Coating Growth Rate Research Articles

Plasma electrolytic oxidation of binary Mg-Al and Mg-Zn alloys

In this study the effect of aluminum and zinc addition on the plasma electrolytic oxidation (PEO) behavior of pure magnesium was investigated. Firstly, binary alloys of Mg-Al and Mg-Zn containing atomically 2, 4 and 8% were produced via induction melting. These alloys were than coated in a silicate based electrolyte at 120min employing an asymmetrical AC power source with a frequency of 50Hz and characterized by scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD) and microhardness tester. Obtained results indicated that compared to PEO of pure magnesium, addition of aluminum into magnesium resulted in a wavy substrate/coating interface, retarded the growth rate and increased microhardness. The addition of zinc into magnesium also retarded the coating growth rate, though not as significantly as aluminum; however, bright precipitates as a continuous part of the substrate were observed in the compact region of the coatings around which oxidation was hindered. No extra phases other than Forsterite and Periclase were found according to the XRD results although some pits in the inner region which might be a mixture of MgO-Al2O3 were detected by cross-sectional SEM-EDS studies. SEM-EDS results also showed that aluminum was incorporated both in outer and inner region of the coatings while zinc was only detected on the regions consisted of bright precipitates in the inner region of the coatings.

Effect of bias potential on the structure and distribution of elements in titanium-nitride coatings obtained by cathodic-arc deposition

It is shown that a change in the pulsed bias potential in the process of the cathodic-arc deposition of titanium nitride has a significant effect on the structure and composition of coatings, as well as on the quantity of the microdroplet fraction. A bias potential of above 50 V leads to a decrease in the coating growth rate, which is related to resputtering of the coating by ions accelerated from the plasma.

Effect of NaAlO2 concentrations on the properties of micro-arc oxidation coatings on pure titanium

TiO2 coating was deposited on a pure titanium substrate using a micro-arc oxidation (MAO) process in an electrolyte solution with the NaAlO2 addition varying from 0 to 0. 4g/l. With increasing NaAlO2 concentration, the working voltage could effectively increase and promoted the growth rate of MAO coatings on pure titanium. The X-ray diffraction (XRD) results indicated that MAO coatings mainly consisted of TiO2(B), rutile, and anatase. The surfaces of the MAO coatings which were formed on pure titanium in the electrolyte without NaAlO2 mainly consisted of micro-pores. The number of the sintered disc and the discharge channel diameter increased with increasing NaAlO2 concentration. The corrosion rates of MAO coatings decreased at the first and then increased.

Prediction Model of Coating Growth Rate for Varied Dip-Angle Spraying Based on Gaussian Sum Model

In automatic spraying of spray painting robot, in order to solve the problems of coating growth rate modeling for varied dip-angle spraying technology, a prediction mode of coating growth rate using the Gaussian sum model is proposed. Based on the Gaussian sum model, a theoretical model for coating growth rate with varied dip-angle spraying is established by using the theory of differential geometry. The coating thickness of the sample points in the distribution range of the coating was obtained by making the experiment of varied dip-angle spraying. Based on the theoretical model, the nonlinear least square method is used to fit the coating thickness of the sample points and the parameter values of the theoretical model are calculated. By analyzing the variation law of the parameters with the spray dip-angle, the prediction model of coating growth rate for varied dip-angle spraying is established. Experiments have shown that the prediction model has good fitting precision; it can satisfy the real-time requirement with varied dip-angle spraying trajectory planning in the offline programming system.

Energy and substance transfer in magnetron sputtering systems with liquid-phase target

The regularities of energy and matter transfer in the magnetron sputtering systems (MSS) with liquid-phase targets were analyzed. For this purpose a mathematical model of heat and erosion processes in a thermally insulated target of MSS has been developed and the problem of energy balance in a substrate and its heating during coating deposition was solved.The model is based on the fact that the flow of atoms from the surface of the liquid-phase target under the action of plasma consists of two independent components: sputtered and vaporized particles. The reliability of this model is confirmed by a good agreement between the calculated and experimental data.Using the model developed the regularities of the rate of the target surface erosion and metal coatings deposition were revealed. We investigated the effect of the ion current power density and material properties of the cathode. The calculations according to this model and experiments show the growth rate of metal coatings reaches from 100 to 1000 nm/s when intense evaporation occurs. It is one or two orders of magnitude higher than the rate of coating deposition when using solid target. The results can be useful for the development of technologies of high-rate coating deposition.

Magnetron deposition of coatings with evaporation of the target

We analyze the potentialities of the plasma in various types of magnetron sputtering systems including pulsed and liquid-target systems for producing intense emission of atoms and high-rate deposition of coatings. For this purpose, a mathematical model of thermal and erosion processes in the target is developed based on the heat conduction equations taking into account first-order phase transitions. Using this model, we determine the parameters of magnetrons for which intense evaporation of atoms from the target surface takes place. It is shown that evaporation leads to an increase in the growth rate of metal coatings by 1–2 orders of magnitude as compared to conventional magnetron systems based only on collisional sputtering.

PEO of rheocast A356 Al alloy: energy efficiency and corrosion properties

PEO coatings were developed under voltage‐controlled mode on a rheocast A356 aluminum alloy without and with pretreatment by conventional anodizing in sulphuric acid. Microstructure and composition of uncoated and coated materials were studied using SEM, EDS and XRD. The anodic precursor oxide layer preserves the microstructure of the alloy because of the presence of secondary phases having a different behavior relative to the matrix during conventional anodizing. After PEO treatment, the coatings consisted of a mixture of α‐Al2O3, γ‐Al2O3, SiO2 and mullite (3Al2O3·2SiO2). The effect of pre‐anodizing on energy efficiency was investigated revealing considerable energy savings with the precursor porous films, because of the faster establishment of the so‐called ‘soft sparking’ regime that promotes faster coating growth rate. The electrochemical corrosion properties were evaluated for uncoated and coated materials using electrochemical techniques (potentiodynamic curves and electrochemical impedance spectroscopy). PEO coated materials showed remarkably reduced susceptibility to corrosion. Copyright © 2015 John Wiley & Sons, Ltd.

Investigation of micro-arc oxidation coating growth patterns of aluminum alloy by two-step oxidation method

The micro-arc oxidation (MAO) process of 2A70 aluminum alloy in different electrolytes containing silicate or phosphate by two-step oxidation was investigated. The growth rate, surface roughness and element distributions of the micro-arc oxidation coatings were characterized. The results show that the coating growth patterns are different in Si and P electrolytes although they both grow inward. The discharge channels formed in Si-electrolyte and P-electrolyte are trumpet-shaped and dumbbell-shaped, respectively. The trumpet-shaped channel is conducive to gather heat and extend the reaction area inward, which leads to a layer growth pattern in P-electrolyte. The dumbbell-shaped channel limits the reactive area but introduces more electrolytes during the cooling process, which results in an island growth pattern in Si-electrolyte. Therefore, a newly formed P-rich layer can be observed between substrate and initially formed Si-rich layer after the second-step oxidation in P-electrolyte, but only localized Si-rich areas could be found when the second-step oxidation was performed in Si-electrolyte.

Effects of electrolytes on the growth behavior, microstructure and tribological properties of plasma electrolytic oxidation coatings on a ZA27 alloy

In this study, plasma electrolytic oxidation (PEO) was successfully applied on a ZA27 alloy to produce continuous and dense ceramic coatings. The effects of the electrolytes (silicate, aluminate and aluminate/borate composite solutions) on the coating growth behavior, microstructure, phase compositions, and tribological properties were investigated. ZnAl2O4, Al2O3, and CuAlO2 can be found in all the three coatings. In addition, the coating produced with silicate solution contains ZnSiO4. During the process of coating, the Al element preferred to react with electrolytes to form the ceramic coatings. The addition of borate solution can accelerate the coating growth rate and be beneficial to produce a denser coating. Coatings produced with silicate and aluminate/borate electrolyte have a structure with porous outlayer, dense layer, and inner barrier layer, whereas the coating produced with aluminate electrolyte only shows a dense monolayer structure. The coating produced with aluminate/borate composite electrolyte achieved the best wear performance, lowest friction coefficient, and highest wear resistance among the three coatings under both low and high normal loads.

High growth rate, wear resistant coatings on an Al–Cu–Li alloy by plasma electrolytic oxidation in concentrated aluminate electrolytes

Plasma electrolytic oxidation of a 2A97 Al–Cu–Li alloy under a pulsed bi-polar constant current regime has been carried out in electrolytes containing 5, 32 and 56gl−1 NaAlO2. Wear resistant coatings, containing relatively large amounts of α-Al2O3, were formed using concentrated solutions. The best protection was obtained with a relatively compact coating, generated with 32gl−1 NaAlO2. The coating growth rate increased from ~1.5 to 11.3μmmin−1 between 5 and 56gl−1 NaAlO2. The high rates indicate that non-Faradaic processes contribute to the coating formation. Increased concentrations of NaAlO2 reduced the energy required for coating formation; e.g. to the range ~1 to 5kWhm−2μm−1 for coatings formed with 32 and 56gl−1 NaAlO2.

Layered double hydroxide-based fire resistant coatings for flexible polyurethane foam

Montmorillonite clay (MMT) is frequently used to create fire resistance in Layer-by-Layer (LbL) applied coatings. This manuscript reports that switching the MMT with layered double hydroxides (LDH) produced a more effective fire resistant coating on flexible foam. Most of the formulations required at most two trilayers (TL) to produce ignition resistant and very low heat release rate polyurethane foam. The best coating (greatest flammability reduction with the least amount of coating) delivered a 41% reduction in peak heat release rate. This was a 2TL, 10% mass coating produced using a coating solution with a high LDH content. This manuscript discusses the impact of the LDH-based coating formulations and number of monolayers on the LbL coating growth rate, composition, and fire resistance.

Timescales and mechanisms of formation of amorphous silica coatings on fresh basalts at Kīlauea Volcano, Hawai'i

Young basalts from Kīlauea Volcano, Hawai'i, frequently feature opaque surface coatings, 1–80μm thick, composed of amorphous silica and Fe–Ti oxides. These coatings are the product of interaction of the basaltic surface with volcanically-derived acidic fluids. Previous workers have identified these coatings in a variety of contexts on Hawai'i, but the timescales of coating development, coating growth rates, and factors controlling lateral coating heterogeneity were largely unconstrained. We sampled and analyzed young lava flows (of varying ages, from hours to ~40years) along Kīlauea's southwest and east rift zones to characterize variation in silica coating properties across the landscape. Coating thickness varies as a function of flow age, flow surface type, and proximity to acid sources like local fissure vents and regional plumes emitted from Kīlauea Caldera and Pu'u Ō'ō. Silica coatings that form in immediate proximity to acid sources are more chemically pure than those forming in higher pH environments, which contain significant Al and Fe. Incipient siliceous alteration was observed on basalt surfaces as young as 8days old, but periods of a year or more are required to develop contiguous coatings with obvious opaque coloration. Inferred coating growth rates vary with environmental conditions but were typically 1–5μm/year. Coatings form preferentially on flow surfaces with glassy outer layers, such as spatter ramparts, volcanic bombs, and dense pahoehoe breakouts, due to glass strain weakening during cooling. Microtextural evidence suggests that the silica coatings form both by in situ dissolution–reprecipitation and by deposition of silica mobilized in solution. Thin films of water, acidified by contact with volcanic vapors, dissolved near-surface basalt, then precipitated amorphous silica in place, mobilizing more soluble cations. Additional silica was transported to and deposited on the surface by silica-bearing altering fluids derived from the basalt interior.

Fabrication of Chromium (III) Oxide (Cr2O3) Coating by Electrophoretic Deposition

Chromium (III) oxide has been widely used as a coating material for corrosion resistance. In this study, electrophoretic deposition (EPD) of nano chromium (III) oxide (Cr2O3) particle (60 nm) was investigated to develop coatings with potential applications of anticorrosive material. The stable suspension of Nano‐Cr2O3 particles were obtained in the mixture of acetylacetone and ethanol containing 0.00025 M nitric acid. The coating growth rate was studied with using different deposition times in the range of 1–30 min at voltages of 50–150 V with various concentrations of suspension. The electrophoretic Cr2O3 coating was sintered at 1000°C and 1200°C for 2 h. The micro‐morphology of coating was qualitatively characterized by focused ion beam scanning electron microscopy (FIB/SEM). The SEM micrographs obviously showed that the electrophoretic Cr2O3 coating has formed a uniform and dense ultrathin layer after sintering at 1200°C. We demonstrated that nano‐Cr2O3 coating could be easily obtained by EPD for the surface modification of metallic materials for potential interest in hard wear‐resistant and/or low‐friction coatings.

Coupled model of coating formation on a cylindrical substrate

A coupled model of coating formation on the surface of a part of a cylindrical shape during deposition from the plasma is proposed. This model takes into account the phenomena of thermal diffusion, diffusive thermal conductivity, and mass transfer under the action of the stress gradient, and the formation of chemical compounds. The coating growth rate is considered to be a given function of the particle velocity and particle concentration near the surface of the growing coating. The problem is solved numerically. It is shown that diffusion cross-fluxes, diffusive thermal conductivity, and thermal diffusion during the growth process reduce the width of the transition zone between the substrate and the coating. This effect becomes most essential if the substrate has a low thermal conductivity. Accounting for stresses arising in the coating-substrate system during the deposition process changes the effective transfer coefficients and significantly affects the result of modeling the distribution of chemical elements and their compounds in the coating.

Fabrication of cobalt oxide (Co3O4) coating by electrophoretic deposition

In this study, electrophoretic deposition (EPD) of nano-cobalt oxide (Co3O4) particle (30nm) was investigated to develop coatings with potential applications in many fields. Nano-Co3O4 particles were suspended in mixture of acetylacetone and ethanol and stable suspensions were obtained. The deposits were characterized qualitatively by X-ray diffraction, SEM observations and EDS analysis. The coating growth rate was varied by different deposition times in the range of 1–30min at voltages of 100–150V. Results of this study demonstrate that EPD is a feasible method of fabricating nano-Co3O4 coating which is of potential interest in gas sensors, heterogeneous catalysts, supercapacitors and many other applications.

Nb–Cr complex carbide coatings on AISI D2 steel produced by the TRD process

Nb–Cr complex carbide coatings were produced onto AISI D2 steel by the thermo-reactive diffusion process to improve the surface hardness and corrosion resistance of this tool steel. The carbide coating treatment was performed using molten borax added with ferroniobium, ferrochrome and aluminum at temperatures of 1,223, 1,293 and 1,363 K during 2, 3, 4 and 5 h. The coating layers were characterized by optical and Scanning Electron Microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy and X-ray fluorescence spectrometry. The coating growth rates were evaluated, and a model of the layer thickness as a function of the treatment time and temperature was established. The hardness of the coating was measured by nanoindentation, and its resistance to corrosion was evaluated with electrochemical tests of potentiodynamic polarization. The produced carbide layers presented a homogeneous thickness as well as an improved hardness and corrosion resistance as compared to the uncoated steel.

Effects of cathodic voltages on structure and wear resistance of plasma electrolytic oxidation coatings formed on aluminium alloy

Plasma electrolytic oxidation (PEO) coatings were prepared on aluminium alloy using pulsed bipolar power supply at constant anodic voltage and different cathodic voltages. The samples were prepared to attain the same coating thickness by adjusting the processing time. The scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and tribometer were employed to investigate the microstructure, element content, phase composition and wear resistance of the coatings respectively. It was found that the coating growth rate enhanced obviously and the coatings exhibited a more compact structure with thicker inner layer and lower surface roughness when the cathodic voltage increased. The coatings were mainly composed of crystalline γ-Al2O3 and amorphous silicate oxides and their relative content changed with the cathodic voltage. The wear resistance of the coatings improved significantly with the increase of cathodic voltage.

Vapor phase epitaxy of monocrystal tungsten coatings

Monocrystal tungsten coatings were obtained by vapor phase epitaxy in a W–WClx–Cl system using Mo single-crystal substrate. The kinetics of the deposition process was studied in a wider temperature and pressure range. As the total pressure was 15.77Pa, the surface kinetics controlled by the deposition process as the temperature was in the range of 1383–1503K. When the deposition temperature was increased up to 1573K, the control mechanism was mass transport limited. When the deposition temperature was maintained at 1673K and the total pressure was 15.77–25.23Pa, the deposition process was mass transport limited. When the total pressure was increased to 42.32Pa, the control mechanism of the deposition process became surface kinetics limited. By basic treatment, namely, ‘supply transport medium as required’, a kinetics model predicting the tungsten coating growth rate was achieved. In the case of lower pressure for the surface kinetics-limited regime, the deposition rate was proportional to pressure. When the pressure was high, the deposition rate was proportional to the half power of pressure. As the pressure increased further, the deposition rate and pressure became kinetically irrelevant. The model predictions were in good agreement with the experimental growth rates, as further proven by the surface morphology analysis of the monocrystal tungsten coatings.

Effect of Yttrium Ion on YSZ-Al<sub>2</sub>O<sub>3 </sub>Composite Coating Formed by PEO on a Cast Al-Si-Cu-Ni Alloy

Y2O3-ZrO2-Al2O3 (YSZ-Al2O3) composite ceramic coating was formed on a cast Al‑12Si-3Cu-2Ni alloy by plus AC plasma electrolytic oxidation (PEO) process, with yttrium ion concentrations varying from 0 to 0.5 g/l and process time from 0 to 45 min. The microstructures of the coatings were investigated by SEM and XRD. The results show that the PEO coating growth behaviour and microstructure were affected greatly by Y ion additions. At a given oxidation time, the coating growth rate accelerated and thickness increased in the PEO process when adding yttrium ions. When the content of Y ions is greater than the optimum value of 0.05 g/l, coating growth gradually decreases. In comparison with a ZrO2-Al2O3 coating, the amount of micro particle in YSZ-Al2O3 coating is less and the ceramic layer is repeatedly deposited by ceramic particles and good binding was achieved between the ceramic coating and the base alloy. The uniform and compact YSZ-Al2O3 composite coating is mainly composed of c-Y0.15Zr0.85O1.93 solid solution, m‑ZrO2, α-Al2O3, γ-Al2O3 and Y2O3 phases. The ZrO2 phase was well stabilized by solid solution containing yttrium element and Al2O3.

Effect of ultrasonic on microstructure and growth characteristics of micro-arc oxidation ceramic coatings on 6061 aluminum alloy

Non-uniform microstructure and low growth rate of ceramic coatings in the mid-to late period of micro-arc oxidation (MAO) are the problems that limit the development of micro-arc oxidation technology. In this paper, the MAO process on 6061 aluminum alloy was carried out under ultrasonic condition, and the effects of ultrasonic vibration on microstructure and growth characteristics of MAO coatings were investigated. The results showed that the ultrasonic action obviously promoted the formation of α-Al2O3 phase, reduced the number of large discharge pores and promoted the increase of the thickness, uniformity and hardness of the coatings. A concept, breakdown resistance, was defined to discuss the results conveniently.