Growth Kinetics Of Coatings Research Articles

Quantitative modeling of oxide growth in plasma electrolytic oxidation of titanium

Plasma electrolytic oxidation (PEO) has been extensively applied in the past to improve tribological properties of titanium and its alloys allowing their widespread use in industrial applications. However, the complete PEO mechanism of action remains unclear. While there are several mathematical models providing useful insights into the PEO process, they do not fully incorporate the effect of applied potential on the outcomes of PEO. In this work, we develop an experimentally informed computational model by expanding on the dielectric breakdown model. We incorporate experimental data to extract key parameters, exploring the iterative dependency of the oxide thickness and ionic current density to predict the oxide growth at various applied external voltages. The model reproduces the PEO process at different potentiostatic conditions and the simulation results for the oxide coating thickness over time is in good agreement with the experimental data, validating the application of Faraday's law and Pyun and Hong's model in describing the coating growth kinetics for the PEO process (Dehnavi et al., 2014/07/25/ 2014; Arrabal et al., 2009/05/30/ 2009; Rakoch et al., 2006/03/01 2006; Ma et al., 2017/10/10/ 2017; Pyun and Hong, 1992/02/01/ 1992; Caire et al., 2007/02/07 2007) [1-6].

Influence of carbon nanotubes on the plasma electrolytic oxidation process of aluminum under “soft” sparking conditions

Plasma electrolytic oxidation of aluminum (PEO) was performed with dispersed multi-walled carbon nanotubes (MWCNTs) into the electrolyte. The effect of the carbon nanotubes concentration, as well as that of the electrical processing conditions, on the PEO process was investigated. From the point of view of the electrical response of the process, results show that the “soft” sparking regime appears earlier in presence of MWCNTs. The process time at which the transition to the “soft” regime occurs decreases linearly with the increase in the MWCNTs concentration. Over a concentration threshold, depending on the applied pulse bipolar current waveform, the PEO process is inhibited. The dispersion of MWCNTs in the electrolyte results in thicker but more porous alumina layers. More specifically, the proportion of the α-alumina phase relative to the γ-alumina phase decreases with the increase in the MWCNTs concentration. Results are explained by considering the increased coating growth kinetics and lower heat accumulation from MWCNTs addition. Finally, the present study gives new insights into the right management of the PEO process under “soft” sparking regime with dispersed carbon-based nanoparticles in the electrolyte.

Development and screening of (Ca-P-Si-F)-PEO coatings for biodegradability control of Mg-Zn-Ca alloys

• Transparent electrolytes containing bioactive agents were developed for PEO of MgZn x Ca y alloys. • AC PEO on MgZnCa and Mg3Zn0.4Ca cast alloys showed a linear coating growth kinetics. • Permeability and insufficient stability of the barrier layer allows undercoating corrosion. • No influence of organic components of α-MEM on Mg alloy/PEO corrosion resistance. PEO coatings were developed on MgZn x Ca y alloys for biomedical applications using different treatment times and a novel transparent electrolyte containing Ca, P, Si and F. Surface characteristics and chemical composition of studied materials were evaluated using scanning electron microscopy, X-ray diffraction and optical metrology. Corrosion investigations were performed for uncoated and coated materials using potentiodynamic polarization, electrochemical impedance spectroscopy and hydrogen evolution measurements in three media (0.9wt.% NaCl, α-MEM and inorganic part of α-MEM). Findings revealed that Zn-rich intermetallics favour the formation of large voids in the coatings. Ca, P, F and Si are successfully incorporated into the coatings. Coatings consist of crystalline phases (MgO, MgF 2 and Ca,Si-rich compounds) and amorphous material. Aminoacids and organic components of α-MEM solution accelerate corrosion of the bare substrates but have no significant influence on the corrosion resistance of coated specimens. Therefore, inorganic α-MEM can be considered as an appropriate medium for fast screening of PEO coatings. The relatively small amount of F content in the barrier layer facilitates early failure of the coatings, which show undercoating corrosion. Zn-rich intermetallic particles in the undercoating crevice induce a more aggressive corrosive environment and enhanced micro-galvanic couple effect with the Mg matrix. This leads to faster degradation of the coated alloys compared to the bare substrates.

Formation Mechanism of Aluminide Diffusion Coatings on Ti and Ti-6Al-4V Alloy at the Early Stages of Deposition by Pack Cementation.

The diffusion coatings were deposited on commercially pure Ti and Ti-6Al-4V alloy at up to 1000 °C for up to 10 h using the pack cementation method. The pack powders consisted of 4 wt% Al (Al reservoir) and 4 wt% NH4Cl (activator) which were balanced with Al2O3 (inert filler). The growth kinetics of coatings were gravimetrically measured by a high precision balance. The aluminised specimens were characterised by means of scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). At the early stages of deposition, a TiO2 (rutile) scale, other than aluminide coating, was developed on both materials at <900 °C. As the experimental temperature arose above 900 °C, the rutile layer became unstable and reduced to the low oxidation state of Ti oxides. When the temperature increased to 1000 °C, the TiO2 scale dissociated almost completely and the aluminide coating began to develop. After a triple-layered coating was generated, the coating growth was governed by the outward migration of Ti species from the substrates and obeyed the parabolic law. The coating formed consisted of an outer layer of Al3Ti, a mid-layer of Al2Ti and an inner layer of AlTi. The outer layer of Al3Ti dominated the thickness of the aluminide coating.

Microstructure formation of silicide coating on Nb–W–Mo–Zr alloy

ABSTRACTSilicide coatings were deposited on the Nb–5W–2.5Mo–1Zr alloy by pack cementation method. The microstructure, phase distribution, growth kinetics and oxidation behaviour of the coatings were studied. The results show that the coatings were mainly composed of an outer NbSi2 layer, an acicular Nb5Si3-dispersed NbSi2 two-phase layer and a Nb5Si3 inner layer, exhibiting a three-layer structure. The temperature and holding time both have a significant effect on the coating formation. It was found that the coating growth kinetics at 1250°C followed a parabolic law. The coating exhibited a relatively good short-term protection to the substrate in static air at 850 and 1250°C. Both scales formed at the two oxidation temperatures consisted mainly of a mixture of SiO2, Nb2O5 and NbO.

COMBINED TECHNOLOGY OF ELECTRO-SPARK ALLOYING AND THERMAL DIFFUSION BORIDING OF STEEL

The process of thermal diffusion boriding is well known and used throughout the world. It has a number of unique properties, such as high hardness and wear resistance under abrasive wear conditions. Continuous boride layers have increased brittleness, which is related with the elastic properties of borides. In this work, we studied the features of the formation of surface layers onlow-carbon steels after electro-spark alloying, and not by continuous processing of the entire surface of the steel, but only sections and subsequent thermal diffusion boriding from powder medium. Such complex technology enables to form discrete regular and irregular layers, which have a number of advantages, in particular, reduced brittleness. The structure and phase composition of the combined coating growth kinetics of the diffusion layer during the thermal diffusion boriding, are investigated. The creation of the considered composite layers with FeB and Fe2B phases with reduced brittleness will significantly expand their area of application, for example, for working conditions with moderate impact loads.

Effect of operating voltage on microstructure and microhardness of NiCoCrAlYTa-Y2O3 composite coatings on single crystal superalloy produced by electrospark deposition

NiCoCrAlYTa-Y2O3 metal matrix composite coatings were prepared on single crystal superalloy by electrospark deposition (ESD) to improve the wear- and oxidation resistance. The effect of operating voltage during ESD on the deposit morphology, coating growth kinetics, microstructure and properties was investigated. The results showed that, with increasing operating voltage, the dimension (both diameter and depth) of single-pulse deposit increased, leading to thinner deposit thickness, thus resulting in higher cooling rate, finer grains, stronger crystal orientation, and increasing microhardness. Whereas the motion of the molten deposit metal became more turbulent, giving rise to the defect of lack of fusion. However, the grain size increased obviously from bottom layer to top layer for 100 V condition, leading to decreasing microhardness. Meanwhile, the mass gain rate increased but the mass transfer coefficient decreased with the increase of operating voltage. The distribution pattern of Y2O3 particles changed during electrospark deposition, from gathering in the boundary between the powders in the electrode to distributing homogeneously in the whole coating, which contributed to the increase of coating microhardness.

Microstructure and growth kinetics of silicide coatings for TiAl alloy

In order to improve the oxidation resistance of TiAl alloy, silicide coatings were prepared by pack cementation method at 1273, 1323, and 1373K for 1-3 hours. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) were employed to investigate the microstructures and phase constitutions of the coatings. The experimental results show that all silicon deposition coatings have multi-layer structure. The microstructure and composition of silicide coatings strongly depend on siliconizing temperatures. In order to investigate the rate controlling step of pack siliconizing on TiAl alloy, coating growth kinetics was analyzed by measuring the mass gains per unit area of silicided samples as a function of time and temperature. The results showed that the rate controlling step was gas-phase diffusion step and the growth rate constant (k) ranged from 1.53 mg2/(cm4·h2) to 2.3 mg2/(cm4·h2). Activation energy (Q) for the process was calculated as 109 kJ/mol, determined by Arrhenius' equation: k = k 0 exp[–Q/(RT)].

The Influence of Cutting Metal Elements on the Structure and Corrosion Resistance of the Zn Coating

The aim of the research was the indication of the causes of a difference in thickness of the zinc coating on the surface shaped by plasma cutting. The research was focused on fittings for overhead power lines - a double eyes links made of steel S355J2. The prepared materials were cut by plasma. Then, links were subjected to mechanical treatment (shot-blasting) and chemical treatment (pickling, rinsing and fluxing). The hot-dip galvanizing process was performed in industrial conditions by applying the constant temperature equal to 457°C, and a dipping time equal to 150 s. The research was conducted on the link’s flat surface (after rolling) and the side surface (after cutting). The (Zn) - coating morphologies and sub-layer thicknesses were evaluated on the basis of metallographic analysis and corrosion tests (acc. to EN ISO 9227). The correlation between the results of corrosion test and coatings morphology was determined.The observed changes are closely related to the outer layer steel structure (HAZ) which subsequently influences the mechanism and zinc coating growth kinetics

The Influence of the Chemical Composition of a Zinc Bath on the Growth Kinetics and Structure of Coatings Obtained on B500SP Steel

In the paper the results of tests conducted to determine the synergistic influence of alloy additions to a zinc bath upon the structure and growth kinetics of coatings obtained on reinforcing steel are presented. The structure and the chemical composition of coatings obtained on B500SP steel have been developed. The tests were carried out in Zn-AlNi, Zn-AlNiBi and Zn-AlNiBiSn baths. It has been determined that the additions of Bi and Sn reduce the coating thickness on B500SP reinforcing steel. The growth kinetics of coatings on the tested reinforcing steel - even though the concentration of Si was within the Sebisty range - did not show reversed temperature dependence, which is characteristic of this type of steel.

The Influence of the Thickness of a Rebar on the Growth Kinetics and Structure of Hot Dip Zinc Coatings

In the paper the results of tests on obtaining zinc coatings on reinforcing steel are presented. The coatings were obtained in a zinc bath on B500SP rebars of various diameters. The structure of coatings obtained on rebars with the diameter of 12, 16, 20 and 32 mm has been developed. The chemical composition of structural components of the coating has been established. The growth kinetics of coatings obtained in the traditional galvanizing process at the temperature of 450°C has been established and the impact of the rebar diameter upon the coating growth curve has been defined. The obtained coatings are continuous and they are quite thick. Moreover, it has been determined that an increase in the rebar thickness induces an increase in the coating thickness.

A comparison of plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte

Plasma electrolytic oxidation of Ti-6Al-4V and Zircaloy-2 alloys in a silicate-hexametaphosphate electrolyte has been studied in detail by various methods, including scanning electron microscopy, transmission electron microscopy and electrochemical techniques. Different coating growth kinetics, formation efficiencies and microstructures have been revealed for the two alloys. From these results, it is suggested that the insulating nature of the coating on Zircaloy-2 promotes the formation of pancake structures, which are formed by strong discharges. In contrast, the semiconducting properties of the coating on the Ti-6Al-4V alloy promote oxygen evolution, reduce the efficiency of coating growth and favour surface or shallow discharges that result in “coral reef” coating features. Anomalous gas evolution, which is more abundant with Zircaloy-2, was observed on both alloys after the appearance of stronger discharges. However, the current density used for the oxygen generation on Zircaloy-2 was still less than that of the Ti-6Al-4V alloy.

The Structure of Coatings Obtained in a ZnAl23Si Bath by the Batch Hot Dip Method

In the paper the results of tests on obtaining ZnAl23Si coatings on low-silicon steel by use of the batch hot dip method have been presented. The growth kinetics of coatings obtained in a ZnAl23 bath with the content of 1% and 2% of Si has been defined. The structure has been developed, the chemical composition of particular structural components of the coating and its phase composition have been established. It has been determined that coatings obtained in the ZnAl23Si bath are continuous and they have uniform thickness. The presence of silicon in the bath allows to reduce excessive coating thickness. The coating is composed of an external layer which is formed by the bath components and of a diffusion layer of the intermetallic FeAl3phase.

The Influence of Si Content in Steel on Growth Kinetics and Structure of Hot Dip ZnAl23Si Coatings

In the paper the results of tests on obtaining coatings in a ZnAl bath of eutoctoid composition on steel with various contents of silicon by use of the batch hot dip method are presented. The growth kinetics of coatings obtained in a ZnAl23 bath with the addition of 1% and 2% of Si on steel with low silicon content (0.01% Si), on Sandelin steel (0.035% Si) and on high-silicon steel (0.27% Si) have been defined. The structure of coatings on tested steels was developed. It has been established that coatings obtained on the Sandelin steel and on high-silicon steel have similar thickness and structure as compared to coatings obtained on low-silicon steel.

Kinetics of Growth and Structure of Coatings Obtained on Sandelin Steels in the High-Temperature Galvanizing Process

In the paper the author presents the results of tests defining the characteristics of behaviour of Sandelin steel in the high-temperature galvanizing process. The growth kinetics of hot-dip zinc coatings on the substrate of 0.05% Si steel in the temperature range of 540-580°C has been established. The structure of the coatings and their phase composition have been developed and the chemical composition of structural components of the coating has be defined. It has been determined that the coating is composed of a compact layer δ1 and an area of a two-phase mixture of δ1 and Zn. The conducted tests confirmed the presence of phase Γ1 , which does not form a continuous layer but it forms individual precipitates which are irregular in shape. The growth kinetics of the coating indicates that an increase in temperature causes a decrease in the coating thickness, which might prove that dissolving processes prevailed over the processes of diffuse coating growth.

The Structure of Coatings Obtained in the Zn-31Al-3Mg Bath on High-Silicon Steel

Coatings obtained in Zn-Al-Mg baths on steel plates by using the continuous method provide effective protection against corrosion. They have greater corrosion resistance than traditional zinc coatings. Even though Zn-Al-Mg coatings have many advantages, they are not made on galvanized products by use of the batch hot dip method. For products galvanized by use of the batch hot dip method coating growth mainly depends on the content of silicon in the steel. An increase in coating thickness is particularly intense in the case of high-silicon steel. In the paper the results of tests on obtaining Zn-Al-Mg coatings on high-silicon steel by use of the batch hot dip method have been presented. The growth kinetics of coatings obtained in the Zn-31Al-3Mg bath on steel with Si content of 0.27% has been defined. The structure has been developed and the chemical composition of particular structural components of the coating has be established. It has been determined that coatings obtained in the Zn-31Al-3Mg bath are continuous and they have uniform thickness. The structure of the coating is compact and the chemical composition points to the presence of phases of the Fe-Al system. The presence of silicon in the steel limits steel reactivity in the tested bath. The course of growth kinetics of the coating is close to parabolic.

Synergistic Influence of the Addition of Al, Ni and Pb to a Zinc Bath upon Growth Kinetics and Structure of Coatings

In this paper the current knowledge about the influence of alloy additions used in galvanizing baths has been analysed. The optimum concentration of Al, Ni and Pb addition has been established. Tests have been conducted to determine the synergistic influence of the addition of AlNiPb in a zinc bath upon the structure and growth kinetics of coatings. The structure and the chemical composition of coatings obtained on steel with low silicon contents and on Sandelin steel have been developed. It has been determined that the synergistic influence of the AlNiPb addition effectively lowers the reactivity of Sandelin steel and it improves zincs flowing off the product surface which results in decreased zinc consumption and better appearance of the coating. The structure of coatings obtained in a Zn-AlNiPb bath on Sandelin steel is similar to the structure of coatings obtained on low-silicon steel.

The Structure and Growth Kinetics of Zinc Coatings on Link Chains Produced of the 23MnNiCrMo5-2 Steel

In the paper the results of tests on obtaining zinc coatings on chains produced of the 23MnNiCrMo5-2 steel of grades C and PW9 have been presented. The growth kinetics of coatings obtained in the traditional galvanizing process at the temperature of 450°C has been established. The structure of coatings obtained on chains of various grades has been developed. The chemical composition of structural components of the coating has be established. It has been determined that the obtained coatings are continuous and they are quite thick. No significant impact of the chain grade upon the structure of coating has been established, however, it has been observed that the thickness of coating slightly decreases as the grade increases.

Wear-resistant coatings formed on Zircaloy-2 by plasma electrolytic oxidation in sodium aluminate electrolytes

Plasma electrolytic oxidation of Zircaloy-2 has been investigated in dilute and concentrated aluminate electrolytes, under a pulsed-bipolar current regime, in order to develop coatings of high wear resistance. Coating growth kinetics, cell potential-time responses and discharging behaviours depend significantly on the electrolyte concentration. The coatings formed in dilute aluminate electrolyte reveal a three-layered structure, with pancake structures at the coating surfaces. “Soft sparks” occur during PEO in dilute aluminate electrolyte, causing a relatively fast growth of the inner layer and resulting in a large amount of alumina-enriched material beneath the pancake structures, and hence an increased wear resistance of the coating. In contrast, more homogenous coatings, free of pancakes, result with the concentrated electrolyte. The main phase in the coatings is t-ZrO2, with γ-Al2O3 also present in coatings formed in the latter electrolyte. The coatings formed in the concentrated electrolyte display a high wear resistance, even for thin coatings formed for short times, which is attributed to the relatively high alumina content of the coatings.

FORMATION OF Ge-Y MODIFIED SILICIDE COATINGS ON Nb-Ti-Si BASE ULTRAHIGH TEMPERATURE ALLOY

Nb-Si base ultrahigh temperature alloy is a promising candidate for high tempera- ture applications due to its high melting point, high strength and relatively low density at elevated temperatures. However, the poor oxidation resistance of this alloy has still be a major barrier to its high temperature applications. Ge and Y modified silicide coatings on an Nb-Ti-Si based ultrahigh temperature alloy have been prepared successfully by co-depositing Si, Ge and Y in order to improve its high temperature oxidation resistance. The structure, constituent phases and formation process of the coatings have been revealed using XRD, SEM and EDS. The results showed that the coatings had obvious layered structure which was composed of a (Nb, X)(Si, Ge)2 (X represents Ti, Cr and Hf elements) outer layer and a (Ti, Nb)5(Si, Ge)4 inner layer. The concentration of Ge in the coating increased with increase in the distance away from the coating surface. Compared with the single Y modified silicide coating, the addition of Ge did not change the coating structrue, but reduced the absorption and deposition of Si and retarded the coating growth. The coatings prepared at 1250 for different time (0—8 h) had similar structures and the coating growth kinetics followed a parabolic law.