolytes in a single stage as well as using dual electrolytes in two stages

Microarc oxidation (MAO) of magnesium is performed in a single stage using an alkaline silicate electrolyte and acidic zirconate electrolyte as well as in two stages, sequentially using the alkaline silicate followed by the acidic zirconate electrolyte. The morphological characteristics, surface roughness, phase content, corrosion behaviour and bioactivity were compared. The surface morphology, thickness, average surface roughness (Ra), and phase composition of the MAO coatings show a strong dependence on the type of electrolytes used and the methodology employed for deposition. Based on the corrosion protective ability in Hank’s balanced salt solution, the uncoated and MAO coated Mg are ranked as follows: Uncoated Mg < MAO coated Mg prepared using alkaline silicate electrolyte < MAO coated Mg prepared using acidic zirconate electrolyte < MAO coated Mg prepared in two stages, sequentially using the alkaline silicate followed by the acidic zirconate electrolyte. Poorly crystalline calcium deficient apatite is formed on all samples after immersion in SBF at 37 °C for 240 h.

Effects of Pulse Duration on Structure and Surface Characteristics of Micro-Arc Oxidation Coatings Formed on Aluminum Alloy

Micro arc oxidation (MAO) of aluminum alloy samples was studied using bipolar pulses with various pulse durations under constant duty cycle. The processing times were chosen as 20, 30 and 40 minutes. The X-ray diffraction (XRD) technique, scanning electron microscope (SEM), surface roughness measurement (Ra) and indentation tests were employed to investigate the phase distribution, microstructure, surface properties and hardness of the coatings, respectively. By increasing the pulse duration and processing time, coatings with different surface roughness, thickness and hardness were fabricated.

Effects of Hypophosphate Concentrations on the Characteristics of Micro-Arc Oxidation Coatings Formed on Biomedical NiTi Alloy

Microarc oxidation coatings on biomedical NiTi alloy were prepared in aluminate electrolytes with and without hypophosphate addition. The effect of hypophosphate concentrations on the characteristics of micro-arc oxidation coatings has been studied. The compositions and surface morphologies of the coatings prepared in different hypophosphate concentrations were determined by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Surface roughness (Ra) and bonding strength of the coatings were measured by surface roughmeter and a direct pull-off test, respectively. The corrosion resistance of the coatings was evaluated in Hank’s solution using potentiodynamic polarization tests. The results show that all of the coatings exhibit a typical porous surface structure and mainly consist of γ-Al2O3 crystal phase. With increasing the hypophosphate concentrations, the elemental contents of Ni, Ti and P increase while Al decreases; the pore sizes and surface roughness of the coatings decrease firstly, reaching a minimum value at 0.01mol/L, and then increase; at the same time, the bonding strength increases up to 60MPa, and then decreases. The corrosion resistance of the coatings decreases with the increase of the hypophosphate concentrations, but all of the coated samples is better than that of the uncoated NiTi alloy.

Effect of Al(OH)3 sol concentration on characteristics of microarc oxidation coatings of titanium alloy

The effect of Al(OH)3 sol on the growth characteristics, microstructure, phase structure and electrochromic properties of the coating fabricated on titanium alloy substrates were investigated by means of changing the Al(OH)3 sol concentration. The results show that the rate of growth of coating thickness gradually increased from slow to fast with the increasing concentration of Al(OH)3 sol and the roughness and size of micropore on the coating surface gradually increased, while its pore density was reduced. The coating was composed of anatase TiO2 when the volume fraction C≤10%. The rutile TiO2 began to emerge at C&gt;10%, and its relative content gradually increased with the increase of the Al(OH)3 sol concentration. When C=40%, the obtained coating was composed of rutile TiO2. The cyclic voltammetry tests in pH 2.0 HCl solution showed that the color of the sample fabricated at C≤20% did not change significantly, with the increase of the Al(OH)3 sol concentration, the color change of the sample became clear. When C=40%, the obtained sample showed good stability and reversibility between blue coloration and bleaching cycles in pH 2.0 HCl solution.

Effect of electrical parameters on characteristics of microarc oxidation coatings of commercially pure titanium in colloid

The effects of the pulse voltage amplitude （U） and duty cycle （d） on the growth characteristics, microstructure, phase component and corrosion resistance of the coating formed by microarc oxidation （MAO） on commercially pure titanium substrates in colloid have been systematically studied. The results show that, with the increase of U or d, the coating thickness increases almost linearly, the pore size and the roughness of the surface increases gradually, while the pore density decreases. The coatings almost entirely consist of dense layer, and they are completely composed of rutile TiO2 except the one formed at the duty cycle of 10% （U=450 V）, which contains a small amount of anatase TiO2 The corrosion resistance test in sulfuric acid solution （30%） shows that the corrosion resistance of the samples is closely related to U and d, with the increase of U or d, the corrosion resistance of the samples increases gradually.

Effects of cathodic voltage pulse duty cycle on characteristics of microarc oxidation coatings of titanium alloy

Under the condition of keeping the bipolar pulse voltage constant, the effect of cathodic pulse voltage duty cycle （dc） on characteristics of microarc oxidation coatings of titanium alloy have been studied by using a multifunction microarc oxidation power supply. Experimental results show that the variations of cathodic and anodic currents with treatment time show obviously several stages, and the beginning and ending time of various stages are closely related to dc. The coating consists mainly of rutile and anatase phases, and there is a sudden change in the rutile phase content at dc=50%. The thickness, morphology and microhardness of the coating strongly depend on dc change. The microhardness valucs of the coatings prepared at dc=50% and dc=70% are the minimum （4.96 GPa） and the highest （6.1 GPa）, respectively. In general, the MAO coatings prepared in the range of 65%≤dc≤80% have higher growth rate and quality.

Effects of sodium tungstate on characteristics of microarc oxidation coatings formed on magnesium alloy in silicate-KOH electrolyte

Oxide coatings on AM60B magnesium alloy were prepared using the microarc oxidation(MAO) technique in silicate-KOH electrolyte with addition of 0-6.0 g/L Na 2WO 4. The MAO processes in base electrolyte with different concentrations of Na 2WO 4 were studied. The microstructure, compositions and mechanical tribological characteristics of the oxide coatings were also investigated by SEM, XRD, XPS, microhardness analysis and ball-on-disc friction testing, respectively. It is found that the addition of Na 2WO 4 into the base electrolyte has direct effect on the characteristics of voltage—time curves and breakdown voltage in MAO process. The number of micropores at top of the coating surface is increased by the addition of Na 2WO 4. The fraction of forsterite Mg 2SiO 4 in the oxide coating increases with increasing concentration of Na 2WO 4 in base electrolytes. Furthermore, the microhardness and wear resistance of oxide coatings are enhanced as well.

Effects of reinforcement phases in magnesium matrix composites on microarc discharge behavior and characteristics of microarc oxidation coatings

The ceramic coatings were synthesized on AZ91 Mg alloy, Al 18B 4O 33w/AZ91 and SiCw/AZ91 Mg matrix composites by microarc oxidation (MAO) technique in an alkali–silicate electrolyte solution. The microarc discharge phenomena of the three substrates during MAO process were described. SEM and XRD were employed to characterize microstructure and phase composition of the coatings. The corrosion resistance of the coatings was evaluated by electrochemical method. The results show that the three substrates have different voltage span of microarc discharge and different voltage evolution trend with increasing period during MAO. As a consequence, mass gain after oxidation and microstructure of coatings are also different for the three substrates. These MAO behaviors are believed to be attributed to the addition of reinforcement phases with different properties. The reinforcement phases in the composites have no influence on the phase composition of MAO coatings. MAO treatment could improve the corrosion resistance of Mg alloy and composites to different extent.