Modification Of Aluminum Oxide Research Articles

FRICTION PROPERTIES OF MАO COATINGS ON ALUMINUM ALLOYS

Aluminum alloys are widely used in mechanical engineering due to their physical and mechanical properties. However, their low wear resistance and burr resistance limit their use in friction nodes. In this regard, parts made of aluminum alloys operating under conditions of sliding friction require surface strengthening, which determines the friction wear parameters. Research works in this direction allow increasing the reliability and service life of friction nodes. Ensuring the compatibility of friction pairs allows reducing the coefficient of friction and, as a result, to increase wear resistance. One of the effective methods of forming wear-resistant coatings on aluminum alloys is the method of micro-arc oxidation (MAO). The use of micro-arc discharges as highly concentrated energy sources to create conditions for high-temperature electrotechnical processes and phase transformations in the surface layer makes it possible to form structures based on high-temperature oxides on the surface of metals, which provides qualitatively new surface properties: high hardness, wear resistance, corrosion resistance in various environments. However, the use of oxide coatings in bearings is mainly due to their high wear resistance and corrosion resistance. Information on the antifriction properties of aluminum oxide-based coatings is limited. The purpose of this work is to determine the coefficient of friction of MAO-coatings on D16 aluminum alloy in a pair with cast iron, steel and MAO-coating under conditions of oil and water lubrication, and to find out the feasibility of using MAO technology to create sliding bearings. The formation of MAO coatings was carried out in an alkaline-silicate electrolyte in the cathodic-anodic mode. The structure and morphology of the working layer of the MAO-coating surface, which works in friction pairs with cast iron, steel and MAO-coating under conditions of oil and water lubrication, was investigated. The benefit of the available porosity of the working layer of the MAO coating, which is 5-10% and contributes to the retention of lubricant in the friction zone, is shown. The phase composition, hardness of the coatings were studied and the coefficient of sliding friction was determined on the SMC-2 friction machine according to the "disk-pad" scheme when using water and oil as lubricants; the load varied from 0.2 to 2 kN, the nominal pressure from 1 to 11 MPa. It was established that the MAO coating has a crystalline structure, the phase composition is high-temperature modifications of aluminum oxide (α-Al2O3 and γ- Al2O3), and the hardness is 18000-20000 MPa. It was established that the coefficient of sliding friction of gray cast iron - MAO-coating, steel - MAO-coating and MAO on MAO decreases to the value of 0.01 - 0.013 under lubrication conditions. It was found that in a pair of MAO-coating - MAO-coating, anti-friction properties are manifested both under conditions of lubrication with oil and water. The low coefficient of friction for the coating-coating pair in the case of water lubrication is explained by the high hydrophilic properties of the oxide contacting surfaces, which makes it possible to recommend them for use in sliding bearings (liners) of hydroturbines. A conclusion was drawn regarding the reasons for the low coefficient of friction of the studied samples. Keywords: D16 aluminum alloy, cast iron, steel, micro-arc treatment, coating, phase composition, crystal structure, aluminum oxides, coating thickness, coating hardness, friction coefficient, antifriction properties

Laser cutting of plasma-sprayed refractory products made of aluminum oxide

The possibility of laser cutting of plasma-sprayed refractory products made of aluminum oxide was investigated. The industrial laser complex KS-5VDM1-3 Navigator (VNITEP, Russia), equipped with an LS-3 ytterbium fiber laser with a power of 3 kW, was used to work out the cutting conditions. It was found that immediately after the completion of the plasma spraying process, the phase composition of the article material is a set of various modifications of aluminum oxide. Subsequent heating of these items leads to a series of polymorphic transformations accompanied by changes in the volume and density of the item material. The process ends when all polymorphic modifications of aluminum oxide pass into the only stable phase α-Al2O3 (corundum). It was shown that laser cutting of plasmasprayed aluminum oxide products is possible only when the process of transition of all its phase modifications to corundum is completely completed. Otherwise, a sharp change in the volume of the product material, caused by polymorphic transformations occurring under conditions of a high temperature gradient that occurs during laser cutting, leads to the destruction of the product. Plasmasprayed product, the material of which is entirely α-modification of aluminum oxide (corundum), is not afraid of high temperature gradients and is easily laser cut without destruction. Cutting data can vary widely. The cutting technology of plasma-sprayed refractory products made of aluminum oxide has been mastered at the Scientific and Educational Center for the Implementation of Laser Technologies at the Vladimir State University. Ill. 3. Ref. 13. Tab. 2.

THERMAL ANALYSIS OF THE CATALYTIC SYSTEM OF PHOSFORMOLYBDENIC HETEROPOLY ACID–ALUMINUM OXIDE

The methods of thermal analysis – thermogravimetry, differential thermal analysis – were used to study the catalytic system phosphor-molybdenum heteropoly acid–aluminum oxide. It is shown that the modification of aluminum oxide has a direct effect on the state of the phosphor-molybdenum heteropoly acid. On the surface of ƞ-Al2O3, phosphomolybdic acid is destroyed by interaction with strong Lewis’s acid sites of the carrier, while on γ-Al2O3 it retains the individuality, probably due to multipoint adsorption on weak Bronsted and Lewis acid sites of the oxide

Рентгеноспектральный анализ активированного трепела Краснослободского месторождения Тамбовской области

The features of the change in the composition of rock-forming minerals during thermal activation of tripoli were established. At temperatures above 500°С, decomposition of clay minerals occurs with the formation of a high-temperature modification of aluminum oxide. In the temperature range of 200-500°С, the ordering of the amorphous opal-cristobalite structure increases due to phase changes in cristobalite, a further increase in temperature leads to its transformation into tridymite.

Picosecond laser surface modification of aluminum oxide with fish-scale structures for cell culture

The surface modification of a layer substrate has received attention for many biomedical applications (e.g., tissue engineering and biosensing). Under an in situ control, we proposed a strategy for surface modification of hard and brittle ceramics with ultrafast laser pulses for cell culture. A type of ultrafast laser based on the picosecond pulse technique was employed to ablate the surface of a pristine aluminum oxide (Al2O3) substrate. The surface of the formed reproducible micro-nanostructures indicated the fish-scale structures, which was analyzed under different laser ablation conditions. Subsequently, the SV40 T-antigen human embryonic kidney 293 (HEK293T) cells were cultured on the laser-ablated Al2O3 substrates. These microstructures and nanostructures can enhance cell proliferation response. The surface effect played a crucial role, by which the surface roughness (Ra) and contact angle (CA) of water droplets on the ablated Al2O3 substrate increased with the increasing laser fluence. The formed fish-scale structures were formed by the parameters of laser ablation where the structural characteristics of the Al2O3 substrate can be remained in the ultrafast laser process. The presented process provides a simple support for developing ablated structures on the Al2O3 substrate for cell culture.

Plasma-Chemical Modification of Facing Composite Material on the Basis of Hollow Glass Microspheres with Decorative Protective Coating

The paper considers plasma-chemical modification of facing composite material on the basis of hollow glass microspheres with decorative protective coatings. It is shown that the technology of obtaining decorative protective coating by the method of plasma reflow is highly efficient and energy-saving, which will allow competition with traditional technologies and significantly reduce the cost of finishing work. Compositions of decorative protective coatings based on high-alumina refractory and liquid glass have been developed. It was revealed that the use of high-alumina refractory crushed material in the intermediate layer makes it possible to reduce the number of microcracks and eliminate the consequences of high-temperature impact on the matrix of fine-grained concrete. The positive effect of hollow glass microspheres on the minimization of thermal shock was established. The results of the investigation of the effect of plasma-chemical modification on the phase composition and structural features of the facing composite material on the basis of hollow glass microspheres with a decorative protective coating are presented. The regularity of layer-by-layer variation of the phase composition and the macro- and microstructure of the decorative protective coating of the facing composite material is established. It is shown that the upper layer is represented by Na–Ca–Al–Si glass, the middle glass ceramic layer is represented by the glass phase and α and β modifications of aluminum oxide, and the lower dehydration layer is represented by dehydration products of hydrosilicates and α and β modifications of alumina. High-temperature action of the plasma jet intensifies thermal diffusion, which in turn leads to a redistribution of oxides along the thickness of the decorative protective coating.

Жидкофазный синтез с элементами низкотемпературной обработки различных модификаций оксида алюминия и исследование их физико-химических свойств

Жидкофазный синтез с элементами низкотемпературной обработки различных модификаций оксида алюминия и исследование их физико-химических свойств

Conditions for the Production of Active Aluminum Oxide Meeting the Requirements of Prospective Chloride Technology

The features of the dehydration and recrystallization processes were studied on samples of aluminum hydroxide obtained by carbonization of an aluminate solution at 40°C and isolated by Bayer’s decomposition. The main differences in the structural transformations of these samples consist in the individual sequence of the formation of polymorphic modifications of oxides in the dehydration of each aluminum hydroxide. The structural transformations of these types of hydroxide and the dynamics of the polymorphism of the decomposition products were studied in the temperature range of 100–1000°С. Data from crystallooptical, X-ray phase, and thermogravimetric methods of analysis showed that the recrystallization of the structure occurs more slowly when calcining the carbonized aluminum hydroxide than in the decomposition method. It is determined that, with the dehydration of the hydroxide, sodium aluminosilicate and carbon reduce the rate of formation of high-temperature modifications of alumina. The reactivity of aluminum oxide obtained by thermal decomposition of chemically pure sulfuric, hydrochloric, and nitric acid crystalline salts of aluminum salts is determined not only by the phase composition but also by the nature of the starting material. Despite the general morphology of the process of formation of aluminum oxide during the decomposition of the investigated crystalline hydrates, the formation of the phase composition of Al2O3 occurs at different temperatures and at different rates. Structural rearrangements during the decomposition of salts occur in the solid phase, which determines the total porosity and the specific surface area of each type of aluminum oxide. A study of the polymorphism of black alumina containing iron and silicon impurities was also carried out on samples obtained after leaching of the mineral part of the carbonaceous rocks with sulfuric, hydrochloric, and nitric acids in the temperature range of 100–1000°C. An inhibitory effect of impurities and a reducing agent on the formation of high-temperature structural modifications of aluminum oxide in the process of heat treatment of sulfuric, hydrochloric, and nitric acid crystalline hydrates of aluminum salts was revealed.

Kinetic Aspects of the Adsorption on Aluminum Oxide Drying Agents Doped with Alkali Metal Ions

Fundamental dynamic (kinetic) aspects of the process in which water vapor interacts with the surface of drying agents that are synthesized on the basis of low-temperature modification of aluminum oxide produced from a pseudoboehmite-containing hydroxide and are modified (doped) with alkali atoms (K, Na). It is shown that the kinetics of adsorption on the samples under study, formed from the fine fraction (0.5–1.0 mm) of aluminum oxide adsorbents, can be described with the Glueckauf equation, which rather well describes the dynamics of water vapor absorption in the course of time. The equation parameters were determined: adsorption rate constants and the equilibrium adsorption capacities (a*). It was found that the alkaline modification of the surface of aluminum oxide adsorbents results in that a* increases (by ~40%) as compared with the unmodified drying agent. A correlation is observed between the equilibrium adsorption capacity of the samples under study and the acid-base properties of the surface.

Chemical surface modification of aluminum oxide nanoparticles with graft copolymer of aluminum oxide and poly(isobutyl vinyl ether) mechanochemically synthesized in vacuum at low temperature

Chemical surface modification of aluminum oxide (AO) nanoparticles with graft copolymer of AO and poly(isobutyl vinyl ether) (PIBVE) (AO-graft-PIBVE) was carried out by ball milling of AO nanoparticles with IBVE in vacuum at low temperature. The AO-graft-PIBVE was evidenced by FT-IR and 1H NMR spectra. The surface modified AO nanoparticles with mono-dispersed particle diameter exhibited structural blue color which was evidenced by photograph, optical spectroscopy (UV-Vis-NIR) and SEM image. The key atoms for the mechanochemical reactions were assigned by the net charge and the distribution of the HOMO and/or LUMO at each atom estimated by molecular orbital calculations. The reaction scheme is proposed; naked Al cations of AO (AO-Al+) were produced by ionic scission of aluminum-oxygen bond by ball milling and trapped at the surface of AO nanoparticles. The AO-Al+ reacted with IBVE, formed a novel bond between the Al-atom of AO-Al+ and the C-atom of IBVE, and resulted in AO-Al-IBVE+. The AO-Al-IBVE+ sequentially reacted to the C-atom of IBVE, and finally resulted in AO-graft-PIBVE. The PIBVE homopolymer was simultaneously produced by the ball milling of AO with IBVE in vacuum at low temperature. The key atoms for the reactions were assigned by the molecular orbital calculations.

Syngas production via the biogas dry reforming reaction over nickel supported on modified with CeO2 and/or La2O3 alumina catalysts

In this study, an investigation of the effects of the modification of aluminum oxide (Al2O3) with various dopants, namely CeO2 and/or La2O3 for Ni based catalysts (Ni/CeAl, Ni/LaAl and Ni/CeLaAl) synthesized via the wet impregnation method at a constant nickel loading (8 wt. %), was performed for the biogas dry reforming reaction. The final catalysts, at their calcined or/and reduced form, were characterized by various techniques, such as XRD, SEM, and N2 adsorption–desorption isotherms. The chemical composition of the catalysts was determined by ICP. Used catalysts were characterized by SEM and RAMAN, in order to establish whether carbon was deposited and its nature. A theoretical model has also been developed, with the aim of assessing the experimental behavior of the fixed bed reactor and the catalysts tested herein. Catalytic performance of all catalysts concerning the biogas dry reforming reaction was studied in order to investigate the effect of the reaction temperature on methane (XCH4) and carbon dioxide (XCO2) conversion, hydrogen (YH2) and carbon monoxide (YCO) yield, and H2/CO molar ratio of the produced gas mixture at the outlet of the reactor. The results indicate that modification of Ni/Al catalysts with CeO2 and/or La2O3 significantly enhances catalytic performance, with catalysts containing lanthanum (i.e., Ni/LaAl and Ni/CeLaAl) performing better than the sample containing only ceria (Ni/CeAl).

Modification of aluminum oxide as a method for controlling its activity and stability in vapor-phase dehydration of glycerol into acrolein

It is shown that promotion of γ-Al2O3 with zirconium, cobalt, boron, lanthanum, and cerium oxides strongly affects its catalytic activity and stability in vapor-phase dehydration of glycerol into acrolein. Modification of gamma-aluminum oxide with acid oxides impairs the stability and activity of the catalyst. Introduction of lanthanum and cerium oxides into the structure of γ-Al2O3 leads to a decrease in the number of strong acid centers, which diminishes the activity of the catalytic system and improves its stability.

Effect of silica on the stability of the nanostructure and texture of fine-particle alumina

The effect of the modification of aluminum oxide with silicon oxide on the stability of fine-particle Γ- and δ-Al2O3 phases upon heat treatment in the wide temperature range of 550–1500°C was studied. It was found that the Γ- and δ-Al2O3 phases modified with silica are thermally stable up to higher temperatures than pure aluminum oxide. This is due to changes in the real structure of the modified samples, specifically, an increase in the concentration of extensive defects stabilized by hydroxyl groups bound to not only aluminum atoms but also silicon atoms. It is likely that Si-OH groups, which are thermally more stable than Al-OH groups, stabilize the microstructure of Γ- and δ-Al2O3 to higher temperatures, as compared with aluminum oxide containing no additives. Simultaneously, an increase in the thermal stability of the modified samples is accompanied by the retention of a high specific surface area and a developed pore structure at higher treatment temperatures.

Shock synthesis of new aluminum oxide modifications using an octogen – aluminum hydride mixture

The results of an x-ray diffraction study of two new modifications of aluminum oxide with tetragonal and hexagonal cells, synthesized by means of in an explosion of an octogen – aluminum hydride mixture (90/10 wt.%), are presented. The parameters of the tetragonal and hexagonal cells are, respectively, a = 7.932(6) A, c = 9.142(5) A, c/a = 1.153, V = 575 A3 and a = 9.140(3) A, c = 3.980(4) A, c/a = 0.453, V = 288 A3. Matrices for transforming one set of cell parameters into the other are presented.

Synthesis of iron aluminates and a new modification of aluminum oxide under shock waves from explosives (Review)

The results of an x-ray diffraction investigation of iron aluminate with unit cell parameter a = 8.090(4) A, cation-defective iron aluminate Fe0.5Al2.23O4 with a = 8.002 A, and a new modification of aluminum oxide synthesized under shock waves from explosives containing aluminum are presented. Aluminum oxide can crystallize in the hexagonal system in a primitive lattice with a = 9.151(1) A, c = 7.945(2) A, V = 576 A3 or in a tetragonal system in a primitive lattice with one-half the volume — a = 7.941(2) A, c = 4.575(1) A, V = 288 A3.

Optimum Polymer - Solid Interface Design for Adhesion Strength: Carboxylation of Polybutadiene and Mixed Silanes Surface Modification of Aluminum Oxide

To understand the optimum design of polymer-solid interfaces for adhesion strength, model polymer-solid interfaces of carboxylated polybutadiene(cPBD) adhered to mixed silane modified Al2O3 surfaces were examined. The cPBD, having various ‒COOH sticker group concentration φ(X) (0 ∼ 10 mol%), was synthesized through high-pressure carboxylation of PBD, while Al2O3 surfaces were modified to have various -NH2 density, φ(Y) (0 ∼ 100 mol%), using self-assembly of mixed amine- and methyl-terminated silanes. The coadsorption kinetic model of the two silanes was analyzed through X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and dynamic contact angle (DCA), which gave the capability of controlling the receptor concentration of aluminum oxide surfaces. The polymer surface chain responses after exposure to various media were understood by measuring contact angle changes of various probe liquids. T-peel tests of the model polymer–solid interfaces, as a function of time and sticker and receptor group concentrations showed much longer time dependence than the characteristic time of a bulk polymer chain. Additionally, the classical equation of interface failure was re-examined to see the effects of deformation rate, annealing temperature, and annealing time. A simple scaling analysis of free energy of an adsorbed polymer on a solid surface was extended to predict the adhesion potential of the model polymer–solid interfaces. From the experiments and theory of adhesive vs. cohesive failure, it was found that there existed an optimum product value r* = φ(X)φ(Y)χ of sticker concentration φ(X), receptor concentration φ(Y), and their interaction strength χ, which was approximately 150 cal/mol for this polymer–solid interface. Below or above this optimum product value r*, the fracture energy of polymer-solid interfaces, G IC, was less than its optimal value, G lc*.

Synthesis of a new modification of aluminum oxide, gahnite, and zincite in detonation

Results of studying the products of explosion of aluminum and octogen charges enclosed by a brass shell under conditions of complete burning out in air are described. Gahnite ZnAl2O4, a new (metastable) modification of aluminum oxide, and small quantities of zinc oxide were identified in the condensed explosion products. The x-ray pattern of the aluminum oxide was indexed in a face-centered cubic lattice with the parameters a = 7.854(1) A, V = 484.4(2) A3. The possible Fedorov groups are Fm3m, F432, and F43m.

Synthesis of a new modification of aluminum oxide with the spinel structure under shock-wave action on gibbsite

A new modification of aluminum oxideγ 1-Al2O3 with the spinel structure was produced under shock-wave impact on gibbsite. The parameter of the face-centered latticea=7.935(1) A. The x-ray pattern and the indexing results are presented. The methods of synthesis are described.

Detonation synthesis of a new modification of aluminum oxide from gibbsite by explosion

The x-ray diffraction analysis of the condensed products resulting from explosion of gibbsite — hexogen mixtures was carried out. It was established that in explosion of the mixture, along withα-Al2O3, another, previously unknown modification of aluminum oxide is formed in nearly the same amount. The x-ray diffractometric data of the new modification are presented.

Raman spectra of nickel and molybdenum compounds on various modifications of aluminum oxide

Raman spectra of nickel and molybdenum compounds on various modifications of aluminum oxide