Influence of cathodic current on composition, structure and properties of Al2O3 coatings on aluminum alloy prepared by micro-arc oxidation process

Alumina coatings on aluminum alloy were prepared by micro-arc oxidation (MAO) process using DC and AC power supplies, respectively. In comparison with the coating deposited by DCMAO, the influence of the cathodic current on the composition, structure and properties of the ACMAO coating was investigated. It is found that the coating deposited by DCMAO is composed of α-Al₂O₃, whereas the coating deposited by ACMAO has a mixture composition of α-Al₂O₃ and γ-Al₂O₃. The results of properties show that compared with the coatings deposited by DCMAO, the ACMAO coatings possess higher density, hardness and corrosion resistance. It can be attributed to that the DCMAO coating is rougher and existing much more micro-cracks in its inner layer. As a result, the adhesion of the DCMAO coating to the substrate decreases.     

Electron energy-loss spectroscopy investigation of Y2O3 films on Si (001) substrate

Electron energy -loss spectroscopy has been used to investigate the interface between a Y₂O₃ film and the silicon substrate. The chemical composition of the interface layer is revealed to be nearly pure amorphous SiO₂. Yttrium silicates are found at the Y₂O₃/SiO₂ interface region. The formation of the interfacial yttrium silicates has been interpreted by the direct chemical reaction between the deposited Y₂O₃ film and the SiO₂ interface layer. The Si L₂₃ and O K edges of yttrium silicates (Y₂SiO₅ and Y₂Si₂O₇) have been calculated by the first-principle full multiple - scattering method. The theoretical results are consistent with the experimental spectra, which confirms the formation of yttrium silicates.     

The effect of micro-structure on the oxidation behavior of a Ni-based superalloy in water vapor plus oxygen

The oxidation behavior of a Ni-based superalloy with polycrystalline, single-crystalline (SC) and nanocrystalline (NC) structures was studied at 1000 °C in water vapor (20.12 vol.%) plus oxygen. The oxidation behavior of the SC alloy was similar to that of the cast alloy, while nanocrystallization increased the corrosion resistance. The oxides scale on the SC alloy and cast alloy consisted of external TiO₂, Cr₂O₃ and internal Al₂O₃, while only external Al₂O₃ scale formed on the NC coating in water vapor plus oxygen. Meanwhile, the morphologies of oxides scale on three samples are significantly different from one another. The effect of micro-structures on the oxidation behavior of this Ni-based superalloy is discussed.     

Characterization of the high temperature oxidation of TBC-coated oxide-dispersed β-NiAl substrates

A Y₂O₃?stabilized ZrO₂ coating on three different oxide-dispersed (Y₂O₃, ZrO₂ and Al₂O₃) β-NiAl substrates was studied to evaluate the spallation behaviour of the coating and its effect on the oxidation behavior of these alloy substrates. Spallation of the coating during cyclic testing at 1200°C primarily occurred at the Al₂O₃ scale-substrate interface and was dependent on the adhesion of the scale to the substrate. Uncoated, Y₂O₃?dispersed β-NiAl had the best Al₂O₃ scale adhesion and the coated β-NiAl substrate had the longest coating lifetime. Al₂O₃?dispersed β-NiAl exhibited the poorest scale adhesion and the coating completely spalled after two 2-hour cycles. For this model-type system, the growth of voids at the metal-scale interface appears to be critical in determining scale adhesion and thus coating lifetime.     

The influence of Al2O3 particulate reinforcement on cyclic stress response and fracture behavior of 6061 aluminum alloy

The cyclic stress response characteristics and cyclic fracture behavior of aluminum alloy 6061 discontinuously reinforced with particulates of Al₂O₃ are presented and discussed. The 6061/Al₂O₃ composite specimens and the unreinforced 6061 aluminum alloy were cyclically deformed using tension-compression loading under constant total strain amplitude control. Both the composite and the unreinforced alloy exhibited softening to failure from the onset of cyclic deformation. The degree of softening was observed to increase at the elevated test temperature for both the composite and the unreinforced counterpart. The intrisic micromechanisms controlling the stress response characteristics during fully-reversed cyclic straining are highlighted and rationale for the observed behavior is discussed. The cyclic fracture behavior of the composite is discussed in terms of the competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic stress response, and test temperature.     

Research on corrosion performance of 6061 aluminum alloy in salt spray environment

Salt spray corrosion test was carried out on 6061 aluminum alloy, and quasi-static tensile test at room temperature was carried out on the sample with universal testing machine. The effect of salt spray corrosion on the mechanical properties of 6061 aluminum alloy was studied by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and electrochemistry. The corrosion rate of 6061 aluminum alloy was quantitatively characterized by different corrosion parameters. It was found that local corrosion of 6061 aluminum alloy occurred in salt spray environment, mainly pitting corrosion and intergranular corrosion. With the increase of corrosion time, the polarization resistance of 6061 aluminum alloy decreases, and the corrosion rate significantly increases. The average corrosion rate and the maximum corrosion rate of 6061 aluminum alloy were characterized by corrosion weight loss and corrosion pit depth. And they can be transformed into each other. The mechanical properties of 6061 aluminum alloy were mainly affected by the depth of corrosion pit. With the increase of corrosion time, the tensile strength and fracture strain decreased, resulting in poor plasticity of the sample. At the same time, the change of elongation of 6061 aluminum alloy can be accurately predicted by the depth of corrosion pit.     

YSZ thin films deposited by e-beam technique

YSZ thin films were grown evaporating cubic and tetragonal phase ZrO₂ stabilized by 8 wt.% of Y₂O₃ (8% of YSZ) ceramic powders by using e-beam deposition technique. Operating technical parameters that influence thin film properties were studied. The influence of substrate crystalline structure on growth of deposited YSZ thin film was analyzed there. The YSZ thin films (1.5-2 μm of thickness) were deposited on three different types of substrates: Al₂O₃, optical quartz (SiO₂), and Alloy 600 (Fe-Ni-Cr). The dependence of substrate temperature, electron gun power, and phase of ceramic powder on thin film structure and surface morphology was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The substrate temperature was changed in the range of 20-600° C (during the YSZ thin film deposition) and its influence on the crystallinity of deposited YSZ thin films was analyzed. It was found that electron gun power and substrate temperature has the influence on the crystallite size, and texture of YSZ thin films. Also, the substrate has no influence on the crystal orientation. The crystallite size varied between 20 and 40 nm and increased linearly changing the substrate temperature. The crystal phase of evaporated YSZ powder has the influence on the structure of the deposited YSZ thin films.     

Whiskers of Al2O3 as reinforcement of a powder metallurgical 6061 aluminium matrix composite

An Al-Mg-Si alloy matrix composite reinforced with 10 vol.% of alumina whiskers (Al₂O₃w) has been processed by powder metallurgy and investigated. The Al₂O₃w were produced as single crystal c-axis alpha-alumina fibres at pre-pilot scale via vapour-liquid-solid (VLS) deposition in a cold-wall air-tight furnace with alumina linings. As far as we know, this is the first report of the utilization of whiskers of Al₂O₃ as reinforcing elements for Al alloys. Tensile tests have been performed on the composite at room and high temperatures. Results show that the AA6061 alloy reinforced with the as-produced Al₂O₃ whiskers has remarkably high mechanical properties at room temperature. This is attributed to the high quality of the Al₂O₃ single crystals and to the strong bonding attained between them and the 6061 alloy matrix.     

Y2O3/(Cu–Me) systems (Me=Al, Ti): interface reactions and wetting

Wetting behavior and interface interaction between Y₂O₃ and Cu–alloys were investigated at 1,423 K. Pure copper does not wet yttria substrate but the wettability is significantly improved by additions of Al and Ti. Different interface structures were observed in the Y₂O₃/(Cu–Al) and Y₂O₃/(Cu–Ti) systems. Relatively deep crater was detected at the interface in the first system, while Cu alloying by Ti led to formation of a flat interface with a thin reaction layer. The results of the wetting experiments and the interface features were well accounted of by thermodynamic analysis of the Y₂O₃/(Cu–Me) systems.     

Alumina coating formed on medical NiTi alloy by micro-arc oxidation

Al₂O₃ coatings were prepared on NiTi alloy by micro-arc oxidation in an aluminate solution. Thin-film X-ray diffraction (TF-XRD) indicated that the coating consisted of only Al₂O₃ crystal phase. Energy dispersive X-ray spectrometer (EDS) showed that there was about 2.53 at.% Ni in the surface layer, which was greatly lower than that of NiTi substrate. Scanning electron microscopy (SEM) showed that the coating exhibited a typical porous surface and excellent adhesive interface between the coating and the substrate. Direct pull-off test showed that the coating had a mean coating-substrate bonding strength of 28 ± 2 MPa. The results of electrochemical impedance spectroscopy (EIS) study and potentiodynamic polarization test indicated that the corrosion resistance of the coated sample was increased by two orders of magnitude compared with uncoated sample.     

Investigation of the aluminum oxide/Si(1 0 0) interface formed by chemical vapor deposition

Thin films of aluminum oxide were deposited using trimethylaluminum and oxygen. The deposition rate was found to decrease with increasing temperature. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the film/substrate interface. When dry O₂ was used during deposition, the film/substrate interface was free of any silicon dioxide or aluminum silicate phase. On annealing the as-deposited films in Ar, a layer of silicon dioxide film formed at the interface. XPS results indicated that the O/Al ratio in the as-deposited films was higher than that in stoichiometric Al₂O₃. However, the ratio was found to decrease in the annealed samples suggesting that excess oxygen present in the deposited films is responsible for the formation of interfacial silicon dioxide layer. Interfacial phase formation was observed in the as-deposited samples, when small amounts of ozone along with oxygen were used as the oxygen precursor.     

Evaluation of interfacial bonding in dissimilar materials of YSZ-alumina composites to 6061 aluminium alloy using friction welding

The interfacial microstructures characteristics of alumina ceramic body reinforced with yttria stabilized zirconia (YSZ) was evaluated after friction welding to 6061 aluminum alloy using optical and electron microscopy. Alumina rods containing 25 and 50 wt% yttria stabilized zirconia were fabricated by slip casting in plaster of Paris (POP) molds and subsequently sintered at 1600 °C. On the other hand, aluminum rods were machine down to the required dimension using a lathe machine. The diameter of the ceramic and the metal rods was 16 mm. Rotational speeds for the friction welding were varied between 900 and 1800 rpm. The friction pressure was maintained at 7 MPa for a friction time of 30 s. Optical and scanning electron microscopy was used to analyze the microstructure of the resultant joints, particularly at the interface. The joints were also examined with EDX line (energy dispersive X-ray) in order to determine the phases formed during the welding. The mechanical properties of the friction welded YSZ-Al₂O₃ composite to 6061 alloy were determined with a four-point bend test and Vickers microhardness. The experimental results showed the degree of deformation varied significantly for the 6061 Al alloy than the ceramic composite part. The mechanical strength of friction-welded ceramic composite/6061 Al alloy components were obviously affected by joining rotational speed selected which decreases in strength with increasing rotational speed.     

Growth and characterization of yttrium oxide films by reactive magnetron sputtering

Yttrium oxide (Y₂O₃) is a promising ceramic material for electronic and optical applications due to its excellent properties. The purpose of this study is to characterize the effects of deposition parameters on the structure and composition of Y₂O₃ films. The films are grown on Si substrates by reactive magnetron sputtering at different substrate temperatures and oxygen pressures. The composition and structure of the films are studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. It is shown that the Y₂O₃ films deposited by reactive magnetron sputtering are mainly cubic phase and polycrystalline. The films are composed of Y-O, Y-O-Si, and Si-O bonds. Increasing substrate temperature induces the monoclinic to cubic phase transition and results in the formation of oxygen vacancies in the film. The preferred growth orientation of Y₂O₃ film is the (110) plane at low temperature, and it changes to the (111) plane at high temperature. The low temperature is preferable for the formation of Y-O bonds. The oxygen pressure influences on the concentration of Y-O bonds significantly. An optimal oxygen partial pressure for the formation of Y-O bonds exists during the film deposition. In addition, the deposited Y₂O₃ films exhibit excellent mechanical properties.     

Interactions between Y2O3–Al mixture studied by solid-state reaction method

Interactions between Y₂O₃–Al mixture studied by solid-state reaction method were investigated in present paper. Interactions between Y₂O₃–Al mixture was characterized by differential thermal and thermogravimetric analyses and X-ray diffraction, Y₂O₃–Al mixture and yttrium aluminum garnet (YAG) powder as final reaction product were characterized by scanning electron microscopy. The results show Al is isolated with Y₂O₃ by aluminum oxide layer in air, and no opportunity of directional reaction between Y₂O₃–Al systems. With temperature increasing to ∼569 °C, aluminum partly turned into transitional aluminas, Y₂O₃ reacts with transitional aluminas instead of aluminum to form yttrium aluminum monoclinic (YAM) and yttrium aluminum perovkite (YAP) phases after calcination at 600 °C, 800 °C separately, and pure YAG powder is obtained after calcination at 1200 °C. From the point of view of reaction temperature, the reaction between Y₂O₃ and transitional aluminas is easier than that of Y₂O₃ and Al or α-Al₂O₃.     

Seed layers on RABiTS tapes for Second-Generation HTS wires

Epitaxial films of CeO₂ and Y₂O₃ on RABiTS metal tapes have been obtained by pulsed laser deposition. The dependence of the film crystal orientation on the temperature of synthesis has been studied. It is established that Y₂O₃ films grow with 100% (100) orientation, whereas a parasitic orientation is present in CeO₂ films obtained in the entire range of growth temperatures. The texture sharpness in the substrate plane amounted to 8°. Electron-microscopic examination showed that an increase in the temperature of synthesis leads to the appearance of cracks on the surface of CeO₂ films, while the surface of Y₂O₃ films remains continuous in the entire range of growth temperatures. Thus, Y₂O₃ films most fully obey the requirements to seed layers in (i) being epitaxial with 100% (100) crystal orientation, (ii) inheriting the substrate texture, and (iii) having a smooth crack-free surface.     

Thin film model systems of ZrO2 and Y2O3 as templates for potential industrial applications investigated by means of electron microscopy

An analytical high-resolution electron microscopy study of Y₂O₃ and ZrO₂ thin films, being relevant oxide model systems for a range of industrial applications, is reported. Both films were deposited on vacuum-cleaved NaCl(001) single crystal planes at varying substrate temperatures. A transition from an amorphous to a well-defined and -ordered structure, exhibiting almost single-crystalline ordering of either body-centered cubic Y₂O₃ or tetragonal ZrO₂, both with uniform electronic structure, has been observed upon raising the substrate temperature from 300 to 573 K. Pronounced crystallographic relationships between the face-centered cubic NaCl structure and the structures of the deposited oxides have been held responsible for the observed epitaxial growth. In summary, the chosen preparation pathway represents an easy and reproducible method to yield well-defined oxide structures at surprisingly low substrate temperatures being at the same time promising model candidates for materials-related (e.g. solid-oxide fuel cell) research. With respect to solid-oxide fuel cell technology, monitoring carbon deposition and reactivity following high-temperature treatments in hydrocarbon-containing gas feeds or the use as templates or supports for model systems of realistic anode materials on metallic or bimetallic basis are envisioned application areas.     

Feasibility of using Y2Ti2O7 nanoparticles to fabricate high strength oxide dispersion strengthened Fe–Cr–Al steels

Addition of Al can improve the corrosion resistance of oxide dispersion strengthened (ODS) steels. However, Al reacts with Y₂O₃ to form large Y–Al–O particles in the steels and deteriorates their mechanical properties. Herein, we successfully prepared Y₂Ti₂O₇ nanoparticles (NPs) by the combination of hydrogen plasma-metal reaction (HPMR) and annealing. Y₂Ti₂O₇ NPs with contents of 0.2 or 0.6 wt.% were then added into the Fe–14Cr–3Al–2W–0.35Ti (wt.%) steel to substitute the conventional Y₂O₃ NPs by mechanical alloying (MA). The Y₂Ti₂O₇ NPs transformed into amorphous-like structure after 96 h MA. They crystallized with a fine size of 7.4 ± 3.7 nm and shared a semi-coherent interface with the matrix after hot isostatic pressing (HIP) of the ODS steel with 0.6 wt.% Y₂Ti₂O₇. With the increasing Y₂Ti₂O₇ content from 0.2 to 0.6 wt.%, the tensile strength of the ODS steel increased from 1238 to 1296 MPa, which was much higher than that (949 MPa) of the ODS steel added with Y₂O₃. The remarkably improved mechanical properties of the Al-containing ODS steels were attributed to the increasing number density of Y₂Ti₂O₇ nanoprecipitates. Our work demonstrates a novel route to fabricate high performance ODS steels with both high mechanical strength and good corrosion resistance.     

Reactive wetting of mullite Al2[Al2 + 2x Si2 − 2x ] O10 − x single crystals by yttrium-aluminosilicate and borosilicate glasses

Mullite, Al₂[Al_{2 + 2x} Si_{2 ? 2x} ]O_{10 ? x} , is introduced as a suitable reactive wetting model system for oxide ceramic compounds. Apparent contact angles on single crystal mullite substrates have been measured from sessile drop experiments involving (i) a highly reactive yttrium aluminosilicate (YAS) glass and (ii) a less reactive borosilicate (BS) glass. The apparent contact angles decrease with crystallographic orientation in the following order: (010) > (100) > (001) independent of glass composition. The surface energy, γ_SV, has been identified as the dominant term controlling reactive wetting with γ_SV (010) < γ_SV(100) < γ_SV(001). This order of surface energy values is rationalized in terms of the high anisotropy of the crystal structure and elastic properties of mullite. The YAS glass reacts stronger with the polycrystalline 3/2 mullite substrate due to the grain boundaries acting as fast diffusion paths. In the YAS/mullite system, analytical electron microscopy shows that for the single crystal 2/1 mullite substrate, a corundum + Y₂Si₂O₇-rich crystalline phase assemblage results upon devitrification while in the case of the polycrystalline 3/2 mullite substrate a 2/1 mullite + Y₂Si₂O₇—rich crystalline assemblage is formed instead. In the less reactive borosilicate system secondary 2/1 mullite microcrystals precipitate at the S/L interface with (i) random orientations established on polycrystalline substrates and (ii) characteristic preferred orientations on the single crystal substrate. A thin Al-rich interdiffusion zone (3 μm) right at the S/L interface is revealed for both glass systems.     

Improving the wear resistance of AA 6061 by laser surface alloying with NiTi

To improve the wear resistance of a popular aluminum alloy AA 6061, a 1.5 mm thick hard surface layer consisting of Ni–Al and Ti–Al intermetallic compounds was synthesized on the alloy by laser surface alloying technique. NiTi powder was preplaced on the aluminum alloy substrate and irradiated with a high-power CW Nd:YAG laser in an argon atmosphere. With optimized processing parameters, a modified surface layer free of cracks and pores was formed by reaction synthesis of Al with Ni and Ti. X-ray diffractometry (XRD) confirmed the main phases in the layer to be TiAl₃ and Ni₃Al. The surface hardness increased from below 100 HV for untreated AA 6061 to more than 350 HV for the laser-treated sample. Accompanying the increase in hardness, the wear resistance of the modified layer reached about 5.5 times that of the substrate.     

Deposition and post-deposition annealing of thin Y2O3 film on n-type Si in argon ambient

Thin (∼5.0 nm) Y₂O₃ films were deposited on n-type Si (1 0 0) substrate using RF magnetron sputtering. Detailed studies on the effects of post-deposition annealing (PDA) temperatures (400, 600, 800, and 1000 °C) in argon ambient on these films were performed by X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), field emission scanning electron microscopy, and atomic force microscopy. Interfacial layer (IL) of SiO₂ in between Y₂O₃ and the Si substrate for sample annealed from 400 to 800 °C had been suggested from the results of FTIR. As for sample annealed at 1000 °C, presence of IL might consist of both Y₂Si₂O₇ and/or SiO₂ through the detection of Y₂Si₂O₇ compound and Si–O chemical bonding from XRD and FTIR analysis, respectively. For as-deposited sample, no detectable chemical functional group at the IL was recorded. Electrical characteristics of the Y₂O₃ films were acquired by fabricating metal-oxide–semiconductor capacitor as test structure. An improvement in the breakdown voltage (V_B) and leakage current density (J) was perceived as the PDA temperature increased. Of the PDA samples, the attainment of the lowest effective oxide charge, interface trap density, total interface trap density, and the highest barrier height at 1000 °C had contributed to the acquisition of the highest V_B and lowest J.