YSZ multi-layered thermal barrier coatings deposited by suspension plasma spray

Accepted Manuscript Functional performance of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings deposited by suspension plasma spray

Satyapal Mahade, Nicholas Curry, Stefan Björklund, Nicolaie Markocsan, Per Nylén, Robert Vaßen PII: DOI: Reference:

S0257-8972(16)31350-0 doi: 10.1016/j.surfcoat.2016.12.062 SCT 21919

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

25 April 2016 25 November 2016 17 December 2016

Please cite this article as: Satyapal Mahade, Nicholas Curry, Stefan Björklund, Nicolaie Markocsan, Per Nylén, Robert Vaßen , Functional performance of Gd2Zr2O7/YSZ multilayered thermal barrier coatings deposited by suspension plasma spray. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2016), doi: 10.1016/j.surfcoat.2016.12.062

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ACCEPTED MANUSCRIPT Functional performance of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings deposited by suspension plasma spray Satyapal Mahade a,*, Nicholas Curry b, Stefan Björklund a, Nicolaie Markocsan a, Per Nylén a, Robert Vaßen c a

Department of Engineering Science, University West, Sweden b Treibacher Industrie AG, Austria c Institute of Energy and Climate Research (IEK-1), Forschungszentrum Jülich GmbH, Germany

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Abstract

7-8 wt% yttria stabilized zirconia (YSZ) is the standard ceramic top coat material used in gas

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turbines to insulate the underlying metallic substrate. However, at higher temperatures

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(>1200 ºC), phase stability and sintering becomes an issue for YSZ. At these temperatures, YSZ is also susceptible to CMAS (calcium magnesium alumino silicates) infiltration. New

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ceramic materials such as pyrochlores have thus been proposed due to their excellent properties such as lower thermal conductivity and better CMAS attack resistance compared

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to YSZ. However, pyrochlores have inferior thermo mechanical properties compared to YSZ. Therefore, double-layered TBCs with YSZ as the intermediate layer and pyrochlore as the top ceramic layer have been proposed. In this study, double layer TBC comprising gadolinium

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zirconate (GZ)/YSZ and triple layer TBC (GZdense/GZ/YSZ) comprising relatively denser GZ

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top layer on GZ/YSZ were deposited by suspension plasma spray. Also, single layer 8YSZ TBC was suspension plasma sprayed to compare its functional performance with the multilayered TBCs. Cross sections and top surface morphology of as sprayed TBCs were analyzed

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by scanning electron microscopy (SEM). XRD analysis was done to identify phases formed in the top surface of as sprayed TBCs. Porosity measurements were made using water intrusion

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and image analysis methods. Thermal diffusivity of the as sprayed TBCs was measured using laser flash analysis and thermal conductivity of the TBCs was calculated. The multi-layered GZ/YSZ TBCs were shown to have lower thermal conductivity than the single layer YSZ. The as sprayed TBCs were also subjected to thermal cyclic testing at 1300 ºC. The double and triple layer TBCs had a longer thermal cyclic life compared to YSZ. The thermo cycled samples were analyzed by SEM. Keywords: Columnar microstructure, Gadolinium zirconate, Multi-layered thermal barrier coating, Suspension plasma spray, Thermal conductivity, Thermal cyclic test, Yttria stabilized zirconia

ACCEPTED MANUSCRIPT 1. Introduction Thermal barrier coatings are applied on metallic components of gas turbine engine (blades, vanes etc.) in order to insulate them from hot gases. The efficiency of a gas turbine can be increased by increasing the operating temperature [1-2]. Improvement in the gas turbine efficiency will also reduce harmful emissions and increase fuel economy [3]. However, with an increase in operating temperature, several issues related to the TBC need to be

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addressed such as phase stability, sintering, CMAS infiltration and enhanced bond coat oxidation.

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YSZ is the standard top coat ceramic material used in gas turbines due to its excellent

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thermal properties. But, at elevated temperatures (>1200 °C), phase stability and sintering becomes an issue for YSZ which can result in shorter service life [4-6]. Above 1200 ºC, YSZ

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also becomes susceptible to CMAS attack, which results in stiffening of the ceramic coating and eventually leading to catastrophic failure of the coating [7-18]. New ceramic materials

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which can address CMAS attack problem are thus desirable. Gadolinium zirconate is one such material with lower thermal conductivity, excellent phase stability and better ability to resist CMAS attack [12-13 , 19]. However, Gd2Zr2O7 (GZ) has a tendency to react with

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alumina (Al2O3) at higher temperature to form GdAlO3 [20], which is not desirable as the

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protective alumina layer (TGO) is being consumed. Also, GZ has a lower fracture toughness than YSZ. Due to these reasons, single layer GZ TBCs showed poor thermal cyclic lifetime compared to single layer YSZ [21]. Therefore, a double layer TBC with YSZ as the

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intermediate layer and GZ as the top layer has been proposed [22-28]. Suspension plasma spray (SPS) makes use of nano or submicron sized powder feedstocks.

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These small sized powders are difficult to be deposited as solid feedstock. Nano sized and submicron sized powders have been shown to have superior thermo mechanical properties compared to micron sized powders [29-31]. Additionally, SPS deposited coatings can mimic the relatively expensive EB-PVD process by generating columnar microstructures [32-34]. Columnar microstructures are believed to improve the strain tolerance of the coating leading to improved thermal cyclic life. SPS is a relatively new TBC processing technique and gadolinium zirconate (GZ) has not been previously studied, besides S.Mahade et al [28]. In this previous study, SPS sprayed GZ/YSZ double and triple layer TBCs were evaluated and compared with a single layer YSZ at

ACCEPTED MANUSCRIPT temperatures below 1200ºC. In this study, single layer YSZ, GZ/YSZ double and GZdense/GZ/YSZ triple layered TBCs were deposited by SPS and evaluated at 1300 °C. The purpose of the dense top GZ layer in the case of triple layer TBC is that it should improve the CMAS & erosion resistance. 2. Experimental Details

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2.1 Specimen preparation Hastelloy-X plates (25mm x 25mm x 1.54mm ) and IN-738 alloy round coupons (30mm

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diameter x 3mm) were used. Before spraying, the substrates were grit blasted using alumina particles to obtain a surface roughness profile of 3 µm on Ra scale. The samples were then air

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blasted to remove alumina particles sticking on the surface of substrates. AMDRY 386-2 (Ni18Co13Cr10Al0.1Y composition with powder size distribution of -88 µm to +18 µm) &

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AMDRY 386-0 (Ni18Co13Cr10Al0.1Y composition with powder size distribution of -63 µm to +5 µm) obtained from Oerlikon Metco, Switzerland, bond coats of NiCrAlY composition with

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an approximate thickness of 230 µm were thereafter deposited by high velocity air fuel (HVAF) process using the M3 gun (Uniquecoat; Richmond, USA). Shorter nozzle (3L2) and a spray distance of 350 mm with nitrogen as the carrier gas was used to deposit the bond coat

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on the substrates.

2.2 Suspension plasma spray parameters For this study, two commercial suspensions and one experimental suspension were provided

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by Treibacher Industrie AG (Althofen, Austria). The commercial suspensions consisted of an 8wt.% YSZ suspension (8YSZ) and a gadolinium zirconate (GZ) suspension. In both cases the

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suspensions had a solids load of 25 wt.% in ethanol and a median particle size D50 of 500 nm. These suspensions have been optimized to produce columnar TBC coatings. The experimental suspension consisted of GZ with a median particle size D 50 of 1 µm suspended in ethanol and a solid load content of 40 wt.%. This experimental suspension was developed to deposit denser, non-columnar SPS coatings. An Axial III plasma gun and Nanofeed 350 suspension feeding system (Northwest Mettech Corp., North Vancouver, Canada) were used for depositing the single, double and triple layer TBCs. Two different spray parameters were chosen for this study, as shown in Table. I. Both sets of parameters cannot be considered fully optimized for coating deposition.

ACCEPTED MANUSCRIPT Deposition of coatings was performed using a combined robot and turntable system from ABB Robotics (Västerås; Sweden). Samples were fixed at the periphery of a 300mm cylindrical fixture with spray processing occurring with fixed rotational speed of the turntable corresponding to the surface speed and step size per revolution for the plasma torch discussed in Table I. Based on the two parameters, three different ceramic architectures were deposited, as

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shown in Fig. 1 with the designed maximum total coating thickness of 300µm. The first coating system comprised of a conventional columnar single layer 8YSZ TBC. The second

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system comprised of a double layer columnar coating (GZ/YSZ) with a 60µm YSZ

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intermediate layer and 240µm GZ outer layer. The third system consisted of a triple layer coating (GZdense/GZ/YSZ) comprising two columnar layers (60 µm YSZ plus 210 µm GZ). A

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third 30 µm GZ outer layer was deposited with the experimental suspension and dense coating parameters. Before deposition of the SPS coating layers, the fixture and samples

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were preheated by 5 strokes of the torch operating without suspension feeding. Deposition of the initial 60µm YSZ layer was performed first; after which 2/3 of the samples were removed from the fixture to form the basis of the multi-layer samples. The remaining 1/3 of

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the samples were coated with a further 240µm of YSZ to complete the single layer system.

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The columnar and dense GZ layers were deposited during separate coating cycles.2.3 Coating Characterization 2.3.1 Metallography

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For the metallographic preparations, in order to avoid pullouts, the sprayed coatings were firstly vacuum cold mounted in a low viscosity epoxy resin. These cold mounted TBCs were

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then sectioned using a slow speed diamond cutting blade (Struers secotom-10 machine). The sectioned samples were cold mounted again using epoxy resin and then subjected to polishing using a Buehler Power Pro 5000 equipment. Details of the polishing steps are discussed elsewhere [28]. The mirror polished TBC specimens were gold sputtered and then observed in SEM (TM 3000, HITACHI, Japan) using back scattered electron mode. Twenty five different cross sectional SEM images at 250X magnification were used to determine the thickness, column density and column width of each layer. For the top surface morphology analysis, carbon tape was applied on the top surface in order to make the TBC conducting.

ACCEPTED MANUSCRIPT 2.3.1.2 Column density measurement Column gaps in the as sprayed TBCs were measured using cross sectional SEM micrographs at a low magnification of 250 X. These micrographs were analyzed in Microsoft paint. A horizontal line across the middle section of the micrograph was drawn and the total number of column gaps intercepting the line were estimated. Twenty five different micrographs at the same magnification were evaluated and the mean value of the column density and

(1)

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standard deviation was calculated using Equ (1).

2.3.1.3 Column width measurement

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The distance between two columns which is hereafter referred to as ‘column width’ was calculated. The procedure for measurement of column width was similar to the column

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density measurement where a horizontal line at approximately half of the coating thickness was drawn in the SEM micrographs. Twenty five cross sectional SEM micrographs at 250X

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were considered for the measurement.

2.3.2 Porosity Measurement

Porosity measurements of the as sprayed TBCs were made using two methods.

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2.3.2.1 Water intrusion method

Free standing coatings of the as sprayed TBCs were obtained after dipping the TBC

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specimens for an hour in an aqueous solution of HCl and HNO3. The free standing coatings were then dried in an oven at a temperature of 150 ºC for 1 hour in order to remove the entrapped liquid. Dry weight of the coatings was measured and later they were immersed in a beaker containing water. The set up was finally placed in a vacuum chamber and the equipment cycled several times to remove air entrapped in the coating. Wet weight of the coatings was measured. Weight of water which infiltrated the accessible pores within the coating was obtained and the volume of water was calculated (Vw). The volume of the coating was also calculated (Vc). Finally, the porosity volume fraction (P) in the coating was calculated according to the Equ (2).

ACCEPTED MANUSCRIPT (2) 2.3.2.2 Image analysis method Porosity content of the as sprayed TBCs was measured by image analysis method using Image J software [35]. Twenty five different SEM micrographs at 15 kV and 300 X magnification in BSE mode were used to obtain the porosity values. A lower magnification of

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300 X was chosen in order to capture the porosity contribution due to column gaps in the microstructure which are otherwise not seen at a higher magnification.

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2.3.3 Apparent density determination

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For the sake of simplicity, actual density (ρa) of as sprayed coatings was calculated after taking into account the porosity values (P) obtained by water intrusion method. Bulk

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densities (ρb) of GZ & YSZ are 6.32 and 6.1 g/cc respectively. The bulk density of GZ/YSZ multilayered coating (6.27g/cc) was calculated according to the thickness proportion of GZ &

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YSZ. Apparent densities of the coating are obtained according to Equ (3). (3)

2.3.4.1 Sample preparation

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2.3.4 Thermal diffusivity & thermal conductivity

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TBCs deposited on thin plates (25 mm x 25mm x 1.54 mm) were waterjet cut to a dimension of 10 mm (diameter) discs. The ceramic surface of these discs were gold sputtered

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(approximately 500 nm thickness) in order to improve energy transfer to the sample and prevent direct transmission of infra-red light though the ceramic layer. Later, gold sputtered

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discs were coated (liquid spray) with a thin graphite layer to impart better absorption and emission properties.

2.3.4.2 Thermal diffusivity Thermal diffusivity of the as sprayed TBCs was measured using NETZSCH 427 Laser Flash Analysis (LFA) equipment (Netzsch Thermophysics, Selb, Germany). For details of the diffusivity model used for measurement see [28]. Thermal diffusivity measurements of the as sprayed TBCs were performed in argon environment (100 ml/min flow rate). After achieving the desired sample temperature, a laser of known pulse width was fired at the rear end of sample and the sensor placed above

ACCEPTED MANUSCRIPT the sample recorded rise in sample temperature. The thermal diffusivity of the coatings was calculated according to Equ (4). (4) Where ‘α’ is the thermal diffusivity (mm2/s), ‘d’ is the ceramic coating thickness (mm) which is known and ‘t0.5’ is the half time taken for rise in temperature measured by the LFA

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equipment. 2.3.4.3 Specific heat capacity

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Specific heat capacity measurements of the as sprayed TBCs were made using NETZSCH 404

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differential scanning calorimetry (Netzsch Thermophysics, Selb, Germany). The as sprayed free standing coatings were powdered and used for DSC analysis. Sapphire was used as the

temperature range of 25 ºC to 1200 ºC.

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2.3.4.4 Thermal conductivity

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reference material and heat capacity of the as sprayed TBCs was measured in the

Thermal conductivity of the as sprayed TBCs was calculated according to Equ (5) at various

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temperature points (25, 200, 400, 600, 800, 1000 and 1190 °C) using the corresponding

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thermal diffusivity and specific heat capacity values obtained at those temperatures. (5)

where ‘λ’ is the thermal conductivity, ‘α’ is the obtained thermal diffusivity, Cp is the

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obtained specific heat capacity and ‘ρ’ is the coating density.

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2.3.5 XRD analysis

Phase analysis of the as sprayed TBCs was carried out by SEIFERT TT 3003 X-Ray Diffractometer. The phases were analyzed using Cu Kα of 1.54 Aº wavelength in two theta range of 20-70 degrees. A slow scan rate with a step size of 0.01 degree and 10 seconds per step was used. 2.3.6 Thermal cyclic test Thermal cyclic test, also known as Burner rig test (BRT), was performed at Forschungszentrum Jülich, Germany. For this test, specifically designed IN-738 alloy substrates of 30 mm diameter x 3mm thickness comprising an aperture at approximately

ACCEPTED MANUSCRIPT half of its thickness in order to accommodate a thermocouple were used. Also, these disc shaped substrates had beveled edges in order to minimize the edge effect. For details of the thermal cyclic test set up see [36]. During the test, top surface of as sprayed TBCs were subjected to a temperature of 1300 ºC whereas the bond coat/substrate temperature was maintained at approximately 1050 ºC using compressed air. A pyrometer was used to measure the top surface temperature whereas a k-type thermo couple was used to measure

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the substrate temperature. Each thermal cycle comprised of 5 minutes of heating using a natural gas burner, followed by 2 minutes of cooling during which the burner was moved

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away automatically from the sample. The thermal cyclic test continued until a visible about 30% spallation of the ceramic top coat was observed after which the samples were assumed

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to have failed and the test was stopped. After taking a photograph, the samples were

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immediately cold mounted to analyze the failure by SEM.

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3. Results & Discussion 3.1 Microstructural analysis

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The cross section and top surface of as sprayed TBCs were analyzed by SEM. Fig. 2(a) shows

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the cross sectional SEM micrograph of the single layer 8YSZ TBC showing a columnar microstructure with an average column density of 13 ± 2 columns/mm and an average column width of 56 ± 15 µm. The total thickness of the ceramic (8YSZ) coating was measured

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to 300 ± 10 µm and a bond coat of 220 ± 10 µm was determined. A better interface between the bond coat and ceramic top coat was observed than reported previously [28], as the

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interface between the bond coat and top coat showed a visible gap in the previously reported work which is absent in the micrographs presented in this work. In the top view, cauliflower resembling microstructure was observed, as shown in Fig. 2(b). Also, in the case of the double layer GZ/YSZ TBC, a columnar microstructure was accomplished, as shown in Fig. 3(a), with columns originating from the underlying YSZ layer and extending into the GZ layer. The column formation, seems thus independent of the spray material. Cross sectional SEM micrographs at higher magnifications showed that the GZ/YSZ interface looked continuous, indicating a minimal thermal contact resistance. The total thickness of ceramic layer was found to be 300 ± 15 µm comprising of approximately

ACCEPTED MANUSCRIPT 240 µm GZ layer on top of 60 µm YSZ layer. Column density was found to be approximately 10 ± 2 columns/mm and the average column width was 45 ± 15 µm. In the top view micrograph, as seen in Fig. 3(b), a cauliflower microstructure similar to the single layer system was observed. Fig. 4(a) shows the cross sectional SEM micrograph of GZ dense/GZ/YSZ triple layer TBC. A columnar microstructure very similar to the double layer GZ/YSZ TBC was observed. The 30 ±

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3 µm thick denser top GZ layer meant for erosion & CMAS resistance is clearly visible. The relatively porous GZ layer was approximately 210 ± 6 µm and the underlying YSZ was The total thickness of the ceramic layer was thus

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approximately 60 ± 6 µm thick.

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approximately 300 ± 15 µm with an average column density of 9 ± 2 columns/mm and average column width of 52 ± 10 µm. As can be seen in the top view SEM micrograph, a

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cauliflower microstructure was created, as seen in Fig 4(b). The microstructure is denser

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compared to the double layer system, as shown in Fig. 3(b).

3.2 Thermal diffusivity

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Thermal diffusivity of the as sprayed TBCs was measured in the temperature range of 25-

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1190 ºC. Five different measurements were made at the same temperature and their mean values along with standard deviation were obtained. Also, the error contribution due to uncertainty in thermal diffusivity measurement by LFA (2%) was considered and the overall

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error in measurement was calculated using law of error propagation. During the thermal diffusivity measurements, multi-layered TBCs were treated as a single coating system and

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their thermal diffusivity was measured. Thermal diffusivity of as sprayed TBCs decreased with an increase in temperature upto 800°C. This could be attributed towards the increased phonon scattering with an increase in temperature. The thermal diffusivity of single layer YSZ system was higher than both the double and triple layer systems at all temperatures, as seen in Fig. 5. Also, in YSZ single layer TBC, thermal diffusivity values increased at above 800 ºC. A similar trend of increase in thermal diffusivity of YSZ above 800 ºC was also reported by Moskal et al [37] and Curry et al [33]. This can be attributed towards two possible reasons. First reason could be due to the fact that radiation becomes significant at temperatures above 800 ºC. The second reason could be due to sintering of the ceramic which causes closure of small pores. The GZ/YSZ coating systems also showed an increase in thermal

ACCEPTED MANUSCRIPT diffusivity above 800 ºC, as seen in Fig. 5. A similar observation of rise in diffusivity above 800 ºC has been reported in the case of suspension plasma sprayed GZ/YSZ TBCs [28]. Also, at 1190°C, the thermal diffusivity values of GZ based TBCs were lower compared to thermal diffusivity values at 25°C. However, for YSZ single layer TBC, the thermal diffusivity value at 1190°C was higher compared to thermal diffusivity value at 25°C. The reason for such an observation is not clearly understood.

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3.3 Porosity measurement

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3.3.1 Water intrusion method

The measured porosity values for the three coating systems are shown in Fig 6. The single

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layer system had a slightly higher porosity content of approximately 21% compared to the double layer GZ/YSZ (approximately 18 %), though both were deposited using identical spray

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conditions. The triple layer GZ dense/GZ/YSZ TBC, had a porosity content of approximately 17%. The results obtained by this measurement method in terms of ranking of the coatings

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differ from the one previously reported [28]. It should be noted that water intrusion method gives a good estimation of open porosity content including the column gaps. However, the method does not take into account the closed porosity within the coating as water cannot

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porosity content [35].

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infiltrate the closed pores. Image analysis was thus also utilized to estimate the total

3.3.2 Image analysis method

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Lower magnification (300 X) SEM micrographs were analyzed in order to ensure that the porosity contribution due to column gaps also was taken into account. The single layer

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system had the highest porosity content of approximately 18 % followed by the double layer GZ/YSZ TBC of approximately 13%. The triple layer TBC had lowest porosity of approximately 12 %, as shown in Fig. 6. The ranking of the coating porosities by this method agrees with the water intrusion method. However, absolute values differ, where image analysis estimates a lower porosity content. A possible explanation could be the low magnification which makes it difficult to capture fine scale porosity. 3.4 Thermal conductivity Thermal conductivity for each coating system was calculated according to Equ. (4), using the porosity values obtained by the water intrusion method. Error bars were calculated by law of

ACCEPTED MANUSCRIPT error propagation after taking into account the experimental uncertainty (accuracy and precision) in measurement of thermal diffusivity, specific heat capacity and density. The thermal conductivity of the single layer YSZ was shown to be higher than the double and triple layer TBCs at all the test temperatures despite of a higher porosity content, the values of thermal conductivity are presented in Fig. 7. One reason for this behavior could be attributed to the difference in crystal structure of GZ & YSZ ceramics which results in

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different ordering of cations and oxygen vacancies and hence the difference in phonon scattering. GZ has approximately 33 mol % of Gd2O3 whereas YSZ has 4-5 mol % of Y2O3

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which results in higher concentration of oxygen vacancies in the GZ crystal structure compared to YSZ. With higher number of oxygen ion vacancies in the crystal structure, a

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higher scattering strength and hence lower the thermal conductivity of the material is found [19, 38]. Another explanation for this observation could be due to the difference in atomic

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weight of the cations. Yttrium, zirconium & gadolinium have atomic weight of 89, 91 & 157 respectively. Larger the difference between atomic weight of cations, higher is the scattering

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strength and lower is its thermal conductivity [19, 38]. The results are in accordance with previous findings [28]. The specific heat capacity of GZ/YSZ is lower than the YSZ, as seen in

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Table II, which can be one of the reasons for the lower thermal conductivity in the double

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and triple layer systems.

It should be noted that the thermal conductivity temperature dependence of the single layer YSZ system follows similar trend as previously reported for APS deposited 8YSZ single

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layer TBCs [19].. In the case of SPS processed GZ/YSZ multi-layered TBCs, thermal conductivity trend observed agrees with the GZ single layered TBCs deposited by APS in [37],

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where the thermal conductivity also decreases with temperature up to 800 ºC. 3.5 Phase analysis

Phase analysis of the top surface of as sprayed TBCs was carried out by XRD. The JCPDS standard was used to identify the peaks. Tetragonal prime phase (t’) (JCPDS 01-70-4430) was found in the case of single layer YSZ, as seen in Fig. 8. It should be noted that phase stability of a TBC in its working temperature range is one of the most important aspect for longer durability. The t’ phase in YSZ remains stable upto 1200 ºC without undergoing any undesirable phase transformation (cubic or monoclinic). In contrast, both the multi-layered TBCs (double & triple layer) showed a cubic (defect fluorite) phase of Gd 2Zr2O7 (JCPDS 01-78-

ACCEPTED MANUSCRIPT 4085). This defect fluorite phase (Fm3m) transforms to pyrochlore phase (Fd3m) at higher temperatures (>1400°C) and this transformation (order-disorder transition) is not associated with significant volume change as the crystal structure (cubic) remains the same [39]. Defect fluorite is a disordered structure with cations in random positions whereas pyrochlore is an ordered variation in which the cations (Gd and Zr) and oxygen vacancies occupy specific positions in the cubic crystal structure.

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3.6 Thermal cyclic fatigue test

The top coat of as sprayed TBCs were subjected to thermal cyclic test at 1300 ºC while

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maintaining the bond coat/ceramic interface at 1050 ºC. The single YSZ system was shown

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to have the lowest thermal cyclic life (43 ± 7 cycles) compared to the other two GZ based coating systems (395 & 521 cycles for double and triple layer TBCs respectively), as seen in

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Fig. 9. New results using slightly modified radial surface temperature profiles indicated even considerably higher lifetime data for the GZ based multi-layered coatings. The reasons for

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this difference as well as the low thermal fatigue life for the single layer system needs further studies. An improvement in thermal cyclic life were also reported in the case of APS sprayed GZ/YSZ double layer TBCs compared to single layer YSZ TBC [21, 40].

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Cross sections of the failed coating systems are shown in Fig. 10(a and c), 11(a and c) and

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12(a and c). SEM analysis after the thermal cyclic test of single layer YSZ revealed that the failure occurred within YSZ at about 60 µm away from the thermally grown oxide (TGO), as seen in Fig. 10(a and c). Mode of failure observed in this case differs from the published

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literature on thermal cyclic test of single layer YSZ where the failure occurred in the TBC close to the TGO [41]. The reason for the observed failure could be the interrupted spraying

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of YSZ where the spray process was halted after depositing 60 µm thick YSZ in order to maintain identical spray conditions for the single, double and triple layer TBCs. Preheating of the surface was done before deposition was continued in order to limit the effects of the interrupted spraying. In previous studies, preheating has been shown the capability of preventing issues due to start and stop during spraying. However, to validate these results, future work would involve the testing of thermal shock performance of YSZ single layer TBC deposited in one step and compare the results with the discretely sprayed YSZ. The thermally grown oxide (TGO) in this case was found to be less than 1 µm. At higher magnifications, as shown in the YSZ Fig. 10(c), the YSZ layer also showed the closure of pores

ACCEPTED MANUSCRIPT close to the horizontal crack propagation. Closure of pores due to sintering leads to the stiffening of ceramic and results in failure. In the case of the failed double and triple layer TBCs, a horizontal crack within the GZ layer close to the GZ/YSZ interface was observed, as seen in Fig. 11(a and c) & 12(a and c). As the strain is dictated by the substrate with a high thermal expansion coefficient (~ 14 x 10-6 (°C)-1) and GZ has a lower fracture toughness (1.02 ± 0.11 MPa.m1/2) than 8YSZ (~ 2 MPa.m1/2) [23], crack propagation at the GZ/YSZ interface

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becomes likely. The standard deviation in the case of BRT life of triple layer TBC was high (approximately 260 cycle). The reason for such a high standard deviation could be due to the

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slight variation (surface temperature profile of the TBC) in the burner rig test conditions. The high magnification failed SEM micrographs of double layer and triple layer TBCs showed

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closure of porosity due to sintering in the GZ layer close to the failure, as shown in Fig 11(c) and Fig. 12(c). A higher TGO thickness of approximately 2 µm was observed in the double

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and triple layer TBCs which is due to the longer lifetimes than the single layer YSZ. Similar failure close to the interface but within the GZ layer has been reported in the case of double

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layer GZ/YSZ TBCs [21, 28]. Conclusion

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In this work, a comparison of the functional performance (thermal conductivity and thermal

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cyclic life) of 8YSZ single layer and multi-layered gadolinium zirconate/YSZ TBCs was conducted. The results showed that; 

Feasible columnar microstructures could be generated for both single and multi-

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layered coating systems.

The multi-layered GZ/YSZ TBCs was shown to have a lower thermal conductivity

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compared to the single layer YSZ coating, despite the fact that the single layer system had a higher porosity content. 

The GZ/YSZ multi-layered TBCs also had a lower thermal diffusivity than the single layer YSZ TBC.

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The GZ/YSZ multi-layered TBCs were shown to have a longer thermal cycling life at 1300 ºC compared to the YSZ single layer TBC which was deposited by interrupted spray conditions.

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A denser top layer in the case of triple layer TBC could be created using a different suspension of same composition with higher median particle size and higher solid

ACCEPTED MANUSCRIPT load content. This denser layer is expected to improve both the CMAS and the erosion resistance of the TBC. This will be investigated in future work. Acknowledgements The authors would like to acknowledge the financial support from KK foundation (Dnr 20140130), Sweden. Also, the authors thank Mr. Toni Bogdanoff from Jönköping University for his assistance in thermal diffusivity measurement by laser flash analysis. The authors also

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would like to thank Dr. Daniel Mack and Mr. Martin Tandler for performing the thermal

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cyclic tests.

References

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ACCEPTED MANUSCRIPT List of Figures Fig. 1 Coating architectures of the investigated samples. Fig.2 SEM micrographs of single layer YSZ TBC (a) cross section (b) top surface morphology. Fig.3 SEM micrographs of double layer GZ/YSZ TBC (a) cross section (b) top surface morphology.

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Fig.4 SEM micrographs of triple layer GZdense/GZ/YSZ TBC (a) cross section (b) top surface morphology.

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Fig.5 Thermal diffusivity versus temperature plot of the as sprayed TBCs.

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Fig.6 Porosity content of the as sprayed TBCs obtained by water intrusion method and image analysis method.

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Fig.7 Thermal conductivity versus temperature plot of the as sprayed TBCs.

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Fig.8 X-ray diffraction patterns of the single, double and triple layer as sprayed TBCs. Fig.9 Thermal cyclic lifetime (BRT) of different TBCs at 1300 °C.

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Fig.10 BRT failed single layer YSZ (a) cross sectional SEM micrograph (b) photograph (c) high

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magnification cross sectional SEM micrograph. Fig.11 BRT failed double layer GZ/YSZ (a) cross sectional SEM micrograph (b) photograph (c) high magnification cross sectional SEM micrograph.

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Fig.12 BRT failed triple layer GZdense/GZ/YSZ (a) cross sectional SEM micrograph (b)

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photograph (c) high magnification cross sectional SEM micrograph.

List of tables

Table .I Spray parameters used for depositing the single and multi-layered TBCs. Table .II Specific heat capacity values of YSZ and GZ/YSZ coating systems

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ACCEPTED MANUSCRIPT Table 1 Spray parameters used for depositing the single and multi-layered TBCs. Columnar Coating

Dense Coating

Stand-off distance (mm)

100

70

Total Gas Flow (l/min)

300

180

Enthalpy (kJ/l)

11.2

12.5

Power (kW)

122

Exit nozzle (inch)

3/8

Atomising Gas Flow (l/min)

20

Suspension Flow Rate (ml/min)

110

Surface Speed (cm/s)

103

Step Size (mm)

5

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Parameters

5 40 103 5

ACCEPTED MANUSCRIPT Table 2. Specific heat capacities of YSZ and GZ/YSZ based coatings Cp (YSZ) J.g-1.K-1 0.5113 0.5501 0.5806 0.61 0.6211 0.6102 0.6051

Cp (GZ/YSZ) J.g-1.K-1 0.4414 0.4555 0.4639 0.4793 0.4806 0.4779 0.4316

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Temperature (°C) 25 200 400 600 800 1000 1200

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Feasible columnar microstructures could be generated for both single and multilayered coating systems.

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The multi-layered GZ/YSZ TBCs was shown to have a lower thermal conductivity compared to the single layer YSZ coating, despite the fact that the single layer system had a higher porosity content. The GZ/YSZ multi-layered TBCs also had a lower thermal diffusivity than the single

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layer YSZ TBC.

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The GZ/YSZ multi-layered TBCs were shown to have a longer thermal cycling life

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compared to the single layer YSZ TBC at 1300 ºC.

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