Electrodeposition of rhenium-base layer as a diffusion barrier between the NiCoCrAlY coating and a Ni-based superalloy

Accepted Manuscript Electrodeposition of rhenium-base layer as a diffusion barrier between the NiCoCrAlY coating and a Ni-based superalloy Reza Ghasemi, Zia Valefi PII:

S0925-8388(17)33666-6

DOI:

10.1016/j.jallcom.2017.10.230

Reference:

JALCOM 43627

To appear in:

Journal of Alloys and Compounds

Received Date: 26 July 2017 Revised Date:

6 October 2017

Accepted Date: 17 October 2017

Please cite this article as: R. Ghasemi, Z. Valefi, Electrodeposition of rhenium-base layer as a diffusion barrier between the NiCoCrAlY coating and a Ni-based superalloy, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.10.230. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Electrodeposition of Rhenium-Base layer as a Diffusion Barrier between the NiCoCrAlY Coating and a Ni-based Superalloy Reza Ghasemi1*, Zia Valefi1 1

Metallic Materials Research Center, Malek Ashtar University of Technology, Tehran, Iran

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* Corresponding author: E-mail address: [email protected] and [email protected] Tel.: +98-021-22923646; fax: +98-021-22923647

Abstract

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The main goal of this study was to evaluate the microstructure and capability of the diffusion barrier Re-base layer between the NiCoCrAlY coating and a Ni-based superalloy. To this end, a

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duplex coating system consisting of the Re-base layer and NiCoCrAlY coating were deposited by the electroplating method and High Velocity Oxygen Fuel (HVOF) spraying, respectively. The mechanism of the deposition process and the effect of deposition time and surface preparation and cleaning on the Faradaic Efficiency (FE), surface morphology, thickness and quality of the

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Re-base diffusion barrier were studied. Also, the inter-diffusion behaviour of the substrateNiCoCrAlY coating with and without the diffusion barrier was investigated by isothermal oxidation at 1000 ˚C. The microstructural features and the diffusion barrier behaviour of the Re-

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base layer were evaluated by a Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS). Criteria related to the properties of diffusion barrier layer based

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on to increase the Faradaic efficiency and the rhenium content in the coating were evaluated. Isothermal oxidation test results showed that the diffusion barrier coating could inhibit the interdiffusion of the elements of the superalloy substrate and significantly decrease the growth rate of the inner β-NiAl depletion zone. Keywords: Diffusion Barrier, Inter-diffusion, Microstructure, Rhenium, NiCoCrAlY

ACCEPTED MANUSCRIPT 1. Introduction The MCrAlY (M = Ni and/or Co) coating has been widely used for the protection of superalloy components operating at high temperatures, such as those in the hot sections of gas turbines

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subject to oxidation and hot corrosion [1–3]. The protection of these coatings against oxidation and hot corrosion can be accomplished by forming and maintaining a slowly-growing and thermally-adherent grown oxide (TGO) on the surface coating [4,5]. However, with prolonging

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the service time, the degradation of MCrAlY coatings occurs owing to the depletion of

aluminium content in the coating; this is not only due to the continuous formation and spallation

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of the alumina surface layer, but also the inter-diffusion between the coatings and the substrate [6–8]. The inter-diffusion of the alloying elements between the superalloy substrate and the coating can cause serious problems. First of all, the inward diffusion of aluminium from the MCrAlY coating, together with the outward diffusion of refractory elements from the superalloy substrate, may lead to the formation of the massive needle topologically close-packed phase

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(TCP) and the secondary reaction zone (SRZ). The formation of the TCP phase and SRZ deteriorate mechanical properties such as creep and the fatigue life time of the superalloy [9–11]. As the second problem, the outward diffusion of other elements, such as Ti, W and Mo, from the

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superalloy substrate can disturb the adhesion of the TGO layer of the MCrAlY coating [9,10].

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Therefore, it is important to prevent or postpone the harmful inter-diffusion by applying an appropriate diffusion barrier between the MCrAlY coating and the superalloy substrate. Extensive efforts have been made to prevent or postpone diffusion by using various types of diffusion barrier layers. In the first generation of diffusion barriers, a tungsten sheet was used between eutectic alloys and the Ni-Cr-Al layer. Although tungsten as a diffusion barrier stopped the outward diffusion of niobium from the δ phase and reduced the inward diffusion of chromium from Ni-Cr-Al, the thickness of tungsten was significantly reduced over time and it could be

ACCEPTED MANUSCRIPT diffused into both Ni-Cr-Al and the eutectic alloy [12]. As the next generation, a TiN layer was used between the MCrAlY coating and a superalloy as a diffusion barrier. The TiN interlayer improved the oxidation resistance of the coating and postponed the deleterious effects of the

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substrate-coating inter-diffusion. But this layer was unstable at above 1000 ˚C [13]. After unsuccessful performance of these coatings, most research has been focused on aluminium-based coatings such as Al-O-N, γ-Al2O3, α-Al2O3 and AlN [14–16]. But one of the problems facing

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these types of coatings at high temperatures may be microstructure changes, such as phase transformation, grain growth and decomposition [17,18].

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The rhenium-based layer, due to the exclusive combination of properties, can be regarded as a new opportunity to create a diffusion barrier. Rhenium is ductile, from sub-zero to high temperature, hard, with 2.6-7.5 GPa; it has the second highest melting point, 3157-3181 ˚C, the third highest modulus of elasticity, 461-471 GPa, and the fourth highest density, 21.00-21.02

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g/cm3 , among all metals. It also has low diffusivity, superior tensile strength, 1000-2500 MPa, and creep-rupture strength over a wide temperature range. Regarding the phase diagram of a ReCr-Ni ternary system at 1423 K (Fig. 1), σ-phase has a melting temperature around 2635 K. The

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σ-phase has both very low solubility and diffusivity of Al, and good high temperature stability particularly with Ni-based alloy substrates [19]. These excellent properties, such as mechanical

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and thermal stability for a long duration at high temperature, make it possible to design parts with thin sections [20].

Two main commercial technologies now widely adapted to deposit Re-base layers are chemical vapour deposition (CVD) and electroplating. CVD is an expensive, complex and energy intensive process. However, electroplating using non-toxic bath chemistries may become a successful alternative to deposit uniform Re coating on the substrate [20].

ACCEPTED MANUSCRIPT Electrolysis of rhenium from their aqueous solutions is very difficult because of the relative nobility of the electrode potential of rhenium. It has recently been reported that the electroplating of the pure Re is accompanied with low Faradaic efficiency (FE˂7%) and poor coating quality.

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However, by adding the appropriate nickel salt to rhenium bath plating, coatings with high rhenium content, high Faradaic efficiency and high quality can be achieved [20–22].

Successful performance of the electroplated Re-base diffusion barrier is related to the quality and

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rhenium content in the layer and this is affected by various factors including substrate

preparation, Faradaic efficiency and deposition parameters such as bath chemical composition,

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deposition time and current. The goals of this work are to understand the deposition mechanism (that is associated with Faradaic efficiency) of Re-base and investigated deposition parameters so that high-quality coatings could be achieved. Then, the effect of the Re-base diffusion barrier on the oxidation resistance of the NiCoCrAlY coating was investigated.

2.1. Materials

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2. Materials and Methods

A Ni-based superalloy disk (Hastelloy X, with composition that is summarized in table1) with 1

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inch diameter was used as the substrate material. Since the bonding between the MCrAlY coating and the substrate is mechanical interlocking, prior to the Re-base layer deposition, the surface of

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the samples were grit-blasted using alumina grit Mesh 36 under the pressure of 5 bar, the distance of 10 cm and the contact angle of 90˚. Re-base diffusion barrier layer was deposited from aqueous solution containing 26.8 g/l ammonium perrhenate (NH4ReO4); 15.4 g/l nickel sulphate (NiSO4), and 1.2 g/l citric acid (C6H8O7). The pH of the electrolyte was adjusted by sodium hydroxide (NaOH) or sulphuric acid (H2SO4). A commercially available NiCoCrAlY powder (Amdry 365-4, Oerlikon Metco, Inc., USA) was used as feed stock for spraying the MCrAlY coating. The morphology and composition of powder are shown in figure 2.

ACCEPTED MANUSCRIPT 2.2. The coating deposition process The duplex coating system consists of a Re-base diffusion barrier and a NiCoCrAlY coating deposited by electroplating and the high velocity oxygen fuel (HVOF) spraying process,

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respectively. Before Re-base film production, electroplating conditions were investigated using a Hull cell. The hull cell test was accomplished in a standard 320 ml cell at the current of 5 A and the temperature of 50-60 ˚C for 6 min. A nickel plate and a brass plate were used as the anode

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and cathode panels, respectively. The pH of the electrolyte was maintained in the range 3 by adding sodium hydroxide or sulphuric acid. Prior to the electroplating, the substrate was

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degreased with ultrasonic cleaning in acetone. In order to descale and create the activated surface, three types of acid including 35 wt. % HCL, commercial Ferro clean, and 30HNO3 + 15HF + 55H2O (volume per cent) were examined. Electrodeposition times of 90, 180, and 270 seconds were selected for the test. After the deposition of the Re-base diffusion barrier, NiCoCrAlY coatings were sprayed using a GTV HVOF system with a kerosene gun, type GTV K2 (GTV

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Verschleiss- Schutz GmbH, Germany). Powder was fed to the gun using the Ar carrier gas. Detailed parameters are listed in table 2.

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2.3. Specimens characterization

Oxidation tests were carried out in air at 1000 ˚C for 200 h to investigate the inter-diffusion

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behaviour. The microstructure of coated and oxidized specimens was investigated by scanning electron microscope (SEM; VEGA \\ TESCAN, Czech Republic). As the TGO layer formed in the oxidation test was brittle and there was the possibility of damage during metallography, a thin layer of nickel was deposited by electroless on the surface of the oxidized specimens. Energy dispersive spectroscopy (EDS) analysis was used to determine the chemical composition of the Re-base layer and the concentration profiles of elements in the coating with and without the

ACCEPTED MANUSCRIPT diffusion barrier. The surface roughness (Ra) of the substrate and Re-base layer were measured by a Mitutoyo Surftest profilometer (Mitutoyo SJ-201P; Japan). 3. Results and Discussion

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3.1. Re-base diffusion barrier analysis In this study, various samples were coated under different operating conditions. In sections 3.1.1 and 3.1.2, the Hull cell test and the electrodeposition mechanism of the rhenium-nickel coating

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are described, respectively. Subsequently, section 3.1.3 describes the effects of the deposition time on the thickness, surface morphology and Faradaic efficiency. Finally, the effect of the

3.1.1. Hull cell test analysis

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substrate surface preparation on the coating quality is investigated in section 3.1.4.

In the electroplating process, the proper parameter of deposition is very important to achieve the appropriate quality and obtain the required function of the deposited film. Therefore, it is

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necessary to obtain the proper parameter of electroplating by regular tests such as the Hull cell test. In the Hull cell test, different useful operating information’s such as temperature, current density, presence of inorganic impurity, the effect of additives on the solution, etc., are

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investigated on one test-piece. The cell is designed so that the cathode is at a pre-defined angle to the anode to generate a spectrum of current densities; hence it is a useful technique for evaluating

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the operating parameters [23–25].

Fig. 3 presents the surface morphology of the layer deposited on the cathode in the Hull cell test at the total current of 5 A for 6 min. It can be seen that due to the trapezoidal shape of the cell, current density was changed, so the left side of the sample was the high current density zone near the anode and the right side was the low current density zone. Zone 1 appeared to be a shiny, smooth and adherent layer, but zone 2 was the blister and rough area. With increasing the

ACCEPTED MANUSCRIPT distance from the cathode, the apparent thickness of the layer was reduced, so that in the zone 6, the layer was not formed. By visualizing the appearance of the layer in Fig. 3, some apparent factors such as surface roughness, glossiness and adherence could be taken into account at the

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numbered zones of 1 to 6; consequently, metallography samples were prepared from the zones numbered as 1, 3 and 4.

The SEMs of the polished cross-section of the coating prepared from zones 1, 3, and 4 are shown

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in Fig. 4. It is clear from these figures that with decreasing the current density, a discontinuous layer of Re-Ni was formed. As the number of the nucleation sites of coating is directly related to

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current density, the position rate was increased with increasing current density in the Hull cell cathode [20]. So, it can be seen from Fig. 4(a) that the coating was formed at zone 1, as shown in Fig. 3, with the thickness of 14 ± 0.5 µm, that show a good quality and uniformity. Accordingly, in order to optimize plating conditions such as surface preparation and the time of deposition, the

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constant current density range of zone 1 was used.

The current density distribution on the cathode in the Hull cell test could be calculated by using

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the following equation:
.  =  (
 +
log )

(1)

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,where C.D is the current density (A/ft2), I is the total current (ampere), L is the distance from the high current density end of panel, and C1 and C2 represent constants indicating the property of the electrolytes bath. Based on the Hull’s equation, C1 and C2 values in different electrolytes are almost similar. So, in the standard 320 mL cell, the current density is usually derived from:
.  =

  

(27.7 − 48.7 log )

(2)

ACCEPTED MANUSCRIPT Fig. 5(a) shows current density-distance (C.D-L) curves obtained from a standard 320 mL cell at the total current of 5 A. Because the optimized layer was formed in zone 1, a current density distribution in this zone is based on Fig. 5(b). As can be seen, the current density range in this

3.1.2. Deposition mechanism of the Re-base diffusion barrier

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zone was 0.31-0.35 A/cm2.

The polished cross section of the Re-Ni electroplated diffusion barrier and its approximate

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chemical composition were investigated by SEM and EDS, respectively. Fig. 6 shows the typical SEM image and EDS analysis of the optimized Re-Ni layer based on the Hull cell test on the

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Hastelloy X superalloy substrate under the current density of 0.34 A/cm2. It was composed of a 5 µm thick Re-Ni layer. It could be seen from Fig. 6(a) that there were no visible crack and discontinuity at the interface of substrate-coating, indicating the good structural integration of the coating. Fig. 6(b) shows the EDS analysis of the Re-Ni diffusion barrier (the point A in Fig.

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6(A)). The chemical composition of the Re-Ni deposit was almost Re-15.5 wt. % Ni. In order to understand the deposition mechanism of the Re-base electroplated diffusion barrier with the chemical composition Re-15.5 wt. % Ni, it is necessary to follow the sequence of events leading

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to the codeposition of the Re-Ni from the plating solution containing rhenium and nickel salts. Compared to other refractory metals such as tungsten and molybdenum, information on the

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electrodeposition of rhenium is very limited, possibility due to the confidentiality of the related projects. So, to explain the deposition mechanism of rhenium, metals with similar properties such as tungsten can be used [26]. Rhenium is the 75th element in the periodic table, in the same group of tungsten. Although chemistries of rhenium and tungsten are different, the stable ion of rhenium in solution is which is isoelectronic with . Hence, some similarities in the electrochemical properties are expected [27].

ACCEPTED MANUSCRIPT Rhenium, due to its very low over-potential for hydrogen evaluation, is difficult to obtain by the electrolysis of its aqueous solution. But, when the low relative concentration of nickel is added to the bath, deposition of rhenium could be facilitated [28]. In the first step, reduction of the divalent

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nickel on the cathode surface, as the rate-determining step in the deposition process, is done as follows: 

!

 + 2" →

"

(3)

"

+  + $  → 

!

+  + 2($)

(4)

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The metallic nickel formed in the first step reduces the  stable ion:

As the ion is the mainly stable ion of rhenium in solution, there is no possibility to deposit metallic rhenium in it directly. But, its chemical reduction to the  solubilized ion (as shown in Eq. 4) makes it possible to deposit metallic rhenium on the cathode surface according to the

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equation (5):

  + 5" + 3$  → " + 6($)

(5)



!

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In the rhenium plating bath, citric acid acts as a complexing agent which can form complex with and  ions. The composition of the Re-Ni is determined by the interaction between the

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rate of the deposition of nickel and rhenium from their complex. Therefore, Re-Ni consists of equal atomic concentrations of nickel and rhenium is expected. But in this study, in the bath with the ration of

() *+

,-./0



= the concentration of rhenium in the Re-Ni coating was nearly 94 atoms. %

(Re-15.5 wt. %Ni). Thus, the presence of nickel in solution is seem to catalyse for increase of the rate rhenium deposition; so by adding nickel in the solution, the partial current density for the deposition of rhenium was increased to five times.

ACCEPTED MANUSCRIPT 3.1.3. The effect of the deposition time on the Re-base diffusion barrier microstructure The effect of the deposition time on the substrate roughness, Faradaic efficiency and the surface morphology of Re-Ni layer was investigated for 90, 180, and 270 seconds under other than

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constant conditions. Since the bonding between the MCrAlY coating and the substrate was mechanical interlocking, prior to the Re-Ni diffusion barrier deposition, the surface of the sample was blasted. Thus, in order to maintain the surface roughness of the substrate, the diffusion

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barrier layer thickness had to be minimized.

Fig. 7 shows the SEM image of the polished cross-section of the coatings deposited under

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different deposition times. As can be seen, the thicknesses of Re-Ni layer at the deposition times of 90,180, and 270 second were 1.5 ± 0.2, 3.5 ± 1, and 6.5 ± 1 µm, respectively. Fig. 7(a) shows the cross-section image of the Re-Ni layer with the thickness of 1.5 ± 0.2µm. It is clear from this image that a discontinuous film of Re-Ni was formed during electrodeposition. This

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discontinuous film could be explained in terms of the model of nucleation and growth of deposits; based on this model, nucleation is beginning at preferential places in which local current density is higher or the force of nucleation is lower. Then, growth from the exiting nuclei is

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favoured at the beginning deposition, but after that, the growth of coarse nuclei is predominant related to the smaller nuclei [27,29]. To fill all roughness uniformly and completely, it is

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necessary to achieve a consistent growth rate. Since, at the deposition time of 90 s, there is not sufficient time for nucleation and growth, the coating cannot be uniformly formed. The results of substrate surface roughness measurement before and after the deposition of the ReNi diffusion barrier are listed in table 3. With increasing the deposition time, the surface roughness of the substrate was decreased, due to the increase in the thickness of the Re-Ni layer. High deposition time during electrodeposition process seemed to promote the nucleation of the grains on the surface that results in increasing of the thickness. On the other hand, the solution

ACCEPTED MANUSCRIPT penetration into the recess of the substrate surface roughness and the formation of coating in these areas, despite the creation of good adhesion to substrate, reduced surface roughness. It has been reported that high quality Re-base layers can be obtained at high Faradaic efficiency.

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The Faradaic efficiency was determined from the chemical composition of the deposit (as calculated by EDS analysis), the mass gained, and the charge passed. The Faradaic efficiency was defined as [20]: 3 45

∑

78 98 : ;8

× 100

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12 =

(6)

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, where W is the weight of the deposit, I is the current of deposition, t is the time of deposition, Ci is the weight fraction of the element (CRe=0.85 and CNi=0.15), Ni is the number of electrons for nickel and rhenium reduction (nRe=7 and nNi=2), Mi is the atomic mass of the element (MRe=186.2 and MNi=58.71 g/mol), and F is the Faraday’s constant (96485 ceqv-1).

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According to the chemical composition of Re-15.5 %wt. Ni coating, the Faradaic efficiency of the deposit for deposition times of 90, 180, and 270 s was obtained to be 66, 62, and 57 per cent, respectively. Reduction of Faradaic efficiency with the increase of deposition time could be

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probably due to hydrogen evolution during deposition on the cathode surface. It seems that detrimental changes in the substrate surface roughness could be due to the thicker Re-Ni layer

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which facilitated the hydrogen evolution rate and consequently, reduced the Faradaic efficiency [30,31].

Fig. 8 shows the surface morphology of the coatings deposited under different deposition times. It can be seen that as the deposition time is increased from 90 to 270 s; micro-cracks appear at the surface (see Fig. 8(f)). The formation of micro-cracks in the surface at the high deposition time is due to the hydrogen embrittlement and the residual stress. In the high deposition time, with

ACCEPTED MANUSCRIPT increasing the thickness of the coating, the internal stress was increased, that leads to cracking of the coating. Also, an undesirable change in the surface roughness was due to the thicker Re-Ni layer which increased the hydrogen evolution rate and increased micro-cracks as well. Also, the

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image of the coating surface showed that with the increase of the deposition time, coating morphology become finer and by reducing the particle size, the layer density is increased [32– 34].

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Thus, by considering factors such as coating uniformity, high Faradaic efficiency, the absence of micro-cracks, the minimum internal stress, the maximum density, and maintenance of the

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substrate surface roughness to provide adhesion for subsequent MCrAlY coating, a Re-Ni layer deposited with the current density of 0.34 A/cm2 and the deposition time of 180 s could be appropriate.

3.1.4. The effect of surface preparation on the Re-base diffusion barrier microstructure

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The adhesion and quality of the electrodeposited Re-Ni diffusion barrier varies with the descaling degree of impurities and the oxides film from the substrate surface. The SEM images of the

shown in Fig. 9.

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polished cross-section of the Re-Ni layers deposited after descaling in different solutions are

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The SEM image of the electrodeposited Re-Ni layer on the superalloy substrate after ultrasonic cleaning in acetone is shown in Fig. 9 (a). It could be seen that a non-uniform layer was formed during electrodeposition. This image indicated that ultrasonic cleaning of the superalloy in the acetone did not remove all of the oxide film on the substrate surface. This oxide film acted as a barrier to the transfer of electron, leading to the non-uniform layer formed on the substrate surface.

ACCEPTED MANUSCRIPT Fig. 9(b) shows the electrodeposited Re-Ni layer on the superalloy substrate after descaling treatment in the commercial Ferro clean solution. As can be seen, the Ferro clean solution caused the severe corrosion of the substrate surface; so not only the oxide film of the substrate surface

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was removed, but also the substrate surface was corroded and damaged. This type of surface condition causes the Re-Ni layer to deposit on a certain area (more on the bump area where local current density is higher) and a large part of the area remains without coating. Due to severe

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corrosion during descaling treatment, the substrate surface was very porous; leading to the loss of the Re-Ni layer during the metallography process and no layer could be observed.

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The SEM images of the electrodeposited Re-Ni layer after descaling treatment in the HCL acid and HF-HNO3 acid solution are shown in Fig. 9 (c) and Fig. 9 (d), respectively. As can be seen, both solutions almost completely dissolved the oxide films, causing activation of the substrate surface and that results layer with the favourite quality. As shown in Fig. 9(c) and 9(d), the effect

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of HCL acid and HF-HNO3 solution on descaling of the substrate is almost the same. But HCL acid was chosen as the descaling solution due to the following reason: a. More solubility of oxide films at the room temperature,

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b. More Solubility of resistant oxide films such as nickel and chromium oxide due to the chlorine ion permeability.

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c. More dissolution rate of the oxide film, related to other acid solutions. 3.2. Oxidation behaviour of the coatings 3.2.1. Oxidation behaviour of the NiCoCrAlY coating without the diffusion barrier The SEM image of the polished-cross section of the NiCoCrAlY coating after 200 h thermal exposure in air at 1000 ˚C is shown in Fig. 10. For the analysis of the TGO layer, the surface of the coating was protected by a Ni electroless layer. After oxidation, three zones including the

ACCEPTED MANUSCRIPT outer β-depletion zone (OBDZ), the intermediate zone (IMZ) and the inner β-depletion zone (IBDZ) were observed in the coating. Also, beneath the coating, an interdiffusion zone (IDZ) was observed in the superalloy substrate.

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In the IMZ of the coating, three-phase microstructures were observed consisting of the dark βphase (NiAl), which is a B2 type ordered superlattice compound with a BCC derivative structure, the continuously bright γ/γ̍ phase, which γ is a nickel rich solid solution of an FCC structure, and

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γ̍-Ni3Al phase, which is a LI2 type ordered superlattice compound with an FCC derivative structure [35,36]. The EDS analysis was performed at both dark and bright areas of the IMZ to

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evaluate the chemical composition of these areas. Figs. 11(a) and 11(b) show the EDS analysis of A and B points that were shown in Fig. 10(b), indicating the dark area containing β (NiAl) and the bright area containing the γ/γ̍ phase based on atomic percentage. Fig. 10(b) shows a higher magnification of the polished cross-section of the OBDZ shown in Fig.

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10(a). After oxidation, a dense and uniform oxide layer (TGO) of predominately Al2O3 was formed on the coating’s surface. EDS analysis confirmed that this layer was Al2O3 (Fig. 11(c)). For the formation and maintenance of the TGO layer, some of the Al in the coating outer part had

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to be diffused to the surface. This could lead to the continuous depletion of Al (β-NiAl phase) at the surface of the coating. In OBDZ, the β-NiAl was dissolved and only α-Cr and γ/γ̍ phase

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remained. EDS from OBDZ also confirmed the depletion of Al from this zone. Fig. 10(c) shows the coating-substrate interface of Fig. 10(a) at a higher magnification after the oxidation test. Similar to OBDZ, after oxidation, IBDZ was formed in the coating close to the coating-substrate interface as some of the Al from the coating was diffuse inward to the substrate. It is interesting that unlike OBDZ, the IBDZ was only composed of γ/γ̍ phase. It is obvious that the inward diffusion of Al occurred due to the Al concentration profile between the coating and

ACCEPTED MANUSCRIPT the substrate; so the Al content in the coating was decreased apparently due to the severe inward diffusion. It can be seen from Fig. 10(c) that a thin discontinuous layer of aluminium was formed between the coating and the substrate. EDS analysis also confirmed that this layer was Al (Fig.

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11(e)). The formation of this layer in the coating-substrate interface could be described according to coating adhesion mechanism. The most obvious and common mechanism of thermally sprayed coating’s adhesion is the mechanical interlocking of the splat to the irregularities of the substrate

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[37,38]. It must be noted that the splat does not contact the substrate over all of its bottom area. According to Fig. 12, the zones in contact are called the welding point (Kudinov, et al.) [39] or

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active zones (Steffens, et al.) [40], corresponding to a small fraction of the splat area (20-30% according to McPherson and Cheang, et al.) [41,42]. Clearly, with Al inward diffusion from the coating to the substrate, some of the Al is accumulated in the discontinuous films at the coatingsubstrate interface.

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During the oxidation test, elements diffusion occurs at the coating-substrate interface due to the difference in the element concentration between coating and the superalloy substrate, resulting in the formation of the IDZ. The phases in IDZ consisted of two kinds of microstructure: the blocky

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grey phase and the granular white phase; both phases were of the carbide type according to the EDS analysis (Fig.13). The blocky and granular- carbide phases were rich in Cr and Mo,

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respectively. So, it could be concluded that the blocky and granular carbides were of M23C6 and M6C types, respectively [43]. EDS analysis of the elemental concentration profiles of the coated sample after oxidation is shown in Fig. 14. The analysis was carried out from the coating top surface to the substrate in the steps of 7.5 µm. Elemental interdiffusion changed the primal chemical composition in the area of the superalloy which was close to the coating-superalloy interface, thus resulting in the microstructure instability of the superalloy. So the outward diffusion of Mo, W and Fe from the

ACCEPTED MANUSCRIPT superalloy to the coating and the inward diffusion of Al, Cr, and Co from the coating to the superalloy tended to produce the M23C6 carbide precipitation, which could be taken as the occasional formation of a depleted zone in the M6C carbide close to the coating-superalloy

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interface. Depending on the size, distribution and type of carbides, they may be beneficial or detrimental to the performance of the superalloy. The Hastelloy X used as the substrate could be invariably strengthened by a combination of carbides and solid-solution hardeners. So, IDZ

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depleted from M6C carbides could decrease the mechanical properties of the substrate. Also, the formation of M23C6 carbide with a large size in the IDZ close to the coating-superalloy interface

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could lead to nucleation of cracks during long service time [43,44]. Also, the outward diffusion of elements such as W and Mo from the superalloy could be detrimental to the adhesion of the TGO layer of the NiCoCrAlY coating [9,10].

3.2.2. The effect of the Re-Ni diffusion barrier on the oxidation of the NiCoCrAlY coating

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Fig. 15 shows the SEM image of the polished cross-section of the NiCoCrAlY coating with the diffusion barrier layer. It was composed of a 240±20 µm thick NiCoCrAlY coating and a 12±2 µm thick Re-Ni diffusion barrier layer. It could be seen that no visible discontinuity was observed

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at the interface of the NiCoCrAlY/Re-Ni/substrate, indicating the better structural integration of the coating system. The SEM image of the polished cross-section of the NiCoCrAlY coating with

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the Re-Ni diffusion barrier after 200 h of thermal exposure in air at 1000 ˚C is shown in Fig. 16. Similar to the NiCoCrAlY coating without the diffusion barrier, in the coating system with the diffusion barrier, three zones including OBDZ, IMZ, IBDZ were observed. Also, beneath the ReNi diffusion barrier, IDZ was observed in the superalloy substrate. A comparison of the thickness of these zones in both NiCoCrAlY coatings with and without the diffusion barrier is presented in table 4.

ACCEPTED MANUSCRIPT The phase microstructure of OBDZ and IMZ in both NiCoCrAlY coatings with and without the diffusion barrier was the same. So after oxidation, IMZ consisted of the β (NiAl) and γ/γ̍ phase and OBDZ included the γ/γ̍ and α (Cr) phases. Also, the chemical composition of the TGO layer

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on the NiCoCrAlY coating with the diffusion barrier was that of Al2O3 (Fig. 17). It was expected that in the long-term operation of the NiCoCrAlY coating without the diffusion barrier, due to the inward diffusion of Al into the substrate, the Al content that is required for formation of Al2O3

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reduced and the mixed oxide of spinel is formed, thereby allowing the NiCoCrAlY coating to be degraded gradually.

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As can be seen in table 4, the OBDZ thickness of the NiCoCrAlY coating was not changed by applying a diffusion barrier. Because this region was due to the formation of TGO layer, and since the TGO layer thickness of both NiCoCrAlY coatings with and without the diffusion barrier was the same, the thickness of OBDZ was not changed.

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According to table 4, the IBDZ thickness in the NiCoCrAlY without and with the diffusion barrier after oxidation was decreased from 23±2 µm to 4±2 µm. The NiCoCrAlY coating with the diffusion barrier presented less IBDZ thickness than the NiCoCrAlY coating without the

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diffusion barrier, showing agreement with the results obtained by the EDS analysis (Fig. 18). As shown in Fig. 16 (c), in the region of NiCoCrAlY coating, inner β-phase depletion occurred, such

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that the surface of the diffusion barrier was the crack. The presence of defects such as voids, pores, grain boundaries and cracks in the diffusion barrier decreased its ability due to the increase of the short-circuit diffusion path. Therefore, it could be concluded that the diffusion barrier effectively prevented the inward diffusion of Al elements from the NiCoCrAlY coating. As shown in Fig. 14. in the NiCoCrAlY coating without the diffusion barrier, an amount of W, Mo, and Fe were present, which was due to the outward diffusion of these elements from the

ACCEPTED MANUSCRIPT superalloy substrate. However, there were no traces of these elements in the NiCoCrAlY coating with the Re-Ni diffusion barrier, but these elements were accumulated beneath the diffusion barrier. Accumulation of refractory metals such as W and Mo beneath the diffusion barrier

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changed the primal chemical composition in the area of the superalloy which was close to the coating, thus resulting in the creation of the depletion zone from M6C carbide. Due to the presence of these elements in the superalloy and the absence of the outward diffusion, the

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superalloy obtained their initial microstructure with appropriate heat treatment. Also, the refractory metals were found to contribute strongly to solid-solution strengthening of the

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superalloy. The loss of these elements in the superalloy could degrade the desired strength of the superalloy. Also, in the IDZ of the coating system with the diffusion barrier, M23C6 carbide was not observed. The reason for this was the prevention of the inward diffusion of Cr from the NiCoCrAlY coating by the diffusion barrier. So it could be concluded that the Re-Ni diffusion barrier prevented the outward diffusion of refractory elements (W, Mo) that improve the

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mechanical properties of the substrate.

Clearly, degradation of the coating occurred by the depletion of the Al reservoir in the coating.

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The Al content in the NiCoCrAlY coating was mainly influenced by the outward diffusion to the coating surface, leading to the formation and spallation of the Al2O3 layer and the inward

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diffusion into the substrate. Therefore, by using the Re-Ni diffusion barrier, the Al reservoir in the NiCoCrAlY coating could be maintained with two mechanisms based on the diffusion barrier. Firstly, the inward diffusion of Al was effectively blocked. Secondary, it has been shown that the outward diffusion of W and Mo into the NiCoCrAlY coating due to oxidization (probably volatilization) into WO3 or MoO2 at high temperatures could break the continuously Al2O3 layer, resulting in the further consumption of Al for the renewed formation of Al2O3 [7,9,15,45,46]. 4. Conclusion

ACCEPTED MANUSCRIPT The goal of this paper was to investigate the microstructure and capability of the diffusion barrier Re-Ni coating between NiCoCrAlY coating and Hastelloy X superalloy; the important results can be summarized as follows:

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1) Re-Ni diffusion barrier was successfully deposited on the superalloy substrate by the optimized parameters in the Hull cell test.

2) A mechanism involving the induced codeposition of Re-Ni diffusion barrier according to the

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addition of Ni+2 to solution enhanced the rate of Re deposition, leading to the in-situ formation of

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the reducing agent Ni.

3) By considering factors such as coating uniformity, high Faradaic efficiency, the absence of micro-cracks, the minimum internal stress, the maximum density and the substrate surface roughness, the current density and deposition time of electrodeposition were optimized.

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4) The diffusion barrier effectively prevented the outward diffusion of the elements, such as W, Mo and Fe, from the superalloy, and the inward diffusion of the elements, such as Al, Cr and Co, from the coating.

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5) The Re-Ni diffusion barrier decreased the thickness of the inner β-depletion zone in the

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NiCoCrAlY coating by preventing the inward diffusion of Al.

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Acknowledgements The authors would like to gratefully appreciate the fruitful collaboration of Mr. masoud mirjani,

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Mr. Hamid Mahmoudi and Mr. Hamed Reza-Gholi.

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Figures captions

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Figure 1- A phase diagram of the ternary Re-Cr-Ni system [19]. Figure 2- SEM micrograph and EDS analysis of the NiCoCrAlY starting powder: (a) powder morphology and (b) powder chemical composition.

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Figure 3- The macroscopic image of the Hull cell test piece.

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Figure 4- SEM micrographs of the polished cross-section of (a) zone 1, (b) zone 2, and (c) zone 3, as shown in Fig.3.

Figure 5- Current density versus distance from the high current density of panel for (a) cathode of Hull cell and (b) zone 1at a higher magnification.

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Figure 6- (a) SEM micrograph and (b) EDS analysis of the polished cross-section of the Re-Ni

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Figure 7- SEM micrographs of the polished cross-section of the Re-Ni diffusion barrier coating at deposition times of (a) 90 s, (b) 180 s, and (c) 270 s.

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Figure 8- SEM micrographs of the surface morphology of the Re-Ni diffusion barrier coating at deposition times of (a, b) 90 s, (c, d) 180 s, and (e, f) 270 s. Figure 9- SEM micrographs of the polished cross-section of the Re-Ni diffusion barrier coating after descaling in the solution of (a) acetone. (b) HCL acid, (c) commercial Ferro clean, and (d) HNO3+Hf acid.

ACCEPTED MANUSCRIPT Figure 10- SEM micrographs of the polished cross-section morphologies of NiCoCrAlY coating at 1000 ˚C after200 h exposure , (a) initial state, (b) OBDZ, and (c) IBDZ and IDZ at a high magnification.

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Figure 11- EDS analysis of (a) A, (b) B, (c) C, (d) D, and (e) E points as shown in Fig. 10. Figure 12- Schematic of the mechanical interlocking of the splat to irregularities of the substrate

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surface.

Figure 13- EDS analysis of (a) A, and (b) B points as shown in Fig. 10(c).

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Figure14- EDS analysis of the elemental concentration profiles of (a) NiCoCrAlY coating with depth from the coating surface to the substrate after oxidation at 1000 ˚C for 200 h; (b) inward diffusion element from coating into substrate, and (c) outward diffusion element from substrate into coating

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Figure 15- SEM micrographs of the polished cross-section of NiCoCrAlY coating with the Re-Ni

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Figure 16- SEM micrographs of the polished cross-section morphologies of NiCoCrAlY coating with the Re-Ni diffusion barrier coating at 1000 ˚C after 200 h exposure, (a) initial state, (b)

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OBDZ, and (c) IBDZ and IDZ at a high magnification. Figure 17- EDS analysis of (a) A, (b) B, (c) C, and (d) points as shown in Fig. 16(b). Figure18- EDS analysis of the elemental concentration profiles (a) of NiCoCrAlY with the diffusion barrier involving depth from the coating surface to the substrate after oxidation at 1000 ˚C for 200 h; (b) inward diffusion element from coating into substrate, and (c) outward diffusion element from substrate into coating

Tables captions

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Table 2- Parameters of the HVOF process.

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Table 1- Chemical composition of Hastelloy X superalloy.

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Table 3- Substrate roughness before and after Re-Ni coating deposition. Table 4- Comparison of the thickness of TGO, OBDZ, IBDZ and IDZ NiCoCrAlY coatings with

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and without the Re-Ni diffusion barrier coating after the oxidation test.

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Tables Table 1: W V 0.90 0.05 Ni Bace -

Table 3:

Condition Before electroplating After electroplating Before electroplating After electroplating Before electroplating After electroplating

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deposition time (s) 90 180

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270

Type of coating

NiCoCrAlY without Diffusion barrier NiCoCrAlY with Diffusion barrier

NiCoCrAlY coating 900 0.5 8 350 100 3

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Parameter Oxygen flow rate (SLPM*) Kerosene flow rate (SLPM) Carrier gas, Ar (SLPM) Spray distance (mm) Powder feed rate(g/min) Spray gun traversing velocity (mm/s) * Standard liter per minute.

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Table 2:

Table 4:

Nb 0.15 Pb 0.001

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Element Si Mn Cr Mo Cu Fe Co Ti Al Wt.% 0.17 0.62 21.5 8.3 0.22 18.5 0.80 ˂ 0.005 0.14 Element C P S Sn Hf Mg Ta Zr B Wt.% 0.06 0.02 0.002 0.003 0.008 0.01 0.03 Trace 0.001

8.85 7.62 8.63 6.75 8.77 5.38

Roughness (Ra) 8.18 7.49 8.29 6.95 8.02 6.82 8.34 7.89 7.01 6.92 6.01 6.35 8.27 8.62 7.81 4.92 5.14 4.58

7.43 7.67 8.02 6.83 8.59 4.87

average 8.05 7.42 7.98 6.57 8.41 4.98

Total thickness of coating (µm)

TGO

OBDZ

IBDZ

IDZ

210 ± 20

4 ± 0.5

9±2

23 ± 2

50 ± 5

240 ± 20

3.5 ± 0.5

9±2

4±2

40 ± 5

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Highlights

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• •

A Re-Ni layer was fabricated as diffusion barrier. Diffusion barrier deposition mechanisms were discussed. Interdiffusion element between MCrAlY coating and superalloy substrate were investigated. Re-Ni layer suppressed the interdiffusion of alloying elements at 1000 °C. Diffusion barrier improved the oxidation resistance of NiCoCrAlY coating.

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