The tribological performance of ultra-hard ceramic composite coatings obtained through microarc oxidation
TL;DR: In this paper, a dense ceramic oxide coating approximately 100 mm thick was prepared on a 7075 Al alloy by microarc oxidation in an alkali-silicate electrolytic solution.
Abstract: A dense ceramic oxide coating approximately 100 mm thick was prepared on a 7075 Al alloy by microarc oxidation in an alkali-silicate electrolytic solution. Coating thickness and surface roughness (R ) were measured during coating formation. The a influence of current density, electrolyte temperature and inter-electrode distance on coating kinetics was investigated. Microstructure and phase compositions were analysed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The microhardness of the coating was also measured. The tribological performance of the coatings was evaluated using a dry sand abrasion test, a solid particle erosion test and a pin-on-disc sliding wear test. In addition, the results are compared to detonation-sprayed alumina (Al O ) coating and bulk Al O . The basic mechanism of microarc coating formation is explained. The material removal 23 2 3 mechanism during solid particle erosion was investigated, and structure–property correlations were established. 2002 Elsevier Science B.V. All rights reserved.
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.
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.
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.
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.
TL;DR: In this paper, optical emission spectroscopy, fast video imaging and coating characterization are employed to investigate AC plasma electrolytic oxidation (PEO) of magnesium alloys and reveal initiation and gradual increase in the number of discharges after 2-4ms of each anodic pulse once a critical voltage was reached.
Abstract: Optical emission spectroscopy, fast video imaging and coating characterization are employed to investigate AC plasma electrolytic oxidation (PEO) of magnesium alloys The findings revealed initiation and gradual increase in the number of discharges after 2–4 ms of each anodic pulse once a critical voltage was reached No discharges were observed during the cathodic half-cycles The lifetimes of discharges were in the range of 005–4 ms A transition in the voltage-time response, accompanied by a change in the acoustic and optical emission characteristics of discharges, was associated with the development of an intermediate coating layer with an average hardness of 270–450 HV 005 The coatings grew at a rate in the range 40–75 µm min − 1 , depending on the substrate composition Regardless of the substrate, the coatings consisted of MgO and Mg 2 SiO 4 , with incorporation of alloying element species Electrolyte species were mainly present in a more porous layer at the coating surface, constituting 20–40% of the coating thickness A thin barrier layer consisting of polycrystalline MgO was located next to the alloy The corrosion rate of the magnesium alloys determined using potentiodynamic polarization in 35 wt% NaCl was reduced by 2–4 orders of magnitude by the PEO treatment
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.
TL;DR: In this article, the properties of oxide films formed on a Ti-6Al-4V alloy by AC plasma electrolytic oxidation (PEO) in aqueous solutions containing aluminate, phosphate, silicate and sulfate anions and some of their combinations are studied by SEM, XRD and microhardness analyses, and by scratch, impact, pin-on-disc friction and potentiodynamic corrosion testing.
Abstract: The paper discusses processing and property aspects of oxide films formed on a Ti–6Al–4V alloy by AC plasma electrolytic oxidation (PEO) in aqueous solutions containing aluminate, phosphate, silicate and sulfate anions and some of their combinations. Structure, composition, mechanical tribological and corrosion resistant characteristics of the films formed are studied by SEM, XRD and microhardness analyses, and by scratch, impact, pin-on-disc friction and potentiodynamic corrosion testing. It is found that the films produced from the aluminate–phosphate electrolyte are dense and uniform and are composed mainly of Al 2 TiO 5 and TiO 2 phases of the rutile form. The films possess a beneficial combination of 50–60 μm thickness, 575 kg/mm 2 hardness and high adhesion and provide a low wear rate (3.4×10 −8 mm 3 /Nm) but a relatively high friction coefficient of μ=0.6–0.7 against steel, caused by material transfer from the counterface. A minimum friction coefficient of μ=0.18 is recorded during the testing of softer rutile–anatase films, 7 μm thick, produced from a phosphate electrolyte. Both of these types of film show good corrosion resistance in NaCl and physiological solutions, where the corrosion current is approximately 1.5 orders of magnitude lower than that of the uncoated substrate. SiO 2 /TiO 2 -based films with 70–90 μm thickness and high bulk porosity produced from silicate and silicate–aluminate electrolytes demonstrate better corrosion behaviour in H 2 SO 4 solution, due to the greater chemical stability of the film phase components in this environment.
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.
TL;DR: In this article, the phase distribution for ceramic coatings formed by microarc oxidation (MAO) on 2024 aluminum alloy was investigated using X-ray diffraction, and the results showed that the ceramic coating mainly consisted of α-Al2O3 and γ-Al 2O3 phases.
Abstract: The phase distribution for ceramic coatings formed by microarc oxidation (MAO) on 2024 aluminum alloy was investigated using X-ray diffraction. The results showed that the ceramic coatings mainly consisted of α-Al2O3 and γ-Al2O3 phases. The percentage of α-Al2O3 gradually increased from the external surface to the interface between the coating and the substrate of samples. The surface layer of coatings mainly contained the γ-Al2O3 phase, and its fraction of the composition remained almost constant with oxidation time. It is believed that the difference in the amounts of α-Al2O3 and γ-Al2O3 phases in the different layers of coatings was caused by the various cooling rates of molten Al2O3, which temporarily existed in the microarc zone.
TL;DR: In this article, the problem of technical and economical optimization of the process of microarc discharge oxidation of high-strength aluminium for the fabrication of oxide ceramic layers for tribotechnical purposes is considered in terms of experimental design.
Abstract: The problem of technical and economical optimization of the process of micro-arc discharge oxidation of high-strength aluminium for the fabrication of oxide ceramic layers for tribotechnical purposes is considered in terms of experimental design. To estimate the effectiveness of the process, a generalized parameter is used which accounts for oxide mass yield as a principal parameter, and mechanical and geometrical characteristics of the layer as restricting parameters. The methods of chemical weight, scanning electron microscopy, optical and durometric analyses are used. The influence of the silicate–alkali electrolyte composition and the amount of electricity carried through the cell on the layer properties is discussed. The response surface of the generalized parameter is plotted with the aid of desirability functions. The area of regimes corresponding to 2–3 g l −1 KOH and 2–3 g l −1 Na 2 SiO 3 electrolyte composition and (2.50–3.33)×10 3 C m −2 of carried electricity is outlined for the most effective fabrication of uniform oxide layers with 165–190 μm thickness and 18–23 GPa hardness.