Mechanisms underlying the formation of thick alumina coatings through the MAO coating technology
TL;DR: In this article, a phenomenological mechanism for the formation of the alumina-based ceramic coatings during the microarc oxidation (MAO) process has been proposed and the surface features of the coatings were studied using scanning electron microscopy.
Abstract: Thick alumina coatings were synthesized on 7075 Al-alloy substrates through the microarc oxidation (MAO) route. Different oxidation times namely 1, 3, 5, 10, 20 and 30 min were employed and the coated samples were subjected to coating thickness and surface roughness measurements. Phase composition of the surface layers of the coatings was evaluated through X-ray diffraction. In addition, the surface features of the coatings were studied using scanning electron microscopy. Influence of coating time on the kinetics of coating formation, surface roughness, microhardness, number and size of the microarc discharge channels was investigated. On the basis of the experimental results, a phenomenological mechanism for the formation of the alumina-based ceramic coatings during the MAO process has been proposed.
Citations
More filters
TL;DR: In this article, the effects of process parameters (i.e., current density and treatment time) on the plasma discharge behavior during the PEO treatment were investigated using optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285-800 nm).
Abstract: In this study, a plasma electrolytic oxidation (PEO) process was used to produce oxide coatings on commercially pure aluminium (1100 alloy) at a pulsed dc power mode. The effects of process parameters (i.e. current density and treatment time) on the plasma discharge behaviour during the PEO treatment were investigated using optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285–800 nm). The elements present in the plasma were identified. Stark shifts of spectral lines and line intensity ratios were utilized to determine the plasma electron concentrations and temperatures, respectively. The plasma electron temperature profile, coating surface morphology and coating composition were used to interpret the plasma discharging behaviour. The different coating morphologies and compositions at different coating surface regions are explained in terms of three types of discharge, which originate either at the substrate/coating interface, within the upper layer, or at the coating top layer. The high spike peaks on the plasma intensity and temperature profiles corresponded to discharges originated from the substrate/coating interface, while the base line and small fluctuations were due to discharges at the coating/electrolyte interface.
461 citations
TL;DR: In this paper, the porosity of surface-connected porosity in plasma electrolytic oxide coatings was detected at levels of the order of 20% by densitometry, mercury porosimetry, helium pycnometry, and high-resolution scanning electron microscopy.
Abstract: Plasma electrolytic oxide coatings are generally assumed to be almost fully dense. However, evidence is presented here for the presence of sub-micrometre, surface-connected porosity in such coatings, on aluminium alloys, at levels of the order of 20%. This evidence comes from densitometry, mercury porosimetry, helium pycnometry, BET adsorption measurements and high-resolution scanning electron microscopy. The very fine scale of the porosity (pore diameter ∼10 to 100 nm), coupled with severe difficulties in making unambiguous microstructural observations, may account for the failure to detect this feature previously. It is pointed out that various measured properties, such as Young’s modulus and thermal conductivity, are consistent with the presence of these relatively high porosity levels. Various other observed characteristics can also be explained on this basis. Finally, a possible mechanistic origin for the porosity is proposed.
358 citations
TL;DR: In this paper, the authors used optical emission spectroscopy (OES) to follow the microdischarges and substrate and electrolyte elements present in the plasma discharge during the coating growth, and to determine plasma electron temperatures.
Abstract: To further our understanding of the plasma electrolytic oxidation (PEO) process, and to aid in the optimization of the process, it is important to identify the mechanisms of coating formation. In the present work, coatings up to 110 μm thick were produced on an AJ62 Mg-alloy substrate using the PEO process. Optical emission spectroscopy (OES) was employed to follow the microdischarges and substrate and electrolyte elements present in the plasma discharge during the coating growth, and to determine plasma electron temperatures. During PEO processing of magnesium, some of the metal cations are transferred outwards from the substrate and react with anions to form ceramic coatings. Also, due to the high electric field in the discharge channels, oxygen anions transfer toward the magnesium substrate and react with Mg 2+ cations to form a ceramic coating. In PEO process, the ceramic coating grows inwards to the alloy substrate and outwards to the coating surface simultaneously. The total coating thickness variation compared with the geometrical dimensions of the uncoated and coated samples were investigated. For the coating growth, there are three simultaneous processes taking place, namely the electrochemical reactions, the plasma chemical reactions and thermal diffusion. Oxygen diffusion occurring during PEO processing is discussed in terms of coating growth mechanisms.
354 citations
TL;DR: In this article, a comprehensive review around mechanisms of PEO coatings fabrication and their different properties is provided, and the coatings properties, affecting parameters and improvement strategies are discussed, including corrosion resistance of coatings, important factors in corrosion resistance and methods for corrosion resistance improvement.
Abstract: Plasma Electrolyte Oxidation (PEO) process has increasingly been employed to improve magnesium surface properties by fabrication of an MgO-based coating. Originating from conventional anodizing procedures, this high-voltage process produces an adhesive ceramic film on the surface. The present article provides a comprehensive review around mechanisms of PEO coatings fabrication and their different properties. Due to complexity of PEO coatings formation, a complete explanation regarding fabrication mechanisms of PEO coatings has not yet been proposed; however, the most important advancements in the field of fabrication mechanisms of PEO coatings were gathered in this work. Mechanisms of PEO coatings fabrication on magnesium were reviewed considering voltage–time plots, optical spectrometry, acoustic emission spectrometry and electronic properties of the ceramic film. Afterwards, the coatings properties, affecting parameters and improvement strategies were discussed. In addition, corrosion resistance of coatings, important factors in corrosion resistance and methods for corrosion resistance improvement were considered. Tribological properties (important factors and improvement methods) of coatings were also studied. Since magnesium and its alloys are broadly used in biological applications, the biological properties of PEO coatings, important factors in their biological performance and existing methods for improvement of coatings were explained. Addition of ceramic based nanoparticles and formation of nanocomposite coatings may considerably influence properties of plasma electrolyte oxidation coatings. Nanocomposite coatings properties and nanoparticles adsorption mechanisms were included in a separate sector. Another method to improve coatings properties is formation of hybrid coatings on PEO coatings which was discussed in the end.
349 citations
TL;DR: In this paper, a study of the electrical characteristics and optical emission spectra exhibited when discharge events take place during plasma electrolytic oxidation processing is made. But the results are limited to a single discharge event, typically tens to hundreds of microseconds, but there is a strong endency for themto occur in cascades that commonly last between several ms and several tens of ms.
Abstract: A study has been made of the electrical characteristics and optical emission spectra exhibited when discharge events take place during plasma electrolytic oxidation processing. Both conventional and small area experimental arrangements have been employed, allowing detailed measurement of durations, and temporal distributions, as well as such characteristics as charge transfer, and power. Individual discharges are of short duration, typically tens to hundreds of microseconds, but there is a strong endency for themto occur in cascades that commonly last between several ms and several tens of ms. The composition, temperature and electron density of the plasma formed during PEO processing are inferred from characteristics of the emission spectra. This confirms that there are two distinct regions of plasma; a lower density peripheral region at ~3500 K, and a higher density core at ~16,000±3500 K. The implications of these results are considered in terms of the interpretation of different types of experimental measurement, and attention is also briefly given to how such behaviour might relate to the mechanisms of growth.
313 citations
References
More filters
TL;DR: The physical and chemical fundamentals of plasma electrolysis are discussed in this article, and the equipment and deposition procedures for coating production are described, and the effects of electrolyte composition and temperature on ignition voltage, discharge intensity and deposited layer thickness and composition are outlined.
Abstract: This paper overviews the relatively new surface engineering discipline of plasma electrolysis, the main derivative of this being plasma electrolytic deposition (PED), which includes techniques such as plasma electrolytic oxidation (PEO) and plasma electrolytic saturation (PES) processes such as plasma electrolytic nitriding/carburizing (PEN/PEC). In PED technology, spark or arc plasma micro-discharges in an aqueous solution are utilised to ionise gaseous media from the solution such that complex compounds are synthesised on the metal surface through the plasma chemical interactions. The physical and chemical fundamentals of plasma electrolysis are discussed here. The equipment and deposition procedures for coating production are described, and the effects of electrolyte composition and temperature on ignition voltage, discharge intensity and deposited layer thickness and composition are outlined. AC-pulse PEO treatment of aluminium in a suitable passivating electrolyte allows the formation of relatively thick (up to 500 μm) and hard (up to 23 GPa) surface layers with excellent adhesion to the substrate. A 10–20 μm thick surface compound layer (1200HV) and 200–300 μm inner diffusion layer with very good mechanical and corrosion-resistant properties can also be formed on steel substrates in only 3–5 min by use of the PEN/PEC saturation techniques. Details are given of the basic operational characteristics of the various techniques, and the physical, mechanical and tribological characteristics of coatings produced by plasma electrolytic treatments are presented.
2,552 citations
TL;DR: In this article, the authors used X-Ray diffraction (XRD) and transmission electron microscopy (TEM) to investigate the coating microstructure, and the coating/substrate interface.
Abstract: Alumina coatings were deposited on Al alloy substrates using an electrolytic plasma technique, based on a dielectric barrier discharge created during anodic oxidation in an aqueous electrolyte. The substrate material (BS Al 6082) was biased anodically with an unbalanced AC high voltage. During processing, a plasma current density of 100 mA/cm2 was used, at which a coating deposition rate of 1.67 μm/min was achieved. Coating abrasive wear and corrosion properties were assessed by conducting dry and wet rubber wheel abrasive tests and potentiodynamic polarization experiments, respectively. X-Ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the coating microstructure, and the coating/substrate interface. The property test results show that the coatings possess excellent abrasive wear and corrosion resistance. XRD analyses indicate that the coatings consist of α- and γ-Al2O3. An amorphous+nanocrystalline inner layer (1.5-μm thick) and a nanocrystalline (50–60 nm) intermediate layer in the coating were observed by TEM. The higher resistance to wear and corrosion can in part be attributed to the presence of these interlayers.
395 citations
TL;DR: In this paper, the formation of metastable phases in plasma- and flame-prepared alumina particles is examined in terms of the classical nucleation theory, rate of transformation of the metastable to stable forms, and the thermal history of the particles during solidification.
Abstract: The formation of metastable phases in plasma- and flame-prepared alumina particles is examined in terms of the classical nucleation theory, rate of transformation of metastable to stable forms, and the thermal history of the particles during solidification. It is suggested that homogeneous nucleation of the solidification of liquid droplets at considerable undercooling results in the formation ofγ-Al2O3 rather thanα-Al2O3 because of its lower critical free energy for nucleation. The phase finally observed depends upon the thermal history of the particles during evolution of the heat of fusion and upon the kinetics of the transformation of the nucleating phase to the stable phase. This means that the cooling rate of the particles is relatively unimportant and under the conditions existing in flames and plasmas, metastable alumina will be formed on solidification. The metastable form will be retained on cooling particles less than approximately 10 μm diameter, but particles larger than this may transform toα-Al2O3 during the solidification exotherm
278 citations
TL;DR: In this article, a plasma electrolysis technique known as micro-arc discharge oxidation (MDO) was investigated; thick and hard oxide ceramic layers were fabricated on BS Al-6082 aluminium alloy by this method.
Abstract: Weight-saving materials are becoming increasingly important, especially in the automotive and aerospace industries. Design engineers would thus like to make more extensive use of light metals such as aluminium, titanium, magnesium and their alloys; however, these materials tend to have poor wear resistance. Previous treatments and coatings applied to aluminium alloys, for example by traditional processes such as hard anodising and thermal spraying, have suffered from the low load support from the underlying material and/or insufficient adhesion, which reduces their durability. Also, although TiN-, CrN- or DLC-coated aluminium alloys (using various PVD methods) can achieve a high surface hardness, in practice they often exhibit poor performance under mechanical loading, since the coatings are usually too thin to protect the substrate from the contact conditions. In the work reported here, a plasma electrolysis technique known as micro-arc discharge oxidation (MDO) was investigated; thick and hard oxide ceramic layers were fabricated on BS Al-6082 aluminium alloy by this method. The phase composition and microstructure of the MDO coatings were investigated by XRD, SEM and EDX analyses. A number of adhesion and tribological sliding and impact wear tests were also performed. It was found that Al–Si–O coatings with a hardness of up to 2400 HV and with excellent wear resistance and load support could be formed. The thickness of the coatings significantly influenced the mechanical properties. In terms of tribological performance, the thicker coatings performed best in sliding, scratch and impact tests whilst thin coatings were also surprisingly effective in both impact and low-load sliding. Coatings of intermediate thickness provided relatively poor performance in all tribological tests.
276 citations
TL;DR: In this paper, phase formation in oxide ceramic coatings on aluminium alloys during plasma electrolytical oxidising has been studied based on a model that considers two mechanisms of oxide formation: electrochemical surface oxidation and plasma chemical oxide synthesis in the discharge channels.
Abstract: Phase formation in oxide ceramic coatings on aluminium alloys during plasma electrolytical oxidising has been studied. The theoretical interpretation was based on a model that considers two mechanisms of oxide formation: electrochemical surface oxidation and plasma chemical oxide synthesis in the discharge channels. Thermodynamic calculations were carried out for both the formation of reaction products, as well as heating and cooling of the discharge channel. The divergence of the calculated and experimental results was estimated to be less than 20%.
229 citations