Plasma Electrolytic Oxidation Ceramic Coatings Research Articles

Phase transformation in plasma electrolytic oxidation coatings on 6061 aluminum alloy

Oxide coatings were produced on a 6061 aluminum alloy using a pulsed unipolar plasma electrolytic oxidation (PEO) process. The effect of electrical parameters including pulse frequency, duty cycle and current density on phase formation in the coatings was revealed using conventional and glancing angle X-ray diffraction. The results show that PEO coatings are mainly composed of γ-Al2O3. Depending on the electrical parameters employed, the coatings can also contain α-Al2O3 and mullite with varying concentrations. Higher current densities and higher duty cycle were found to favor the formation of mullite. Under the experimental conditions used, the ratio of the integrated XRD peaks for α- and γ-Al2O3 varied from 0 to about 0.6, indicating that the relative content of α-Al2O3 in the PEO coatings varied over a wide range. Longer pulse on-times and higher current densities promoted the gamma to alpha-alumina phase transformation. Depth profiling of PEO ceramic coatings using glancing angle XRD with different incident beam angles revealed that mullite was more concentrated in the top surface of the coatings. No significant variation in α-Al2O3 concentration across the coatings could be concluded in this study, unlike the results of some other studies.

Effects of Alloying Elements on Structure of Plasma Electrolytic Oxidation Ceramic Coatings on Aluminum Alloys

Alloying elements have vital effect on the formation of ceramic coatings. In order to research the effects of Cu, Mg, Si and Zn on plasma electrolytic oxidation (PEO) coatings, experiments of PEO were carried out on aluminum and its alloys in sodium silicate-sodium hydroxide solutions. Its microstructure and phase composition were analyzed by scanning electron microscope and X-ray diffraction. Results showed that, high content of Cu, Si and Zn in alloys suppressed process of some reactions, Mg element had different effects on high-temperature phase transformation of alumina ceramics while its weight percentage varied.

Structure and Corrosion Resistance of PEO Ceramic Coatings on AZ91D Mg Alloy Under Three Kinds of Power Modes

Power mode is one of the most important factors in the plasma electrolytic oxidation (PEO) technique, which greatly influences the structure and performances of the prepared ceramic coatings. The aim of this work was to investigate effects of three kinds of power modes (constant voltage (Mode 1), constant current (Mode 2), and constant power (Mode 3)) on the structure and corrosion resistance of the PEO ceramic coatings containing Ca and P on AZ91D Mg alloy. The phase composition, morphology, and the element distribution of the coatings was studied using X‐ray diffraction, scanning electron microscopy, and energy‐dispersive spectroscopy, respectively. The electrochemical impedance spectrum was measured to evaluate the structure and the corrosion resistance. The polarizing curves were measured to evaluate the corrosion resistance. The results show that the coatings are of porous structure and mainly composed of MgO. The thickness and surface roughness of three kinds of coatings abide by the following sequences: Mode 1 > Mode 2 > Mode 3. By comparison, “equivalent circuit” mode Rs{Q1[R1(Q2R2)]} is the most proper to reflect the structure of such coatings, including the surface roughness, the outer layer pore structure, and the inner layer structure. The coatings prepared under Mode 1 are of maximum thickness and high density, and therefore have the best general corrosion resistance. And the coatings prepared under Mode 3 are of the smallest surface roughness and higher density, which leads to the best pitting corrosion resistance.

Effect of PEO power modes on structure and corrosion resistance of ceramic coatings on AZ91D Mg alloy

The aim of this work is to investigate the effects of different power modes [constant current (CC), constant voltage (CV) and constant power (CP)] on the structure and corrosion resistance of the plasma electrolytic oxidation ceramic coatings containing Ca and P on AZ91D Mg alloy. The phase composition, morphology and element distribution of the coatings were studied by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The corrosion resistance of the coatings in the simulated body fluid was examined by electrochemical impedance spectroscopy and polarisation curve methods. The results showed that the coatings were of porous structure and mainly composed of MgO. The thickness, surface roughness, the size of the surface micropores and the amount of MgO were all increased with the cell voltage under the same power mode or increased in the sequence CV>CC>CP under the different power modes. The corrosion resistance of coatings was related to the structure characteristics, which were determined by different power modes. Among the different power modes, the corrosion resistance of the coatings is generally increased in the sequence CV>CP>CC, and under the same mode, the proper increase in the cell voltage is liable for improving the corrosion resistance of the coatings.

TEM analysis and wear resistance of the ceramic coatings on Q235 steel prepared by hybrid method of hot-dipping aluminum and plasma electrolytic oxidation

The hybrid method of PEO and hot-dipping aluminum (HDA) was employed to deposit composite ceramic coatings on the surface of Q235 steel. The composition of the composite coatings was investigated with X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The cross-section microstructure and micro-hardness of the treated specimens were investigated and analyzed with scanning electron microscopy (SEM) and microscopic hardness meter (MHM), respectively. The wear resistance of the ceramic coatings was investigated by a self-made rubbing wear testing machine. The results indicate that metallurgical bonding can be observed between the ceramic coatings and the steel substrate. There are many micro-pores and micro-cracks, which act as the discharge channels and result of quick and non-uniform cooling of melted sections in the plasma electrolytic oxidation ceramic coatings. The phase composition of the ceramic coatings is mainly composed of amorphous phase and crystal Al2O3 oxides. The crystal Al2O3 phase includes κ-Al2O3, θ-Al2O3 and β-Al2O3. The grain size of the κ-Al2O3 crystal is quite non-uniform. The hardness of the ceramic coatings is about HV1300 and 10 times higher than that of the Q235 substrate, which was favorable to the better wear resistance of the ceramic coatings.

Corrosion Properties of Plasma Electrolytic Oxidation Ceramic Coatings on an A356 Alloy Tested in an Ethanol-Gasoline Fuel (E85) Medium

Ceramic oxide coatings were prepared on an aluminum A356 alloy by a plasma electrolytic oxidation (PEO) technique under unipolar, bipolar and duplex unipolar/bipolar current modes. Cross-sectional morphologies of the coatings were studied using a scanning electron microscope (SEM). The corrosion behavior of the coated and uncoated samples was evaluated in ethanol-gasoline E85 fuels through potentiodynamic polarization and zero resistance ammeter (ZRA) testing methods. The results indicated that all the coatings had a better corrosion resistance compared to the uncoated substrate. The unipolar current mode created the PEO coating with a thicker coating microstructure and thus a better corrosion resistance, compared to a bipolar current mode. The duplex treatments of unipolar/bipolar or bipolar/unipolar current modes produced the best performance of the coatings against galvanic corrosions caused by a steel/Al coupling in the E85 fuel medium.

Effects of Sodium Fluoride on Structure and Corrosion Resistance of Plasma Electrolytic Oxidation Ceramic Coatings on Magnesium Alloys

Abstract F− is used widely as an additive in the electrolyte solutions during the plasma electrolytic oxidation (PEO) of Mg alloys. The goal of this work was to investigate effects of sodium fluoride (NaF) on the structure and corrosion resistance of the ceramic coatings containing Ca and P on AZ91D (UNS M11916) Mg alloy using the PEO technique. The phase composition, morphology, and the element distribution of the coatings were studied using x-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The corrosion resistance of the coatings in the simulated body fluid (SBF) was examined using electrochemical impedance spectroscopy (EIS) and the polarizing curve methods. The results show that the coating was composed mainly of magnesium oxide (MgO), which was increased with a dosage of NaF. Ca and P also existed in the coating. The coating had a two-layer structure: the outer layer was loose and porous and the inner layer was comparatively dense. There were many microholes on the ...

Effects of cathode pulse at low frequency on the structure and composition of plasma electrolytic oxidation ceramic coatings

The aim of this work is to investigate the effects of the cathode pulse under the low working frequency on the structure and the composition of the ceramic coatings on Ti–6Al–4V alloys by plasma electrolytic oxidation (PEO). Ceramic coatings were prepared on Ti alloy by pulsed bi-polar plasma electrolytic oxidation in NaAlO 2 solution. The phase composition, morphology, and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy, and energy distribution spectroscopy. The coating was mainly composed of a large amount of Al 2TiO 5 and a little α-Al 2O 3 and rutile TiO 2. Increasing the cathode pulse, the amount of rutile TiO 2 was increased while the amount of Al 2O 3 was decreased; and decreasing the cathode pulse, the amount of Al 2O 3 was increased while the amount of rutile TiO 2 was decreased. The thickness of the coatings was increased and then decreased with the increase of the cathode pulse. The grain sizes of Al 2TiO 5 were increased with the cathode current densities, but changed little with the cathode pulse width. The grain size of α-Al 2O 3 was decreased with the decrease of the cathode pulse, while the grain size of TiO 2 was increased with the increase of the cathode pulse. The proper cathode pulse was helpful to reduce the roughness and to increase the density of the coatings.

Growth characteristics of plasma electrolytic oxidation ceramic coatings on Ti–6Al–4V alloy

The aim of this work is to discuss the growth characteristics of the ceramic coatings on Ti alloy by plasma electrolytic oxidation (PEO) technique. Ceramic coatings were prepared on Ti alloy by plasma electrolytic oxidation in different electrolyte solutions under different pulse modes. The composition and the structure of the coatings were investigated by X-ray diffraction and scanning electron microscopy (SEM), respectively. The amount of the dissolved titanium into the electrolytes during PEO process was measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES). The structure and the composition of the coatings were related to the mode of the spark discharge during PEO process. (a) Under the pulsed single-polar mode: In Na 3PO 4 solution, the spark discharge was mainly due to the breakdown of the oxide film, and the coatings prepared were porous and mainly structured by the Ti from the substrate. In K 4ZrF 6–H 3PO 4 and NaAlO 2–Na 3PO 4 solutions, the main mode of the spark discharge was the breakdown of the oxide film at the initial stage, and then changed into the breakdown of the vapor envelope, and the coatings were rough and thick, and mainly structured by the elements from the electrolyte. (b) Under the pulsed bi-polar mode in NaAlO 2–Na 3PO 4 solution, the spark discharge may be mainly due to the breakdown of the oxide film, the coatings prepared were dense in inner layer and loose in outer layer, and structured by the elements from both the substrate and the electrolyte. Besides, the ICP-AES analyses showed that the amount of the dissolved titanium in the electrolyte during PEO process was more under the breakdown of the oxide film than under the breakdown of the vapor envelope, which was consistent with the changes of the structure of the coatings. Cathode pulse in the pulsed bi-polar mode increased the amount of the dissolved titanium in the electrolyte, compared with the pulsed single-polar one.

Microstructure, temperature estimation and thermal shock resistance of PEO ceramic coatings on aluminum

Plasma electrolytic oxidation (PEO) is a novel surface technique for producing ceramic coatings on valve metals and their alloys. In this paper, microstructure, temperature estimation and thermal shock resistance of the ceramic coatings formed on pure aluminum by hetero-polar pulsed current ceramic synthesizing system were investigated. Results show that the coating roughness becomes greater and the plasma discharge channel populations in the ceramic coatings decrease while the pores enlarge with PEO treatment time. In addition, the surface morphology of the ceramic coating indicates that melting and solidifying process happen alternately in the process. The results also show that the coatings have a good thermal shock resistance. Moreover, the temperature and the temperature rise rate of the plasma discharge channels were investigated, and a formula for calculating the temperature and the temperature rise rate of the plasma discharge channels at any time during the PEO process was deduced.

Surface Morphology and Discharge Channel Temperature Estimation of PEO Ceramic Coatings on Aluminum

The surface roughness and morphology of the ceramic coatings fabricated on industrially pure aluminum by the plasma electrolytic oxidation (PEO) technique with a hetero-polar pulsed current ceramic synthesizing system were investigated. A model for the process was established. Based on this a formula was deduced to estimate the temperature rise rate at the corresponding locations before the plasma discharge channels have formed during the PEO process, and the channels’ temperature when the plasma discharges occur. Results show that the PEO ceramic coatings grow by an alternating coating melting and solidifying way; The coating roughness becomes greater and the plasma discharge channel population in the ceramic coatings decreases while the pores enlarge with PEO treatment time and current. With the deduced formula, the PEO alumina coating on industrially pure aluminum was taken as an example for validation of the model.

Structure and mechanical properties of ceramic coatings fabricated by plasma electrolytic oxidation on aluminized steel

Ceramic coatings were formed by plasma electrolytic oxidation (PEO) on aluminized steel. Characteristics of the average anodic voltages versus treatment time were observed during the PEO process. The micrographs, compositions and mechanical properties of ceramic coatings were investigated. The results show that the anodic voltage profile for processing of aluminized steel is similar to that for processing bulk Al alloy during early PEO stages and that the thickness of ceramic coating increases approximately linearly with the Al layer consumption. Once the Al layer is completely transformed, the FeAl intermetallic layer begins to participate in the PEO process. At this point, the anodic voltage of aluminized steel descends, and the thickness of ceramic coating grows more slowly. At the same time, some micro-cracks are observed at the Al 2O 3/FeAl interface. The final ceramic coating mainly consists of γ-Al 2O 3, mullite, and α-Al 2O 3 phases. PEO ceramic coatings have excellent elastic recovery and high load supporting performance. Nanohardness of ceramic coating reaches about 19.6 GPa.