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 Al₂O₃/FeAl interface. The final ceramic coating mainly consists of γ-Al₂O₃, mullite, and α-Al₂O₃ phases. PEO ceramic coatings have excellent elastic recovery and high load supporting performance. Nanohardness of ceramic coating reaches about 19.6 GPa.     

Formation of titania composite coatings on carbon steel by plasma electrolytic oxidation

Titania composite coatings were prepared on carbon steel by plasma electrolytic oxidation in silicate electrolyte and aluminate electrolyte with titania powers doping in the electrolytes. The microstructure of the coatings was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The properties of the coatings including bond strength, thickness, thermal shock resistance and corrosion resistance varying with the quantities of titania powers in the electrolytes were studied. Investigation results revealed that the coating obtained in silicate electrolyte was composed of anatase-TiO₂, rutile-TiO₂ crystal phases and some Fe, Si, P elements; coating obtained in aluminate electrolyte consisted of anatase-TiO₂, Al₂TiO₅ and some Fe, P elements. Coatings obtained in two types of electrolytes show porous and rough surface. With increasing the concentration of titania powers in the electrolytes, the coating surface first became more compact and less porous and then became more porous and coarse. The bond strength and thickness were not strongly affected by concentration of titania powers in electrolytes. The valves were 23 MPa and for 66 μm for coatings obtained in aluminate electrolyte, and 21 MPa and 35 μm for coatings obtained in silicate electrolyte. Coatings obtained in silicate electrolyte showed a little better thermal shock resistance than those obtained in aluminate electrolyte and the best coatings were obtained with middle concentration of titania powers in the electrolytes. All coated samples showed better corrosion resistance than the substrate in 3.5 wt% NaCl solution. The best coatings were also obtained with middle concentration of titania powers doping in both electrolytes whose corrosion current density was decreased by 2 orders of magnitude compared with the substrate.     

In situ formation of low friction ceramic coatings on carbon steel by plasma electrolytic oxidation in two types of electrolytes

In situ formation of ceramic coatings on Q235 carbon steel was achieved by plasma electrolytic oxidation (PEO) in carbonate electrolyte and silicate electrolyte, respectively. The surface and cross-section morphology, phase and elemental composition of PEO coatings were examined by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). The bond strength of the coating was determined using a direct pull-off test. The hardness as well as tribological properties of the ceramic coating was primarily studied. The results indicated that the coating obtained in carbonate electrolyte was Fe₃O₄, while the coating achieved from silicate electrolyte was proved to be amorphous. Both kinds of coatings showed coarse and porous surface. The Fe₃O₄ coatings obtained in carbonate electrolyte showed a high bonding strength to the substrate up to 20 ± 2 MPa and the value was 15 ± 2 MPa for the amorphous coatings obtained in carbonate electrolyte. The micro hardness of the amorphous coating and the Fe₃O₄ coating was 1001 Hv and 1413 Hv, respectively, which was more than two and three times as that of the Q235 alloy substrate (415 Hv). The friction coefficient exhibited by amorphous coating and Fe₃O₄ coating was 0.13 and 0.11, respectively, both lower than the uncoated Q235 substrate which ranged from 0.17 to 0.35.     

Preparation of ceramic coating on Ti substrate by plasma electrolytic oxidation in different electrolytes and evaluation of its corrosion resistance: Part II

The aim of this work is to discuss the growth characteristics and corrosion behavior of the prepared ceramic coatings on titanium by plasma electrolytic oxidation (PEO) technique in different electrolytes. PEO process was carried out on titanium under constant voltage regime using a pulse power supply. Three kinds of electrolytes, phosphate, silicate and borate based solutions, were used to evaluate the influence of electrolyte composition on the structure, surface morphology, phase composition and corrosion behavior of prepared ceramic oxide films (titania). The phase composition of the coatings was investigated by X-ray diffraction. Scanning electron microscopy was employed to evaluate the growth and surface morphology of coatings. Elements of coatings were investigated with energy dispersive spectrometer. Corrosion behavior of the coatings was also examined by potentiodynamic polarization and electrochemical impedance spectroscopy. The spark voltage of oxide films had a significant effect on the surface morphology, size and homogeneity of micro-pores, thickness and corrosion properties of 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 Na₃PO₄ 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 K₄ZrF₆-H₃PO₄ and NaAlO₂-Na₃PO₄ 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 NaAlO₂-Na₃PO₄ 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.     

Preparation of ceramic coating on Ti substrate by Plasma electrolytic oxidation in different electrolytes and evaluation of its corrosion resistance

Ceramic oxide coatings (titania) were produced on Ti by micro-arc oxidation in different aluminate and carbonate based electrolytes. This process was conducted under constant pulsed DC voltage condition. The effect of KOH and NaF in aluminate based solution was also studied. The surface morphology, growth and phase composition of coatings were investigated using scanning electron microscope and X-ray diffraction. Corrosion behavior of the coatings was also examined by potentiodynamic polarization and electrochemical impedance spectroscopy. It was found that the sparking initiation voltage (spark voltage) had a significant effect on the form and properties of coatings. Coatings obtained from potassium aluminate based solution had a lower spark voltage, higher surface homogeneity and a better corrosion resistance than the carbonate based solution. Addition of NaF instead of KOH had improper effects on the homogeneity and adhesion of coatings which in turn caused a poor corrosion protection behavior of the oxide layer. AC impedance curves showed two time constants which is an indication of the coatings with an outer porous layer and an inner compact layer.     

Preparation and characterization of a novel Si-incorporated ceramic film on pure titanium by plasma electrolytic oxidation

A Si-incorporated bioactive ceramic film was prepared on pure titanium by plasma electrolytic oxidation (PEO) in a new bath containing Ca²⁺, H₂PO₄⁻ and SiO₃²⁻. The film was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscope (XPS). The apatite-induced ability of PEO film was evaluated by soaking in a simulated body fluid (SBF) for various periods. The results showed that Si-incorporated PEO film present a porous microstructure, the pore size is around 1–5 μm. The film mainly consists of anatase and rutile and a small amount of CaHPO₄ and CaO, besides, bioactive compounds such as CaSiO₃ and SiO₂, also exist in the Si-incorporated PEO film. After immersion in SBF for 28 days, not only the surface layer but also the pores inside the Si-incorporated PEO film were completely filled by apatite crystals, whereas on the surface of a benchmark PEO film free of Si just present small piles of apatite crystals. Silicon incorporated into the PEO film provided more heterogeneous nucleation sites for apatite deposition and hence increased remarkably bioactivity of the PEO film.     

Characterization of calcium containing plasma electrolytic oxidation coatings on AM50 magnesium alloy

An attempt was made to produce calcium containing plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy using an alkaline electrolyte. This study was performed in three alkaline electrolytes containing calcium hydroxide and sodium phosphate with three different mass ratios viz., 1:2.5, 1:5 and 1:7.5. All the three coatings produced were found to contain Ca and P in appreciable amounts. The concentration of P was found to be higher in the coatings obtained in the electrolytes with higher concentration of phosphate ions. Even though all the three coatings were found to be constituted with magnesium oxide and magnesium phosphate phases, X-ray diffraction analyses revealed that the phase composition was influenced by the phosphate ion concentration/conductivity of the electrolyte. Further, the PEO coating obtained in the 1:7.5 ratio electrolyte was found to contain di-calcium phosphate (monetite) and calcium peroxide phases, which were absent in the other two coatings. Potentiodynamic polarization studies performed in 0.1 M NaCl solution showed that the coatings obtained from the 1:5 ratio electrolyte possessed a superior corrosion resistance, which is attributed to the combined effect of thickness, compactness and phase/chemical composition of this coating.     

Effect of surface roughness on leakage current and corrosion resistance of oxide layer on AZ91 Mg alloy prepared by plasma electrolytic oxidation

The influence of the surface roughness of Mg alloys on the electrical properties and corrosion resistance of oxide layers obtained by plasma electrolytic oxidation (PEO) were studied. The leakage current in the insulating oxide layer was enhanced by increasing the surface roughness, which is a favorable characteristic for the material when applied to hand-held electronic devices. The variation of corrosion resistance with surface roughness was also investigated. The corrosion resistance was degraded by the increasing surface roughness, which was confirmed with DC polarization and impedance spectroscopy. Pitting corrosion on the passive oxide layer was also analyzed with a salt spray test, which showed that the number of pits was not affected by the surface roughness when the spray time reached 96 h.     

Fabrication of functionally gradient nanocomposite coatings by plasma electrolytic oxidation based on variable duty cycle

Plasma electrolytic oxidation (PEO) was applied on the surface of commercially pure titanium substrates in a mixed aluminate-phosphate electrolyte in the presence of silicon nitride nanoparticles as suspension in the electrolyte in order to fabricate nanocomposite coatings. Pulsed current was applied based on variable duty cycle in order to synthesize functionally gradient coatings (FGC). Different rates of variable duty cycle (3, 1.5 and 1%/min), applied current densities (0.06-0.14 A/cm²) and concentrations of nanoparticles in the electrolyte (2, 4, 6, 8 and 10 g l⁻¹) were investigated. The nanopowder and coated samples were analyzed by atomic force microscope, scanning electron microscope and transmission electron microscope. The influence of different rates of variable duty cycle (or treatment times) on the growth rate of nanocomposite coatings and their microhardness values was investigated. The experimental results revealed that the content of Si₃N₄ nanoparticulates in the layer increases with the increase of its concentration in the plasma electrolysis bath. Nanocomposite coatings fabricated with lower rate of variable duty cycle have higher microhardness with smoother microhardness profile.     

Plasma electrolytic oxidation coating on AZ91 magnesium alloy modified by neodymium and its corrosion resistance

Ceramic coatings on the surfaces of Mg-9Al-1Zn (AZ91) magnesium alloy and Mg-9Al-1Zn-1Nd magnesium alloy (AZ91 magnesium alloy modified by neodymium, named as AZ91Nd in this paper) are synthesized in aluminate electrolyte by plasma electrolytic oxidation (PEO) process, respectively. X-ray diffraction and X-ray photoelectron spectroscopy analyses show the PEO coating on the Mg-9Al-1Zn-1Nd alloy comprises not only MgO and Al₂O₃, which are found in the coating on the AZ91 alloy, but also a trace amount of Nd₂O₃. Microstructure observations indicate the addition of Nd can decrease the sizes of β phases and form Al₂Nd intermetallics in the AZ91 alloy. The fine β phases can effectively restrain the formation of unclosed-holes and greatly decrease the sizes of pores in the coating during the PEO process. In addition, the Al₂Nd intermetallics can be completely covered due to the lateral growth of the PEO coatings formed on the α and β phases. As a result, the coating on the AZ91Nd alloy possesses a dense microstructure compared with that on the AZ91 alloy. The following corrosion tests indicate the corrosion resistance of the PEO coating on the AZ91Nd alloy is evidently higher than that of the PEO coating on the AZ91 alloy.     

Characteristics of sealed plasma electrolytic oxidation coatings with electrochemical impedance spectroscopy

<正>Plasma electrolytic oxidation(PEO) coatings are prepared on aluminium with graphite powders added into the electrolyte.The scanning electron microscopy(SEM) coupled with an energy dispersive x-ray analysis system(EDX) is used to characterize the surface and the cross-section morphologies of the coatings.The electrochemical impedance spectroscopy(EIS) is used not only to evaluate the corrosion resistance but also to analyse the structure of the coating. Results show that graphite powders are embedded in the PEO coating.The corrosion resistances of both the inner barrier and the outer porous layer are greatly improved,and the EIS could give some valuable detailed information about the coating structure.     

Microstructure characteristic of ceramic coatings fabricated on magnesium alloys by micro-arc oxidation in alkaline silicate solutions

Micro-arc oxidation (MAO) of AZ31B magnesium alloys was studied in alkaline silicate solutions at constant applied current densities. The microstructure, phase composition and elemental distribution of ceramic coatings were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDX). There are two inflections in the voltage-time response, three regions were identifiable and each of the regions was almost linear. The pores with different shapes distributed all over the coating surface, the number of the pores was decreasing, while the diameter was apparently increasing with prolonged MAO treatment time. There were also cracks on the coating surface, resulting from the rapid solidification of the molten oxide. The ceramic coating was comprised of two layers, an outer loose layer and an inner dense layer. The ceramic coating was mainly composed of forsterite phase Mg₂SiO₄ and MgO; the formation of MgO was similar to conversional anodizing technology, while formation of Mg₂SiO₄ was attributed to a high temperature phase transformation reaction. Presence of Si and O indicated that the electrolyte components had intensively incorporated into coatings.     

Investigation of the voltage and time effects on the formation of hydroxyapatite-containing titania prepared by plasma electrolytic oxidation on Ti-6Al-4V alloy and its corrosion behavior

Producing titania and hydroxyapatite (HA) bioceramic coating on titanium alloys increases corrosion resistance and biocompatibility of these alloys. Plasma electrolytic oxidation (PEO) is one of the effective techniques for producing this type of coating. This method produces coatings with enough thickness and appropriate adhesion. In this study, titania and HA were directly produced on Ti-6Al-4V by applying PEO process in a Ca- and P-containing electrolyte by changing voltage and time parameters. Morphology and cross section, chemical composition and elements of coatings were investigated by scanning electron microscope, X-ray diffraction and energy dispersive spectroscopy, respectively. Corrosion behavior of the samples was also examined by potentiodynamic polarization and electrochemical impedance spectroscopy. The results indicated that the formation of HA phase with titania needs a minimum voltage below which HA is not formed. By increasing the operation time, the amount of the formed HA increased. Also, the sample coated at 500 V and 15 min showed the best corrosion behavior in Ringer's solution.     

Characterization of oxide coatings formed on tantalum by plasma electrolytic oxidation in 12-tungstosilicic acid

Oxide coatings were formed on tantalum by plasma electrolytic oxidation (PEO) process in 12-tungstosilicic acid. The PEO process can be divided into three stages with respect to change of the voltage-time response. The contribution of electron current density in total current density during anodization results in the transformation of the slope of voltage-time curve. The surface morphology, chemical and phase composition of oxide coatings were investigated by AFM, SEM-EDX, XRD and Raman spectroscopy. Oxide coating morphology is strongly dependent of PEO time. The elemental components of PEO coatings are Ta, O, Si and W. The oxide coatings are partly crystallized and mainly composed of WO₃, Ta₂O₅ and SiO₂. Raman spectroscopy showed that the outer layer of oxide coatings formed during the PEO process is silicate tungsten bronze.     

Microstructure and corrosion resistance behavior of ceramic coatings on biomedical NiTi alloy prepared by micro-arc oxidation

In this paper, ceramic coatings were prepared on biomedical NiTi alloys by micro-arc oxidation (MAO) in constant voltage mode. The current density-time response was recorded during the MAO process. The microstructure, element distribution and phase composition of the coatings prepared at different MAO treatment times were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), thin-film X-ray diffraction (TF-XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion behavior of the coatings in 0.9% NaCl solution was evaluated by the potentiodynamic polarization test. It is found that the coatings become more compact with increasing the MAO treatment time, and the growth rate of coating decreases. The results of TF-XRD, EDS and XPS indicate that the coatings are composed of a large amount of γ-Al₂O₃ and a little α-Al₂O₃, TiO₂ and Ni₂O₃. The Ni content of the coatings is about 3 at.%, which is greatly lower than that of NiTi substrate. The bonding strength of coating-substrate is higher than 40 MPa for all the samples in this study. The corrosion resistance of the coatings is about two orders of magnitude higher than that of the uncoated NiTi alloy.     

Influence of Co(CH3COO)2 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation

Ceramic thermal protection coatings on Ti6Al4V alloy were achieved by micro-arc oxidation (MAO) in the presence of Co(CH₃COO)₂. The morphology, crystallographic structure and chemical composition of the coating were characterized by various techniques. The thermal emission of the coating was measured by Fourier transform spectrometer apparatus. The bonding strength between the coating and substrate was studied, together with the thermal shock resistance of the coating. The results indicate that the content of Co in the coating layer significantly affects its thermal emissivity. Higher concentration of Co(CH₃COO)₂ in electrolytes leads to more Co ions into the coating, which enhances the emissivity of the coating. All the coatings show bonding strength higher than 10 MPa. In addition, the coating remains stable over 40 cycles of thermal shocking. The coating formed at 4 g/L Co(CH₃COO)₂ displays an average spectral emissivity value more than 0.9 and bonding strength about 10.4 MPa.     

Effects of ultrasound on the evolution of plasma electrolytic oxidation process on 6061Al alloy

The plasma electrolytic oxidation (PEO) process of 6061Al alloy was carried out under the conditions with and without the assistance of ultrasound, respectively. The effects of ultrasound on the evolution of voltage, micro-discharge, morphology and composition of PEO coatings were investigated. The results show that ultrasound can greatly decrease the dielectric breakdown voltage of the coatings, increase the number of micro-discharges while decrease their average size, promote the evolution of micro-discharges, decrease the number and the average size of residual discharge pores in the coatings after 30 min of the process, promote the homogeneous distribution of elements and the formation of α-Al₂O₃ and 3Al₂O₃⋅2SiO₂ in the coatings.     

Effect of current density on the microstructure and corrosion behaviour of plasma electrolytic oxidation treated AM50 magnesium alloy

Plasma electrolytic oxidation (PEO) of an AM50 magnesium alloy was accomplished in a silicate-based electrolyte using a DC power source. Coatings were produced at three current densities, i.e. 15 mA cm⁻², 75 mA cm⁻², and 150 mA cm⁻² and were characterised for thickness, roughness, microstructural morphology, phase composition, and corrosion resistance. Even though the 15 min treated coatings produced at higher current density levels were thicker, they showed poor corrosion resistance when compared to that of the coatings obtained at 15 mA cm⁻². Short-term treatments (2 min and 5 min) at 150 mA cm⁻² yielded coatings of thickness and corrosion resistance comparable to that of the low current density coatings. The superior corrosion resistance of the low thickness coatings is attributed to the better pore morphology and compactness of the layer.     

Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation

The paper reports on a systematic investigation into the effects of process parameters on the growth kinetics and associated changes in the structure, phase composition and mechanical properties of surface layers formed on Ti–6Al–4V alloy by plasma electrolytic oxidation (PEO) treatment in 0.05–0.2 mol l⁻¹ solutions of sodium aluminate. Methods of gravimetric, SEM and XRD analysis, as well as microhardness and scratch testing, are employed to investigate mass transfer and phase-structure transformations in the surface layer. The probable mechanisms of layer formation are discussed, which comprise electrochemical oxidation of the Ti-electrode by OH⁻ anions, complimented by chemical precipitation of Al(OH)₃ and plasma-induced transformations in the surface discharges. Running with a total yield efficiency of 20–30%, these processes lead to the formation of predominantly the Al₂TiO₅ phase with heterogeneous precipitation of Al₂TiO₅·TiO₂ and 3Al₂TiO₅·Al₂O₃ eutectics. Al- and Ti-enriched constituents of this structure show hardnesses of 1050–1480 and 300–845 H_K, 0.02, respectively. The layer growth rate increases with increasing electrolyte concentration, providing a maximum thickness of over 60 μm and a surface roughness (R_a) of 3–4 μm. Increasing the electrolyte pH from 12.0 to 12.8 results in smoothing and thickening of the surface layer but a lower sample weight gain, associated with an enhancement of the Ti electro-oxidation process. Morphological changes during PEO formation of the surface layer include gradual transformation of the original fine grained but porous structure into a dense, fused morphology which is adversely affected by discharge-induced thermal stresses, causing a degradation of the layer adhesion strength.