Properties Of Alumina Coatings Research Articles

Effect of current density and polytetrafluoroethylene on the properties of micro‐arc oxide coating of pure aluminum

AbstractMicro‐arc oxidation (MAO) is a surface treatment technology that enhances the surface properties of valves by creating a ceramic oxide layer on the metal surface. The goal of this study is to investigate the influence of current density on the properties of aluminum coatings during preparation and to improve the tribological properties of MAO/PTFE self‐lubricating films on the coating surface. The characterization of the coating was performed using X‐ray photoelectron spectroscopy, X‐ray diffraction, Raman spectra, and energy dispersive spectroscopy. The roughness, hardness, and elastic modulus of the coatings were tested using atomic force microscopy and nanoindentation. Tribological experiments were conducted to evaluate the tribological properties of the coatings. The experimental results show that the friction coefficient (COF), roughness, hardness, and elastic modulus of the MAO coating increase with the increase of current density. Additionally, the friction coefficient of the MAO composite coating significantly decreases after the addition of polytetrafluoroethylene (PTFE), improving the service life and application range of the metal coating. These findings are expected to promote the development of valve metal in various application fields.

Properties of Alumina Coatings Deposited by Detonation Spraying

Coatings deposited by detonation spraying equipment CCDS2000 using alumina powder are studied. CCDS2000 is characterized by a computer control system, robot compatible spraying unit (gun barrel), a portable chiller, two powder feeders, and other peculiar properties. This installation allows to deposit coatings on complex shape surfaces and thin-walled parts under optimal conditions. Studies of the coating properties included measurements of coating microstructure, porosity, microhardness, adhesion, cohesion, abrasive and erosive wear, and dielectric properties (specific resistivity and dielectric strength). The detonation sprayed coatings have an adhesion of 60-70 MPa, cohesion of 100 MPa, microhardness of 1500 HV100, porosity of about 1% (measured on microsections of coatings using OLYMPUS Stream Image Analysis software). Impregnation of coatings with silicone oil showed that the real open porosity of coatings is up to 6%. Dielectric strength of the ceramic layer with the thickness of 200 μm exceeds 30 kV/mm. Specific resistivity depends on atmospheric humidity and when the relative humidity is less than 60%, the specific resistivity is greater than 1013 Ω·cm.

Properties of flame-powder sprayed aluminum coatings reinforced with particles of carbonaceous materials


 
 
 The article presents the results of research on tribological properties of aluminum coatings, reinforced with particles of carbon nanotubes Nanocyl NC 7000 in quantities of 0.5 wt.% and 1 wt.% as well as carburite (elemental carbon) in an amount of 0.5 wt.%, flame-powder sprayed on a non-alloy structural steel grade S235J0 according to EN 10027-1.The coating properties were assessed based on macro and microscopic metallographic examinations, chemical composition tests, microhardness measurements and abrasion and erosive wear resistance tests. The obtained results were compared with the results obtained for samples with coatings made of aluminum powder – EN AW 1000 series.
 
 

Microstructural evolution of alumina coatings by a novel long laminar plasma spraying method

A super long and stable laminar plasma jet was used in the atmospheric plasma spray process in this work. Microstructures evolution and properties of alumina coatings that were obtained using a long spraying distance ranging from 200 mm to 350 mm in an atmospheric environment were studied in this work. These processes were carried out by using a total gas flow rate of 14 slpm with 70% nitrogen and 30% argon in volume and output power of 25.4 kW (current of 160 A). A long particle residence time was obtained in this study, which compared with other current atmospheric plasma spray methods. The microstructures of coatings showed the multi-island protrusions on the top surfaces and overlapped lamellar splats at the fracture surfaces. The microstructural evolution was significantly affected by the heating and motion behaviors of alumina particles when these were flowing in the long laminar plasma jet at different spraying distances.

Comparative Study of Mechanical and Tribological Properties of Alumina Coatings Formed on 5754 Aluminium Alloy under Various Conditions

In this paper we present the results of the comparative study of the performance of two sulfuric and oxalic anodizing process already retained, based on the superposition of the surface responses. For this purpose two Doehlert experimental designs with three variables (temperature, current density, sulphuric acid concentration) and other three variables (temperature, current density, oxalic acid concentration) were realized. Three responses were studied, namely: growth rate, Vickers microhardness, wear rate after friction test of the anodic oxide layer. A comparative study based on the surface responses was carried out. Compared with the sulphuric acid bath, it was found that the oxalic acid bath affords low growth rates, high wear resistance and high microhardness but less ductile layers. The observed mechanical properties of the oxide layers can be related to their morphology revealed by the scanning electron microscopy and optical observations and their chemical composition determined by the glow-discharge optical emission spectroscopy.

Measuring mechanical properties of plasma-sprayed alumina coatings by nanoindentation technique

AISI 1020 steel substrate is coated with alumina as feedstock material using plasma spraying process in order to correlate the microstructural features with mechanical properties of coating. The present work focuses on the effects of microstructural inhomogeneity on mechanical properties of alumina coating through nanoindentation technique. Young’s modulus and hardness of the alumina coating are analytically evaluated. Indentation stress–strain curves are generated from the experimentally obtained load–displacement curves to characterise the mechanical properties of the coating. The results have shown large variation in hardness and Young’s modulus of alumina due to microstructural inhomogeneity of the coating.

Preparation and Properties of Alumina Coatings as Tritium Permeation Barrier by Plasma Electrolytic Oxidation

The alumina coatings as tritium permeation barrier were deposited on 316L stainless steel substrates by plasma electrolytic oxidation. The effects of plasma electrolytic oxidation on the coatings were investigated, and phase, surface morphology and thickness were characterized by XRD, SEM and eddy current method, respectively. After scratch adhesion test, thermal shock resistance test and tritium permeability test, the optimized coatings were obtained. The results show that the phase, the surface morphology and the thickness are affected by current density, voltage and PEO (plasma electrolytic oxidation) duration time. The current density and voltage can change the phase structure of coatings, higher current density propels the transformation of Al→Al2O3 and higher voltage propels the transformation of γ-Al2O3→α-Al2O3. With increasing voltage and duration time, the pore size becomes bigger and the quantity is less so that the surface morphology is worse. With current density increasing to appropriate value (9 A/dm2), surface morphology becomes good. After a series of tests, the coatings, which are obtained at 6 A/dm2, 300 V, 30 min, exhibit good performances with good film-substrate cohesion, thermal shock resistance, and the tritium permeation resistance of coated sample is improved by 3 orders of magnitude higher than that of the 316L stainless steel bulk.

Improvement of structural and mechanical properties of alumina coatings by incorporation of TiO2 and α-Al2O3 nanoadditives

Alumina coatings embedded with different nanoadditives were fabricated on aluminium alloy by microarc oxidation (MAO). Incorporation of nanograins into the prepared coatings was accomplished by dispersing nanoadditives into different electrolytes during the MAO process. Our results show that nanograins are successfully embedded in the ceramic coatings, and the embedded coatings are compact and have lower porosity. The mechanical properties of the nanograin embedded coatings such as hardness, adhesion and wear resistance are consequently improved, and the samples prepared in aluminate electrolyte with α-Al2O3 nanoadditive have better mechanical properties than those prepared in other electrolytes. Our results also show that the mechanical properties of MAO coatings are closely related to the surface structure. The introduction mechanism of nanograins into the ceramic coatings resulted from the reactions occurring in the microarc discharge channels such as diffusion and electrophoresis, which is believed to improve the structure of the prepared coatings.

Structure and properties of nanosized coatings deposited onto polymers

Results of studies aimed at developing a new approach to measuring stress-strain properties of nanosized solids (strength, yield stress, and the value of plastic deformation at uniaxial tension) are generalized. This approach is based on the analysis of the parameters of microrelief arising upon the deformation of polymer films with thin coatings. It is demonstrated for the first time that the stress-strain properties of aluminum coatings deposited onto Lavsan substrates depend on the level of stresses in the substrate, the value of its deformation, and the thickness of the coating. The evolution of these parameters is related to the strain hardening of metal and the effect of nanostructuring of crystalline materials in the range of small thicknesses. When precious metal (Au, Pt) nanosized films are deposited onto polymers by ion-plasma sputtering, in the course of metal deposition, polymer surface layers interact with cold plasma. Stress-strain properties of polymer surface layers modified by plasma are quantitatively estimated for the first time. The model is proposed that makes it possible to take into account the contribution of the properties of precious metal and plasma-modified polymer surface layer to the strength of the coating.

Strength Properties of Thin Alumina Coatings on 4043 Aluminum Alloy

Alumina (Al2O3) coatings in the thickness of 10μm and 15μm were prepared on the 4043 Al-Si alloy substrates by electroplating technology. Multiple mechanical properties of alumina coatings were evaluated by various techniques: the long distance microscope, Charge Coupled Device microphotographics, acoustic emission and strain foils. For the electroplating alumina coatings, the in-plane tensile strength is 68.1 MPa, the cohesive strength is 96.2 MPa and the adhesive strength is over 22.7 MPa. Three different directional microcracks, which are relative to the tensile directional and corresponding to two fail modes in the alumina (coatings)/4043 Al-alloy (substrate) system, were found.

A comparative study of V2O5/AC and V2O5/Al2O3 catalysts for the selective catalytic reduction of NO by NH3

Abstract The influence of two different washcoats deposited on ceramic honeycombs (alumina and carbon) on the catalytic and mechanical properties of catalysts are examined in this work. A series of carbon and alumina washcoated samples was prepared through a dip-coating method and subsequently cured and treated to reach a suitable chemistry surface. Carbon-coated monoliths present a high washcoat homogeneity, high integrity and a suitable chemistry surface that favour NO reduction at low temperatures. The carbon washcoat was bonded perfectly to the ceramic monoliths. The high adherence between both materials, checked by vibrational tests, provides higher resistance indexes against axial compression and prevents crack propagation. Both facts are a consequence of the high mechanical strength of the catalyst. On the other hand, alumina coatings present better temperature resistance, although their adherence to ceramic monoliths is not as good as in carbon-coated samples. The properties of alumina coatings are strongly dependent on the slurry viscosities, which influence the integrity of the washcoat, mechanical properties and even NO conversion. Both series of catalysts have shown a good NO conversion under on-board conditions, although the key parameters of each washcoat are completely different.

Thermal-sprayed dielectric coating e.m. properties evaluation via free-space measurements at microwave frequencies

At present, there is a lack of knowledge about the electromagnetic (e.m.) properties of alumina coatings in a wide microwave frequency range. In this paper, complex permittivity of thermal sprayed alumina coatings were evaluated via free-space measurements. XRD, microstructural and microhardness characterisations were performed too. The results showed that the real part of permittivity is consistent with that found for bulk alumina, being the difference due to the complexity of the coating, which is composed by multiple phases originated during the deposition process. The bond coat measurements showed that it is possible to recognise an anisotropic texture of the coating.

Structure and properties of aluminum coatings obtained by the cold gas-dynamic spraying method

Structure and properties of aluminum coatings deposited onto steel substrates by the cold gas-dynamic spraying (CGS) method were examined. Aluminum CGS coatings fundamentally differ from their thermal counterparts as they enable the formation of heavy-duty layers of metal particles on substrates at temperatures below 500 K. A dense, low-porosity coating is found to form, tightly bound to the base metal. The adhesion strength is shown to weakly depend on the thickness of the sprayed coating due to the compressive stress present in the surface layer. A qualitative model for the coating formation process is proposed.

Effect of conditions of formation on the physicomechanical properties of detonation-deposited Al2O3 coatings

This paper examines detonation-deposited alumina coating that are suitable for insulating current-conducting elements and which can be employed in the manufacture of gas turbine components. Detonation-deposition was performed using classified and heat treated G-O, ChDA, and industrial M20 powders. M-1 copper specimens were used for wear tests and VT3-1 titanium alloy, type 3 steel, brass, and aluminum specimens were used for transverse rupture tests. The results of an investigation into the effect of coating thickness on the strength properties of alumina coatings and titanium and steel bases and on the adhesion between these components are presented. The adhesion of titanium bases is five times stronger than that of aluminum bases. For satisfactory operation, the coating thickness should be 0.1-0.15 mm.

Mechanism of formation and properties of aluminum coatings on high-temperature strength nickel alloys

1. The aluminum content, the thickness, the structure, and the composition of aluminum coatings obtained by the circulation method are determined to a significant degree by the activity of the impregnation process. 2. As the result of regulation of the activity of the process, coatings with different properties may be obtained on the same material with the same temperatures and pressure. Consequently, the activity of the aluminizing process is an important technical parameter which must be selected based on the composition of the alloy and the requirements for its service properties. 3. Despite the somewhat lower heat resistance under isothermal conditions, for operation in nonisothermal creep under nonsteady conditions coatings with 24–28 wt.% Al in the NiAl β-phase formed according to the relatively inactive mechanism are preferable.

Mechanical properties of aluminum coatings on heat-resistant steels

Mechanical properties of aluminum coatings on heat-resistant steels

The precipitation and some properties of alumina coatings on rutile

AbstractAlumina was precipitated on to rutile in four different ways to provide a series of alumina‐coated rutiles. There were two main types of precipitate. Type A with alumina precipitated in an acid medium and type B with alumina precipitated at pH 8. The two types of coating differed in appearance, surface area and in the nature of alumina present. For the type A coatings, the precipitate was irregular with much interstitial material and consisted of amorphous alumina of surface area < 100 m2 g−1. The type B coatings were compact and almost invisible and had surface areas > 140 m2 g−1. Alumina gels were precipitated under similar conditions to the type A and type B coatings and compared with these coatings to see if the presence of rutile had modified the type of alumina that formed.

Effect of processing parameters on some properties of detonation-deposited alumina coatings

1. The properties of alumina coatings are determined by both the kinetic and thermal parameters of the particle being sprayed. 2. The composition of the detonation products appears to exert a pronounced influence on the coating formation process.