Microstructure and 3D microtomographic characterization of porosity of MAO surface layers formed on aluminium and 2214-T6 alloy

A fine characterization of the microstructure of the MAO coatings formed on aluminium (purity 99,999%) and its alloy 2214-T6 was observed using field emission gun scanning electron microscopy (FEG-SEM) with energy dispersive spectrometry (EDS) and X-ray microtomography (XMT). In addition, the influence of current frequency (pure aluminium) and the precipitates of substrate were investigated (case of 2214-T6 alloy).The MAO surface layers formed on aluminium and its alloy 2214-T6 can be divided into two parts: the outer layer with high porosity (so-call porous layer) and the inner layer with low porosity (so-call dense layer). However, the porosity of the inner layer increases toward the MAO layer/substrate interface. It is found that, both the thickness and the porosity increase when the current frequency decreases. The porosity of the MAO coating can be attributed to discharges formation in the vicinity of the MAO coating/substrate interface.In the particular case of 2214-T6 aluminium alloy (3.9% Cu), with coarse non-valve metal rich precipitates aligned perpendicular to the surface of the work electrode; the transformation of precipitates under discharge effect can provoke the formation of big channels, which go through the MAO surface layer. This last one induces local formation of a layer rich electrolyte species toward the substrate.     

Microstructure characterization of as-fabricated and 475 °C annealed U-7 wt.% Mo dispersion fuel in Al-Si alloy matrix

High-density uranium (U) alloys with an increased concentration of U are being examined for the development of research and test reactors with low enriched metallic fuels. The U-Mo fuel alloy dispersed in Al-Si alloy has attracted particular interest for this application. This paper reports our detailed characterization results of as-fabricated and annealed (475 °C for 4 h) U-Mo dispersion fuels in Al-Si matrix with a Si concentration of 2 and 5 wt.%, named as “As2Si”, “As5Si”, “An2Si”, “An5Si” accordingly. Techniques employed for the characterization include scanning electron microscopy and transmission electron microscopy with specimen prepared by focused ion beam in situ lift-out. Fuel plates with Al-5 wt.% Si matrix consistently yielded thicker interaction layers developed between U-Mo particles and Al-Si matrix, than those with Al-2 wt.% Si matrix, given the same processing parameters. A single layer of interaction zone was observed in as-fabricated samples (i.e., “As2Si”, “As5Si”), and this layer mainly consisted of U₃Si₃Al₂ phase. The annealed samples contained a two-layered interaction zone, with a Si-rich layer near the U-Mo side, and an Al-rich layer near the Al-Si matrix side. The U₃Si₅ appeared as the main phase in the Si-rich layer in “An2Si” sample, while both U₃Si₅ and U₃Si₃Al₂ were identified in sample “An5Si”. The Al-rich layer in sample “An2Si” was amorphous, whereas that in sample “An5Si” mostly consisted of crystalline U(Al,Si)₃, along with a small fraction of U(Al,Si)₄ and U₆Mo₄Al₄₃ phases. The influence of Si on the diffusion and reaction in the development of interaction layers in U(Mo)/Al(Si) is discussed in the light of growth-controlling mechanisms and irradiation performance.     

Microstructure, dielectric and ferroelectric properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 (NBTB) and 0.05BiFeO3-0.95NBTB ceramics: Effect of sintering atmosphere

0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ (NBTB) and 0.05BiFeO₃-0.95NBTB (BF-doped NBTB) lead-free ceramics were prepared by solid state reaction method. The ceramics were sintered at 1180 °C for 2 h in O₂ and N₂. All ceramics exhibited a single phase of perovskite structure. Relative amount of tetragonal phase was related to the sintering atmospheres. Both grain size and shape were influenced by the sintering atmospheres. Sintering the ceramics in N₂ weakened their dielectric anomalies corresponding to the transition from ferroelectric phase to the so-called “intermediate phase”. When the NBTB and BF-doped NBTB ceramics were sintered in N₂, their maximum dielectric constant and the degree of diffuseness of the transition from the “intermediate phase” to paraelectric phase increased, but their Curie temperatures decreased. The difference in dielectric properties of the ceramics sintered in different atmospheres was closely related to the difference in oxygen vacancy concentration. The correlation between ferroelectric properties and sintering atmospheres is associated with a competing effect among oxygen vacancy concentration, A-site vacancy concentration and percentage of tetragonal phase.     

Formation of dicalcium phosphate dihydrate on magnesium alloy by micro-arc oxidation coupled with hydrothermal treatment

A layer containing dicalcium phosphate dihydrate (DCPD) and β-Ca₃(PO₄)₂ was prepared on magnesium alloy by hydrothermal treatment of micro-arc oxide (MAO) layer. The biocorrosion resistance of the oxide layers before and after hydrothermal treatment was analyzed by anodic polarization and electrochemical impedance spectroscopy (EIS) in Hank’s solution. The prepared MAO layers consisted mainly of MgO and MgAl₂O₄, and Ca and P inside the oxide layers existed with amorphous phase. Hydrothermal treatments not only made the amorphous Ca and P change into DCPD and β-Ca₃(PO₄)₂ crystals, but also improved the biocorrosion resistance of magnesium alloys, especially the pitting corrosion resistance.     

Cyclic oxidation behavior and thermal barrier effect of YSZ-(Al2O3/YAG) double-layer TBCs prepared by the composite sol-gel method

Novel YSZ (6 wt.% yttria partially stabilized zirconia)-(Al₂O₃/YAG) (alumina-yttrium aluminum garnet, Y₃Al₅O₁₂) double-layer ceramic coatings were fabricated using the composite sol-gel and pressure filtration microwave sintering (PFMS) technologies. The thin Al₂O₃/YAG layer had good adherence with substrate and thick YSZ top layer, which presented the structure of micro-sized YAG particles embedded in nano-sized α-Al₂O₃ film. Cyclic oxidation tests at 1000 °C indicated that they possessed superior properties to resist oxidation of alloy and improve the spallation resistance. The thermal insulation capability tests at 1000 °C and 1100 °C indicate that the 250 μm coating had better thermal barrier effect than that of the 150 μm coating at different cooling gas rates. These beneficial effects should be mainly attributed to that, the oxidation rate of thermal grown oxides (TGO) scale is decreased by the “sealing effect” of α-Al₂O₃, the “reactive element effect”, and the reduced thermal stresses by means of nano/micro composite structure. This double-layer coating can be considered as a promising TBC.     

The air oxidation of an austenitic Fe-Mn-Cr stainless steel for fusion-reactor applications

The oxidation in air of an austenitic Fe-Mn-Cr steel containing 17.8 Mn, 9.5 Cr, 1.0 Ni, 0.27 C, and 0.03 N was studied over the range 700–1000°C. Oxidation of surface-abraded samples at low temperatures, 700–750°C, resulted in only Mn ₂O₃ containing dissolved chromium, except at corners, where large nodules containing spinel and manganowustite formed. The Mn₂O₃ layer grew into the substrate forming a globular-type film. This growth mode was the result of slow interdiffusion in the alloy after the cold-worked surface layer had been recrystallized and/or consumed, as evidenced by the formation of a ferrite layer subjacent to the scale and by the instability of the planar interface. No internal oxidation was observed beneath the Mn₂O₃ film at either 700 or 750°C. Samples oxidized in thehigh-temperature region, 800–1000°C, exhibited vastly different behavior, forming thick stratified scales at long times (24 hr), the scales consisting of a very thin outer layer of Mn₂O₃ (with appreciable iron in solution), Fe-Mn spinel beneath the outer layer, and a thick inner layer of manganowustite and a chromium-containing spinel. No chromium was found in the outer two layers. A thin layer of nearly pure Fe₂O₃ formed between Mn₂O₃ and the outer spinel. Quasiparabolic kinetics were observed. The high-temperature rates were about 10³ to 10⁴ times greater than at low temperatures at the transition temperature. The rapid rates at high temperatures were attributed to manganowustite growth. However, oxidation of an electropolished sample at 750°C, from which the superficial cold-worked layer had been removed, formed scales similar to those observed at high temperatures at comparable rates. A difference by a factor of over 10⁴ existed between the oxidation rate of the electropolished sample and the surface-abraded sample at 750°C. The much slower oxidation rate of the latter is attributed to greatly enchanced manganese diffusion through the high dislocation-density, cold-worked layer. Short-time tests at 800°C revealed an incubation period during which a thin protective layer of Mn²O₃ formed. The incubation period corresponded to the recrystallization time of the cold-worked layer. Subsequently, nodular growth occurred which was associated with internal oxidation. The nodules, consisting of spinel and manganowustite, eventually linked up to form a thick, stratified scale. Comparison of the scale structures with calculated phase diagrams of composition versus oxygen activity (at constant temperature), showed that the protective films formed at low temperatures were due to kinetics factors, involving enhanced manganese diffusion through the cold-worked layer, rather than to thermodynamics. A model for the breakdown of protective films is proposed which involves internal oxidation.     

An electrochemical and superficial assessment of the corrosion behavior of AA 2024-T3 treated with metacryloxypropylmethoxysilane and cerium nitrate

The present work aims at studying the corrosion behavior of treatments based on the deposition of layers of metacryloxypropylmethoxysilane (MAOS) and/or cerium nitrate on aluminum alloy 2024 T-3 (AA2024-T3). The corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS) during immersion in 0.1 M Na₂SO₄ and NaCl solutions. Atomic force microscopy (AFM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were also used to perform a surface analysis before and after the treatments. The electrochemical results show that cerium nitrate, when present between two layers of MAOS (sandwich-type deposited layer), improves the corrosion resistance. This can be attributed to the presence of an internal layer rich in silicium and cerium and another external MAOS layer, which further improves the barrier effect of the layer.     

低碳钢在海水中的阴极电化学行为

采用电化学技术结合XRD分析, 研究了A3碳钢在海水中的阴极电化学行为, 探讨了锈层在阴极过程中的作用. 碳钢表面生成的锈层由内锈层和外锈层组成, 内锈层主要组成相为γ-FeOOH, α-FeOOH, β-FeOOH以及Fe₃O₄与γ-Fe₂O₃的混合物. 浸泡126 d时, 外锈层主要由γ-FeOOH组成; 浸泡364 d由γ-FeOOH, α-FeOOH, Fe₃O₄和 γ-Fe₂O₃组成. 不同锈层在阴极过程中所起的作用不同. 外锈层主要作用是阻碍溶解氧到达金属表面, 内锈层除此之外还可以参与还原反应, 加速阴极反应. 提出了一个评价锈层参与还原反应程度的参数α, 在浸泡不同时期锈层参与还原反应的比例不同, 浸泡前7 d, α值上升比较明显, 随后增加比较缓慢, 浸泡168 d后基本稳定. 探讨了内、外锈层组分的变化以及锈层各组分间的相互作用.     

Formation of subsurface crevices in aluminum alloy 2024-T3 during deposition of cerium-based conversion coatings

The processing variables that contributed to the formation of subsurface crevices under cerium-based conversion coatings on AA 2024-T3 were investigated. Focused ion beam milling revealed the presence of subsurface crevices underneath a small fraction (∼ 10%) of coated areas, typically in areas with large cracks through the coatings. A solution of sodium chloride and H₂O₂ etched AA 2024-T3 and produced features similar to subsurface crevices, which confirmed that crevices formed during deposition due to the composition of the coating solution. Using sodium nitrate in place of sodium chloride resulted in no etching of the substrate. Thus, coatings free of subsurface crevices could be produced by using cerium nitrate instead of cerium chloride in the coating solution. Electrodeposited coatings, even those deposited from solutions containing chloride ions and H₂O₂, were also free of subsurface crevices. As a result, subsurface crevices are not inherent to cerium-based conversion coatings, but rather were formed due to certain process parameters, specifically the presence of chloride ions and hydrogen peroxide in the coating solution.     

Influence of High-Temperature Annealing of the Textured Metal Ni–W Substrate on the Structural Properties of Seed Layer in HTS 2G tapes

The surface reconstruction of the textured metal tapes at the temperatures typical for the formation of the seed buffer Y₂O₃ layer of HTS 2G tapes have been revealed and studied for the first time. The influence of a terrace structure of the substrate surface on the characteristics of the texture of the seed Y₂O₃ layer has been shown. This effect is critically important to the deposition of buffer layers on the moving tape and allows to expand the temperature range of growth, in which the full inheritance of substrate texture by the seed Y₂O₃ layer is occurred.     

The influence of sodium tungstate concentration and anodizing conditions on microarc oxidation (MAO) coatings for aluminum alloy

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.     

Microstructure and apatite-forming ability of the MAO-treated porous titanium

Porous titanium was treated by micro-arc oxidation (MAO) in the aqueous electrolytes containing 0.1 and 0.2 M NaOH. The microstructure (including morphology, phase component, element composition and chemical species) and in vitro apatite-forming ability of the oxidized films formed on the inner-pore walls of porous titanium were investigated. It is found that continuous thin films with pore sizes of 20-60 nm are formed in both electrolytes. The films consist of an amorphous TiO₂ outmost layer, a coexisted intermediate layer of amorphous TiO₂ and rutile, and a Ti₂O₃ bottom layer, and tightly bond to the titanium substrate without any cracks. In vitro bioactivity assessment shows that both MAO films possess high apatite-forming abilities. It is also found that, compared with the film formed in the 0.1 M NaOH-containing electrolyte, the film formed in the 0.2 M NaOH-containing electrolyte has a higher roughness and more nanopores which help shorten apatite induction time. It is expected the MAO-formed bioactive porous titanium will not only be beneficial to bone ingrowth into the porous structure, but also be beneficial to achieve a tough chemical bonding at the bone/implant interface.     

Effect of voltage on microstructure and corrosion resistance of microarc oxidation coatings on CP-Ti

The surface modification of commercially pure titanium (CP-Ti) by microarc oxidation (MAO) under different voltages was investigated using 1%H₃PO₄ solution as an electrolyte. The microstructure, phase composition and elemental distribution of ceramic coatings were investigated using scanning electron microscopy (SEM) and X-ray diffraction. The corrosion behaviour of the coating was also examined by potentiodynamic polarisation testing in a 3·5 wt-%NaCl solution. Micropore oxide films were formed on all the sample groups by MAO. The thickness and micropore size of the MAO coating increased with the increasing voltage. Energy dispersive X-ray spectroscopy results indicate that Ti, O and P became incorporated into the MAO coatings. At a low voltage of 250 V, the MAO coatings were composed of amorphous, P₂O₅, TiP₂O₇ and titania phases (rutile and anatase). Variation of treatment voltages increased the ceramic coatings from an amorphous structure to a phase structure, and the P₂O₅ phase disappeared. The corrosion potential Φ_corr of the MAO sample shifted towards nobler directions, and the corrosion density I_corr fell significantly compared with that of the bare CP-Ti. Corrosion testing showed that the sizes of the micropore of the MAO samples obviously decrease, and the MAO surface becomes smooth.     

Micro-arc oxidation of TC4 substrates to fabricate TiO2/YAG:Ce compound films with enhanced photocatalytic activity

This paper introduces a process for “in situ” preparing TiO₂ photocatalytic film compounded with YAG:Ce³⁺ semiconductor upon titanium alloy by using micro-arc oxidation (MAO). The surface morphology, chemical compositions, phase structures and photocatalytic properties of the films were characterized and measured by field emission gun scanning electron microscope (FEG-SEM), energy-dispersive X-ray spectrometer (EDS), X-ray diffractometer (XRD), electro-chemical workstation and UV-vis spectrophotometer. The results show that the YAG:Ce³⁺ semiconductor particles which were added in the electrolyte had been homogenously compounded within the TiO₂ film during MAO. Compared with the pure TiO₂ film, the compounded film exhibited much larger specific surface area, stronger absorption in the visible light and higher photo-generated current density, which improves the photocatalytic property markedly. It is expected that MAO will provide a simple, economic and promising approach for preparing a superior photocatalytic TiO₂ film.     

Influence of Al-La Cocementation on the Oxidation Behavior of Ti3SiC2-Base Ceramic

Al–La thermal diffusing was conducted on Ti₃SiC₂-base ceramic by the pack-cementation method. The microstructure, phase and oxidation resistance of the diffusion layer were characterized. The complete aluminide coatings have not been obtained. Al and La penetrated into the Ti₃SiC₂ substrate quickly, distributing into the whole interior of the specimen after cementation at 1100°C for 4 hr. Al existed as a solid solution and dispersed particles of AlLa₃. Oxidation of cemented Ti₃SiC₂ at 1100°C in air for 20 hr formed a single continuous Al₂O₃ layer with a small amount of TiO₂ grains on the outer layer. Compared with Ti₃SiC₂, the parabolic rate constant of the cemented Ti₃SiC₂ was decreased by two orders of magnitude, which means that the cementation treatment remarkably improves the oxidation resistance of Ti₃SiC₂. Al distributed in Ti₃SiC₂ acted as a reservoir to supply enough Al for the formation of a continuous Al₂O₃ scale during the oxidation process.     

Hot Corrosion Behavior of HVOF Sprayed Coatings on ASTM SA213-T11 Steel

Cr₃C₂-NiCr, NiCr, WC-Co and Stellite-6 alloy coatings were sprayed on ASTM SA213-T11 steel using the HVOF process. Liquid petroleum gas was used as the fuel gas. Hot corrosion studies were conducted on the uncoated as well as HVOF sprayed specimens after exposure to molten salt at 900 °C under cyclic conditions. The thermo-gravimetric technique was used to establish the kinetics of corrosion. XRD, SEM/EDAX and EPMA techniques were used to analyze the corrosion products. All these overlay coatings showed a better resistance to hot corrosion as compared to that of uncoated steel. NiCr Coating was found to be most protective followed by the Cr₃C₂-NiCr coating. WC-Co coating was least effective to protect the substrate steel. It is concluded that the formation of Cr₂O_3, NiO, NiCr₂O₄, and CoO in the coatings may contribute to the development of a better hot-corrosion resistance. The uncoated steel suffered corrosion in the form of intense spalling and peeling of the scale, which may be due to the formation of unprotective Fe₂O₃ oxide scale.     

Plasma spraying of Al2O3–WO3 composite coatings

Al₂O₃–WO₃ oxide composites are of considerable interest as solid acid catalysts in the petrochemical industry. Typically they consist of a monolayer of WO₃ dispersed over a high surface area alumina support. The aim of this work was to form a single layer Al₂O₃–WO₃ composite coating by plasma spraying. Ammonium metatungstate was dispersed through porous alumina particles and transformed to WO₃ by heat treatment. WO₃ doped powders were plasma sprayed using “low” and “high” enthalpy plasma systems. Plasma spraying led to dramatic changes in the WO₃ oxide to form metallic tungsten, a metastable Al₂O₃–W solid solution and the mixed oxide Al₂(WO₄)₃. The mechanisms accounting for these transformations are discussed.     

Influence of anions in phosphate and tetraborate electrolytes on growth kinetics of microarc oxidation coatings on Ti6Al4V alloy

The growth kinetics of microarc oxidation (MAO) coatings on Ti6Al4V alloy was studied by designing an electrolyte with low content and high content, using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and potentiodynamic polarization. The results showed that increased the spark intensity and dissolved most of the oxides at high temperatures. Then, a thicker barrier layer at the coating/substrate interface was produced, which increased the polarization resistance of the coating. at a low concentration also promoted the uniform growth of the MAO coating and the formation of hat-shaped holes in the outer deposition layer. The thickness of the MAO coatings obtained in Na₂B₄O₇ electrolytes exhibited an exponential increase with time at spark discharge stage, while that of the MAO coating obtained in phosphate–tetraborate electrolytes showed a linear trend as the content increased.     

Wear and oxidation resistance of Al2O3 particle dispersed Al3Ti composite with a nanostructure prepared by pulsed electric current sintering of mechanically alloyed powders

Al₃Ti-matrix composite layers containing Al₂O₃ particles were formed on Ti substrate by pulsed electric current sintering (PECS) of mechanically alloyed (MA) powders to improve the wear and oxidation properties of the Ti substrate. Reducing the grain size of each element by MA makes the combustion synthesis of Al₃Ti possible at a lower temperature. The grain size formed by the combustion synthesis of Al–Ti–Al₂O₃ powder mechanically alloyed for 720 ks was about 10 nm and its growth during sintering was suppressed by the existence of Al₂O₃. The densification behavior of the powder was investigated quantitatively. The obtained Al₃Ti/Al₂O₃ composite layer showed better wear and oxidation resistance than the monolithic Al₃Ti layer.     

Chromia Coating on Iron Formed from CrO3 in Ozone

Ozone gas was blown over Cr₂O₃ powder painted on an iron substrate. About one half the Cr₂O₃ transformed to CrO₃ at 473 K. Subsequently, the liquid CrO₃ was decomposed thermally at 723 K to Cr₂O₃ which adhered to the substrate. The dense layer had the corundum structure of (Cr,Fe)₂O₃ which formed at the Cr₂O₃/substrate interface. Without the ozone-gas treatment, the layer was porous, and a thick spinel phase (Cr,Fe)₃O₄ formed; it was not effective for oxidation resistance. The ozone treatment enhanced the oxidation resistance below 1000 K.