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High temperature oxidation resistance of plasma sprayed and surface treated YSZ coating on Hastelloy X
Accepted Manuscript High temperature oxidation resistance of plasma sprayed and surface treated YSZ coating on Hastelloy X
A. Karimi, R. Soltani, M. Ghambari, H. Fallahdoost PII: DOI: Reference:
S0257-8972(17)30458-9 doi: 10.1016/j.surfcoat.2017.05.002 SCT 22321
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
24 December 2016 16 April 2017 1 May 2017
Please cite this article as: A. Karimi, R. Soltani, M. Ghambari, H. Fallahdoost , High temperature oxidation resistance of plasma sprayed and surface treated YSZ coating on Hastelloy X, Surface & Coatings Technology (2017), doi: 10.1016/j.surfcoat.2017.05.002
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ACCEPTED MANUSCRIPT
High temperature oxidation resistance of plasma sprayed and surface treated YSZ coating on Hastelloy X
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School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran
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A. Karimia*, R. Soltania*, M. Ghambaria, H. Fallahdoostb
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Corresponding author. Email address: [email protected]
Abstract
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In This research increasing oxidation resistance of YSZ coated Hastelloy X by surface remelting was the main goal. Tungsten Inert Gas Welding (TIG) process was used to melt and solidify
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surface layers to close open porosities and seal the surface against penetration of oxidizing gas. To achieve the best surface conditions, the minimum amount of porosity and residual stresses in
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the melting zone, parameters such as current intensity, scanning velocity, distance from surface, electrode angles of TIG system and number of remelting passes were selected as main variables.
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Different microstructures and remelted zone geometries in each sample were obtained. After isothermal oxidation at 1100 °C for 40, 80, 120, 160 and 200 hours, the thickness of Thermally
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Grown Oxide (TGO) layer was investigated. The results showed that in an optimum thickness of remelted area, the thinnest TGO layer was achieved. The minimum thickness of TGO layer was observed when the melting process was carried out twice on the YSZ coating and not more than that. Keywords: Thermal barrier coatings; GTAW; Surface remelting; Oxidation test; Thermally Grown Oxide.
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1. Introduction Applying appropriate surface treatment processes on air plasma sprayed (APS) thermal barrier
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coatings based on yttria-stabilized zirconia (YSZ) to increase the cycle life of Thermal barrier
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coating (TBCs), have been a target to many researches during last decades [1-3]. These surface treatments have been applied to hot work components of gas turbine engines to ameliorate
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working durability and resistance to oxidation in order to protect valuable superalloy substrate from hot corrosion or erosion and also progress energy efficiency of engines at elevated
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temperature due to its outstanding thermal insulation performance [4, 5]. TBC systems generally
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contain three individual parts: (i) thermal insulation agent called topcoat (TC), (ii) the superalloy substrate and (iii) an adhesive agent between the topcoat and the substrate called bondcoat (BC)
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[6, 7].TBCs are exposed to high temperature oxidation that is unavoidable during the long-term
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service, leads to form an interface layer. Thermally grown oxidation (TGO) is a thin layer which forms between the topcoat and the bondcoat during high temperature operation. This layer is
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mainly composed of metallic oxides like Al2O3, NiO, Chromia ((Cr, Al)2O3), Spinel (Ni(Cr,
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Al)2O4) and other oxides according to composition of topcoat and bondcoat. Formation of TGO layer is mainly due to diffusion and reaction of oxygen and TC/BC during extensive thermal exposure [8, 9]. Because of great difference between thermal expansion coefficient and elastic modulus of TGO with vicinity layers, increasing the thickness of TGO layer during hot work will develop residual stresses at the interface of TGO/TC and TGO/BC and intensify thermal mismatch in the TBC systems. Spalling off and delamination which result from crack initiation and propagation at the interface of TGO with TC and BC lead to coating failure [10]. 2
ACCEPTED MANUSCRIPT Furthermore, even with high-tech surface remelting, oxygen diffuses through YSZ in ionic path because of high density of oxygen vacancies at high temperatures. Coatings applied by APS method usually have lamellar structure accompanied by high volume fraction of interconnected porosities which can ease oxygen diffusion through topcoat [11-13]. Thus, investigating the
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parameters which have influence on TGO layer growth rate will lead to assess and predict
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durability of thermal exposure components with TBCs. different kinds of surface-sealing
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treatments have been suggested to improve the surface condition of YSZ topcoat in order to restrain TGO growth such as surface remelting, sol-gel process, chemical surface densifying and
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phosphating sealing [14, 15]. Surface remelting processes seems to be more applicable among
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other surface-sealing treatments, because the output dense layer at the YSZ topcoat can decrease the conductivity of oxygen and subsequently decelerate the TGO growth rate [16]. Although
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inappropriate surfaces remelting process makes some cracks perpendicular to the TC after
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solidification, by proceeding 2 or 3 times remelting, a homogenous remelted surface will form which is resistant to oxygen diffusion.
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Generating cracks after remelting and solidification is expected to some extent. Increasing the
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amount of cracks could increase interconnected paths for oxygen transfusion and consequently destruct the remelted layer [17]. Using GTAW method makes remelting process more
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controllable through varying parameters. Besides, more efficiency with low cost, accessibility of equipment, simplicity of technology as well as a lot of variable parameters is expected via GTAW compared with laser techniques for surface remelting [18, 19]. Despite advanced surface treatments, TBCs are still suffering from high temperature oxidation, corrosion and other damages at elevated temperatures. It seems that more investigation needed to invent new sealing-surface process. 3
ACCEPTED MANUSCRIPT In this research, a novel surface remelting process was applied to the TBCs by a modified GTAW (Gas Tungsten Arc Welding) device [18]. X-ray analysis was conducted to recognize the important elements/phased in the coatings. Three main parameters of GTAW remelting process (applied current intensity, scanning speed and number of remelting passes) were altered to
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investigate the influence of each parameter on remelted area depth and consequently thickness of
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the formed TGO layer after thermal exposure up to 1100o C for up to 200 h. Then, scanning
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electron microscopy (SEM) equipped with Energy-dispersive X-ray (EDS) investigation was
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performed to analyze the outcome composition and thickness of TGO layer.
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2. Experimental procedures
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2.1. Sample preparation
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A nickel-based superalloy (Hastelloy X) sheet was cut into the dimensions of 15×15×3 mm as substrate. This alloy with good strength at high temperatures is widely used in gas turbine
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engines, industrial furnaces applications and chemical process components. Chemical
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composition of Hastelloy X is brought in Table 1. Ni-22Cr-10Al-1Y (AMDRY 962-OerlikonMetco) powder with a particle size distribution of -106+53 μm and 8 wt.% Y2O3 stabilized ZrO2
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(204 NS- Oerlikon- Metco) powder with a particle size of -125+11 μm were used as bond and topcoat feedstock, respectively. Air plasma sprayed coating process was done using triple cathode gun (Northewest Mettech Crop., Canada) with axial powder feed. Parameters of the APS process are shown in Table 2. The average achieved thickness of YSZ topcoat and NiCrAlY bondcoat were approximately 350 µm and 150 µm, respectively. Figure 1 shows a typical cross section of specimens demonstrating these two layers.
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ACCEPTED MANUSCRIPT 2.2. Remelting process Since Thermal barrier coatings are not conductive they can’t be melted with TIG system with transferred arc. To overcome this problem, the TBC specimens were surface remelted by modified twin head GTAW technique [18]. The modification has done to conventional GTAW
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apparatus to enable remelting of different materials or coatings containing low thermal and
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electrical conductivity or even non-conductive compositions. Schematic of the modified GTAW
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is shown in Figure 2. In this method, thermal heat was produced via electric arc glowing between two tungsten electrodes on the top of the coating with a distance and no involvement of the
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coating surface in formation and stabilization of the arc. Thermal heat produced by this method
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which could be controlled via changing GTAW parameters, is transferred into the surface via convention and radiation. Molten material and electrodes’ tips are protected by argon gas from
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oxidation.
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This apparatus contains a pair of modified GTAW electrode, an automatic workbench to move
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the sample with precise velocity and a controller that is connected to the transformer device. Parameters involved in the outcome thermal heat could be categorized into three groups,
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electrical, process and electrode parameters. These three groups of parameters are listed in Table
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3. Based on the initial set of experiments, it was decided to change three items from this table; current intensity, scanning velocity and number of melting passes. According to variable parameters, different specimens were prepared to oxidation test. Codes of remelted samples are listed in Table 4. 2.3. Oxidation process In order to investigate oxidation behavior, all the specimens including surface remelted and nonremelted ones, were put into a high temperature electrical furnace with controlled heating and 5
ACCEPTED MANUSCRIPT cooling rates. Isothermal oxidation was performed in static air at atmospheric pressure. All samples were heated to 1100 o C from ambient temperature with the heating rate of 3 o C/min. the effect of process parameters on the formation and thickness of TGO layer, as a failure mechanism of coating, was investigated. The samples were cooled down to room temperature in
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the furnace after 40 h, 80 h, 120 h, 160 h and 200 h. The thickness of TGO layer was selected as
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a scale of oxidation behavior. 2.4. Analysis process
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To analyze different phase formation in the coats, an X-ray diffractometer Equinox 3000 (Inel, France) was used with Cu Kα radiation (λ = 1.54056) over 2Ɵ ranges from 20o to 80o with a step
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scan of 0.03 o/s was used. The surface and cross sections of all specimens were studied using
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scanning electron microscopy (JSM-6360, JEOL, Japan) equipped with energy-dispersive X-ray spectroscopy (EDS) in normal mode and secondary electron (SE) mode. Thickness
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measurements of remelted area and TGO layer were gained according to ASTM: B748-
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thin layers in images.
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90(2016). Furthermore, Digimizer and Photoshop softwares are used to precise measurements of
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3. Results and discussion 3.1. Microstructure SEM micrographs of as-sprayed and surface remelted samples with varying parameters of current intensity and number of remelting passes are demonstrated in Figure 3. As-sprayed YSZ with rough surface and lots of cracks which is coated over NiCrAlY bondcoat is shown. By remelting YSZ topcoat with current intensity above 75 A, numbers of cracks and pores are decreased and uniformity is considerably increased. The Fewer microcracks in the zirconia based 6
ACCEPTED MANUSCRIPT topcoat seals the coatings surface which will results in inferior oxygen diffusion to the bond coat leading to a thinner TGO layer, consequently [20]. Applying remelting process two times or more in a row could efficiently decrease microcracks and increase the depth of remelting area compared with one pass remelting. Because most of the microcracks and pores would be closed
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after first remelt, the heat could be transferred to the lower layers of YSZ at the second pass of
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remelting. Remelting YSZ coating more than two times was performed however, a smooth low
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crack surface was never achieved due to high energy input leading to surface destruction. Furthermore, increasing current intensity causes a growth in remelted area depth as it is seen in
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cross-section views.
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Figure 4 indicated the effect of electrical current intensity and scanning speed on the remelted
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area depth formed on the specimens. Remelting depths are reported as a percentage of the whole YSZ topcoat thickness .As it is clear, with increasing the current, the remelting depth percentage
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increases. Current intensity values lower than 75 A didn’t form any remelted area. Besides, the
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heat produced by current intensity higher than 110 A destroy the YSZ topcoat. According to the results, it seems that varying scanning velocity in the range of 100 to 200 mm/min does not have
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any sensible change on remelting depth. Applied current for GTAW surface remelting process is
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the most important parameter affecting the depth of remelted area. Obtaining uniform remelted surface under approximately 8% remelted depth with GTAW process seems practically unachievable. However, with increasing applied current the percentage of remelting depth area could be increased leading to an increase in uniformity. On the other hand, the thicker remelted depth, the higher could be thermal conductivity which is inappropriate in TBCs performance and oxidation resistance. Thus, specimens with thicker remelted area may not always form thinner TGO layer or better oxidation resistance. 7
ACCEPTED MANUSCRIPT Figure 5 represents the influence of remelting passes on the depth of remelted area in four different applied current values and scanning speed of 200 mm/min. It is seen that with increasing current, remelted depth will increase in any times of remelting. Applying two or more remelting process on YSZ topcoat is usually carried out to achieve more homogenous surface
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which result in deeper remelting area. Repeating surface remelting process causes closing pores
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and cracks on surface, leads to easily transfer heat throughout the coat. Subsequently, by
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applying 105 A after three passes of remelting process, a remelting depth of approximately 60%
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YSZ topcoat could be achieved. 3.2. XRD analysis
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XRD patterns of as-sprayed YSZ topcoat and remelted coatings are presented in Figure 6.
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Totally, ZrO2 shows three polymorphs known as monoclinic, tetragonal and cubic phase. Tetragonal phase has two types, non-transformable and transformable phase which named as t´
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and t, respectively. In plasma sprayed YSZ coatings, due to very high solidification rates, the
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majority of the phases are non-transformable tetragonal phase (t´) [3, 21]. At ambient temperature, monoclinic phase is stable in zirconium oxide and phase transition to tetragonal and
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cubic is done at higher temperatures (<1170 oC).Transformation of cubic to tetragonal and then
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to monoclinic phase is carried on with volume expansion and large stresses which leads to destruct the coat upon cooling from high temperatures (2370 oC). However, doping zirconia with other oxides e.g. yttrium oxide stabilizes tetragonal and cubic phases to some extent. Thus, in order to estimate the quality of proceeded YSZ topcoats, volume fraction of each phase could be calculated. To obtain volume fraction of monoclinic phase equation (1) was suggested, where Xm is the integrated intensity ratio expressed as equation (2) [3, 22]: 8
ACCEPTED MANUSCRIPT (1)
(2)
and
represent the intensity of monoclinic and non-
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,
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deformable tetragonal phase peak obtained from XRD patterns.
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Where
Furthermore, by obtaining the mole fraction ratio Mc/Mt/ the volume fractions of all phases could
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be calculated. The mole fraction ratio of cubic phase and tetragonal can be obtained by equation
(3)
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(3) [22].
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Where Ic represent the intensity of cubic phase. The calculated all three phases volume fraction of selected samples are summarized in Table 5. Because of different cooling rates, percentage of
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monoclinic, tetragonal and cubic phases are different in specimens. As it was expected, surface
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remelting of sample with current intensity of 105 A, showed the highest content of monoclinic phase which may consequently lead to highest volume expansion due to increased energy input.
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This may result in more cracks and lower oxidation resistance. Cubic phase content in the specimens is not vital in estimating vulnerability of a coating due to phase transformation of cubic to tetragonal since it takes a lot of time and also is carried out in temperatures higher than working temperatures. Thus, with decreasing monoclinic phase the probability of crack formation due to volume change from phase transformation is decreased.
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3.3. TGO layer Oxygen passed through YSZ topcoat reacts with elements of BC, forming an oxide layer called
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thermally grown oxide (TGO) layer at elevated temperatures. The growth of this layer introduces
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stress at the interface of TC and BC responsible for crack initiation leading to spallation of coating at the interface. Residual stress produced due to difference in thermal expansion between
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TGO and vicinity layers (NiCrAlY bondcoat and YSZ topcoat) are released as tensile stress parallel to the direction of TGO stream result in cracking near interface zone and spalling off the
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coating [23, 24]. Thus, thicker TGO layer undoubtedly results in greater stresses causing coating
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destruction. Figure 7 shows SEM image and EDS analysis of C100 R1 S200 sample crosssection view after 200 h oxidation. As it is shown, the formed TGO layer is mainly consisted of
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chromium, aluminum, nickel and oxygen. A trace of aluminum oxide is also seen below TGO
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layer which may show diffusion of oxygen element even beneath the surface of bondcoat. Topcoats and bondcoats applied by air plasma spray have rough surfaces with microcracks and
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cavities. Existence of pores and cracks in YSZ coatings produced by APS are inevitable [25].
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However, they could partially decrease thermal conductivity. Generally, transportation of oxygen from YSZ topcoat has two main mechanisms. One way is the diffusion of oxygen in the gas phase (gas permeation) through interconnected cracks; the other one is oxygen ion migration through the YSZ lattice [26]. As it is shown in
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ACCEPTED MANUSCRIPT Figure 8, YSZ topcoat is a good electrolyte for oxygen ion conduction in high temperatures but this amount of transfusion is minor compared to gas permeation before the meeting point of two curves. At the meeting point which represents a specific temperature, oxygen flux through YSZ topcoat is equal in two mechanisms. Working temperatures of TBCs are usually lower than the
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meeting temperatures of two transportation mechanisms. Thus, in service condition TGO growth
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is more affected by gas permeation mechanism than ionic diffusion. This fact clearly verifies the
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importance of surface sealing treatments on oxidation behavior of TBCs in extremely hot work service. Irregular thickness of TGO layer (Figure 9) is influenced by two main issues; lamellar
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structure of APS coating and uneven surface of bondcoat. Besides, there are many random cracks
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and pores which cause variation in oxygen diffusion and subsequently non-uniform TGO layer. Uneven growth of TGO layer makes precise thickness measurement difficult. In 20 different
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positions the thickness of the layer in cross-section view of SEM images were measured and the
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average was reported for each sample as it is typically shown in Figure 9 for the C100 R1 S200 and C105 R1 S200 samples after 160 h of oxidation at 1100 ˚C. After 40 h oxidation at 1100 oC,
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TGO layer was formed at the interface of topcoat and bondcoat for all specimens. Figure 10
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represent the thickness of TGO layer as a function of remelting depth and oxidation time for samples remelted with only one pass. Clearly, TGO thickness in the non-remelted samples (0%
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of remelting depth) is the thickest one in every oxidation time. This graph confirms that applying remelting process could decrease TGO layer thickness efficiently. As it is clear from Figure 10 with increasing remelted depth, the TGO thickness decreases to some extent and then increases gradually. The minimum amount of TGO thickness in all oxidation times after one pass of remelting was obtained at current intensity of 100 A, scanning speed of 200 mm/min and remelting depth of 38% (colored zone). In remelting depths deeper than 38%, TGO thickness
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ACCEPTED MANUSCRIPT gets thicker in all the oxidation times. Applying higher currents causes more energy input leading to deeper remelting area. This results in an increase of surface crack density (Figure 3), due to higher residual stresses. In this research, remelted depth of 38% seems to be an optimal melting thickness which higher than that the density of cracks will be very high and again TGO
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thickness increases significantly to eliminate sealing effect of remelting.
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Figure 11 demonstrated TGO thickness as a function of remelting depth and oxidation times. The minimum values for TGO thickness was achieved in 39% remelted depth in every oxidation
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times except in 40 h oxidation for samples remelted twice. This trend was seen in samples remelted one time however, after two times remelting in a row, the minimum thickness of TGO
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was achieved at current of 95 A. It should be noted that TGO thickness is decreased with one
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more surface remelting compared to one time remelting. The microstructure of samples with double times remelting (Figure 3) is more uniform with less surface cracks. It seems that the first
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remelting pass preheats the surface for the second pass and subsequently the cooling rate
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decreased and less thermal shock was tolerated by the coating. SEM cross-section images of non-remelted and remelted samples are shown in Figure 12 with the focus on TGO layer after
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200 h of oxidation test. With increasing remelting depth area, the thickness of TGO layer
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decreases. At the depth of 38% for samples remelted once and 39% for samples remelted twice the minimum amount of TGO thicknesses were achieved. After obtaining optimum amount of remelting depth, the effect of residual stress and crack density overcome the effect of increased remelting area depth in oxygen transfusion. Thus, TGO layer will be thicker in all samples after more hours of oxidation. With increasing the thickness of TGO layer, the amount of accumulated residual stress and the rate of crack initiation and propagation around the layer are enlarged.
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4. Conclusions Surface remelting process employing GTAW was conducted on the YSZ thermal barrier coatings in order to seal the surface openings and therefore minimize the thickness of thermally
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grown oxides layer as a function of oxidation behavior. In this research, current intensity,
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scanning velocity and number of remelting passes in GTAW process were varied and below results were obtained:
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1) Surface remelting of TBCs was done using GTAW method with applied current intensity
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in the range of 75 to 110 A. In currents below 75 A smooth remelted areas was not form and upper 110 A remelted area formed with lots of cracks due to excessive energy input.
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2) Surface remelting seems to have efficient role on sealing the coat however, applying
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more than two passes in a row showed no sensible change due excessive stress impose to underlying layers leading to an increase in crack density.
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3) Minimum thicknesses of TGO layer was formed in samples with 38% to 40% of YSZ
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remelted depth. The thinnest TGO layer was formed in sample remelted twice with 95 A
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current and scanning velocities of 200 mm/min.
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ACCEPTED MANUSCRIPT Reference
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ACCEPTED MANUSCRIPT Table Captions: Table 1- Chemical composition of Hastelloy X in wt. %. Table 2- Parameters of APS process applied to produce the bondcoat and the topcoat. Table 3- The parameters of GTAW affecting remelting process of the topcoat.
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Table 4- Sample coding according to variable parameters of remelting process.
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Table 5- Phase content of the as-sprayed and GTAW surface remelted samples.
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ACCEPTED MANUSCRIPT Tables:
Table 1 Cr
Mo
Mn
C
Si
Fe
Balance
23
10
1
0.15
1
20
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Ni
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Table 2 Bondcoat (NiCrAlY)
Topcoat (YSZ)
Current (A)
620
620
Voltage (V)
60-70
60-70
Primary gas flow, Ar (L/min)
50
Secondary gas flow, H2 (L/min)
15
Spray distance (mm)
100
Powder feed rate (g/min)
25
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Parameters
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Table 3 Value/Type
Electric current type
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Current intensity (A)
70-110
Voltage (v)
Remelting passes
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Direction
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Scanning speed (mm/min)
Process parameters
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Argon flow rate (L/min)
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100, 150, 200 1, 2, 3 Perpendicular 7 Air cooling
Cooling of electrode
Water cooling
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Cooling of substrate
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Electrode diameter (mm)
3.2
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1
Distance from surface (mm)
1
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Electrode parameters
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Electrical parameters
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GTAW parameters (unit)
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Table 4 Symbol
Parameter
Current intensity (A) Remelting passes Scanning speed (mm/min)
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70,75,80,85,90,95,100,105,110 1,2,3 100, 150,200
C70 R1 S150
Current intensity : 70
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Table 5 Samples Phase
GTAW surface remelted
As-sprayed C95 R1 S200
C105 R1 S200
C95 R2 S200
content Tetragonal
84.9
92.4
89.8
92.3
Monoclinic
8.1
4.6
5.2
Cubic
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(non-remelted)
3.7 4
ACCEPTED MANUSCRIPT Figure Captions:
Figure 1- Typical SEM image of applied thermal barrier coating via APS technique. Figure 2- Schematic of the modified GTAW apparatus used in this research.
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Figure 3- SEM images of top view and cross-section view of non-remelted and remelted samples with different parameters of GTAW (scanning speed was selected to 200 mm/min).
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Figure 4- Remelting depth percentage as a function of current intensity in three scanning speeds.
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Figure 5- Remelting depth percentage as a function of remelting passes for different currents.
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Figure 6- XRD pattern in the range of 20 o to 80o of a) as-sprayed YSZ, b) C95 R1 S200, c) C105 R1 S200 and d) C95 R2 S200.
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Figure 7- SEM image of formed TGO layer after 200 h oxidation for C100 R1 S200 sample along with elemental distribution of O, Al, Ni, Cr and Y.
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Figure 8- Schematic of oxygen transfusion through YSZ topcoat in two ways, a) ionic diffusion and b) gas permeation [27].
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Figure 9- SEM images for cross-section view of a) C100 R1 S200 and b) C105 R1 S200 after 160 h isothermal oxidation test showing TGO layer; thickness was measured in several positions.
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Figure 10- The trend of TGO thickness as a function of remelting depth percentage in 5 different remelting depths and oxidation times.
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Figure 11- The trend of TGO thickness as a function of remelting depth percentage in 5 different remelting depths and oxidation times for samples surface remelted twice.
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Figure 12- SEM micrographs of a) non-remelted, b) C95 R1 S200, c) C105 R1 S200 and d) C95 R2 S200 samples after 200 h oxidation.
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights Surface remelting process was done on YSZ coat using modified GTAW method. Modified GTAW supply varying parameters to acquire different qualities of remelt. TGO Thickness was measured after oxidation to compare coat sealing of samples.
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In an optimum thickness of remelted depth, the thinnest of TGO layer was achieved.
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A. Karimi, R. Soltani, M. Ghambari, H. Fallahdoost PII: DOI: Reference:
S0257-8972(17)30458-9 doi: 10.1016/j.surfcoat.2017.05.002 SCT 22321
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
24 December 2016 16 April 2017 1 May 2017
Please cite this article as: A. Karimi, R. Soltani, M. Ghambari, H. Fallahdoost , High temperature oxidation resistance of plasma sprayed and surface treated YSZ coating on Hastelloy X, Surface & Coatings Technology (2017), doi: 10.1016/j.surfcoat.2017.05.002
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.
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High temperature oxidation resistance of plasma sprayed and surface treated YSZ coating on Hastelloy X
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School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran
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A. Karimia*, R. Soltania*, M. Ghambaria, H. Fallahdoostb
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Corresponding author. Email address: [email protected]
Abstract
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In This research increasing oxidation resistance of YSZ coated Hastelloy X by surface remelting was the main goal. Tungsten Inert Gas Welding (TIG) process was used to melt and solidify
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surface layers to close open porosities and seal the surface against penetration of oxidizing gas. To achieve the best surface conditions, the minimum amount of porosity and residual stresses in
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the melting zone, parameters such as current intensity, scanning velocity, distance from surface, electrode angles of TIG system and number of remelting passes were selected as main variables.
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Different microstructures and remelted zone geometries in each sample were obtained. After isothermal oxidation at 1100 °C for 40, 80, 120, 160 and 200 hours, the thickness of Thermally
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Grown Oxide (TGO) layer was investigated. The results showed that in an optimum thickness of remelted area, the thinnest TGO layer was achieved. The minimum thickness of TGO layer was observed when the melting process was carried out twice on the YSZ coating and not more than that. Keywords: Thermal barrier coatings; GTAW; Surface remelting; Oxidation test; Thermally Grown Oxide.
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1. Introduction Applying appropriate surface treatment processes on air plasma sprayed (APS) thermal barrier
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coatings based on yttria-stabilized zirconia (YSZ) to increase the cycle life of Thermal barrier
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coating (TBCs), have been a target to many researches during last decades [1-3]. These surface treatments have been applied to hot work components of gas turbine engines to ameliorate
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working durability and resistance to oxidation in order to protect valuable superalloy substrate from hot corrosion or erosion and also progress energy efficiency of engines at elevated
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temperature due to its outstanding thermal insulation performance [4, 5]. TBC systems generally
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contain three individual parts: (i) thermal insulation agent called topcoat (TC), (ii) the superalloy substrate and (iii) an adhesive agent between the topcoat and the substrate called bondcoat (BC)
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[6, 7].TBCs are exposed to high temperature oxidation that is unavoidable during the long-term
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service, leads to form an interface layer. Thermally grown oxidation (TGO) is a thin layer which forms between the topcoat and the bondcoat during high temperature operation. This layer is
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mainly composed of metallic oxides like Al2O3, NiO, Chromia ((Cr, Al)2O3), Spinel (Ni(Cr,
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Al)2O4) and other oxides according to composition of topcoat and bondcoat. Formation of TGO layer is mainly due to diffusion and reaction of oxygen and TC/BC during extensive thermal exposure [8, 9]. Because of great difference between thermal expansion coefficient and elastic modulus of TGO with vicinity layers, increasing the thickness of TGO layer during hot work will develop residual stresses at the interface of TGO/TC and TGO/BC and intensify thermal mismatch in the TBC systems. Spalling off and delamination which result from crack initiation and propagation at the interface of TGO with TC and BC lead to coating failure [10]. 2
ACCEPTED MANUSCRIPT Furthermore, even with high-tech surface remelting, oxygen diffuses through YSZ in ionic path because of high density of oxygen vacancies at high temperatures. Coatings applied by APS method usually have lamellar structure accompanied by high volume fraction of interconnected porosities which can ease oxygen diffusion through topcoat [11-13]. Thus, investigating the
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parameters which have influence on TGO layer growth rate will lead to assess and predict
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durability of thermal exposure components with TBCs. different kinds of surface-sealing
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treatments have been suggested to improve the surface condition of YSZ topcoat in order to restrain TGO growth such as surface remelting, sol-gel process, chemical surface densifying and
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phosphating sealing [14, 15]. Surface remelting processes seems to be more applicable among
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other surface-sealing treatments, because the output dense layer at the YSZ topcoat can decrease the conductivity of oxygen and subsequently decelerate the TGO growth rate [16]. Although
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inappropriate surfaces remelting process makes some cracks perpendicular to the TC after
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solidification, by proceeding 2 or 3 times remelting, a homogenous remelted surface will form which is resistant to oxygen diffusion.
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Generating cracks after remelting and solidification is expected to some extent. Increasing the
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amount of cracks could increase interconnected paths for oxygen transfusion and consequently destruct the remelted layer [17]. Using GTAW method makes remelting process more
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controllable through varying parameters. Besides, more efficiency with low cost, accessibility of equipment, simplicity of technology as well as a lot of variable parameters is expected via GTAW compared with laser techniques for surface remelting [18, 19]. Despite advanced surface treatments, TBCs are still suffering from high temperature oxidation, corrosion and other damages at elevated temperatures. It seems that more investigation needed to invent new sealing-surface process. 3
ACCEPTED MANUSCRIPT In this research, a novel surface remelting process was applied to the TBCs by a modified GTAW (Gas Tungsten Arc Welding) device [18]. X-ray analysis was conducted to recognize the important elements/phased in the coatings. Three main parameters of GTAW remelting process (applied current intensity, scanning speed and number of remelting passes) were altered to
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investigate the influence of each parameter on remelted area depth and consequently thickness of
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the formed TGO layer after thermal exposure up to 1100o C for up to 200 h. Then, scanning
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electron microscopy (SEM) equipped with Energy-dispersive X-ray (EDS) investigation was
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performed to analyze the outcome composition and thickness of TGO layer.
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2. Experimental procedures
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2.1. Sample preparation
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A nickel-based superalloy (Hastelloy X) sheet was cut into the dimensions of 15×15×3 mm as substrate. This alloy with good strength at high temperatures is widely used in gas turbine
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engines, industrial furnaces applications and chemical process components. Chemical
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composition of Hastelloy X is brought in Table 1. Ni-22Cr-10Al-1Y (AMDRY 962-OerlikonMetco) powder with a particle size distribution of -106+53 μm and 8 wt.% Y2O3 stabilized ZrO2
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(204 NS- Oerlikon- Metco) powder with a particle size of -125+11 μm were used as bond and topcoat feedstock, respectively. Air plasma sprayed coating process was done using triple cathode gun (Northewest Mettech Crop., Canada) with axial powder feed. Parameters of the APS process are shown in Table 2. The average achieved thickness of YSZ topcoat and NiCrAlY bondcoat were approximately 350 µm and 150 µm, respectively. Figure 1 shows a typical cross section of specimens demonstrating these two layers.
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ACCEPTED MANUSCRIPT 2.2. Remelting process Since Thermal barrier coatings are not conductive they can’t be melted with TIG system with transferred arc. To overcome this problem, the TBC specimens were surface remelted by modified twin head GTAW technique [18]. The modification has done to conventional GTAW
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apparatus to enable remelting of different materials or coatings containing low thermal and
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electrical conductivity or even non-conductive compositions. Schematic of the modified GTAW
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is shown in Figure 2. In this method, thermal heat was produced via electric arc glowing between two tungsten electrodes on the top of the coating with a distance and no involvement of the
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coating surface in formation and stabilization of the arc. Thermal heat produced by this method
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which could be controlled via changing GTAW parameters, is transferred into the surface via convention and radiation. Molten material and electrodes’ tips are protected by argon gas from
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oxidation.
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This apparatus contains a pair of modified GTAW electrode, an automatic workbench to move
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the sample with precise velocity and a controller that is connected to the transformer device. Parameters involved in the outcome thermal heat could be categorized into three groups,
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electrical, process and electrode parameters. These three groups of parameters are listed in Table
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3. Based on the initial set of experiments, it was decided to change three items from this table; current intensity, scanning velocity and number of melting passes. According to variable parameters, different specimens were prepared to oxidation test. Codes of remelted samples are listed in Table 4. 2.3. Oxidation process In order to investigate oxidation behavior, all the specimens including surface remelted and nonremelted ones, were put into a high temperature electrical furnace with controlled heating and 5
ACCEPTED MANUSCRIPT cooling rates. Isothermal oxidation was performed in static air at atmospheric pressure. All samples were heated to 1100 o C from ambient temperature with the heating rate of 3 o C/min. the effect of process parameters on the formation and thickness of TGO layer, as a failure mechanism of coating, was investigated. The samples were cooled down to room temperature in
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the furnace after 40 h, 80 h, 120 h, 160 h and 200 h. The thickness of TGO layer was selected as
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a scale of oxidation behavior. 2.4. Analysis process
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To analyze different phase formation in the coats, an X-ray diffractometer Equinox 3000 (Inel, France) was used with Cu Kα radiation (λ = 1.54056) over 2Ɵ ranges from 20o to 80o with a step
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scan of 0.03 o/s was used. The surface and cross sections of all specimens were studied using
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scanning electron microscopy (JSM-6360, JEOL, Japan) equipped with energy-dispersive X-ray spectroscopy (EDS) in normal mode and secondary electron (SE) mode. Thickness
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measurements of remelted area and TGO layer were gained according to ASTM: B748-
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thin layers in images.
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90(2016). Furthermore, Digimizer and Photoshop softwares are used to precise measurements of
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3. Results and discussion 3.1. Microstructure SEM micrographs of as-sprayed and surface remelted samples with varying parameters of current intensity and number of remelting passes are demonstrated in Figure 3. As-sprayed YSZ with rough surface and lots of cracks which is coated over NiCrAlY bondcoat is shown. By remelting YSZ topcoat with current intensity above 75 A, numbers of cracks and pores are decreased and uniformity is considerably increased. The Fewer microcracks in the zirconia based 6
ACCEPTED MANUSCRIPT topcoat seals the coatings surface which will results in inferior oxygen diffusion to the bond coat leading to a thinner TGO layer, consequently [20]. Applying remelting process two times or more in a row could efficiently decrease microcracks and increase the depth of remelting area compared with one pass remelting. Because most of the microcracks and pores would be closed
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after first remelt, the heat could be transferred to the lower layers of YSZ at the second pass of
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remelting. Remelting YSZ coating more than two times was performed however, a smooth low
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crack surface was never achieved due to high energy input leading to surface destruction. Furthermore, increasing current intensity causes a growth in remelted area depth as it is seen in
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cross-section views.
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Figure 4 indicated the effect of electrical current intensity and scanning speed on the remelted
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area depth formed on the specimens. Remelting depths are reported as a percentage of the whole YSZ topcoat thickness .As it is clear, with increasing the current, the remelting depth percentage
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increases. Current intensity values lower than 75 A didn’t form any remelted area. Besides, the
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heat produced by current intensity higher than 110 A destroy the YSZ topcoat. According to the results, it seems that varying scanning velocity in the range of 100 to 200 mm/min does not have
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any sensible change on remelting depth. Applied current for GTAW surface remelting process is
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the most important parameter affecting the depth of remelted area. Obtaining uniform remelted surface under approximately 8% remelted depth with GTAW process seems practically unachievable. However, with increasing applied current the percentage of remelting depth area could be increased leading to an increase in uniformity. On the other hand, the thicker remelted depth, the higher could be thermal conductivity which is inappropriate in TBCs performance and oxidation resistance. Thus, specimens with thicker remelted area may not always form thinner TGO layer or better oxidation resistance. 7
ACCEPTED MANUSCRIPT Figure 5 represents the influence of remelting passes on the depth of remelted area in four different applied current values and scanning speed of 200 mm/min. It is seen that with increasing current, remelted depth will increase in any times of remelting. Applying two or more remelting process on YSZ topcoat is usually carried out to achieve more homogenous surface
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which result in deeper remelting area. Repeating surface remelting process causes closing pores
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and cracks on surface, leads to easily transfer heat throughout the coat. Subsequently, by
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applying 105 A after three passes of remelting process, a remelting depth of approximately 60%
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YSZ topcoat could be achieved. 3.2. XRD analysis
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XRD patterns of as-sprayed YSZ topcoat and remelted coatings are presented in Figure 6.
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Totally, ZrO2 shows three polymorphs known as monoclinic, tetragonal and cubic phase. Tetragonal phase has two types, non-transformable and transformable phase which named as t´
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and t, respectively. In plasma sprayed YSZ coatings, due to very high solidification rates, the
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majority of the phases are non-transformable tetragonal phase (t´) [3, 21]. At ambient temperature, monoclinic phase is stable in zirconium oxide and phase transition to tetragonal and
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cubic is done at higher temperatures (<1170 oC).Transformation of cubic to tetragonal and then
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to monoclinic phase is carried on with volume expansion and large stresses which leads to destruct the coat upon cooling from high temperatures (2370 oC). However, doping zirconia with other oxides e.g. yttrium oxide stabilizes tetragonal and cubic phases to some extent. Thus, in order to estimate the quality of proceeded YSZ topcoats, volume fraction of each phase could be calculated. To obtain volume fraction of monoclinic phase equation (1) was suggested, where Xm is the integrated intensity ratio expressed as equation (2) [3, 22]: 8
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(2)
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represent the intensity of monoclinic and non-
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deformable tetragonal phase peak obtained from XRD patterns.
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Where
Furthermore, by obtaining the mole fraction ratio Mc/Mt/ the volume fractions of all phases could
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be calculated. The mole fraction ratio of cubic phase and tetragonal can be obtained by equation
(3)
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(3) [22].
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Where Ic represent the intensity of cubic phase. The calculated all three phases volume fraction of selected samples are summarized in Table 5. Because of different cooling rates, percentage of
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monoclinic, tetragonal and cubic phases are different in specimens. As it was expected, surface
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remelting of sample with current intensity of 105 A, showed the highest content of monoclinic phase which may consequently lead to highest volume expansion due to increased energy input.
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This may result in more cracks and lower oxidation resistance. Cubic phase content in the specimens is not vital in estimating vulnerability of a coating due to phase transformation of cubic to tetragonal since it takes a lot of time and also is carried out in temperatures higher than working temperatures. Thus, with decreasing monoclinic phase the probability of crack formation due to volume change from phase transformation is decreased.
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3.3. TGO layer Oxygen passed through YSZ topcoat reacts with elements of BC, forming an oxide layer called
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thermally grown oxide (TGO) layer at elevated temperatures. The growth of this layer introduces
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stress at the interface of TC and BC responsible for crack initiation leading to spallation of coating at the interface. Residual stress produced due to difference in thermal expansion between
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TGO and vicinity layers (NiCrAlY bondcoat and YSZ topcoat) are released as tensile stress parallel to the direction of TGO stream result in cracking near interface zone and spalling off the
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coating [23, 24]. Thus, thicker TGO layer undoubtedly results in greater stresses causing coating
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destruction. Figure 7 shows SEM image and EDS analysis of C100 R1 S200 sample crosssection view after 200 h oxidation. As it is shown, the formed TGO layer is mainly consisted of
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chromium, aluminum, nickel and oxygen. A trace of aluminum oxide is also seen below TGO
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layer which may show diffusion of oxygen element even beneath the surface of bondcoat. Topcoats and bondcoats applied by air plasma spray have rough surfaces with microcracks and
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cavities. Existence of pores and cracks in YSZ coatings produced by APS are inevitable [25].
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However, they could partially decrease thermal conductivity. Generally, transportation of oxygen from YSZ topcoat has two main mechanisms. One way is the diffusion of oxygen in the gas phase (gas permeation) through interconnected cracks; the other one is oxygen ion migration through the YSZ lattice [26]. As it is shown in
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ACCEPTED MANUSCRIPT Figure 8, YSZ topcoat is a good electrolyte for oxygen ion conduction in high temperatures but this amount of transfusion is minor compared to gas permeation before the meeting point of two curves. At the meeting point which represents a specific temperature, oxygen flux through YSZ topcoat is equal in two mechanisms. Working temperatures of TBCs are usually lower than the
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meeting temperatures of two transportation mechanisms. Thus, in service condition TGO growth
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is more affected by gas permeation mechanism than ionic diffusion. This fact clearly verifies the
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importance of surface sealing treatments on oxidation behavior of TBCs in extremely hot work service. Irregular thickness of TGO layer (Figure 9) is influenced by two main issues; lamellar
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structure of APS coating and uneven surface of bondcoat. Besides, there are many random cracks
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and pores which cause variation in oxygen diffusion and subsequently non-uniform TGO layer. Uneven growth of TGO layer makes precise thickness measurement difficult. In 20 different
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positions the thickness of the layer in cross-section view of SEM images were measured and the
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average was reported for each sample as it is typically shown in Figure 9 for the C100 R1 S200 and C105 R1 S200 samples after 160 h of oxidation at 1100 ˚C. After 40 h oxidation at 1100 oC,
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TGO layer was formed at the interface of topcoat and bondcoat for all specimens. Figure 10
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represent the thickness of TGO layer as a function of remelting depth and oxidation time for samples remelted with only one pass. Clearly, TGO thickness in the non-remelted samples (0%
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of remelting depth) is the thickest one in every oxidation time. This graph confirms that applying remelting process could decrease TGO layer thickness efficiently. As it is clear from Figure 10 with increasing remelted depth, the TGO thickness decreases to some extent and then increases gradually. The minimum amount of TGO thickness in all oxidation times after one pass of remelting was obtained at current intensity of 100 A, scanning speed of 200 mm/min and remelting depth of 38% (colored zone). In remelting depths deeper than 38%, TGO thickness
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ACCEPTED MANUSCRIPT gets thicker in all the oxidation times. Applying higher currents causes more energy input leading to deeper remelting area. This results in an increase of surface crack density (Figure 3), due to higher residual stresses. In this research, remelted depth of 38% seems to be an optimal melting thickness which higher than that the density of cracks will be very high and again TGO
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thickness increases significantly to eliminate sealing effect of remelting.
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Figure 11 demonstrated TGO thickness as a function of remelting depth and oxidation times. The minimum values for TGO thickness was achieved in 39% remelted depth in every oxidation
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times except in 40 h oxidation for samples remelted twice. This trend was seen in samples remelted one time however, after two times remelting in a row, the minimum thickness of TGO
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was achieved at current of 95 A. It should be noted that TGO thickness is decreased with one
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more surface remelting compared to one time remelting. The microstructure of samples with double times remelting (Figure 3) is more uniform with less surface cracks. It seems that the first
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remelting pass preheats the surface for the second pass and subsequently the cooling rate
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decreased and less thermal shock was tolerated by the coating. SEM cross-section images of non-remelted and remelted samples are shown in Figure 12 with the focus on TGO layer after
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200 h of oxidation test. With increasing remelting depth area, the thickness of TGO layer
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decreases. At the depth of 38% for samples remelted once and 39% for samples remelted twice the minimum amount of TGO thicknesses were achieved. After obtaining optimum amount of remelting depth, the effect of residual stress and crack density overcome the effect of increased remelting area depth in oxygen transfusion. Thus, TGO layer will be thicker in all samples after more hours of oxidation. With increasing the thickness of TGO layer, the amount of accumulated residual stress and the rate of crack initiation and propagation around the layer are enlarged.
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4. Conclusions Surface remelting process employing GTAW was conducted on the YSZ thermal barrier coatings in order to seal the surface openings and therefore minimize the thickness of thermally
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grown oxides layer as a function of oxidation behavior. In this research, current intensity,
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scanning velocity and number of remelting passes in GTAW process were varied and below results were obtained:
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1) Surface remelting of TBCs was done using GTAW method with applied current intensity
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in the range of 75 to 110 A. In currents below 75 A smooth remelted areas was not form and upper 110 A remelted area formed with lots of cracks due to excessive energy input.
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2) Surface remelting seems to have efficient role on sealing the coat however, applying
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more than two passes in a row showed no sensible change due excessive stress impose to underlying layers leading to an increase in crack density.
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3) Minimum thicknesses of TGO layer was formed in samples with 38% to 40% of YSZ
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remelted depth. The thinnest TGO layer was formed in sample remelted twice with 95 A
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current and scanning velocities of 200 mm/min.
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[1] C. Zhao, M. Zhao, M. Shahid, M. Wang, W. Pan, Restrained TGO growth in YSZ/NiCrAlY thermal barrier coatings by modified laser remelting, Surf Coat Technol (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.05.015 [2] M.G. Gok, G. Goller, Microstructural evaluation of laser remelted gadolinium zirconate thermal barrier coatings, Surf Coat Technol 276 (2015) 202-209. [3] C. Zhu, P. Li, A. Javad, G.Y. Liang, P. Xiao, An investigation on the microstructure and oxidation behavior of laser remelted air plasma sprayed thermal barrier coatings, Surf Coat Technol 206 (2012) 3739-3746. [4] X.H. Zhong, Y.M. Wang, Z.H. Xu, Y.F. Zahng, J.F. Zhang, X.Q. Cao, Hot-corrosion behaviors of overlay-clad yttria-stabilized zirconia coatings in contact with vanadate–sulfate salts, Journal of the European Ceramic Society 30 (2010) 1401-1408. [5] X. Zhong, Y. Wang, Z. Xu, Y. Zhang, J. Zhang, X. Cao, Influence of laser‐glazing on hot corrosion resistance of yttria‐stabilized zirconia TBC in molten salt mixture of V2O5 and Na2SO4, Materials and corrosion 60 (2009) 882-888. [6] J. Toscano, R. Vaben, A. Gil, M. Subanovic, D. Naumenko, L. Singheiser, W.J. Quadakkers, Parameters affecting TGO growth and adherence on MCrAlY-bond coats for TBC's, Surf Coat Technol 201 (2006) 3906-3910. [7] A. Gilbert, K. Kokini, S. Sankarasubramanian, Thermal fracture of zirconia–mullite composite thermal barrier coatings under thermal shock: A numerical study, Surf Coat Technol 203 (2008) 91-98. [8] W.R. Chen, X. Wu, B.R. Marple, D.R. Nagy, P.C. Patnaik, TGO growth behaviour in TBCs with APS and HVOF bond coats, Surf Coat Technol 202 (2008) 2677-2683. [9] Y. Li, C.J. Li, Q. Zhang, G.J, Yang, C.X. Li, Influence of TGO composition on the thermal shock lifetime of thermal barrier coatings with cold-sprayed MCrAlY bond coat, J Therm Spray Techn 19 (2010) 168-177. [10] E. Shillington, D.R. Clarke, Spalling failure of a thermal barrier coating associated with aluminum depletion in the bond-coat, Acta Mater 47 (1999) 1297-1305. [11] M.R. Dorfman, D. Sporer, P. Meyer, Thermal Spray Technology Growth in Gas Turbine Applications (2013). [12] R. Ghasemi, R. Shoja-Razavi, R. Mozafarnia, H. Jamali, Laser glazing of plasma-sprayed nanostructured yttria stabilized zirconia thermal barrier coatings, Ceram Int, 39 (2013) 9483-9490. [13] M.R. Loghman-Estarki, R.S. Razavi, H. Edris, H. Jamali, Life time of new SYSZ thermal barrier coatings produced by plasma spraying method under thermal shock test and high temperature treatment. Ceram Int 40 (2014) 1405-1414. [14] L. Pin, V. Vidal, F. Blas, F Ansart, S Duuard, J. Bonino, Y.L Maoult, P. Lours, Optimized sol–gel thermal barrier coatings for long-term cyclic oxidation life, J Eur Ceram Soc 34 (2014) 961-974. [15] G. Pulci, J. Tirillo, F. Marra, F. Sarasini, A. Bellucci, T. Valente, C. Bartuli, High temperature oxidation of MCrAlY coatings modified by Al2O3 PVD overlay, Surf Coat Technol 268 (2015) 198-204.
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[16] D. Wang, Z. Tian, L Shen, Z. Liu, Y Huang, Effects of laser remelting on microstructure and solid particle erosion characteristics of ZrO2–7wt% Y2O3 thermal barrier coating prepared by plasma spraying, Ceram Int 40 (2014) 8791-8799. [17] T. Sadowski, P. Golewski, Proposed Experimental Investigations of TBCs Systems, Loadings in Thermal Barrier Coatings of Jet Engine Turbine Blades (2016) Springer 6789. [18] J. Iwaszko, K. Kudła, M. Szafarska, Remelting treatment of the non-conductive oxide coatings by means of the modified GTAW method, Surf Coat Technol 206 (2012) 28452850. [19] J. Iwaszko, Surface remelting treatment of plasma-sprayed Al2O3+ 13wt.% TiO2 coatings, Surf Coat Technol 201 (2006) 3443-3451. [20] A. Keyvani, M. Saremi, M.H. Sohi, Z. Valefi, A comparison on thermomechanical properties of plasma-sprayed conventional and nanostructured YSZ TBC coatings in thermal cycling, J Alloy Compd 541 (2012) 488-494. [21] D.R. Clarke, C.G. Levi, Materials design for the next generation thermal barrier coatings. Annual Review of Materials Research 33 (2003) 383-417. [22] T.N. Ridge, Sintering, phase stability and thermal conductivity of plasma-sprayed Gd1O3stabllized zro; Mohamed N. Rahaman and Jacob R. Gross, University of Missouri-Rolla, Department of Mate. in Advances in Ceramic Coatings and Ceramic-Metal Systems: A Collection of Papers Presented at the 29th International Conference on Advanced Ceramics and Composites, Jan 23-28, 2005, Cocoa Beach, FL, Ceramic Engineering and Science Proceedings, Vol 26. 2009: John Wiley & Sons. [23] X. Song, F. Meng, M. Kong, Y. Wang, L. Huang, X. Zheng, Y. Zeng, Thickness and microstructure characterization of TGO in thermal barrier coatings by 3D reconstruction, Mater Charact 120 (2016) 244-248. [24] W. Zhu, M. Cai, L. Yang, J.W. Guo, Y.C. Zhou, C. Lu, The effect of morphology of thermally grown oxide on the stress field in a turbine blade with thermal barrier coatings, Surf Coat Technol 276 (2015) 160-167. [25] A.M. Limarga, S. Widjaja, T.H. Yip, Mechanical properties and oxidation resistance of plasma-sprayed multilayered Al2O3/ZrO2 thermal barrier coatings. Surf Coat Technol, 197 (2005) 93-102. [26] Y. Bai, Z.H. Han, H.Q. Li, C. Xu, Y.L. Xu, C.H. Ding, J.F. Yang, Structure–property differences between supersonic and conventional atmospheric plasma sprayed zirconia thermal barrier coatings, Surf Coat Technol 205 (2011) 3833-3839. [27] R.B. Heimann, Classic and advanced ceramics: from fundamentals to applications. (2010) John Wiley & Sons.
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ACCEPTED MANUSCRIPT Table Captions: Table 1- Chemical composition of Hastelloy X in wt. %. Table 2- Parameters of APS process applied to produce the bondcoat and the topcoat. Table 3- The parameters of GTAW affecting remelting process of the topcoat.
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Table 4- Sample coding according to variable parameters of remelting process.
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Table 5- Phase content of the as-sprayed and GTAW surface remelted samples.
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Table 1 Cr
Mo
Mn
C
Si
Fe
Balance
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10
1
0.15
1
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Ni
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Table 2 Bondcoat (NiCrAlY)
Topcoat (YSZ)
Current (A)
620
620
Voltage (V)
60-70
60-70
Primary gas flow, Ar (L/min)
50
Secondary gas flow, H2 (L/min)
15
Spray distance (mm)
100
Powder feed rate (g/min)
25
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Parameters
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50 15 100 40
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Table 3 Value/Type
Electric current type
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Current intensity (A)
70-110
Voltage (v)
Remelting passes
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Direction
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Scanning speed (mm/min)
Process parameters
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Argon flow rate (L/min)
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100, 150, 200 1, 2, 3 Perpendicular 7 Air cooling
Cooling of electrode
Water cooling
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Cooling of substrate
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Electrode diameter (mm)
3.2
Distance between electrodes (mm)
1
Distance from surface (mm)
1
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Electrical parameters
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GTAW parameters (unit)
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Table 4 Symbol
Parameter
Current intensity (A) Remelting passes Scanning speed (mm/min)
C R S
70,75,80,85,90,95,100,105,110 1,2,3 100, 150,200
C70 R1 S150
Current intensity : 70
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Variable parameter
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Remelting passes : 1
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Scanning speed : 150
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Table 5 Samples Phase
GTAW surface remelted
As-sprayed C95 R1 S200
C105 R1 S200
C95 R2 S200
content Tetragonal
84.9
92.4
89.8
92.3
Monoclinic
8.1
4.6
5.2
Cubic
7
3
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(non-remelted)
3.7 4
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Figure 1- Typical SEM image of applied thermal barrier coating via APS technique. Figure 2- Schematic of the modified GTAW apparatus used in this research.
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Figure 3- SEM images of top view and cross-section view of non-remelted and remelted samples with different parameters of GTAW (scanning speed was selected to 200 mm/min).
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Figure 4- Remelting depth percentage as a function of current intensity in three scanning speeds.
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Figure 5- Remelting depth percentage as a function of remelting passes for different currents.
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Figure 6- XRD pattern in the range of 20 o to 80o of a) as-sprayed YSZ, b) C95 R1 S200, c) C105 R1 S200 and d) C95 R2 S200.
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Figure 7- SEM image of formed TGO layer after 200 h oxidation for C100 R1 S200 sample along with elemental distribution of O, Al, Ni, Cr and Y.
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Figure 8- Schematic of oxygen transfusion through YSZ topcoat in two ways, a) ionic diffusion and b) gas permeation [27].
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Figure 9- SEM images for cross-section view of a) C100 R1 S200 and b) C105 R1 S200 after 160 h isothermal oxidation test showing TGO layer; thickness was measured in several positions.
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Figure 10- The trend of TGO thickness as a function of remelting depth percentage in 5 different remelting depths and oxidation times.
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Figure 11- The trend of TGO thickness as a function of remelting depth percentage in 5 different remelting depths and oxidation times for samples surface remelted twice.
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Figure 12- SEM micrographs of a) non-remelted, b) C95 R1 S200, c) C105 R1 S200 and d) C95 R2 S200 samples after 200 h oxidation.
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Figure 12
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights Surface remelting process was done on YSZ coat using modified GTAW method. Modified GTAW supply varying parameters to acquire different qualities of remelt. TGO Thickness was measured after oxidation to compare coat sealing of samples.
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In an optimum thickness of remelted depth, the thinnest of TGO layer was achieved.
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