Sol–gel Al2O3/ZrO2 duplex films for protection of a γ-TiAl based alloy against high temperature oxidation

Al₂O₃/ZrO₂ duplex films were deposited on a γ-TiAl based alloy by sol–gel processing starting from aluminum isopropoxide (Al(OC₃H₇)₃) and zirconium (IV) oxychloride octahydrate (ZrOCl₂ · 8H₂O) as raw materials. Isothermal oxidation at 900 and 1,000 °C in 0.1 MPa O₂ and cyclic oxidation at 1,000 °C in air of the coated and uncoated specimens were performed to investigate the effect of the duplex films on the oxidation behavior of the γ-TiAl alloy. The results of the isothermal oxidation tests indicated that the parabolic rate constants of the alloy were decreased due to the applied thin film. Additionally, the present film exhibited a beneficial effect on the cyclic oxidation resistance of the alloy in air. The duplex film could restrain the growth of TiO₂, causing an increase of the Al₂O₃ content in the oxide mixture and thus decreased the oxidation rate.     

High‐temperature oxidation behavior of Ni‐based superalloys with Nb and Y and the interface characteristics of oxidation scales

Ni‐based superalloys with niobium (Nb) or/and yttrium (Y) were prepared by vacuum melting. The oxidation kinetics of these alloys was studied by thermogravimetry at 800 °C for 100 h in static air. Morphology of oxides was studied using SEM, and the composition was analyzed by X‐ray diffraction. Energy‐dispersive X‐ray spectrometer was employed to examine the linear element distribution of the cross section of the oxidation films. Results showed that the oxidation kinetics all followed a parabolic law at different stages. The oxide films were mainly comprised of Cr₂O₃, NiCr₂O₄, Al₂O₃ and TiO₂. All the oxide films exhibited layered structure owing to different oxidation stages. With the addition of Nb or Y, the high‐temperature oxidation resistance of the superalloy was improved significantly and the surface morphology of the oxidation film was ameliorated. The comprehensive effect of Nb and Y was remarkable in improving the high‐temperature oxidation resistance of Ni‐based alloys. Copyright © 2014 John Wiley & Sons, Ltd.     

Oxidation behavior of rare earth oxide-coated Fe20Cr and Fe20Cr5Al alloys

This article presents the influence of surface additions of nanocrystalline rare earth (RE) oxides CeO₂, La₂O₃, and CeO₂ + La₂O₃ on the isothermal oxidation behavior of Fe20Cr and Fe20Cr5Al at 1000 °C. Thermogravimetric studies revealed parabolic kinetics in all cases and the scale thickness on specimen surfaces varied with the nature of RE oxide. The oxidation resistance of specimens coated with two RE oxides was significantly higher than those coated with either one of the two oxides. The marked increase in the oxidation resistance of the alloys coated with two RE oxides is due to optimization of RE ion radius and RE oxide grain size/shape.     

Study of the effect of laser treatment on the initial oxidation behaviour of Al‐coated NiCrAlY bond‐coat

In this study, the initial oxidation behaviour of laser‐treated Al/NiCrAlY bond‐coat is investigated. Two approaches, (i) Al film sputtering on the surface of bond‐coat and (ii) laser treatment, have been taken to enhance the oxidation resistance of NiCrAlY bond‐coat. Experimental results showed that after laser treatment, the Al/NiCrAlY bond‐coat exhibited a columnar dendritic microstructure without cracks and voids. A dense and continuous α‐Al₂O₃/Cr₂O₃ multilayer was found to form on the bond‐coat surface. Results on the cyclic oxidation at 1200 °C (for time ≤ 204 h) revealed that the laser‐treated Al/NiCrAlY bond‐coat exhibited better oxidation resistance compared to as‐sprayed NiCrAlY, Al/NiCrAlY and laser‐remelted NiCrAlY bond‐coat. The formation of θ‐Al₂O₃, NiO, Cr₂O₃ and NiCr₂O₄ spinel oxides was observed to be suppressed due to the preformed α‐Al₂O₃ scale during initial oxidation on the surface of laser pre‐oxidized Al/NiCrAlY bond‐coat. Copyright © 2013 John Wiley & Sons, Ltd.     

含O2高温高压CO2环境中3Cr钢腐蚀产物膜特征

采用高温高压反应釜分别开展3Cr钢在CO2和O2共存、单独CO2和单独O2三种气体条件下的腐蚀实验,利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、能量色散X射线能谱(EDS)和电化学方法研究了3Cr钢在高温高压含有O2的CO2环境中的腐蚀产物膜特征.结果表明,在含有O2的CO2的条件下,3Cr钢表面腐蚀产物膜疏松多孔,主要成分为FeCO3、Fe3O4和Fe2O3,腐蚀产物中未见明显Cr元素富集,3Cr钢表现出点蚀的腐蚀形态.3Cr钢在高温高压含O2的CO2腐蚀条件下内外膜层电阻(Rf1、Rf2)和电荷传递电阻Rt均比仅含有CO2腐蚀环境的低,双电层电容(Cdl)和内外膜层电容(Cf1、Cf2)均比仅含有CO2腐蚀环境的高.含有O2的CO2条件下,其保护性显著低于单一CO2条件下形成的腐蚀产物膜.提出了在含O2的CO2气体条件下,3Cr钢表面存在由多种物质组成的腐蚀产物,这导致腐蚀产物疏松多孔,不会形成单一CO2条件下存在的显著提高腐蚀产物膜保护性的Cr(OH)3层,从而促进了3Cr钢的析氢腐蚀和酸性介质中的吸氧腐蚀的机理.     

Formation of chromium composite electrochemical coatings from sulfate oxalate solutions based on Cr(III)

The mechanism responsible for the inclusion of Al₂O₃ and SiC nanoparticles, the mixture of Nb₂N and Ta₂N (1: 1), MoS₂, Cr₂O₃, and SiO₂ with diverse electric conductivity, hydrophilicity, and resistance to solution components in chromium deposits from the sulfate–oxalate suspension solutions based on Cr(III) was studied. The main factors that determine the formation of chromium composite electrochemical coatings, their composition, and surface morphology were determined. The film on the surface of the growing deposit of the intermediates of the reduction of chromium ions plays the key role in the formation of composite coatings from Cr(III) and Cr(VI) sulfate–oxalate suspension solutions. The film can play the role of a structural mechanical barrier that hinders the incorporation of particles in the deposit, or it can fix the particles on the electrode surface by creating hydroxo bridges with chemisorbed hydroxide compounds on the particle surface.     

Improvement of the oxidation behavior of Ti1-xAlxN hard coatings by optimization of the Ti/Al ratio

The oxidation behavior of cubic Ti_1-xAl_xN films was improved by decreasing the Ti/Al ratio from 50/50 in the direction of the phase transition between cubic and hexagonal structure. Metastable, polycrystalline, single-phase Ti_1-xAl_xN films were deposited on high speed steel (HSS) substrates by reactive magnetron sputtering ion plating (MSIP). The composition of the bulk was determined by electron probe microanalysis (EPMA), the crystallographic structure by thin film X-ray diffraction (XRD). A Ti_1-xAl_xN film with a Ti/Al atomic ratio of 38/62 was deposited in cubic NaCl structure, whereas a further decrease of the Ti/Al ratio down to 27/73 led to a two-phase film with both cubic and hexagonal constituents. The Ti_0.38Al_0.62N film was oxidized in synthetic air for 1 h at 800?°C. The oxidic overlayer was analyzed by X-ray photoelectron spectroscopy (XPS) sputter depth profiling, EPMA crater edge linescan analysis, and secondary neutrals mass spectroscopy (SNMS). Scanning electron microscopy (SEM) micrographs of the cross sectional fracture were taken for morphological examination. With higher Ti content, the Ti_1-xAl_xN formed a TiO_2-x rich sublayer beneath an Al₂O₃ rich toplayer, whereas the oxide layer on the Ti_0.38Al_0.62N film consisted of pure Al₂O₃. The thickness of the oxide layer was determined to 60–80 nm, about a quarter of the oxide layer thickness detected on Ti_0.5Al_0.5N films. The absence of a TiO_2-x sublayer was also confirmed by XRD. The results show a distinct improvement of the oxidation resistance of cubic Ti_1-xAl_xN films by increasing the Al content from x = 0.5 to 0.62, whereas a further increase leads to the hexagonal structure, which is less suitable for tribological applications due to its tendency to form cracks during oxidation.     

The influence of initial surface conditions on field crystallization of anodic aluminum oxide films determined by synchrotron X‐ray diffraction

X‐ray diffraction measurements were performed using synchrotron radiation at the SPring‐8 facility and electrochemical techniques to investigate the effect of polishing methods and storage conditions on the crystal structure of air‐formed oxide films and anodic oxide films formed on highly pure aluminum. Storage in an N₂ environment hinders local film breakdown during anodizing, and it was established that the X‐ray diffraction measurements showed the presence of a γ‐Al₂O₃ in the anodic oxide film formed on mechanically polished (MP) specimens. Formation of γ‐Al₂O₃ during anodizing was inhibited by electropolishing because of the removal of the work‐hardened layer that was formed on the MP by electro‐polishing. The X‐ray diffraction results do not show clear differences in the influence of the polishing method on the crystal structure of air formed oxide film. This is due to the very fast oxidation rate of the air‐formed oxide film and very long storage times for the X‐ray measurements. The anodic oxide film formed on aluminum, which has a very flat surface, shows color and the color depended on grain orientation. The electrochemical impedance of the MP specimen is slightly lower than that of the mechanically and then electrochemically polished specimen at the middle frequency range. This impedance difference may be due to formation of γ‐Al₂O₃ in the amorphous anodic oxide film and the thickness of the film. Copyright © 2010 John Wiley & Sons, Ltd.     

Radiation-induced catalytic reduction of chromium(VI) in aqueous solution containing TiO2, Al2O3 or SiO2 fine particles

Reduction of chromium (VI), Cr(VI) in aqueous neutral or basic solution was promoted by γ-ray irradiation in the presence of oxide particles such as TiO₂, Al₂O₃ or SiO₂. The oxide particles behaved as a catalyst, and the efficiency of the Cr(VI) reduction increased with an increase of the irradiation dose irrespective of the initial Cr(VI) concentration. The insoluble Cr(III) oxide formed through the Cr(VI) reduction also acted as the catalyst.     

Achieving ultra-low friction of a-C:H film grown on 9Cr18Mo steel for industrial application via programmable high power pulse magnetron sputtering

Owing to the high hardness and hydrogen passivation of carbon bonds, hydrogenated diamond-like carbon (a-C:H) film has shown promising potential to achieve ultra-low friction and wear on steel surfaces. Here, a-C:H film was successfully deposited on 9Cr18Mo steel via programmable high power pulse magnetron sputtering and potential application for industrial was evaluated. The a-C:H films against different mating materials of GCr15 steel balls, Al₂O₃, Si₃N₄, ZrO₂, and a-C:H-coated GCr15 balls all showed ultra-low friction under a normal load of 5 N in a dry ambient air environment. Among them, self-mating tribo-system a-C:H films on steel surfaces and a-C:H-coated steel balls achieve best friction performance; the principal reason is that both contacting surfaces coated with a-C:H film have the lower electron affinities compared with other tribo-systems. However, the differences of coefficient of friction (COF) for uncoated-GCr15, Al₂O₃, ZrO₂, Si₃N₄, and a-C:H(GCr15) balls can be attributed to different sizes of clustering in wear debris. This work provides new insights on synthesis and industry application of the a-C:H films with ultra-low friction properties.     

Catalytic activity and physicochemical properties of Ni-Au/Al<Subscript>3</Subscript>CrO<Subscript>6</Subscript> system for partial oxidation of methane to synthesis gas

This work is focused on the role of gold and Al₃CrO₆ support for physicochemical properties, and catalytic activity of supported nickel catalysts in partial oxidation of methane (POM). Catalysts, containing 5% Ni and 5% Ni-2% Au active phases dispersed on mono- (Al₂O₃, Cr₂O₃) and bi-oxide Al₃CrO₆ support, were investigated by TPR, BET and XRD methods, and the activity tests in POM reaction were carried out. Bimetallic Ni-Au catalysts dispersed on Al₃CrO₆ support remained highly stable and active. The amorphous binary oxide Al₃CrO₆ can stabilize considerable amount of Cr⁴⁺, Cr⁵⁺, and Cr⁶⁺ species in Ni-Au/Al₃CrO₆ catalyst network during its calcination in the air. Nickel supported on binary oxide Ni/Al₃CrO₆ can form Ni(III)CrO₃ bi-oxide phase in reductive conditions. During TPR H2 reduction of Ni-Au/Al₃CrO₆ catalyst chromium(II) oxide Cr(II)O phase is observed. After POM reaction the existence of bimetallic Au-Ni alloy was experimentally confirmed on mono-oxide Al₂O₃ support surface, but its formation was not identified on bioxide Al₃CrO₆ support. Published in Russian in Kinetika i Kataliz, 2009, Vol. 50, No. 1, pp. 149–156. The article is published in the original. Based on a report at the VII Russ. Conf. on Mechanisms of Catalytic Reactions (with international participation), St. Petersburg, July 2–8, 2006.     

Oxidation of 3- and 4-methylpyridines on modified vanadium oxide catalysts

The partial oxidation of 3- and 4-methylpyridines on V₂O₅ and vanadium oxide catalysts doped with TiO₂, Al₂O₃, and ZrO₂ was studied. The catalytic activities of the studied catalysts were correlated with the calculated proton affinities of the vanadyl oxygen. A possible mechanism of the surface stages of the partial oxidation of 3- and 4-methylpyridines on the vanadium oxide catalysts was discussed.     

The negative adsorption of chromium atoms on alloy-oxide film boundaries during oxidation in air and anodic passivation of Fe-Cr and Ni-Cr alloys

Equilibrium of Cr atoms between the surface layer and bulk of a binary alloy was analyzed. The Gibbs adsorption equation was used to obtain the dependence of the adsorption activity of atoms in the surface layer on their activity in the bulk. An approximate thermodynamic method was used to calculate the adsorption of Fe (Ni) and Cr atoms in the surface layers of Fe-Cr and Ni-Cr alloys. According to calculations, there was negative adsorption, X _Cr ≪ 1, in the surface layer of the alloys caused by a large difference between the Gibbs surface energies of Cr and Fe (or Ni). The negative adsorption of Cr shifted chemical reaction equilibria on the alloy-oxide film boundary both in oxidation in air and in anodic passivation, 3FeO (NiO) + 2Cr = Cr₂O₃ + 3Fe(Ni), toward oxide film enrichment in the FeO (or NiO) oxide. A unified method for calculating the composition of oxide films on alloys was used for both processes. The method was based on the use of the initial data on the Gibbs surface energy of metals constituting alloys. The calculated oxide film compositions were close to the experimental X-ray photoelectron spectroscopy data.     

Time-on stream behavior of Pd-, Co-, and Mn-zeolite catalysts supported on metal blocks in high-temperature methane oxidation

Stability of the Pd-, Co-, and Mn-zeolite catalysts supported on metal blocks was studied in high-temperature methane oxidation. The temperature regions were found in which the starting catalysts exhibit stable performance. The temperature was determined at which a partial deactivation is followed by stabilization of catalysts in reaction environment. In terms of specific activity, the partially deactivated Pd-zeolite catalyst is several times more active than conventional oxidation catalysts Pd/Al₂O₃, Pt/Al₂O₃, and the most active oxide CeO·6Al₂O₃.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2075–2078, October, 2004.     

Growing kinetics and structural characterization of oxide film formed on La-doped Co-40Cr alloy

The isothermal and cyclic oxidizing kinetics of Co-40Cr alloy and its lanthanum ion-implanted samples were studied at 1000℃ in air by thermal-gravity analysis (TGA). Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM)) were used to examine the oxidized film's morphology and the structure after oxidation. Secondary ion mass spectrum (SIMS) method was used to examine the binding energy change of chromium caused by La-doping and its in?uence on formation of Cr2O3 film. Laser Raman spectrum was used to examine the tress changes within oxidized films. It was found that lanthanum implantation remarkably reduced the isothermal oxidizing rate of Co-40Cr and improved the anti-cracking and anti-spalling properties of Cr2O3 film. The reasons were that the implanted lanthanum reduced the grain size and internal stress of Cr2O3 oxide, increased the high temperature plasticity of oxidized film. Lanthanum mainly existed in the outer surface of Cr2O3 film in the forms of fine La2O3 and LaCrO3 spinel particles.     

Microstructural characterization of oxide layers formed on Ni–20Cr–8Al alloy foam using transmission electron microscopy

Microstructural characterization was carried out during the pre‐oxidation of Ni–20Cr–8Al alloy foam using transmission electron microscopy (TEM). During the pre‐oxidation at 1000 °C for 1, 30, and 60 min in air, the sequential formations of NiO, NiCr₂O₄, Cr₂O₃, and α‐Al₂O₃ 1‐μm‐thick oxide layers were, respectively, characterized. Initially, during pre‐oxidization, the layers formed abnormally in an island growth mode, but they grew to be morphologically uniform after 30 min. Pores were found after only 1 min in the middle region of the oxide layers, near the Cr₂O₃ layer, and then these developed into critical micro‐cracks after 60 min. Copyright © 2017 John Wiley & Sons, Ltd.     

Formation of Cr(VI) compounds during the thermal decomposition of amorphous chromium hydroxide

Chromium hydroxide (CH) was prepared by the reduction of purified sodium chromate using starch. CH was then used to prepare chromium oxide (Cr₂O₃). Results of thermogravimetric, X-ray diffraction, Fourier transform infrared spectroscopy, and chemical analyses suggested that a small amount of the initial Cr(VI) content speeds up the oxidation and reduction reactions, thereby promoting the purity and crystallization of Cr₂O₃. Cr(VI) in CH induced the evolution of CH and the formation of Cr(VI) containing compounds including CrO₃, NaCr(CrO₄)₂, Cr₃O₈, and Cr₅O₁₂ at low sintering temperature. Furthermore, homogeneous Cr₂O₃ nanoparticles with 99 % purity and particle size of 50 nm were obtained.     

Preparation of compact Al2O3 film on metal for oxidation resistance by polyvinylpyrrolidone

Oxidation resistance of metal at high temperature can be improved by an environmentally friendly solution deposition approach. Stable precursor solution with high oxide concentration, favorable viscosity and low surface tension was prepared using aluminum sec-butoxide (ASB) and polyvinylpyrrolidone (PVP) as starting raw materials. Alumina sol-gel films were deposited onto metal by spin-coating followed by heat treatment. When PVP was added according to an amount of 50 mg/mL into a sol with an ASB/H₂O molar ratio of 1:35, the as-obtained sol exhibited favorable gelation time and viscosity. The surface tension of the alumina sol with PVP was examined to be lower by 32% than the sol (ASB:H₂O = 1:100) without PVP. TG-DTA analyses show the densification of the alumina gel film with PVP was progressed within a wide temperature range from 200 to 650 °C. Crack-free Al₂O₃ film with a thickness up to 1.5 μm was successfully produced on metallic substrate by three spin-coating cycles. SEM and XRD analyses revealed the gel film transformed into compact α-Al₂O₃ material after calcined at 1,000 °C for 0.5 h. The weight gained by the samples during firing at 1,000 °C indicated that the Al₂O₃ coating film could reduce the rate of oxidation by ∼81%. The hardness of the Al₂O₃ film coated metal was higher by 260% than the uncoated metal that was calcined at 1,000 °C for 0.5 h. It was confirmed by adhesion test that both the alumina/PVP hybrid film and the as-produced α-Al₂O₃ coating film had strong adhesion.     

High temperature oxidation of chromium with nanometric ceria sol–gel coating

Isothermal oxidation behavior of chromium with and without nanometric sol-gel CeO2 coating is studied at 1000℃ in air. Scanning electron microscopy （SEM） and transmission electron microscopy （TEM） are used to examine the surface morphology and microstructure of their oxide films. It is found that ceria coating greatly improves the anti-oxidation property of chromium. Laser Raman spectrometer and X-ray diffraction spectrometer （XRD） are also used to study the stress level in oxide films formed on ceria-coated and ceria-free Cr. The difference in oxidation behavior is mainly attributed to the fact that ceria greatly reduces the growth speed and grain size of Cr2O3 film, and this fine grain-sized Cr2O3 film probably has better high temperature plasticity, i.e. oxide film can relieve parts of compressive stress by means of creeping. XRD and Raman testing results both show the stress declination due to nano-CeO2 application, and their deviation is analyzed conceming to the rare earth effect.     

Effect of inert components (Y2O3, Al2O3, and Ga2O3) on the chemistimulating effect of the Sb2O3 activator of GaAs thermal oxidation

Chemistimilated thermal oxidation of gallium arsenide was studied using Sb₂O₃ activator oxide in compositions with Ga₂O₃, Al₂O₃, and Y₂O₃ inert components. For Sb₂O₃-Y₂O₃ compositions, the thickness of the resulting oxide layer on GaAs was found to be a linear function of composition over the enter range of the compositions. For antimony oxide compositions with Ga₂O₃ and Al₂O₃ inert components, nonadditivities were observed near the component ordinates. For the Sb₂O₃-Ga₂O₃ system, the chemistimulating efficiency noticeably weakened at low concentrations of the inert component. The linear trend observed for this system within 0–60 mol % Sb₂O₃ is additively determined by the oxide layer thickness on GaAs in the presence of Sb₂O₃ and in the absence of activator. In the presence of inert Al₂O₃, the chemistimulating effect was enhanced near the Al₂O₃ ordinate and the resulting function was nonadditive with respect to the thicknesses reached in the presence of the individual components.