Microarc Discharge Research Articles

Effect of Na2SiO3 concentration on microstructure and corrosion resistance of MAO coatings prepared on Al-Mg-Sc alloys

PurposeThe purpose of this paper is to investigate the effect of Na2SiO3 concentration on the microstructure and corrosive properties of microarc oxidation (MAO) coating on Al-Mg-Sc alloy and explore microstructure evolution rule of Al substrate in the contact area.Design/methodology/approachThe Na2SiO3 concentration in electrolytes influenced the microstructure and corrosion behavior of MAO coatings. Instantaneous high temperature and high pressure due to microarc discharge caused annealing treatment. The corrosive behavior of the MAO coating was featured with polarization curves and electrochemical impedance spectrum in 3.5 Wt.% NaCl solution.FindingsThe substrate in the contact area existed the instantaneous annealing treatment, which caused obvious recrystallization. The coating prepared in electrolyte containing 7 g/L Na2SiO3 exhibited the highest protective properties in 3.5 Wt.% NaCl solution.Originality/valueMAO treatment could increase the corrosion resistance by producing a protective layer on the Al-Mg-Sc alloy surface at a suitable Na2SiO3 concentration and microstructure evolution rule of Al substrate in the contact area was obtained.

Production of Hydrogen From Heavy Hydrocarbons

The mechanism of the effect of an arc discharge on heavy hydrocarbon raw materials is studied in the work. The mechanism of formation of charged particles in micro-discharges with recessed electrodes is shock ionization. As a result of the interaction of hydrocarbon raw materials with an arc discharge, long hydrocarbon molecules are broken down, light hydrocarbons such as ethylene, hydrogen, and methane are formed. In the case of the organization of a micro-arc discharge, a gentle thermal regime also provides a gentle splitting of long hydrocarbon molecules, and in this case gasoline fractions of oil are formed in greater quantities compared to the case of exposure to raw materials by a conventional arc discharge.

Разноразмерная пористость и теплопроводность оксидных слоев, сформированных плазменно-электролитическим оксидированием на силумине АК12Д

Oxide layers formed by plasma-electrolytic oxidation (PEO) are characterized by a sufficiently high porosity, which influences almost the whole complex of service characteristics. However, the known data on the integral porosity of PEO-produced layers are rather contradictory, and the nature of the pore size distribution in these layers remains understudied. As a result of processing the images of the layer cross-section produced in a wide magnification range (scanning electron microscopy – SEM, image-based analysis), the authors obtained the pore size distribution in the range of 10 nm to 10 µm, which is sufficiently well described by a lognormal distribution function (pore geometry was approximated by a spherical shape). Such distribution pattern indicates the nature of pore formation, which can be related to the thermally activated process of gas emission from a liquid melt, the volume and average temperature of which, in their turn, depend on the micro-arc discharge energy. The paper presents the results of identifying the oxide layer phase composition and crystallites sizes by the X-ray crystallography method. Comparing the results of X-ray spectral microanalysis and X-ray crystallography, the amorphous component phase composition was evaluated. Using the stationary method and the method of pulsed laser heating, the authors determined the thermal conductivity of the initial oxide layer and the layer after the removal of its highly-porous outer part. The porosity values obtained experimentally and calculated based on the analysis of SEM-images and the results of determining the phase composition, including amorphous phases, allowed evaluating the oxide layer thermal conductivity with the help of four known analytical models. The results of calculating the thermal conductivity using the Loeb model showed good convergence with the experimental results obtained in this work. The modeling demonstrated that the size of crystallites influence the oxide layer thermal conductivity much less than the porosity and amorphous phase.

Nanostructured PEO-coatings on silumin as environmental catalysts

The process of the formation of mixed oxide coatings on a high-silicon aluminum alloy in the cobalt-containing pyrophosphate electrolyte by the plasma-electrolytic oxidizing (PEO) method has been studied. It has been shown that AK12M2MgN chemical composition heterogeneity causes the consumption of part of the anode current to homogenize the treated surface, which is reflected in minimizing the content of doping components at the initial processing stage. It has been established that the growth of mixed oxides Al2O3·CoOx relative mass is a function of time with the maximum at 55 min. The chemical, phase composition and surface morphology of the formed oxide layer depend on the oxidizing time. The catalytic component content in the surface oxides varies from 0.2 to 23.3 at.% with an increase in processing time from 10 to 60 min. Maximum cobalt incorporation into the oxide layer occurs at PEO of 35–50 min, while the silicon content in the surface layers does not exceed 2 at.%, which is favorable for the catalytic properties. The cobalt oxide, preliminary Co3O4, incorporation in alumina α-Al2O3 matrix is visualized by the blue-violet color spheroid surface structures at the sites of micro-arc discharges. The mixed alumina and cobalt oxides layer are characterized by a developed micro globular surface consisting of conglomerate spheroids with an average size of 1–2 µm. There are some amorphous phases in the structure of mixed oxides due to non-equilibrium PEO conditions. The set of detected factors is a prerequisite for the high catalytic properties of oxide coatings. A promising field of Al2O3·CoOx systems application is an intra-cylinder catalysis in the internal combustion engines.

Improvement of oxide layers formed by plasma electrolytic oxidation on cast Al Si alloy by incorporating TiC nanoparticles

The effect of titanium carbide nanoparticles (TiC NPs) on the structure, chemical and phase composition of the oxide layers obtained by plasma electrolytic oxidation (PEO) on the aluminum-silicon alloy (7.5 wt% Si) was investigated. A significant increase in hardness and effective elastic modulus (~1.4 times), wear resistance (~3 times) and corrosion resistance (~10 times) along with a substantial increase of the layer thickness (about 30%) were found to be connected with chemically inert incorporation of TiC NPs into the oxide layer, while the volume fraction of the incorporated particles in the oxide layer was only about 1%. Improvement of the layer properties was concerned with a decrease in the number of pores and cracks, an increase in the layer crystallinity, and a shift in the phase composition towards more stable phases such as mullite. The assumed mechanism of the TiC NPs effect is a decrease in the breakdown voltage of vapor-gas bubbles (VGB) due to sparking (local breakdowns) from cathodic electrolyte onto the TiC nanoparticles incorporated into the microchannel walls. Sparking to these particles can promote ionization of the gaseous medium, providing earlier microarc discharge through the entire VGB. It results in a substantial increase in the number of microarc discharges, which then initiate larger volumes of melted splash metal and subsequently larger volumes of oxide layer. The higher average temperature results in a decrease in the defectiveness of the macrostructure, the removal of gas from the layer, the growth of crystallites, the removal of residual strains, and a stabilization in the phase composition.

Plasma-chemical synthesis of germanium nanotubes

The paper describes the synthesis of germanium nanotubes in an argon microarc. The article presents the results of two experiments. The evaporation of germanium atoms was carried out from the surface of single-crystal germanium, which served as the anode of the microarc discharge. The synthesis of nanotubes was observed on the surface of a molybdenum cathode in the first experiment and on the surface of a graphite rod in the second experiment. The diameters of the obtained germanium nanotubes reach 800-900nm.

Wear and corrosion resistant coatings prepared on LY12 aluminum alloy by plasma electrolytic oxidation

Although aluminum alloys are widely used in the aerospace industry, their relatively poor wear and corrosion resistance have hampered some applications and reduce the lifetime of the components. In this work, by using a zinc-containing phosphate electrolyte, the corrosion resistance, surface hardness, and wear resistance of the coatings produced by plasma electrolytic oxidation (PEO) are simultaneously improved. The micro-arc discharge is optimized by changing the phosphate concentration instead of introducing hard phase cations to improve the crystallinity of the zinc-doped alumina coating. Our results reveal that by using a concentration of (NaPO3)6 of 50 g/L, the hard phase of crystralline alumina and corrosive-resistant phase of Zn3(PO4)2 are produced in concert. In the salt spraying test, the lifetime of the PEO coating is over 5000 h and the electrochemical corrosion current density is reduced by about 2 orders of magnitude confirming the improved corrosion resistance and durability. Moreover, the surface hardness increases from 400 HV for the coating prepared using the conventional phosphate electrolyte to 711.8 HV and the wear rate also drops from 8.5 × 10−4 mm3·(N·m)−1 to 2.0 × 10−5 mm3·(N·m)−1. This strategy can guarantee the aluminum alloy PEO coating with simultaneous excellent wear and corrosion resistance to cope with the complex service environment.

Impact of microarc carburizing and boriding of steel on diffusion layer structure

Intensification of carburizing and boriding of steel parts is achieved by microarc surface alloying. For carburizing, the parts are immersed in coal powder followed by electric current passing. For boriding, a coating with diffusant is used. Acceleration of diffusion is achieved by action of microarc discharges on the steel surface. The aim of this work was to study the effect of diffusion parameters on thickness, structure, and phase composition of coatings. The samples were made of 20 steel. Surface current density was 0.45 – 0.53 A/cm2. Duration of the process was 2 – 8 min. At the beginning of heating, temperature of the samples increases, and then stabilizes at 930 – 1250 °C due to cessation of micro-formation during combustion of coal particles. After carburizing, a eutectoid mixture is formed on the surface, then, the zone with ferrite-perlite structure is located, which transfers into the original structure. The maximum layer thickness (60 – 390 microns) is reached after 6 – 7 min of heating and then does not increase due to a decrease in the carbon potential during combustion of coal particles. Similar relationship is obtained when boriding: the maximum layer thickness (60 – 340 microns) is reached after 6 – 7 min and then does not increase due to depletion of diffusant source in the coating. At current density of 0.45 A/cm2, the layer consists of a base (a dispersed ferrite-carbide mixture) containing fine inclusions of iron borides and boron carbide. At current densities of 0.49 and 0.53 A/cm2, the layer has heterogeneous structure, with areas of high-hard boride eutectic located at the base. At high current density, diffusion of carbon and boron along the grain boundaries forms Fe – C – B triple eutectic. At lower current density, surface temperature is lower than eutectic formation temperature, so heterogeneous coating structure is not formed. The work results make it possible to choose modes of microarc heating to obtain the required parameters of diffusion layer.

Effect of External Factors on Diffusion Acceleration Mechanism During Steel Microarc Surface Impregnation

The considerable duration of steel chemical heat treatment (CHT) processes makes it necessary to intensify them. For this purpose, it is advisable to use microarc surface impregnation that provides acceleration of CHT in all stages. During microarc alloying steel products are immersed in a metal container filled with coal powder, followed by passage of an electric current in the circuit. Microarc discharges occur within the powder and are concentrated around the product with formation of a microarc halo and rapid heating to the diffusion saturation temperature. When the coal powder is heated a saturated atmosphere based on carbon monoxide is formed within the container that leads to steel product carburization. Preliminary application of a coating containing a diffusant to the surface provides preparation of multi-component coatings. This technology provides acceleration of CHT in all stages. The surface being treated is affected by microarc discharges that lead to accelerated formation of active diffusant atoms and ions and their movement towards a treated surface. On transfer into the metal there is an effect of electrotransport whose operating factors are a force for action of an electric field on the diffusant ions, and the strength of the interaction of atoms and diffusant ions with directional flow of conduction electrons that drives them in the direction of their own movement (“electron wind”) as well as heat transfer, whose impact is described by a vacancy mechanism (Wirts model), the mechanism of entrainment of ions by electrons under the action of a temperature gradient, as well as the mechanism of diffusing atom attraction by phonons of the crystal lattice (“phonon wind”). Experimental studies are conducted for the intensity of diffusion flow depending on the direction of the electric current for various alloying elements, as well as a calculated estimate of the values of these forces. It is found that the main factor determining intensification of diffusion saturation during microarc alloying is the “electron wind” strength. Calculated values of alloying element diffusion coefficients for various versions of microarc heating correspond to the scheme proposed for acting forces.

Selection the measuring system for registering the metal parts temperature at microarc heating and its thermophysical analysis

Measuring the temperature of metal parts during thermal and thermochemical treatment is necessary for the control of the correctness of the technological process. Currently, new methods of thermochemical processing of parts under high-energy conditions, such as microarc surface alloying, are proposed. Heating is carried out by the action of microarc discharges on the surface of parts immersed in coal powder when an electric current flows. It reduces the processing time from a few hours to a few minutes, but makes it difficult to measure the temperature, because heating occurs in dynamic mode. The aim of the work was to select a measuring system for recording the metal parts’ temperature during microarc heating, and the thermophysical analysis of the heating process. The S-type sensor was selected. The measuring system also includes a digital multimeter and a personal computer. The density of the electric current on the surface of the samples was 0.53 A/cm2 The registered heating curve consists of several stages. At the first stage the heating speed was about 8 °C/s. At the second stage it increases to 16 °C/s, but then decreases to 3.5 °C/s. Then the temperature increases monotonously with an average speed of 9 °C/s. At the fourth stage the sample temperature is stabilized at 1250 °C. The thermophysical process’ model of the heating metal parts under the influence of microarc discharges was proposed. The analysis showed that the calculated data coincide with the experimental data.

Study into initial stages of plasma electrolytic oxidation of magnesium alloy

In the present work, a coating growth during plasma electrolytic oxidation on magnesium alloy was investigated. We observed the initial stages: electrochemical anodizing accompanied by anodic dissolution; oxidation with low power spark discharges; oxidation with microarc discharges. The study discusses the evolution of morphology, porosity, elemental composition in comparison with the PEO electrical characteristics and optical emission of microdischarges. The kinetic model, which is consistent with the observed mechanism of coating growth, is constructed.

An experimental analysis of the influence of electrolyte compositions, current density and duration of the micro­arc oxidation process on the structural­phase state and properties of VT3­1 titanium alloy

It was determined that in an electrolyte containing 1.75 g/L KOH+1 g/L Na2SiO3+2 g/L NaAlO2, with an increase in current density from 15 A/dm2 to 50 A/dm2, the phase composition of the coating changes. In the three-phase state (aluminum titanate, rutile, and amorphous-like phase), with increasing j, instead of an amorphous-like phase, a crystalline mullite phase appears. The hardness of the coating increases from 5400 MPa to 12500 MPa. It was found that, in combination with aluminum titanate, mullite is the basis for achieving high hardness in the coating. The formation of a ceramic micro-arc oxide coating on the surface of the VT3-1 titanium alloy makes it possible to reduce the dry friction coefficient by more than 5 times to f=0.09.The effect of electrolysis conditions during micro-arc oxidation of the VT3-1 alloy (titanium-based) on the growth kinetics, surface morphology, phase-structural state, and physical and mechanical characteristics (hardness, coefficient of friction) of oxide coatings was studied. It was found that the process in the mode of micro-arc discharges is stably implemented on the VT3-1 alloy in an alkaline (KOH) electrolyte with additions of sodium aluminate (NaAlO2) and liquid glass (Na2SiO3). This makes it possible to obtain coatings up to 250 μm thick. In this case, a linear dependence of the coating thickness on the time of the MAO process is observed. The growth rate of the coating increases with increasing current density. The highest growth rate was 1.13 μm/min. It was revealed that in an electrolyte containing 1 g/L KOH+14 g/L NaAlO2 with an increase in the duration of oxidation from 60 to 180 minutes, the relative content of the high-temperature phase, rutile, increases. In the coatings obtained in the electrolyte 1.75 g/L KOH+1 g/L Na2SiO3+2 g/L NaAlO2, with an increase in the duration of the MAO process, the relative content of the amorphous-like phase decreases and the content of the crystalline phase of mullite (3Al2O3·2SiO2) increases

Effect of tungsten doping on the performance of MAO coatings on a Ti6Al4V drill pipe

A tungsten (W)-doped microarc oxidation (MAO) coating on a TC4 titanium (Ti) alloy drill pipe was prepared by adding sodium tungstate (Na2WO4) into electrolytes. The effects of adding different concentrations on the electrical conductivity of the electrolyte and the performance and structure of MAO coatings on the titanium alloy drill pipe were studied. The electrical conductivity of the solution was tested using Ohm’s law. The surface morphologies of the coatings were observed by scanning electron microscopy. The element contents of the MAO coatings were analyzed by energy-dispersive spectroscopy. The hardness of the coatings was measured by using a microhardness tester. The phase composition was analyzed by X-ray diffraction, and the corrosion resistance was evaluated by polarization curves. The results show that orthotungstate (WO4 2−) begins to form tungsten trioxide (WO3) when the temperature of the microarc discharge area is above 1314 K. With increasing sodium tungstate doping concentration, the conductivity of the solution increases first and then decreases, and the hardness and corrosion resistance of the coatings also show an identical trend. Tungsten doping slightly affects the phase composition of the MAO coating, and it is helpful in improving the hardness and corrosion resistance of the coatings. The optimum doping concentration is 3 g/l in this work.

Plasma-Chemical Decomposition of Hydrocarbons on the Basis of the Micro-Arc Discharge with Disc Electrodes Rotating in the Bulk of Raw Materials

An innovative plasma-chemical hydrocarbon processing reactor has been developed. It is based on the interaction of nonequilibrium gas-discharge plasma with hydrocarbons in the bulk of a raw material. The results of experiments have shown that nonequilibrium gas-discharge plasma of micro-arc discharge, initiated in the bulk of hydrocarbon raw materials, is a unique tool for their processing. It is shown that this method allows not only decomposing hydrocarbons into light fractions, but also simultaneous obtaining fullerenes and nanotubes.

Synthesis, corrosion, and wear resistance of a black microarc oxidation coating on pure titanium

Black microarc oxidation (MAO) ceramic coatings were prepared on TA2 pure titanium by adding the colorant [Cu(NH3)4]2+ to a Na2SiO3 base solution. The microstructures of the MAO coatings were characterized by scanning electron microscopy, energy dispersive X-ray spectrometry, and X-ray photoelectron spectroscopy. The corrosion resistance and wear resistance of the coatings were evaluated through potentiodynamic polarization and friction tests. [Cu(NH3)4]2+ was absorbed into the pores of the microarc discharge to form black copper oxide under high temperature and pressure. Cu content in the MAO coating increased with [Cu(NH3)4]2+ concentration. The wear resistance and hardness of the prepared black ceramic coatings were remarkably improved in terms of self-sealing microstructure. However, the corrosion resistance of the black coatings was approximately one or two orders of magnitude lower than that of the MAO coating without Cu doping. This study shows that the proposed method is a promising method for preparing decorative coating or absorbing light coating on a titanium surface.

PECULIARITIES OF COBALT CONTAINING OXIDE COATINGS FORMATION ON SILUMIN

The process of mixed oxide coatings formation on a high-silicon aluminum alloy in a cobalt-containing pyrophosphate electrolyte by the plasma-electrolytic oxidation (PEO) method is studied. It was shown that AL25 chemical composition heterogeneity causes the consumption of a part of the anode current to homogenize the treated surface, which is reflected in minimizing the content of doping components at the initial processing stage. It was established that the growth of mixed oxides Al2O3·CoxOy relative mass is a function of time with a maximum at 55 minutes. The chemical, phase composition and surface morphology of the formed oxide layer depend on the oxidation time. The catalytic component content in the surface oxides varies from 0.2 to 23.3 at.% with an increase in processing time of 10 to 60 minutes. Maximum cobalt incorporation into the oxide layer occurs at PEO of 35–50 minutes, while the silicon content in the surface layers does not exceed 2 at.%, which is favorable for the catalytic properties. The cobalt oxide, preferably Co3O4, incorporation in the alumina α-Al2O3 matrix is visualized by the blue-violet color steroidal surface structures in the sites of micro-arc discharges. The alumina cobalt oxide mixture layers are characterized by a developed micro-globular surface which consists of spheroid conglomerate with an average size of 1–2 microns. There are some amorphous phases in the structure of mixed oxides due to non-equilibrium PEO conditions. The set of detected factors is a prerequisite for the high catalytic properties of oxide coatings. A promising field of Al2O3·CoxOy systems application is intra-cylinder catalysis in internal combustion engines.

The Effect of Additives of Ni, Co, or Fe Sulfates in the Alkali-Silicate Electrolyte for Microarc Oxidation on the Characteristics of Coatings Formed on Titanium

The effect of nickel, cobalt, and iron sulfates’ additives in the alkali-silicate electrolyte for microarc oxidation on the element composition and electrochemical behavior of MAO-coatings formed on titanium has been studied. It has been established that these additives were incorporated into an oxide layer. The increase of the content of these components in the electrolyte resulted in the decrease of their content (along with silicon) in the coating and the increase of the titanium content. This dependence can be related to the MAO process mechanism specifics and, first of all, to changes in temperature–time characteristics of microarc discharges.

Microstructure of BaCO3 and BaTiO3 coatings produced on titanium by plasma electrolytic oxidation

BaCO3 and BaTiO3 layers were coated on titanium by plasma electrolytic oxidation at 5 mA/cm2 in 0.5 M Ba(CH3COO)2 and 2 M NaOH electrolyte. Coatings were investigated by scanning electron microscopy, X-ray and electron diffraction, and high-resolution transmission electron microscopy (HRTEM). Processing for 1 h required low voltages producing a BaCO3 coating along with a thin TiO2 interlayer (~60 nm). The coating is composed of orthorhombic BaCO3 nanorods that were vertically oriented at the bottom layer close to Ti substrate and horizontally oriented in the top layer close to the surface. Processing for 4 h required higher voltages resulting in generation of microarc discharges and discharge channel formation. These conditions produced a tetragonal BaTiO3 coating along with a broader TiO2 interlayer at the Ti interface composed of rutile and brookite nanostructures. It is suggested that the discharge channels promote the oxidation of the substrate along with the migration of titanium ions to the coating facilitating the BaCO3 to BaTiO3 transformation. The HRTEM evidence suggests that the latter transformation occurs via an epitaxial relationship existing between the BaTiO3 and BaCO3 crystal structures. Based on the present evidence, a structural model is proposed to describe the BaTiO3 growth from the BaCO3 lattice.

Compact −300 kV dc inverted insulator photogun with biased anode and alkali-antimonide photocathode

This contribution describes the latest milestones of a multiyear program to build and operate a compact $\ensuremath{-}300\text{ }\text{ }\mathrm{kV}$ dc high voltage photogun with inverted insulator geometry and alkali-antimonide photocathodes. Photocathode thermal emittance measurements and quantum efficiency charge lifetime measurements at average current up to 4.5 mA are presented, as well as an innovative implementation of ion generation and tracking simulations to explain the benefits of a biased anode to repel beam line ions from the anode-cathode gap, to dramatically improve the operating lifetime of the photogun and eliminate the occurrence of micro-arc discharges.

Effect of NaF additive on the micro/nano-structure and properties of the microarc oxidation coating on Ti6Al4V alloy

Micro/nano-structured coatings with antibacterial function were prepared by microarc oxidation (MAO) treatment on Ti6Al4V alloy in a silicate/phosphate electrolyte with a NaF additive. The microstructure, phase composition, and corrosion resistance of the coatings modified by adding NaF (0.15–0.5 M) were examined using scanning/transmission electron microscopy, energy dispersive spectroscopy, atomic force microscopy, X-ray diffraction, and potentiodynamic polarization. The results showed that the incorporation of F ion reduced the threshold voltage for electrons avalanche on the surface film of the Ti alloy, and increased the intensity and lifetime of discharge. MAO coatings with 100–500 nm nano-pores and 1–20 [Formula: see text]m micro-pores were formed by the modification of the NaF additive. The F ions promoted microarc discharge as well as phase transformation from the metastable anatase to the stable rutile phase. The F ions also promoted the generation of penetration cracks and bubbles in the coating. The surface roughness, phase content, and thickness of the coating were enhanced by the NaF additive. However, the corrosion resistance of the coating first increased and then decreased with the increasing F ion concentration, reaching a maximum when the NaF content was 0.25 M.