Oxidation Ceramic Coatings Research Articles

Investigation of the Characteristics of Ceramic Coatings Obtained by Microarc Oxidation on Direct and Alternating Currents in an Alkaline Silicate Electrolyte

This work presents the results of studies of structural and physicomechanical characteristics of oxide-ceramic coatings obtained on aluminum by microarc oxidation in an alkaline silicate electrolyte with various contents of silicate ions using alternating and direct currents.

Effects of the sources of calcium and phosphorus on the structural and functional properties of ceramic coatings on titanium dental implants produced by plasma electrolytic oxidation

Plasma Electrolytic Oxidation (PEO) is as a promising technique to modify metal surfaces by application of oxide ceramic coatings with appropriate physical, chemical and biological characteristics. Therefore, objective of this research was to find the simplest settings, yet able to produce relevant bioactive implant surfaces layers on Ti implants by means of PEO. We show that an electrolyte containing potassium dihydrogen phosphate as a source of P and either calcium hydroxide or calcium formate as a source of Ca in combination with a chelating agent, ethylenediamine tetraacetic acid (EDTA), is suitable for PEO to deliver coatings with desired properties. We determined surface morphology, roughness, wettability, chemical and phase composition of titanium after the PEO process. To investigate biocompatibility and bacterial properties of the PEO oxide coatings we used microbial and cell culture tests. The electrolyte based on Ca(OH)2 and EDTA promotes active crystallization of apatites after PEO processing of the Ti implants. The PEO layers can increase electrochemical corrosion resistance. The PEO can be potentially used for development of bioactive surfaces with increased support of eukaryotic cells while inhibiting attachment and growth of bacteria without use of antibacterial agents.

Role of incorporation of ZnO nanoparticles on corrosion behavior of ceramic coatings developed on AZ31 magnesium alloy by plasma electrolytic oxidation technique

Improvement of corrosion resistance is one of the main goals in developing plasma electrolytic oxidation (PEO) ceramic coatings on AZ31 magnesium alloys. In this study, ceramic coatings were deposited on AZ31 magnesium alloys by PEO technique in electrolytes containing 10 g/l sodium phosphate (Na3PO4) and 1.5 g/l sodium hydroxide (NaOH) at different concentrations of ZnO nanoparticles and current density of 7.1 A/dm2 for 10 min. To investigate the corrosion behavior of the samples, they were placed in Hank's solution, while Tafel polarization and electrochemical impedance spectroscopy (EIS) were used. X-ray diffraction (XRD) together with field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS) techniques were employed to investigate the coatings. XRD results showed that the prepared PEO layers mainly consist of ZnO, Mg, and Mg3(PO4)2 phases. Corrosion test results showed that the use of ZnO nanoparticles improved the corrosion resistance of PEO ceramic coatings in Hank's solution. In contrast, the best result was obtained by using the electrolyte containing 6 g/l ZnO nanoparticles with a corrosion current density of 0.08 µA/cm2 and corrosion potential of -1.4615 VAg/AgCl. Also, EIS results showed the corrosion resistance of the mentioned coating as 1.05 × 106 Ω.cm2.

The effect of electrolyte composition on the plasma electrolyte oxidation and phase composition of oxide ceramic coatings formed on 2024 aluminium alloy

Purpose: Purpose of this work is to analyse the process of synthesis of oxide ceramic coatings in plasma electrolytes on 2024 aluminium alloy and to form an electrolyte which allows to reduce energy consumption for the coating formation. Design/methodology/approach: The oxide ceramic coatings were synthesized on 2024 aluminium alloy. The coatings were formed by the alternate application of anode and cathode pulses to the sample. X-ray diffraction analysis of coatings was performed on a DRON-3.0 X-ray diffractometer using CuKα radiation. The thickness of the coatings was determined using a CHY TG-05 thickness gauge. The porosity of the coatings was investigated by analysing the micrographs of the plasma electrolyte oxidation (PEO) coatings obtained on a scanning electron microscope at ×500 magnification using the image processing technique. Findings: The electrolyte with 5 g/l H2O2 additive have been elaborated as an optimal composition for synthesis of a coating with an increased content of corundum (α-Al2O3) as compared to a coating synthesized in the same mode in the 3KOH+2Na2SiO3 electrolyte without H2O2. This synthesis mode allows obtaining a coating with a high corundum content at low energy consumption. Research limitations/implications: For further optimization of the synthesis modes, it is necessary to analyse the influence of the phase composition and porosity of the obtained oxide ceramic coatings on their microhardness, wear resistance, and corrosion resistance. Practical implications: Based on the developed modes of synthesis of the coatings, it will be possible to obtain wear and corrosion resistant oxide ceramic coatings with predetermined functional properties and to reduce energy consumption for their formation. Originality/value: Methods for accelerating the formation of coatings have been proposed and tested, in particular, by adding various amounts of hydrogen peroxide to the electrolyte. The content of oxides in the obtained coatings, in particular, their ratios at various concentrations of hydrogen peroxide in the electrolyte, were determined by X-ray phase analysis. The modes of synthesis of the coatings were developed which allow obtaining a continuous coating without cracks with simultaneous decreasing porosity from 4.32% to 3.55–3.53%.

Corrosion tests of oxideceramic coatings formed by microarc oxidation

The results of experimental studies of the corrosion resistance of aluminum and its alloys modified with ceramic coatings based on the microarc oxidation method in some aggressive environments are presented. The mechanism of destruction of the coating is considered. Recommendations on increasing corrosion resistance are given.

Corrosion Tests of Oxide-Ceramic Coatings Formed by Microarc Oxidation

The results of experimental studies of the corrosion resistance of aluminum and its alloys modified with ceramic coatings based on the microarc oxidation method in some aggressive environments are presented. The mechanism of destruction of the coating is considered. Recommendations on increasing corrosion resistance are given.

Introduction to Plasma Electrolytic Oxidation—An Overview of the Process and Applications

Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO), is an innovative method in producing oxide-ceramic coatings on metals, such as aluminum, titanium, magnesium, zirconium, etc. The process is characterized by discharges, which develop in a strong electric field, in a system consisting of the substrate, the oxide layer, a gas envelope, and the electrolyte. The electric breakdown in this system establishes a plasma state, in which, under anodic polarization, the substrate material is locally converted to a compound consisting of the substrate material itself (including alloying elements) and oxygen in addition to the electrolyte components. The review presents the process kinetics according to the existing models of the discharge phenomena, as well as the influence of the process parameters on the process, and thus, on the resulting coating properties, e.g., morphology and composition.

Microstructure and wear resistance of micro-arc oxidation ceramic coatings prepared on 2A50 aluminum alloys

With a bipolar pulse power, wear-resistant ceramic coatings with high micro-hardness were in-situ fabricated on 2A50 aluminum alloy using micro-arc oxidation method in silicate electrolyte. The effect of cathodic voltage on microstructure, phase structure, micro-hardness and wear properties was investigated by changing cathodic voltage from 0 to −200 V. The phase structure, microstructure, micro-hardness and wear resistance were determined by XRD, SEM, HVS-1000 Vicker's micro-hardness tester and tribology tester. The results show that the phase of the obtained coatings is mainly composed of γ-Al2O3. The coating consists of the inner dense layer and the outer loose layer. As the cathodic voltage increases, the quantity and size of the micro-pores in the coatings first decrease and then increase. Micro-arc oxidation can greatly strengthen 2A50 aluminum alloy, the micro-hardness increases from 75 HV0.5 to 1321 HV0.5 after a micro-arc oxidation under a cathodic voltage of −100 V. The friction coefficient of the ceramic coatings are in the range of 0.35–0.55, and the coating obtained under −100 V exhibits the best wear resistance. The wear mechanism is deduced to be the combination of abrasive and adhesive wear.

Aluminum Oxide Ceramic Coatings on 316l Austenitic Steel Obtained by Plasma Electrolysis Oxidation Using a Pulsed Unipolar Power Supply

AISI 316 steel has good corrosion behavior and high-temperature stability, but often prolonged exposure to temperatures close to 700 °C in aggressive environments (e.g., in boilers and furnaces, in nuclear installations) can cause problems that lead to accelerated corrosion degradation of steel components. A known solution is to prepare alumina ceramic coatings on the surface of stainless steel. The aim of this study is to obtain aluminum oxide ceramic coatings on 316L austenitic steel, by Plasma Electrolysis Oxidation (PEO), using a pulsed unipolar power supply. The structures obtained by PEO under various experimental conditions were characterized by XPS, SEM, XRD, and EDS analyses. The feasibility was proved of employing PEO in NaAlO2 aqueous solution using a pulsed unipolar power supply for ceramic–like aluminum oxide films preparation, with thicknesses in the range of 20–50 μm, and a high content of Al2O3 on the surface of austenitic stainless steels.

Characterization of ceramic oxide coatings prepared by plasma electrolytic oxidation using pulsed direct current with different duty ratio and frequency

In this study, plasma electrolytic oxidation (PEO) was performed on the commercial aluminum 5083-O alloy using pulsed direct current (DC) power source. The coating process was carried out in KOH 2 g/L + Na2SiO3 2 g/L electrolyte with the systematic change of frequency (100–2000 Hz) and duty ratio (20–80%). The coating was characterized by scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS), and potentiodynamic polarization experiment. The coating layers showed different growth behavior and chemical composition by frequency and duty ratio. The PEO coating improved the corrosion resistance of Al 5083-O from a minimum of three times to a maximum of one order. In conclusion, it was found that the frequency and duty ratio had a considerable influence on the coating properties of Al 5083-O in DC power mode.

Microstructure and Wear Properties of Micro Arc Oxidation Ceramic Coatings.

The interaction effect of micro arc oxidation (MAO) parameters on the microstructure and wear properties was investigated. The results showed that the electric current and oxidation time significantly influenced the thickness and grinding crack width of the ceramic coatings within the range of the selected parameters, and the interaction effect of the electrical parameters was not obvious. The surface morphology, cross-section morphology, and element distribution of the coatings were observed using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results showed that ceramic coatings with γ-Al2O3 and α-Al2O3 formed, which enhanced the coating performance. After that, the microhardness and wear resistance were tested. Under the optimal process, the microhardness of a coating section was up to 1200 HV0.1, and the friction coefficient was just 0.3. When wear occurred, the volcanic microstructures experienced extrusion and deformation, and then peeled off under shear stress, which led to the formation of a grinding crack. The main failure modes of the micro arc oxidation coatings were abrasive wear and spalling failure.

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.

High-temperature oxidation behaviors of plasma electrolytic oxidation coating on hot-dip aluminized HP40Nb alloy

In the current investigation, HP40Nb heat-resistant alloy was successfully aluminized. The procedure of aluminizing consisted of pack aluminizing and subsequent hot-dip aluminizing (HDA) process. Plasma electrolytic oxidation (PEO) ceramic coatings were also synthesized on the surface of hot-dip aluminized HP40Nb alloys. PEO coatings were made using aluminate (mixture of 2 g/L KOH, 5 g/L NaAlO2, and deionized water) and silicate (mixture of 2 g/L KOH, 5 g/L Na2SiO3, and deionized water) electrolytes at 440 V for 10 min. To evaluate the high-temperature oxidation behavior of PEO coatings, interrupted isothermal oxidation at 1100 °C for 100 h was employed. The surface and cross-sectional morphologies before and after high-temperature oxidation test, chemical composition, and phase identification of the coatings were characterized by field emission scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The results revealed that the PEO coating using aluminate electrolyte could increase the oxidation resistance of HP40Nb alloy, more than pack aluminized, hot-dip aluminized samples, and also the PEO coating using silicate electrolyte. The PEO coating using silicate electrolyte contains relatively big surface cracks, which can be responsible for oxide spallation during high-temperature oxidation. The weight gain of samples shows that oxidation resistance of hot-dip aluminized specimen, improved 1.2 times by PEO treatment in aluminate electrolyte. The Kirkendall voids may have a deteriorated effect on the anti-oxidation performance of hot-dip aluminized and PEO coated samples.

Study on Preparation, Microstructure and Properties of Micro-Arc Oxidation Ceramic Coating on AZ91 Magnesium Alloy in Phosphate Electrolyte

The micro-arc oxidation behavior of AZ91 magnesium alloy in alkaline phosphate electrolyte was studied. The effect of phosphate on the microstructure and properties of micro-arc oxide film was investigated and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive spectrum (EDS) and salt spray test. The results show that phosphate concentration has an important effect on the surface morphology, phase structure, thickness and corrosion resistance of micro-arc oxidation film. With the increase of phosphate concentration from 5g/L to 30g/L, the film quality decreases, the surface roughness increases, the thickness decreases and the corrosion resistance decreases. So in phosphate electrolysis the lower phosphate concentration in the liquid system is helpful to the formation of better film quality.

Wear Resistance of Plasma-Sprayed Coatings in Intensive Abrasive Wear Conditions

The abrasive wear resistance of plasma-sprayed oxide ceramic (Al2O3 and Cr2O3) and clad cermet ((Ti–Cr–C)–30% Ni and (TiC)–30% Ni) coatings was studied. The wear characteristics of the coatings such as wear rates, friction forces, friction coefficients, and wear groove depth determined with two methods were compared. The wear test methods included simulation of the friction process using loose abrasive particles and reciprocal ball-on-disk friction using a diamond indenter. The plasma-sprayed TiC-based coatings showed the lowest wear rate and can be applied to protect equipment parts subjected to intensive abrasive wear. The wear loss of the (Ti–Cr–C)–30% Ni and (TiC)–30% Ni coatings determined by friction against loose abrasive particles was 10–17 μm, while the wear loss of the oxide ceramic coatings was 20–42 μm, being 2–2.5 times higher on average.

Elemental Composition and Microhardness of the Coatings Prepared on Faced Aluminum Alloys by Plasma Electrolytic Oxidation in a Silicate–Alkaline Electrolyte

Owing to their high microhardness, corrosion resistance, and heat resistance, the oxide ceramic coatings formed by plasma electrolytic oxidation (PEO) find ever-growing usage for various purposes in many industries and, in particular, for maintenance. One of the methods widely used for the maintenance of aluminum alloys products is argon-arc facing. As the materials for facing, alloys of various grades, such as AK9M2, AK5, and AMr6, are used. The electrolytes widely used for PEO are aqueous silicate–alkaline electrolytes, in particular, KOH—Na2SiO3. To find a correlation between the chemical composition of an oxidized faced aluminum alloy and the microhardness of coatings formed on the alloy, X-ray spectrometry analysis is performed. The performed analysis and the determined distribution of chemical elements across the coating thickness allow us to estimate the mechanism of plasma electrolytic oxidation and the correlation between the electrolyte composition, the chemical composition of faced aluminum alloy, and the element composition of the hardened layer.

Long time corrosion test of AZ31B Mg alloy via micro-arc oxidation (MAO) technology

Micro-arc oxidation (MAO) ceramic coatings of AZ31B Mg alloy were formed in phosphate-silicate based electrolyte with the addition of K2ZrF6. The effect of treatment time on physical and chemical properties of the coatings were thoroughly investigated. The results showed that the micropores on the surface of the coatings increased on size and decreased on porosity with the increasing MAO treatment time. The ZrO2 was inclined to absorbed into the outer porous layer than that of the inner dense layer. The corrosion study presents that the corrosion resistance increased with the increasing treatment time, which was proved directly proved by 150 h salt immersion test. The potentiodynamic polarization test proved that the coating with the MAO treatment time of 40 min is of the most superior corrosion resistance. In addition, the long time electrochemical impedance spectroscopy (EIS) test of the coating formed in the electrolyte with the treatment time of 40 min showed that, the corrosion resistance of the coating decreases in the initial immersion stage (64 h) little by little is because of the MgO decomposition in the outer porous layer. With the immersion time increased, the blocking effect of the outer porous layer postpone the deterioration of the inner dense layer.

Cordunum pistons increase diesel engine economy and reliability

Taking into account the oil resources depletion the requirements to fuel consumption of internal combustion engines are now increasing as well as to their reliability and durability. With the continual increase in the number of internal combustion engines in operation, along with the problem of parts of the cylinder piston group wearing out has caused exhaust from such engines to be one of the main source of harmful pollutant emissions in cities. Therefore, environmental requirements have in turn increased dramatically. The engine resource and its efficiency largely depend on the process of fuel combustion in the combustion chamber. Experimental studies aimed to improve the working process on diesel engines by piston insulation have shown an effective decrease in fuel consumption by reducing heat loss and more complete fuel combustion. When oxide ceramic coatings were used on the piston and cylinder head, the maximum power increased and the specific fuel consumption decreased. However ceramic coatings are not widely used due to their peeling. We have developed a technology for the galvanic plasma treatment of pistons, which made it possible to obtain on the pistons surface made of aluminum alloys a ceramic corundum layer with high adhesion to the base metal that does not peel and has electret properties. In 1993, pistons with a corundum surface layer were installed in a shunting diesel locomotive and life-time running tests were conducted. Such pistons increased wear resistance, reduced the wear of cylinder liners, increased the strength of the annular jumpers, and were not prone to burnouts and scuffing. They provided an increase in the resource of the cylinder-piston group of the diesel engine by more than 125 thousand engine hours. The paper provides an analysis of the effect of corundum pistons thermal insulation on significant increasing the, engine power and fuel consumption reduction. Basing on experimental bench studies of a gasoline engine, a tractor diesel engine and long-term operational life tests of diesel engines, an attempt had been made to explain the reasons for the improvement in the engines’ efficiency.

Improving the Antifriction Properties of Ceramic Coatings Obtained by the Method of MAO on Aluminum Alloys

The problem of increasing the antifriction properties of oxide-ceramic coatings obtained by microarc oxidation on the surface of aluminum alloys is considered. This problem was solved by the impregnation of oxide-ceramic coatings with solutions of polymers (oligomers), which, upon further heat treatment and polymerization, give the oxide-ceramic coatings antifriction properties. Laboratory tests of the tribotechnical properties of the obtained polymer–ceramic coatings were carried out. The areas of possible practical application of the obtained polymer-ceramic coatings are considered.

Plasma electrolytic oxidation ceramic coatings proceed by porous anodic film

Ceramic coatings were performed on aluminum through plasma electrolytic oxidation (PEO) in a Na5P3O10 solution with various concentrations of NaOH. It was revealed that the concentration of NaOH affected the types of anodic films and the activity of plasma discharges. Emphatic investigation was performed on the coatings fabricated in the electrolyte with addition of 1 g L−1 NaOH, focusing on the coating microstructure evolution and coating corrosion resistance. Some coatings were detached from the substrate, showing that the oxide particles on the anodic film C/S interface underwent profile variations from hemisphere to polygon, and its interpore distance decreased over time. The formations of pores of anodic films and plasma discharge of the PEO coatings were both easy to occur around the existing pores and the existing discharge channels. Fractured cross-section showed that a pancake-like structure corresponded to a discharge channel, a cavity, and a barrier layer protrusion whose area was similar to that of the corresponding pancake-like structure. The data of potentiodynamic polarization measurements showed that the sample treated with PEO for 10 min possessed the lowest icorr value of 4.97E-8 A cm−2, the most positive Epit value of −0.63 V, and the highest Rp value of 2.66E5 Ω cm2. EIS data of 10 min sample exhibited the highest R2 value of 1.61E6 Ω cm2, demonstrating the best corrosion resistance among these samples.