Alexey Kossenko

Effect of time on the formation of hydroxyapatite in PEO process with hydrothermal treatment of the Ti-6Al-4V alloy

A titania layer containing calcium and phosphate with rough and porous structure was prepared by plasma electrolytic oxidation (PEO) and hydrothermal treatment (HT) at different time treatment. The most corresponding to the stoichiometry of hydroxiapatite ratio of Ca: P in the oxide layer can be achieved by the optimization of the electrolyte composition and the main parameters of PEO. While at the stage of PEO hydroxiaptite precursors are formed with only residual quantity of the hydroxyapatite, the subsequent hydrothermal treatment results in the formation of a much more pronounced hydroxyapatite phase.

Coating Formation on Ti-6Al-4V Alloy by Micro Arc Oxidation in Molten Salt

Micro Arc Oxidation (MAO) is an electrochemical surface treatment process to produce oxide protective coatings on some metals. MAO is usually conducted in an aqueous electrolyte, which requires an intensive bath cooling and leads to the formation of a coating containing impurities that originate in the electrolyte. In the current work, we applied an alternative ceramic coating to the Ti-6Al-4V alloy using the MAO process in molten nitrate salt at a temperature of 280 °C. The obtained coating morphology, chemical and phase composition, and corrosion resistance were investigated and described. The obtained results showed that a coating of 2.5 µm was formed after 10 min of treatment, containing titanium oxide and titanium‒aluminum intermetallic phases. Morphological examination indicated that the coating is free of cracks and contains round, homogeneously distributed pores. Corrosion resistance testing indicated that the protective oxide coating on Ti alloy is 20 times more resistive than the untreated alloy.

Electric and Hydraulic Properties of Carbon Felt Immersed in Different Dielectric Liquids

Electroconductive carbon felt (CF) material, having a permeable structure and significant electroconductive surface, is widely used for electrodes in numerous electrochemical applications such as redox flow batteries, fuel cells, electrochemical desalination apparatus, etc. The internal structure of CF is composed of different lengths of carbon filaments bonded together. This structure creates a large number of stochastically oriented and stochastically linked channels that have different lengths and cross sections. Therefore, the CF hydraulic permeability is similar to that of porous media and is determined by the internal empty volume and arrangement of carbon fibers. Its electroconductivity is ensured by the conductivity of the carbon filaments and by the electrical interconnections between fibers. Both of these properties (permeability and electrical conductivity) are extremely important for the efficient functioning of electrochemical devices. However, their influences counter each other during CF compressing. Increasing the stress on a felt element provides supplementary electrical contacts of carbon filaments, which lead to improved electrical conductivity. Thus, the active surface of the felt electrode is increased, which also boosts redox chemical reactions. On the other hand, compressed felt possesses reduced hydrodynamic permeability as a result of a diminished free volume of porous media and intrinsic channels. This causes increasing hydrodynamic expenditures of electrolyte pumping through electrodes and lessened cell (battery) efficiency. The designer of specific electrochemical systems has to take into account both of these properties when selecting the optimal construction for a cell. This article presents the results of measurements and novel approximating expressions of electrical and hydraulic characteristics of a CF during its compression. Since electrical conductivity plays a determining role in providing electrochemical reactions, it was measured in dry conditions and when the CF was immersed in several non-conductive liquids. The choice of such liquids prevented side effects of electrolyte ionic conductivity impact on electrical resistivity of the CF. This gave an opportunity to determine the influences of dielectric parameters of electrolytes to increase or decrease the density of interconnectivity of carbon fibers either between themselves or between them and electrodes. The experiments showed the influence of liquid permittivity on the conductivity of CF, probably by changing the density of fiber interconnections inside the felt.

Special Features of Oxide Layer Formation on Magnesium Alloys during Plasma Electrolytic Oxidation

The process of the oxidation of magnesium alloys in a silicate electrolyte during plasma electrolytic oxidation is investigated. An anomalous form of the chronogram of the formation voltage of the oxide layer in the electrolytes with the highest silicate concentration (approximately 0.15 M Na2SiO3 · 5H2O) is detected. X-ray diffraction analysis, scanning electron microscopy with energy dispersive X-ray spectroscopy analysis, and thickness gauges are used to characterize the surface microstructure, phase composition, and thickness, respectively. Mechanisms for the initial period of PEO and the “insular” growth were described. During the “insular” growth, islands consisting of vitrified components of the electrolyte are growing on the original smooth surface.

A mathematical model of powder components oxidation during thermal spray process

A model developed on the basis of kinetic principles of mass transfer allows performing computer simulation of the oxidation of powder materials during the thermal spray process. Such simulation enables one to determine the oxidation degree of the powder. The calculation is based on determining quasi-stationary oxygen diffusion flow on a flying particle. Calculations performed for various spray powders and various flammable gases demonstrate a significant decrease of the oxidation degree with the growth of the particle diameter and density of a spray powder. The calculation results were confirmed by experiments.