Evaluation of alumina incorporated combined ceramic layer thermal barrier coating

Evaluation of alumina incorporated combined ceramic layer thermal barrier coating

    Evaluation of alumina incorporated combined ceramic layer thermal barrier coating Pritee Purohit, S.T. Vagge PII: DOI: Reference: S0...

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    Evaluation of alumina incorporated combined ceramic layer thermal barrier coating Pritee Purohit, S.T. Vagge PII: DOI: Reference:

S0257-8972(16)31016-7 doi: 10.1016/j.surfcoat.2016.10.022 SCT 21667

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

16 August 2016 6 October 2016 7 October 2016

Please cite this article as: Pritee Purohit, S.T. Vagge, Evaluation of alumina incorporated combined ceramic layer thermal barrier coating, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.10.022

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ACCEPTED MANUSCRIPT Evaluation of alumina incorporated combined ceramic layer thermal

Department of Metallurgy and Materials science, College of Engineering, Pune (An

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Pritee Purohita*, S. T. Vaggea

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barrier coating

Autonomous Institute of Govt. of Maharashtra), Maharashtra State, India

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Abstract

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In present work the coatings having lanthanum-titanium-aluminium oxide that is LaTi2Al9O19 (LTA) in combination with yttria stabilized zirconia (YSZ) and alumina (Al2O3), were

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developed using plasma spray method. Thus the top coat comprises of LTA/YSZ/Al2O3

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ceramic top layer. The coatings were tested for type I hot corrosion in presence of Na2SO4

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and NaCl salts in 3:1 mass proportion at 900°C for 100 h. LTA 150 (having LTA top coat of thickness 150 µm) as-sprayed and annealed samples have shown excellent hot corrosion resistance upto 100 h. XRD patterns indicate that the LTA and Al2O3 phases were retained

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even after 100 h of isothermal hot corrosion with slight decrease in their peak intensity. Hot corrosion of alumina incorporated LTA coating resulted in the formation of LaAlO3, Na2Al2O4, NaAlO2.

Keywords: Ceramic, Plasma spray, Thermal barrier coating, Hot corrosion, Alumina, Parabolic rate constant

*Corresponding author. Tel.: +919145231814 Fax: +91-20-25507000 E-mail address: [email protected] (P. M. Purohit), [email protected] (S. T. Vagge)

ACCEPTED MANUSCRIPT 1. Introduction The performance and efficiency of a gas turbine and aero engine increases with

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increase in the inlet temperature to the turbine. By using the thermal barrier coatings, the

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operating temperature of gas turbine can be increased [1-5]. Thermal barrier coatings consist

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of an insulating ceramic top coat layer applied over a metallic bond coat [6]. The gas turbine engine is provided with the material systems capable of survival in harshest environments.

temperatures, and thermal degradation [7].

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Engine components are subjected to rigorous mechanical loading conditions, high

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The best compromise among these different requirements is presently offered by partially stabilized zirconia, 6 to 8 wt. % Y2O3-ZrO2 (Y-PSZ), deposited either by the air

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plasma spray (APS) technique or by electron beam physical vapor deposition (EB-PVD) [8]. With the increasing operating temperature, the thermo-mechanical and thermo-physical

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properties of thermal barrier coating (TBC) changes. Also, sintering and creep process affects the thermal fatigue resistance and performance of TBC [9]. After Al depletion and thickening

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of thermally grown oxide (TGO) layer, other oxides such as Ni and Co containing spinels are formed [10]. The delamination and spallation of TBCs usually occur at TGO scales, the main cause is the oxidation of bond coat. The different thermo-mechanical properties such as coefficients of thermal expansion of ceramic layer, bond coat and alloy substrate, affects the long term stability of TBC [11]. The coatings with different substrate alloys and different preparation techniques have different microstructure and properties [12]. Studies have been done to search a method to reduce the internal oxidation of the bond coat which is the main reason for TGO growth. Alumina layer acts as an oxygen barrier and 2

ACCEPTED MANUSCRIPT retards the further bond coat oxidation [13]. Al2O3/YSZ coatings have shown good spallation and oxidation resistance and increased densification and phase transition rate [14]. In

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oxidation test, the composite coatings of alumina as a top coat and the mixed YSZ alumina

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layer, showed better resistance [15]. Studies proved that the alumina incorporated NiCrAlY

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bond coat have better hot corrosion resistance than the YSZ incorporated NiCrAlY bond coat [16].

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A number of ceramic materials have been suggested in the last decade as new TBC materials. It covers aluminates, doped zirconia, perovskites, pyrochlores, and fluorite [17]. As

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La2Zr2O7 (LZ) is having lower thermal conductivity than YSZ, it was proposed as a promising material [18]. The cyclic oxidation behavior of the double ceramic layer (DCL)

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coatings was studied by Zhenhua Xu. et al. The DCL coating shown better oxidation

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resistance compared to single ceramic layer (SCL) [19]. LZ and LZ7C3 coatings were

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developed by EB-PVD and the hot corrosion behavior is studied, it has shown the best hot corrosion resistance with least degradation and spallation [20]. LZ3Y coatings prepared by EB-PVD have shown poor resistant to the attack of molten mixture of Na2SO4 +V2O5 [21].

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The DCL LZ/8YSZ TBCs has better thermal shock resistance ability at 1200°C and 1000°C. Thus DCL coating may be an important development direction [22]. LZ pyrochlore as a top coat material has revealed excellent high-temperature capability and high thermal stability [23]. H. Dong et. al, developed La2Ce2O7 (LC) coating using APS by using La2Ce2.5O8 powder. The LC/YSZ coating has thermal cycling life 40% more than YSZ coating at 1320°C [24]. Thermal conductivity of DCL coatings having top layer of 50% La2Zr2O7 is reduced and 50% Gd2Zr2O7 increased [25]. Pyrochlore LZ is having lower

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ACCEPTED MANUSCRIPT thermal conductivity and good sintering resistance compared to YSZ. But it is having short life due to thermal expansion mismatch and higher thermal stresses generating from it. The

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LZ coating have low thermal expansion coefficient (TEC) which leads to higher thermal

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stresses [26].

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Recently, lanthanum-titanium-aluminium oxide that is LaTi2Al9O19 (LTA) has been proposed as TBC material, which is having excellent phase stability from room temperature

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to 1600°C. It is combination of rare earth element and ceramic that is Lanthanum oxide La2O3, titanium oxide TiO2 and aluminium oxide Al2O3. But due to its relative low fracture

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toughness it is having short life. This difficulty can be solved by using a double LTA/YSZ ceramic layer [27].

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Studies revealed that alumina incorporated coatings give excellent performance and LTA/YSZ combination have an excellent hot corrosion resistance. Yet combination of

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LTA/YSZ along with Al2O3 is not reported in the open literature. Use of Al2O3 and rare earth oxide that is lanthanum-titanium-aluminium oxide (LTA) as a top coat material with YSZ is a

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novel approach. In present work the hot corrosion behavior of the thermal barrier coating having Al2O3 incorporated LTA/YSZ combined top coat was investigated. Thus, the top coat comprised of LTA/YSZ/Al2O3 ceramic top layer. The isothermal hot corrosion performance of the plasma sprayed coatings was evaluated at 900°C using field emission scanning electron microscopy and x-ray diffraction. Results obtained are plotted, tabulated and discussed.

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ACCEPTED MANUSCRIPT 2. Materials and Methods 2.1. Coatings development

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Inconel 718 was used as a substrate material for developing the coatings. The chemical composition of Inconel 718 (ASTM B670 UNS N07718) provided by the supplier is shown

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in Table 1 [28]. Specimens of 10 mm x 10 mm x 4 mm were cut using wire cut electrical discharge machining (EDM). The edge effect of the specimen was eliminated by grinding the

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edges. Using alumina powder the specimens were grit blasted before spraying. From the data available in previous literature the LTA powder was prepared using La2O3-99%, TiO2-99%,

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and Al2O3-99% having size 325 µm. These powders were dried for removal of moisture at 200°C for 10 h, mixed together and then ball milled for 5 h. Mixed powder was dried at

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200°C for 5 h and calcined at 1500°C for 24 h. It was compacted at 400 MPa and sintered at

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1600°C for 72 h [27]. Finally, sintered mass crushed and sieved with 90 µm sieve. The

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fluidity was maintained around 38 to 50 gm/min. A 40 kW Metco thermal plasma spray unit having plasma spray F4 gun was used for developing coating. Thermal grade powders having fluidity in the range of 38 to 50 gm/min

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were selected for coating. The particle size of powders was kept 90 µm. The thickness of the LTA coatings was maintained in the range of 320-370 μm [10]. LTA powder was preheated at 150°C before spraying. The coating parameters such as arc current, feed rate and spray distance were selected based on the data provided by the suppliers of these powders (Sigma Aldrich India). Coating parameters generally depend on the melting temperatures and the particle size of the powders used for the coating. The large size particles and high melting temperature needs a high arc current to melt the particles

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ACCEPTED MANUSCRIPT adequately before they are deposited onto the substrate [29]. Accordingly, it can be seen that LTA and Al2O3 needs high arc current than NiCrAlY as they exhibit a much higher melting

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temperature. The coating parameters are described in Table 2.

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While selecting combination of coating layers in double ceramic layer coating, the

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material of top layer should have a low thermal conductivity with high phase stability, and act as a thermal insulator to protect the inner layer [30]. Two combinations of LTA/Al2O3/YSZ

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coatings were finalized with varying topcoat thickness of LTA, 1. LTA 100 with 100 µm thickness, 2. LTA 150 with 150 µm thickness. The conventional YSZ coatings were

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developed for comparison. In all of the above mentioned coatings bond coat of NiCrAlY of 90 μm thickness was sprayed on substrate. In LTA 100 and LTA 150, 30 μm thick layer of

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Al2O3 and 150 μm thick layer of YSZ as bottom ceramic layer was sprayed successively on

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bond coat. Then a top ceramic layer of LTA was sprayed of thickness 100 μm and 150 μm on LTA 100 and LTA 150 specimens respectively. In conventional YSZ coating YSZ was

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sprayed as top ceramic layer of 200 µm thickness after the bond coat. The top coat and bond coat thicknesses were selected in such way that total coating thickness will not exceed 500

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μm, with the bond coat and top coat thicknesses less than 150 μm and 300 μm respectively [10]. The compositions of coatings are shown in Table 3 and Fig. 1. Annealing of LTA 150 and LTA100 coatings was done at 1050°C for 5 h in resistance heated muffle furnace for recrystallization of coating [27]. Annealed LTA 100 and LTA 150 samples were mentioned as LTA 100A and LTA 150A respectively.

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ACCEPTED MANUSCRIPT 2.2. Hot corrosion tests In present work, the type I hot corrosion tests were conducted based on earlier

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published work and by referring standards ISO 17224:2015 and JIS Z2292-2004 [38,39]. Hot

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corrosion test was conducted at 900°C in presence of Na2SO4 and NaCl salts in 3:1weight

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proportion. Na2SO4 + 25% NaCl salt solution was prepared by mixing Na2SO4 and NaCl salts in 3:1 weight proportion in distilled water [31]. The specimen weight was recorded and it is

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preheated for 1 h at 250°C, as it provides good adhesion of the salt to the specimen. A small air spray gun was used to apply the salt solution on all sides of the specimen. The salt

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solution was applied on the specimens’ surface. To remove the moisture, the salt coated specimens were kept in a furnace for 2 h at 120°C. Approximately 3-5 mg/cm2 salt was

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coated on the specimen. The weight of the specimen was recorded as initial weight of sample.

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The salt coated specimens were kept in the furnace for hot corrosion test. To check the reproducibility, two sets of coatings were tested for hot corrosion under same condition and

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the average of two reading was reported. A resistance heated tubular furnace, (make: Heat and control system) with ±1°C accuracy was used for the experiments.

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An isothermal hot corrosion test was carried out at 900°C for 20, 45, 75 and 100 h in presence of Na2SO4 + 25wt. % NaCl salt. Specimens were coated with salt and kept in a furnace at 900°C in four different trays. First tray was taken out after completion of 20 h, second after 45 h, and third after 75 h and the forth tray after 100 h. The weight of the specimen was measured after isothermal oxidation to study weight gain per unit surface area with respect to time.

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ACCEPTED MANUSCRIPT 2.3. X-ray diffraction analysis (XRD) The coatings were examined using x-ray diffraction Technique by using BRUKER D8

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X-Ray diffraction instrument. The x-ray diffractograms were obtained using Cu Kα radiation

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in the range of 2θ from 15 to 70°. Various phases obtained were identified using the X’Pert

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high Score plus software package.

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2.4. Field Emission Scanning Electron Microscope (FESEM) analysis Surface morphology of as-sprayed and hot corroded samples was examined using

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FESEM (model Carl Zeiss Sigma). The SEM images were taken in the range of 3 kV to 15 kV. Energy Dispersive X-Ray analysis (EDX) was carried out to determine the elemental

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percentage using point method and line scan method.

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ACCEPTED MANUSCRIPT 3. Results

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3.1. Hot corrosion kinetics

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Hot corrosion kinetics of LTA 100, LTA 100A, LTA 150, LTA 150A and conventional

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(YSZ) samples was studied for 20, 45, 75 and 100 h. The plot of weight gain per unit surface area observed in mg/cm2 vs. time in hour of hot corrosion is shown in Fig. 2. It shows that, the hot corrosion weight gain increased with decrease in LTA thickness. The hot corrosion

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weight gain followed the trend like,

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LTA 100 with weight gain of 0.09 mg/cm2 > YSZ with weight gain of 0.07 mg/cm2 > LTA 150 with weight gain of 0.069 mg/cm2 > LTA 100A with weight gain of 0.057 mg/cm2 >

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LTA 150A with weight gain of 0.02 mg/cm2

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These coatings were further examined for hot corrosion kinetics by plotting (weight

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gain/area)2 vs. time as shown in Fig. 3. The figure reveals a good fit between (weight gain/area)2 and time. From this it can be predicted that the coatings follow the equation for

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hot corrosion kinetics. ∆w/A = (kp. t)1/2 + Co

Where, ∆w/A is the weight gain per unit surface area (mg/cm2) at time t (s), Co is a constant and kp is the parabolic rate constant (mg2 cm-4 s-1). The parabolic rate constants (kp) was calculated from the above fit are summarized in Table 4. LTA 150A coating reveals a kp value of 4.32 x 10-6 mg2 cm-4 s-1. With decrease in the thickness of LTA top layer, the rate constant was increased. Same order is observed in weight gain data [29]. Conventional

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ACCEPTED MANUSCRIPT samples have also shown negligible mass gain but cracks were developed after 45 h hot corrosion. The coating was completely spalled off after 100 h.

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3.2. X-Ray Diffraction analysis

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LTA 150 coatings were survived upto 100 h, therefore to understand the phases developed XRD analysis of LTA 150, LTA 150A, LTA 100 and LTA 100A samples was done before and after 20, 45, 75 and 100 h hot corrosion at 900°C is shown in Fig. 4, 5, 6 and

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7 respectively. It shows the presence of LTA phases in all samples before and after hot corrosion. It indicates that LTA phases remained even after 100 h hot corrosion in presence

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of Na2SO4 and NaCl salts in 3:1weight proportion at 900°C in both as-sprayed and annealed samples. After annealing, the XRD phases have shown the presence of LTA and Al2O3. After

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hot corrosion, the XRD pattern of all coatings revealed the presence of several aluminates like NaAlO2, Na2Al2O4, LaAlO3 and also presence of Al2O3 and LTA. The incorporated

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alumina layer in coating gives rise to NaAlO2, Na2Al2O4 and LaAlO3. But in LTA 100 phases of NaAlO2 were observed after 20 h and in LTA 150 observed after 45 h. Phases of Na2Al2O4

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were observed after 45 h in LTA 100 and in LTA 150 they were observed after 75 h. The larger thickness of LTA 150 coating is the reason behind it. After hot corrosion the decrease in the intensity and broadening of LTA peaks, observed due to the phase change of LTA from tetragonal to monoclinic (t’ to m) and separation or dissolution of LTA phase into Al2O3 and other oxides [26-27]. This indicates that the decrease in crystalline nature of LTA phase with increase in hot corrosion exposure time. The formation of different phases is discussed later.

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ACCEPTED MANUSCRIPT 3.3. Surface morphology

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3.3.1. Surface morphology of LTA 150

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Surface morphology of LTA 150 as-sprayed and annealed coatings after 100 h of hot

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corrosion is shown in Fig. 8a and 8b. An exposure to hot corrosion beyond 75 h has led to an increase in number of micro cracks on specimen surface in case of LTA 150 as-sprayed samples. The surface oxides are appeared much coarse and rough compared to the coatings.

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Visible cracks were appeared after 100 h of hot corrosion [32]. Annealed LTA 150 sample have shown better hot corrosion performance. The porosity was retained till 100 h. No

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sintering was observed in microstructure. In case of both samples oxides like NaAlO2 and Na2Al2O4 formed on surface after subjecting to hot corrosion. XRD patterns showed peaks of

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theses oxides. Also, EDX analysis was done at different locations shown in Fig. 8a and 8b and the elemental composition is shown in Table 5. EDX analysis of LTA 150 as-sprayed and

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annealed coatings at various points showed the presence of elements like Al, O, La, Ti, Na, Cr and also Cl. It evidences the formation of oxides, chlorides and spinels containing Al and

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Cr. In Fig. 8a point 1 may contains LaAlO3, Na2Al2O4 and NaAlO2, point 2 may contain LTA, Na2Al2O4 and Point 3 may contain, NaAlO2 and LTA. The same is validated through EDX and XRD. In Fig. 8b, point 2 shows the presence of Al2O3 as the EDX shows point 1 to 3 rich in Al and O. Annealed sample shows higher amount of Al and O and proves protection ability of coating.

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ACCEPTED MANUSCRIPT 3.3.2 Surface morphology of LTA 100 Surface morphology of LTA 100 annealed coating after 100 h isothermal hot corrosion

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is shown in Fig. 8c. The annealed samples shown good resistance to hot corrosion and were

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survived till 100 h. The microstructure shows number of pores and fine cracks. EDX of

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annealed samples showed the presence of elements like O, Na, Al, S, Ti, Zr and La. Higher percentage of Al and O represents formation of protective oxide layers of Al2O3. No traces of

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Cl found in annealed samples.

The as-sprayed coating was completely spalled off upto 100 h of hot corrosion. To

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analyze the reason of spallation, SEM and EDX analysis was carried out on the spalled off surface. Morphology of spalled off surface of coating is shown in Fig. 8d. The retained layer

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is of bond coat and incorporated alumina layer. It shows the presence of some dark gradient Al2O3 layer and some bright areas of different oxides. Elemental composition of LTA 100

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annealed specimen and spalled of surface after 100 h isothermal hot corrosion obtained from EDX analysis (wt. %) is shown in Table 5. It showed the presence of O, Na, Al, S, Cl, Ti, Cr,

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Ni, Y, Zr and La. Presence of Cl evidences the formation of metal chlorides. The dark region at point 2 in Fig. 8d is showing the presence of Al and O in higher percentage and Cr and Ni in small percentage. Percentage of Cr and Ni is higher at the white region and witnesses the presence of oxides such as Cr2O3, and spinels of Ni and Cr like NiCr2O4 shown in point 1 and 3 of Fig. 8d [33]. Some traces of S and Na are also observed which indicates the penetration of molten salt from top ceramic layer. The metal chlorides, oxides like NaAlO2 and Na2Al2O4 and spinels are contributing to weight gain. The detail reactions are discussed later. 12

ACCEPTED MANUSCRIPT 4. Discussion The present study shows that LTA 150 alumina incorporated coatings showed the best

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performance in the molten salt of Na2SO4 and NaCl salts in 3:1weight proportion at 900°C

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for 100 h. Hot corrosion tendency retards with the increase in top LTA ceramic layer

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thickness. Kinetic data shows that the hot corrosion kinetics of coating followed a parabolic rate. It indicates that the process is having controlled diffusion of corrosive products. The

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XRD analysis shows the formation of phases like LTA, Al2O3, LaAlO3, Na2Al2O4 and NaAlO2 in LTA 150 annealed coating after 100 h hot corrosion. As shown by SEM, EDX and

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XRD, LTA suffered slight degradation during hot corrosion test in contact with molten salt of Na2SO4 and NaCl in 3:1weight proportion at 900°C for 100 h [34]. Therefore, both LTA 100

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and LTA 150 coatings were intact upto 75 h in the severe hot corrosion atmosphere. LTA 100 annealed samples were survived till 100 h. But LTA 100 as-sprayed sampled

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completely spalled off upto 100 h. The thickness of LTA top layer played important role in increasing the coating life. Samples with higher top layer thickness 150 µm (LTA 150)

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survived till 100 h and annealing has also contributed towards increasing hot corrosion resistance of both LTA 100 and LTA 150. Incorporated alumina layer helped in annealed sample to retain protective Al2O3 layer. From EDX it is proved that the annealed samples shown higher percentage of Al and O. Al2O3 protective layer prevents/reduces further diffusion of corrosive species into coating [35]. From the SEM micrographs, XRD patterns and EDX it is observed that different oxides containing La, Ti, Al, Na and Cr were formed after hot corrosion. The possible reactions are mentioned below.

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ACCEPTED MANUSCRIPT The rate of formation of Na2SO4 is equal to the rate of decomposition; equation (1) summarizes the same [20,36]. (1)

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Na2SO4 → Na2O + SO3

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In type I hot corrosion, due to basic fluxing the sulfate salt reacted with the protective scales of Al2O3 to form aluminate as shown in reaction (2) to (4) [36].

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Al2O3 + Na2O → 2NaAlO2

Al2O3 + Na2SO4 → 2NaAlO2 + SO3

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Al2O3 + Na2SO4 → Na2Al2O4 + SO3

(2) (3) (4)

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Due to incorporated alumina the protective layer of Al2O3 was retained even after these

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reactions.

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In LTA 150 and LTA 100 samples LaAlO3, NaAlO2 and TiO2 were formed after slight degradation of LTA. LaAlO3 is among the 7 binary compounds mentioned in phase

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equilibrium diagram of the system A12O3-La2O3-TiO2 [37]. The chemical reactions of LTA in the hot corrosion mechanism require detailed knowledge of the phase equilibrium in the system of salts and LTA. However, it is not available in the open literature. X. Zhou et al. [34], have explained possible chemical reactions between V2O5 and LTA through combining V2O5–Al2O3. Final corrosion products were composed of Al2O3, LaVO4 and TiO2 [34]. LTA is the only ternary compound of La2O3 and TiO2 to Al2O3. According to the ternary Al2O3– La2O3–TiO2 phase diagram [37] and the chemical composition ratio, the decomposed La and Ti-rich compounds on the coating surface is possibly a solid solution of La2/3TiO3 and

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ACCEPTED MANUSCRIPT LaAlO3 [17]. Also, the phases observed in XRD and EDX of present study were the combination of Al and Na. Therefore, the possible reaction is proposed here in equation (5)

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(5)

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LaTi2Al9O19 + 4Na2O → LaAlO3 + 2TiO2 + 8NaAlO2

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[17].

In case of LTA 100 as-sprayed coatings the thickness of LTA top layer was 100 µm, which was less than LTA 150 (150 µm) coating. With increase in the test duration along with

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slight degradation of LTA the molten salt infiltrated through the pores and micro-cracks of the 100 µm ceramic top layer of LTA 100 samples and seeped down by the edge effect,

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approached the bond coat and resulted in failure of coating [27].

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NaCl does not play any direct role in hot corrosion. Sulfates and chlorine were the

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products of the reactions between NaCl in the air. It is shown in equation (6) [36]. (6)

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4NaCl + 2SO3 + O2 → 2Na2SO4 + 2Cl2

In both LTA 100 and LTA 150 as-sprayed samples traces of Cl was detected in

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elemental analysis. The main reason for spallation of LTA 100 is the same. Different metal chlorides was formed due to availability of Cl. Al + 3/2Cl2 → AlCl3

(7)

Cr + 3/2Cl2 → CrCl3

(8)

The chlorides formed were volatile and they further diffuse outward through the openings, pores and again oxidize.

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2CrCl3 + 3/2O2 → Cr2O3 + 3Cl2

(10)

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2AlCl3 + 3/2O2 → Al2O3 + 3Cl2

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Due to repetition of reactions (7) to (10) the coatings were subjected severe hot

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corrosion [29]. NaCl reacted with SO3 as shown in reaction (6) to form Cl2. This Cl2 forms metal chlorides of Al, Cr as shown in reactions (7) to (8). The newly formed chlorides diffuse outward through the openings, pores and again oxidize to form Al2O3 and Cr2O3 [29]. This is

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the main reason of the weight gain of LTA 100 as-sprayed samples upto 0.09 mg/cm2. The incorporated alumina layer was also consumed in these reactions. The repetition of reactions

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(7) to (10) has accelerated the hot corrosion and spallation tendency of LTA 100 coating. Thermal expansion-mismatch between oxides, metal chlorides and Cr-rich, Ni-rich spinels

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NiCr2O4, NiAl2O4 identified by EDX, XRD and SEM resulted in cracking and complete spallation of coating. It can be concluded from the above discussion that the LTA 150

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coatings with top ceramic layer having thickness 150 µm of LTA survived and LTA 100 with top ceramic layer of LTA having thickness 100 µm failed in molten salt of Na2SO4 and NaCl

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salts in 3:1weight proportion at 900°C after 100 h.

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5. Conclusions

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It can be concluded from the present study that LTA 150 alumina incorporated coatings

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showed the best performance in the molten salt of Na2SO4 and NaCl salts in 3:1weight proportion at 900°C for 100 h.

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1. The hot corrosion weight gain followed the trend,

LTA 100 with weight gain of 0.09 mg/cm2 > YSZ with weight gain of 0.07 mg/cm2 >

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LTA 150 with weight gain of 0.069 mg/cm2 > LTA 100A with weight gain of 0.057 mg/cm2 > LTA 150A with weight gain of 0.02 mg/cm2

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2. Hot corrosion of LTA 150 coating resulted in the formation of LaAlO3, Na2Al2O4 and

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NaAlO2.

3. The thick top LTA layer and negligible degradation of LTA in molten salt contributed

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towards better hot corrosion resistance of LTA 150 coating. Negligible weight gain of 0.02 mg/cm2 was observed in LTA 150 annealed coating.

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4. LTA 100 as-sprayed coating failed after 100 h with the highest weight gain of 0.09 mg/cm2 due to the formation of metal chlorides. Acknowledgement The authors would like to express sincere gratitude towards Prof. V. S. Raja for providing an opportunity and facility to carry out experimental work in Aqueous Corrosion Laboratory, IIT Bombay and for his valuable guidance. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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hafnia based thermal barrier coatings, Surf. Coat. Technol. 108-109 (1998) 114-120. [10]. N. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas turbine engine

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applications, Sci. 296 (2002) 280-284

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[11]. C.H. Lee, H.K. Kim, H.S. Choi, H.S. Ahn, Phase transformation and bond coat

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[12]. X. Ren, F. Wang, X. Wang, High temperature oxidation and hot corrosion behaviors of the NiCr-CrAl coating on a nickel based superalloy, Surf. Coat. Technol. 198 (2005) 425-431.

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ACCEPTED MANUSCRIPT [14]. C. Ren, Y.D. He, D.R. Wang, Cyclic oxidation behaviour and thermal barrier effect of YSZ–(Al2O3/YAG) double-layer TBCs prepared by the composite sol-gel method Surf.

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[15]. A. Keyvani, M. Saremi, M.H. Sohi, Oxidation resistance of YSZ-alumina composites

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[16]. G. Sreedhar, MD. MasroorAlam, V.S. Raja, Hot corrosion behavior of plasma sprayed

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YSZ/Al2O3 dispersed NiCrAlY coatings on Inconel-718 superalloy, Surf. Coat.

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[17]. X. Xie, H. Guo, S. Gong, H. Xu, Thermal cycling behavior and failure mechanism of

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LaTi2Al9O19/YSZ thermal barrier coatings exposed to gas flame, Surf. Coat. Technol.

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[18]. H. Dai, X. Zhong, J. Li, Y. Zhang, J. Meng, X. Cao, Thermal stability of doubleceramic-layer thermal barrier coatings with various coating thickness, Mater. Sci. Eng.

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[19]. Z. Xu, L. He, R. Mu, X. Zhong, Y. Zhang, J. Zhang, X. Cao, Double-ceramic-layer thermal barrier coatings of La2Zr2O7/YSZ deposited by electron beam-physical vapor deposition, J. Alloys Compd. 473 (2009) 509-515. [20]. Z. Xu, L. He, R. Mu, S. He, G. Huang, X. Cao, Hot corrosion behavior of rare earth zirconates and yttria partially stabilized zirconia thermal barrier coatings, Surf. Coat. Technol. 204 (2010) 3652-3661. 20

ACCEPTED MANUSCRIPT [21]. Z. Xu, L. He, R. Mu, S. He, G. Huang, X. Cao, Hot corrosion behavior of La2Zr2O7 with the addition of Y2O3 thermal barrier coatings in contacts with vanadate–sulfate

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salts, J. Alloys Compd. 504 (2010) 382-385.

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[22]. L. Wang, Y. Wang, X.G. Sun, J.Q. He, Z.Y. Pan, C.H. Wang, Thermal shock behavior

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of 8YSZ and double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying, Ceram. Int. 38 (2012) 3595-3606. Ramachandran,

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[23]. C.S.

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Ananthapadmanabhan,

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Viswabaskaran, Influence of the intermixed interfacial layers on the thermal cycling

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behavior of atmospheric plasma sprayed lanthanum zirconate based coatings, Ceram.

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Int. 38 (2012) 4081-4096.

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[24]. H. Dong, D. Wang, Y. Pei, H. Li, P. Li, W. Mab, Optimization and thermal cycling

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behavior of La2Ce2O7 thermal barrier coatings, Ceram. Int. 39 (2013) 1863-1870. [25]. K. Bobzin, N. Bagcivan, T. Brogelmann, B. Yildirim, Influence of temperature on phase stability and thermal conductivity of single and double-ceramic-layer EB–PVD

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TBC top coats consisting of 7YSZ, Gd2Zr2O7 and La2Zr2O7, Surf. Coat. Technol. 237 (2013) 56-64.

[26]. X. Xie, H. Guo, S. Gong, H. Xu, Lanthanum–titanium–aluminum oxide: A novel thermal barrier coating material for applications at 1300ºC, J. Eur. Ceram. Soc. 31 (2011)1677-1683. [27]. X. Xie, H. Guo, S. Gong, H. Xu, Hot corrosion behavior of double-ceramic-layer LaTi2Al9O19/YSZ thermal barrier coatings, Chin. J. Aeronaut. 25 (2012) 137-142. 21

ACCEPTED MANUSCRIPT [28]. ASTM B670-07(2013), Standard Specification for Precipitation-Hardening Nickel Alloy (UNS N07718) Plate, Sheet, and Strip for High-Temperature Service, ASTM

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International, West Conshohocken, PA, 2013, www.astm.org

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[29]. G. Sreedhar, V.S. Raja, Hot corrosion of YSZ/Al2O3 dispersed NiCrAlY plasma-

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sprayed coatings in Na2SO4-10 wt. % NaCl melt, Corros. Sci. 52 (2010) 2592-2602. [30]. X.Q. Cao, R. Vassen, F. Tietz, D. Stoever, New double-ceramic-layer thermal barrier coatings based on zirconia–rare earth composite oxides, J. Eur. Ceram. Soc. 26 (2006)

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247-251

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[31]. D. Mudgal, S. Singh, S. Prakash, Cyclic Hot corrosion behavior of Superni 718, Superni 600, and Superco 605 in sulfate and chloride containing environment at 900°C,

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Metallogr. Microstruct. Anal. 4 (2015) 13-25.

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[32]. M.Z. Mehrizi, G. Eisaabadi, R. Beygi, Hot corrosion behavior of CoWSi/WSi2 coating exposed to Na2SO4 + NaCl salt at 900°C, Ceram. Int. 42 (2016) 3959-3964.

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[33]. S.M. Jiang, H.Q. Li, J. Ma, C.Z. Xu, J. Gong, C. Sun, High temperature corrosion behavior of a gradient NiCoCrAlYSi coating II: Oxidation and hot corrosion, Corros.

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Sci. 52 (2010) 2316-2322. [34]. X. Zhou, Z. Xu, L. He, J. Xu, B. Zou, Hot corrosion behavior of LaTi2Al9O19 ceramic exposed to vanadium oxide at temperatures of 700-950°C in air, Corros. Sci. 104 (2016) 310-318 [35]. R.A. Mahesh, R. Jayaganthan, S. Prakash, Evaluation oh hot corrosion behavior of HVOF sprayed NiCrAl coating on superalloys at 900°C, Mater. Chem. Physics. 111 (2008) 524-533. [36]. S. Bose, High temperature coatings, First edition, Butterworth-Heinemann, 2007

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ACCEPTED MANUSCRIPT [37]. S. Skapin, D. Kolar, D. Suvorov, X-ray Diffraction and Microstructural Investigation of the Al2O3-La2O3-TiO2 system, J. Am. Ceram. Soc, 76 (9) (1993) 2359-62.

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[38]. ISO 17224:2015, Corrosion of metals and alloys-Test method for high temperature

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corrosion testing of metallic materials by application of a deposit of salt, ash, or other

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substances.

[39]. JIS Z2292-2004, Methods for high-temperature corrosion test of metallic materials by

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salt coating

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ACCEPTED MANUSCRIPT Table 1 Chemical composition of Inconel 718 (wt. %) Ni

Cr

Fe

Co

Nb

Ti

Mn

V

Si

C

S

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54.975 17.656 17.213 0.050 5.466 1.069 0.124 0.033 0.169 0.015 0.011 0.002

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ACCEPTED MANUSCRIPT Table 2 Parameters used for plasma spray coating method Current (A) 650 600 700 660

Voltage (V)

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NiCrAlY YSZ LTA Alumina

Injector Distance (mm) 6 6 6 6

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1. 2. 3. 4.

Parameters Nozzle Injector (mm) Diameter (mm) 6 1.5 6 1.5 6 1.5 6 1.5

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Powder

49 51 52 52

Feed Rate (kg s-1) 750 667 500 500

Spray Distance (mm) 102 102 102 102

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Table 3

-

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Top LTA layer 100 μm Top LTA layer 150 μm -

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Layer 4 LTA (100 µm) LTA (150 µm)

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Layer 3 YSZ (150 µm) YSZ (150 µm)

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Remark

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Composition Layer 1 Layer 2 NiCrAlY Al2O3 LTA 100 (90 µm) (30 µm) NiCrAlY Al2O3 LTA 150 (90 µm) (30 µm) NiCrAlY YSZ YSZ (90 µm) (200 µm) Coating Type

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Compositions of coatings obtained by plasma spray method using F4 gun

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Table 4

Specimen

kp (mg2 cm-4 s-1)

1

LTA 100

8.76 x 10-5

2

LTA 150

6.98 x 10-5

3

YSZ

7.21 x 10-5

4

LTA 100A 4.52 x 10-5

5

LTA 150A 4.32 x 10-6

S.

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Hot corrosion rate constant (kp) for coated specimens exposed to hot corrosion in presence of Na2SO4 and NaCl salts in 3:1weight proportion at 900°C for 100 h

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Table 5

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Elemental composition of coatings obtained from EDX analysis (Wt. corrosion. Specimen Point Elements O Na Al S Cl Ti LTA 150 1 43.52 3.22 28.06 1.30 0.03 10.54 (Fig. 8a) 2 31.58 1.25 28.36 0.19 0.00 8.13 3 34.22 2.49 25.86 1.01 0.00 8.08 LTA 150 A 1 35.59 0.47 33.77 0.00 0.00 8.83 (Fig. 8b) 2 51.86 0.46 40.22 0.19 0.00 3.40 3 29.08 0.02 24.88 0.03 0.00 7.00 LTA 100 A 1 50.81 0.26 44.88 0.11 0.00 2.28 (Fig. 8c) 2 16.47 0.12 13.51 1.32 0.00 11.78 3 46.60 0.47 36.94 0.13 0.00 4.98 4 0.11 0.00 2.32 48.87 0.36 41.20 LTA 100 1 11.47 0.33 10.49 0.03 0.11 0.04 (Fig. 8d) 2 33.00 0.00 39.81 0.02 0.07 0.94 3 37.22 0.55 32.15 0.11 0.92 0.14

%) after 100 h hot

Cr 1.41 0.38 2.30 1.17 0.00 0.50 0.00 3.06 1.35 1.05 12.78 6.60 7.86

Ni 0.13 0.49 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 46.09 1.42 17.80

La 6.64 10.12 10.62 5.35 1.76 3.70 1.31 7.95 2.79 1.28 0.00 0.25 0.09

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ACCEPTED MANUSCRIPT Figure captions

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obtained by plasma spray method a. LTA100, b. LTA150, c. YSZ

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Figure 1. Diagram showing the types of coating layer combination along with thickness

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Figure 2. Weight gain plots for all types of coatings after exposed to Na2SO4 and NaCl salts in 3:1weight proportion at 900°C after 100 h

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Figure 3. (Weight gain/area)2 vs. Time plots for all types of coatings after exposed to Na2SO4 and NaCl salts in 3:1weight proportion at 900°C after 100 h

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Figure 4. X-Ray diffraction patterns of LTA 150 as sprayed coating after 20 h, 45 h, 75 h and

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100 h hot corrosion at 900°C

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Figure 5. X-Ray diffraction patterns of LTA 150 annealed coating after 20 h, 45 h, 75 h and

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100 h hot corrosion at 900°C

Figure 6. X-Ray diffraction patterns of LTA 100 as sprayed coating after 20 h, 45 h, 75 h and

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100 h hot corrosion at 900°C

Figure 7. X-Ray diffraction patterns of LTA 100 annealed coating after 20 h, 45 h, 75 h and 100 h hot corrosion at 900°C Figure 8. a: Surface morphology of LTA 150 as sprayed after 100 h hot corrosion. b: Surface morphology of LTA 150 annealed after 100 h hot corrosion. c: Surface morphology of LTA 100 annealed after 100 h hot corrosion. d: Surface morphology of LTA 100 as sprayed spalled off surface after 100 h hot corrosion

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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Figure 7.

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ACCEPTED MANUSCRIPT cps/eV

(a)

12

2

10

8

3

P

S

La Ti

Cl

4

2

Microcracks 0

Pores

1

2

35

30

1

25

2

15

10

3

Na

Al

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5

Cr S La Cl Ti Zr O

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20

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5 keV

Ni

6

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cps/eV

(b)

Cr

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Al

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Cr La Cl Ti Ni 6 S O Na

Y Zr S Cl

La Ti

7

8

9

10

Cr

Y

0

cps/eV

Oxides

(c)

1

2

3

4

5 keV

35

6

7

8

9

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Cr Cl La S Ti 15 Zr O

Na

Al

Y Zr S Cl

La Ti

Cr

Y

10

1

4

5

0 cps/eV

1

2

3

4

5 keV

6

7

8

9

10

Spinels containing Ni, Cr

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25

12

3

10

8 Zr La S Ti Fe ClC O Na 6 Nb Cr Ni

1

Al

Zr Y S Nb

La Ti

Cl

Cr

Fe

Ni

Y

4

2

Al2O3

2

0 1

2

3

4

5 keV

6

7

8

9

10

Figure 8.

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ACCEPTED MANUSCRIPT Highlights: 

Alumina incorporated LTA/YSZ combined ceramic layer coatings were developed using plasma spray method The top layer of LTA with thickness of 100 μm and 150 μm was applied on YSZ

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Coatings were tested for isothermal hot corrosion in presence of Na2SO4 and NaCl

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layer

salts at 900° 

LTA 150 samples have shown excellent hot corrosion resistance with weight gain less

Hot corrosion of LTA 150 coating resulted in formation of LaAlO3, Na2Al2O4 and

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NaAlO2

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than 0.02 mg/cm2

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