Improvement of surface porosity and properties of alumina films by incorporation of Fe micrograins in micro-arc oxidation

Micro-arc oxidation (MAO) is an effective approach to improve the properties of aluminum and its alloy by forming ceramic films on the surface. However, the oxide layers often have a porous surface structure, which exhibits relatively high friction coefficients. In this work, in order to enhance the surface and mechanical properties of the films produced by micro-arc oxidation, Al₂O₃ coatings embedded with Fe micrograins of different thicknesses were produced on aluminum alloys by adding Fe micrograins into the electrolyte during MAO. Compared to the Al₂O₃ coatings without Fe micrograins, the MAO Al₂O₃ coatings with Fe micrograins are much denser and harder, and the wear resistance is also improved significantly. The enhancement can be attributed to the enhancement of the surface structure and morphology of the MAO Al₂O₃ coatings with embedded Fe micrograins.     

X-ray photoelectron spectroscopy of nano-multilayered Zr-O/Al-O coatings deposited by cathodic vacuum arc plasma

Nano-multilayered Zr-O/Al-O coatings with alternating Zr-O and Al-O layers having a bi-layer period of 6-7 nm and total coating thickness of 1.0-1.2 μm were deposited using a cathodic vacuum arc plasma process on rotating Si substrates. Plasmas generated from two cathodes, Zr and Al, were deposited simultaneously in a mixture of Ar and O₂ background gases. The Zr-O/Al-O coatings, as well as bulk ZrO₂ and Al₂O₃ reference samples, were studied using X-ray photoelectron spectroscopy (XPS). The XPS spectra were analyzed on the surface and after sputtering with a 4 kV Ar⁺ ion gun. High resolution angle resolved spectra were obtained at three take-off angles: 15°, 45° and 75° relative to the sample surface.It was shown that preferential sputtering of oxygen took place during XPS of bulk reference ZrO₂ samples, producing ZrO and free Zr along with ZrO₂ in the XPS spectra. In contrast, no preferential sputtering was observed with Al₂O₃ reference samples. The Zr-O/Al-O coatings contained a large amount of free metals along with their oxides. Free Zr and Al were observed in the coating spectra both before and after sputtering, and thus cannot be due solely to preferential sputtering.Transmission electron microscopy revealed that the Zr-O/Al-O coatings had a nano-multilayered structure with well distinguished alternating layers. However, both of the alternating layers of the coating contained of a mixture of aluminum and zirconium oxides and free Al and Zr metals. The concentration of Zr and Al changed periodically with distance normal to the coating surface: the Zr maximum coincided with the Al minimum and vice versa. However the concentration of Zr in both alternating layers was significantly larger than that of Al. Despite the large free metal concentration, the Knoop hardness, 21.5 GPa, was relatively high, which might be attributed to super-lattice formation or formation of a metal-oxide nanocomposite within the layers.     

Effect of different alloyed layers on the high temperature oxidation behavior of newly developed Ti2AlNb-based alloys

The application of titanium aluminide orthorhombic alloys (O-phase alloys) as potential materials in aircraft and jet engines was limited by their poor oxidation resistance at high temperature. The Ti₂AlNb-based alloys were chromised (Cr), chromium-tungstened (Cr-W) and nickel-chromised (Ni-Cr) by the double glow plasma surface alloying process to improve their high temperature oxidation resistance. The discontinuous oxidative behavior of Cr, Cr-W and Ni-Cr alloyed layers on Ti₂AlNb-based alloy at 1093 K was explored in this study. After exposing at 1093 K, the TiO₂ layer was formed on the bare alloy and accompanied by the occurrence of crack, which promoted oxidation rate. The oxidation behavior of Ti₂AlNb-based alloys was improved by surface alloying due to the formation of protective Al₂O₃ scale or continuous and dense NiCr₂O₄ film. The Ni-Cr alloyed layer presented the best high-temperature oxidation resistance among three alloyed layers.     

EPR of Transition-Metal Ions in Microplasma Coatings on the Aluminum Alloy D16

The influence of microplasma oxidation (MPO) conditions on the creation of the Al₂O₃ coatings on aluminum alloy D16 was studied by the electron paramagnetic resonance (EPR) technique. Aluminum alloy D16 contains alloyed transition metals such as Cu, Mn, Fe, Ni, and Ti incorporated into the coating structure during MPO. EPR data on transition-metal ions allow estimating the appearance of a high-temperature phase of Al₂O₃ due to the difference in incorporation coefficients of transition-metal ions in low- and high-temperature Al₂O₃ phases of MPO coatings. These data show that no high-temperature Al₂O₃ phases were created during anodic MPO. Authors' address: Vladimir A. Nadolinny, Institute of Inorganic Chemistry, Lavrentyev prospect 3, Novosibirsk 630090, Russian Federation     

Study of microstructure and phase evolution of hot-dipped aluminide mild steel during high-temperature diffusion using electron backscatter diffraction

Mild steel was coated by hot-dipping into a molten aluminum bath. The microstructure and phase evolution in the aluminide layer during diffusion at 750 °C in static air were analyzed by electron backscatter diffraction (EBSD). The results showed that the aluminide layer of the as-coated specimen consisted of an outer aluminum topcoat, minor FeAl₃ and major Fe₂Al₅, respectively. Also, Fe₂Al₅ possessed a tongue-like morphology, which caused corresponding serration-like morphology in the steel substrate. A portion of the peaks of serration-like substrate were isolated, after short exposure at 750 °C, and accompanied by the formation of voids, which continued to appear with further exposure at 750 °C. As the aluminum topcoat was consumed, FeAl₃ phase disappeared and left an aluminide layer of Fe₂Al₅ phase. After 60 min of exposure, FeAl₂ and FeAl phases formed at the interface between Fe₂Al₅ and the steel substrate. With increasing exposure time, the voids condensed and the serration-like morphology disappeared, while FeAl₂ and FeAl phases kept growing. After prolonged exposure, the aluminide layer was composed of FeAl₂ and FeAl and possessed a flat interface between FeAl and steel substrate.     

Fracture and debonding behavior of coated brittle alumina layer on ductile aluminum substrate wire

The fracture and debonding behavior of the Al₂O₃ layer coated on a ductile aluminum substrate wire was studied experimentally and analytically. When tensile strain was applied, the brittle Al₂O₃ coating layer showed multiple cracking perpendicular to the tensile axis. After the multiple cracking, compressive fracture of the Al₂O₃ layer arose in the circumferential direction when the layer was thinner than around 30 μm, while interfacial debonding between the Al₂O₃ layer and aluminum substrate arose when it was thicker. Such a difference in behavior between thin and thick layers could be accounted for by the difference in the layer thickness-dependence of the tensile radial stress at the interface and the compressive hoop stress of the Al₂O₃ layer calculated by the finite element method; the former stress increases while the latter one decreases with increasing layer thickness.     

The mechanism of oxide whisker growth and hot corrosion of hot-dipped Al-Si coated 430 stainless steels in air-NaCl(g) atmosphere

The mechanisms of oxide whisker growth and hot corrosion of 430 stainless steel (430SS) and aluminide 430 stainless steel hot-dipped in a Al-10 wt.%Si molten bath (430HDAS) were studied at 750 and 850 °C in air mixed with 500 and 990 vppm NaCl_(g). The results showed that the loose Cr₂O₃ scale which formed on the 430SS could not prevent the corrosion of 430SS in a 500 vppm NaCl_(g) atmosphere, resulting in the formation of Fe₂O₃ scale. Fe₂O₃ whiskers grew at the grain boundary of the Fe₂O₃ scale. However, no Fe₂O₃ whiskers formed on the Fe₂O₃ scale while 430SS was exposed in a 900 vppm NaCl_(g) atmosphere. During the initial high-temperature corrosion of 430HDAS in a 500 vppm NaCl_(g) atmosphere, a dense Al₂O₃ scale formed on the surface of the specimens. Also, Al₂O₃ whiskers grew on the Al₂O₃ scale. As exposure time increased, cyclic chlorination/oxidation degraded the protective aluminide layer and caused the formation of Fe₂O₃ scale and Fe₂O₃ whiskers. The morphology of Fe₂O₃ whiskers formed at 750 °C is more slender than those formed at 850 °C. The formation and growth of both Fe₂O₃ and Al₂O₃ whiskers may be attributed to the chloridation of both the steel substrate and aluminide layer, accelerating the diffusion rate of metallic ions in the oxide scales.     

Interfacial reactions and oxidation behavior of Al2O3 and Al2O3/Al coatings on an orthorhombic Ti2AlNb alloy

The uniform and dense Al₂O₃ and Al₂O₃/Al coatings were deposited on an orthorhombic Ti₂AlNb alloy by filtered arc ion plating. The interfacial reactions of the Al₂O₃/Ti₂AlNb and Al₂O₃/Al/Ti₂AlNb specimens after vacuum annealing at 750 °C were studied. In the Al₂O₃/Ti₂AlNb specimens, the Al₂O₃ coating decomposed significantly due to reaction between the Al₂O₃ coating and the O-Ti₂AlNb substrate. In the Al₂O₃/Al/Ti₂AlNb specimens, a γ-TiAl layer and an Nb-rich zone came into being by interdiffusion between the Al layer and the O-Ti₂AlNb substrate. The γ-TiAl layer is chemically compatible with Al₂O₃, with no decomposition of Al₂O₃ being detected. No internal oxidation or oxygen and nitrogen dissolution zone was observed in the O-Ti₂AlNb alloy. The Al₂O₃/Al/Ti₂AlNb specimens exhibited excellent oxidation resistance at 750 °C.     

Characterization of water exposed plasma sprayed oxide coating materials using XPS

The surface compositions and oxidation states of non-exposed and water exposed plasma sprayed oxide coatings were studied using X-ray photoelectron spectroscopy (XPS). Coating materials were TiO₂, Al₂O₃ and Cr₂O₃ and their mixtures. Water exposures were performed for free standing coating disks at mild electrolyte (1 mmol NaCl solution) at pH 4, 7 and 9. The exposure time was two weeks.It was observed that pure plasma sprayed TiO₂ material was chemically stable over whole experiment pH range and only slight surface hydroxylation was observed for this material.In case of plasma sprayed Al₂O₃ materials the surface O/Al ratio increased considerably during water exposure especially at exposure pH 7. This was probably result of surface conversion to hydrous form. No surface oxidation state changes were observed for this material.The non-exposed Cr₂O₃ materials contained both Cr(III) and Cr(VI) oxides. The water exposures increased the surface oxygen and Cr(VI) contents at the expense of Cr(III). The most probable reason for that was the dissolution of surface Cr(VI) oxide phase during water exposures and the (re)adsorption of dissolved Cr(VI) species back to the surface.     

Surface Modification of Al2O3 Fiber with Binary Nanoparticles using a Dry-Mechanical Coating Technique

Integrating materials with different functionalities into a composite material to obtain synergetic properties has generated considerable interest in various scientific and technical fields. In this study, a dry-mechanical coating process was used to fix nanosized Al₂O₃ and CuO particles directly onto the surface of Al₂O₃ fiber substrates by employing high shear and compression forces. The resulting composite materials showed good dispersion and homogeneous distribution of Al₂O₃ and CuO nanoparticles. Important coating parameters, including initial particle loadings and processing times were investigated for their effects on coating characteristics and product properties. The experimental results showed that the product surface area increased with higher nanoparticle loadings. The degree of dispersion and homogenous distribution of Al₂O₃ nanoparticles with CuO nanoparticles increased with the processing time. Additionally, the crystalline phase of raw materials was preserved during the coating process under the conditions studied in this work.     

Fabrication of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al structure by nitric acid oxidation at room temperature

A thick Al₂O₃/aluminum (Al) structure has been fabricated by oxidation of Al with 68wt% and 98wt% nitric acid (HNO₃) aqueous solutions at room temperature. Measurements of the Al₂O₃ thickness vs. the oxidation time show that reaction and diffusion are the rate-determining steps for oxidation with 68wt% and 98wt% HNO₃ solutions, respectively. Observation of transmission electron micrographs shows that the Al₂O₃ layer formed with 68wt% HNO₃ has a structure with cylindrically shaped pores vertically aligned from the Al₂O₃ surface to the Al₂O₃/Al interface. Due to the porous structure, diffusion of HNO₃ proceeds easily, resulting in the reaction-limited oxidation mechanism. In this case, the Al₂O₃/Al structure is considerably rough. The Al₂O₃ layer formed with 98wt% HNO₃ solutions, on the other hand, possesses a denser structure without pores, and the Al₂O₃/Al interface is much smoother, but the thickness of the Al₂O₃ layer formed on crystalline Al regions is much smaller than that on amorphous Al regions. Due to the relatively uniform Al₂O₃ thickness, the leakage current density flowing through the Al₂O₃ layer formed with 68wt% HNO₃ is lower than that formed with 98wt% HNO₃.     

Effect of the deposition parameters on the structure and physicochemical properties of protective Al2O3 coatings

Three series of Al₂O₃ coating samples are fabricated by microarc oxidation under various deposition conditions and are studied by scanning electron microscopy (SEM) in combination with energy-dispersive x-ray spectroscopy (EDXS), Rutherford backscattering, and X-ray diffraction. Defects and pores in the coatings are analyzed by positron annihilation spectroscopy at room temperature without vacuum. No nanometer pores are detected in the coatings. When changing the electrolyte-plasma oxidation conditions, one can change the concentration and the ratio of the types of vacancy defects in these Al₂O₃ coatings.     

Influence of preparation technology on the microstructure and anti-oxidation property of SiC-Al2O3-mullite multi-coatings for carbon/carbon composites

Oxidation protective SiC-Al₂O₃-mullite multi-coatings for carbon/carbon (C/C) composites were prepared with a two-step pack cementation process. The influence of preparation temperature and SiO₂/Al₂O₃ ratio of the pack powder on the phase, microstructure and oxidation resistance of the multi-coatings were investigated. It showed that the multi-coatings that contained mullite could be produced at 1700-1800 °C. A denser coating surface was acquired with the decrease of SiO₂/Al₂O₃ ratio in the pack chemistries while a little damnification to the interface of the coating and C/C substrate. The as-prepared coating could effectively protect C/C composites from oxidation at 1600 °C for 81 h.     

钛铝合金高温氧化机理电子理论研究

为了从电子层面揭示钛铝合金高温氧化的物理本质,采用递归法与Castep相结合的方式, 计算了原子埋置能、亲和能、结合能等电子结构参数,探索合金氧化机理.研究表明: 氧在钛中有较大固溶度,氧原子可以在钛表面的基体内聚集,逐步向深层扩散. 氧与钛具备较强的亲和力,能形成钛的氧化膜.钛基体中铝原子间具有相互吸引力, 能形成铝的原子团簇.铝原子团簇中的钛原子间相互排斥与铝形成化合物. 铝、钛与氧的亲和能相近,不易发生铝的优先氧化,而是同时生成钛的氧化物和铝的氧化物. Al₂O₃比TiO₂的结合能略低,因而更加稳定,铝在TiO₂中有较大的固溶度, 能替换其中的钛形成更稳定的Al₂O₃氧化物.     

Sonochemical synthesis of supersaturated Ga–Al liquid-alloy fine particles and Al3+-doped γ-Ga2O3 nanoparticles by direct oxidation at near room temperature

In this study, we investigated the fabrication of supersaturated gallium (Ga)–aluminum (Al) liquid alloy and Al³⁺-doped γ-Ga₂O₃ nanoparticles (NPs) at near room temperature (60 °C) using sonochemical and sonophysical effects. Supersaturated Ga–Al liquid alloy microparticles (D_av = 1.72 µm) were formed and stabilized at 60 °C by the thermal nonequilibrium field provided by sonochemical hot spots. Compared with liquid Ga, supersaturated Ga–Al liquid alloy was rapidly oxidized to a uniform oxide without Al₂O₃ or Al deposition. Thus, ultrafine Al³⁺-doped γ-Ga₂O₃ NPs were obtained after only 1 h of ultrasonic irradiation at 60 °C. The oxidation of liquid Ga was remarkably accelerated by alloying with metallic Al and ultrasonic irradiation, and the time was shortened. The average diameter and surface area of the γ-Ga₂O₃-based NPs were 59 nm and 181 m²/g, respectively. Compared with γ-Ga₂O₃, the optical bandgap of the Al³⁺-doped γ-Ga₂O₃ NPs was broadened, and the thermal stability improved, indicating Al³⁺-doping into the γ-Ga₂O₃ lattice. However, the lattice constant of γ-Ga₂O₃ was almost unchanged with or without Al³⁺-doping. Al³⁺ was introduced into the defect sites of Ga³⁺, which were massively induced in the defective spinel structure during ultrasonic processing. Therefore, sonochemical processing, which provides nonequilibrium reaction fields, is suitable for the synthesis of supersaturated and metastable materials in metals and ceramics fields.     

Design of Some Oxide/Metal Composite Supports and Catalysts

Textural, mechanical and catalytic properties of porous composite materials Al₂O₃/Al, MeO _x (Me)/Al₂O₃/Al with metal particles homogeneously distributed in the alumina matrix were studied. These materials were prepared by mixing the powdered components with aluminum followed by hydrothermal treatment and calcination. The macroporous structure was shown to be controlled by the size of large (>microns) particles in starting blends. The mesoporous structure is primarily determined by the properties of alumina formed by dehydration of hydroxide produced in turn via aluminum oxidation by water. The mechanical strength of porous cermets is determined by the number and properties of contacts between micron-size components of composites. Improved catalytic performance of composites is ensured by the developed macroporous structure providing enhanced mass transfer inside the cermet granules.     

Double-ceramic-layer thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/La2Ce2O7 deposited by electron beam-physical vapor deposition

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and La₂Ce₂O₇ (LC) were deposited by electron beam-physical vapor deposition (EB-PVD). The composition, interdiffusion, surface and cross-sectional morphologies, cyclic oxidation behavior of DCL coating were studied. Energy dispersive spectroscopy and X-ray diffraction analyses indicate that both LZ7C3 and LC coatings are effectively fabricated by a single LZ7C3 ingot with properly controlling the deposition energy. The chemical compatibility of LC coating and thermally grown oxide (TGO) layer is unstable. LaAlO₃ is formed due to the chemical reaction between LC and Al₂O₃ which is the main composition of TGO layer. Additionally, the thermal cycling behavior of DCL coating is influenced by the interdiffusion of Zr and Ce between LZ7C3 and LC coatings. The failure of DCL coating is a result of the sintering of LZ7C3 coating surface, the chemical incompatibility of LC coating and TGO layer and the abnormal oxidation of bond coat. Since no single material that has been studied so far satisfies all the requirements for high temperature applications, DCL coating is an important development direction of TBCs.     

Atmospheric reactive plasma sprayed Fe-Al2O3-FeAl2O4 composite coating and its property evaluation

In the present study, Fe-Al₂O₃-FeAl₂O₄ composite coatings were successfully deposited by reactive plasma sprayed Al/Fe₂O₃ agglomerated powder. Phase composition and microstructure of the coatings were determined by XRD and SEM. The results indicated that the composite coatings were principally composed of three different phases, i.e. FeAl₂O₄ phase as main framework, dispersed ball-like Fe-rich phase, and small splats of Al₂O₃ phase, and it was thought that the in situ synthesized metal phase was helpful to toughen the coating matrix. According to the results of the indentation and frictional wear tests, the composite coating exhibited excellent toughness and anti-friction properties in comparison with conventional Al₂O₃ monophase coating, though its microhardness value was a little lower than that of Al₂O₃ coating. The formation mechanism and the toughening mechanism of the composite coating were clarified in detail.     

Preparation and properties of composite ceramic coating containing Al2O3–ZrO2–Y2O3 on AZ91D magnesium alloy by plasma electrolytic oxidation

A composite ceramic coating containing Al₂O₃–ZrO₂–Y₂O₃ was successfully prepared on AZ91D magnesium alloy by plasma electrolytic oxidation (PEO) technique in an alkaline aluminate electrolyte. The morphology, elemental and phase composition, corrosion behavior and thermal stability of the uncoated and coated samples were studied by environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectrometer (EDS), X-ray diffractometer (XRD), electrochemical corrosion test, high temperature oxidation test and thermal shock test. The results showed that the composite ceramic coating was composed of Al₂O₃, c-ZrO₂, t-ZrO₂, Y₂O₃ and some magnesium compounds, such as MgO, MgF₂ and MgAl₂O₄. After PEO treatment, the corrosion potential of AZ91D alloy was increased and the corrosion current density was significantly reduced. Besides, the coated magnesium alloys also showed excellent high temperature oxidation resistance and thermal shock resistance at 500 °C environment.     

Microstructure and mechanism of Al2O3–ZrO2 eutectic coating prepared by combustion-assisted thermal explosion spraying

Thin Al₂O₃–ZrO₂ eutectic coating was produced by combustion-assisted thermal explosion spraying using a mixture of Al and Zr(NO₃)₄ powders. The cross-sectional microstructure of the coating shows successively amorphous, cellular, and dendrite structure along the direction perpendicular to the Cu substrate. The formation mechanism of the coating structure was analyzed based on the experimental results, and the morphology of different crystal structure was observed by SEM and TEM.