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Effect of temperature and ceramic bonding on BC oxidation behavior in plasma-sprayed thermal barrier coatings
Accepted Manuscript Effect of temperature and ceramic bonding on BC oxidation behavior in plasma-sprayed thermal barrier coatings
Kaveh Torkashvand, Esmaeil Poursaeidi PII: DOI: Reference:
S0257-8972(18)30554-1 doi:10.1016/j.surfcoat.2018.05.069 SCT 23444
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
24 March 2018 26 May 2018 29 May 2018
Please cite this article as: Kaveh Torkashvand, Esmaeil Poursaeidi , Effect of temperature and ceramic bonding on BC oxidation behavior in plasma-sprayed thermal barrier coatings. Sct (2017), doi:10.1016/j.surfcoat.2018.05.069
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ACCEPTED MANUSCRIPT Effect of Temperature and Ceramic Bonding on BC Oxidation Behavior in Plasma-Sprayed Thermal Barrier Coatings Kaveh Torkashvanda, Esmaeil Poursaeidia,* a
Mechanical Engineering Department, Faculty of Engineering, University of Zanjan, Iran
Abstract
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This study was conducted to investigate formation and growth of thermally grown oxide
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(TGO) and mixed oxide clusters of Chromia, Spinel, and Nickel oxide (CSN) under the influence of temperature and the absence of top coat (TC). For this purpose, 21 plasma-
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sprayed thermal barrier coating (TBC) samples underwent isothermal loading at various
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temperatures with different time exposures and the effect of temperature and time exposure on the formation of TGO and CSN oxides were explored. It was concluded that the minimum
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temperature for formation of TGO and CSNs were 900 and 1040℃, respectively, and the minimum time exposure for CSNs to start the formation process was 12 h. Furthermore, an
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isothermal loading at 1070℃ was applied on 5 samples without TC to study the formation
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trend of TGO growth and found that the growth rate of TGO is lower when TC exists in the samples. In order to explain the behavior of TGO growth in this state, a finite element model
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was executed and the applied stresses to TGO from bond coat (BC) were calculated. Results show that the order of stresses in the state with TC is much lower than the state without TC,
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which consequently leads to TGO separation in the latter state and, as a result, a weaker adhesion to the BC and lower TGO growth rate. Keywords: Bond coat oxidation, Ceramic bonding effect, Temperature effect, isothermal oxidation,
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Corresponding author. Tel.: +98 91 22133496; fax: +98 26 34467231; E-mail addresses: [email protected] (E. Poursaeidi)
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ACCEPTED MANUSCRIPT Thermal barrier coating
1.
Introduction Given the necessity of increasing the temperature of the gas entering the turbines to
enhance its efficiency, it is known that the thermal barrier coatings play a key role in protecting the components from hot gasses and thus prolonging their life [1–4]. A thermal
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barrier coating (TBC) system usually consists of four layers; ceramic top coat (TC), thermally
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grown oxide (TGO), metal bond coat (BC), and a superalloy substrate [5,6]. Meanwhile, in plasma sprayed TBCs, given the difference in the properties of the various layers, the
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complexity of the interfaces geometry and the formation and growth of various oxides, they
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are subjected to an intense and complicated loading [6–10]. As a TBC system is exposed to the high-temperature environment, the formation and growth of the TGO and other oxides
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will be inevitable [3,11]. Considering the literature, the separation of the thermal barrier coatings usually initiates within these oxides [12–14]. Stresses in the TBC system increase by
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increasing mixed oxides and TGO thickness [15,16].
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Temperature is one of the major contributors to the growth of oxides and the stress distribution in the system of thermal barrier coatings [15,17,18]. A few experiments have
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been conducted to investigate the effect of temperature on BC oxidation. For instance,
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Renusch et al. investigated the value of diffusion coefficient for bond coat aluminum diffusion by performing an isothermal oxidation in a temperature range of 950-1100℃ [19]. Czech et al. investigated TGO formation in the TC/TC interface of an EB-PVD TBC system by conducting an isothermal experiment at 900, 950, and 1000℃ [20]. In this regard, cooling plays a key role given the need to increase the temperature for enhancing the performance of high-temperature matters in turbines, on one hand, and for prolonging the life of components and their coating, on the other hand. Meanwhile, given the influence that temperature has on 2
ACCEPTED MANUSCRIPT the formation of oxides, knowing the critical temperatures is important in order to determine the optimum cooling temperature. Hence, in this project, through a detailed study, the effect of temperature and time exposure on the formation and growth of oxides is investigated. In the presence of the ceramic layer, TGO is a combination of two separate layers; one is
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in the vicinity of the bond coat with bigger grain sizes and the other is close to the TC with smaller grains [21–23]. Stiger et al. conducted an experiment to compare TGO growth with
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and without TC and concluded that in the presence of TC, TGO growth rate is faster [23].
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There have been a few studies on the how TGO layer grows without the presence of ceramic layer [21] and to the best of our knowledge there is no detailed study and explanation on the
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effect of ceramic coat on bond coat oxidation. Considering the effect of bond coat on the
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formation of TGO and other oxides, knowing more about the oxidation behavior of the bond coat is of utmost importance.
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In this study, the effect of temperature and time exposure on the formation and growth of
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TGO and other oxides was investigated by conducting an isothermal loading at various temperatures. Furthermore, by performing an isothermal loading with different time
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exposures, the behavior of bond coat oxidation without the presence of the ceramic top coat was studied. The formation and growth of TGO and mixed oxides including Chromium (Cr, Nickel oxide
NiO (CSN) were investigated and
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Al)2O3, Spinel Ni(Cr, Al)2O4, and
compared with the state of a complete TBC system (with TC); and the difference was discussed. In the next section, the procedure of experimental method including sample preparation isothermal tests is provided. After that, results are presented and discussed and at last a list of highlighted results are rendered.
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Experimental Procedure
2.1. Samples Preparation To prepare the required samples, disk shape samples with a diameter of 25 mm and a thickness of 3 mm were extracted from the root of a Ni-based superalloy turbine blade.
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The commercial NiCrA1Y powder with the specifications demonstrated in Table 1 was chosen as the bond coat coating. The thickness of this layer is considered as 100+20 μm. To
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coat the top coat layer with a thickness of 300+20 μm, we chose the ZrO2-8 wt.% Y2O3
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powder with the specifications shown in Table 1.
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Table 1. Specifications of materials used for various layers and the thickness of layers. Substrate GTD111 3 [mm] -
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Material Thickness Size of particles
Bond coat
Top coat
NiCrAlY 100 [micron] 60-100 [micron]
ZrO2–8wt.% Y2O3 300 [micron] 45-100 [micron]
The mass composition of powder for BC and TC is provided in Table 2.
Ni Balance
O2 0.02
Y 0.95
Hf 1.72
Fe 0.05
Si 0.45
Ti 0.05
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Al 0.08
Cr 21.02
Y 7.64
Zr Balance
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ZrO2 - 8wt. % Y2O3
Al 9.92
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NiCrAlY
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Table 2. The composition of top and bond coat layers
In order to apply the coating, after degreasing, the samples were grit-blasted by Al2O3 particles. Next, to apply the bond coat layer, they were placed on mechanical fixtures on a rotating table. Having the bond coat layer applied, the samples that were supposed to be under a loading without the presence of the ceramic layer were discarded and the rest of the samples were again placed on the fixtures and the top coat layer was implemented. The parameters involved in the coating process were set in accordance with Table 3 (same as our 4
ACCEPTED MANUSCRIPT previous studies). Table 3. The values of parameters used in coating [5,24].
2.2. Isothermal Oxidation at Various Temperatures
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Values BC (315-310), TC (330-333) BC (42), TC (45) 20 2 7
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Parameter Arc current [A] Voltage [V] Pressure of primary gas Ar [psi] Pressure of secondary gas H2 [psi] Feeding rate [g.min-1]
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When it comes to the discussion of high temperature, which is an accelerator parameter of oxidation phenomenon, it is necessary to investigate the impact of the value of temperature
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as an accelerator on the formation and growth speed of TGO and mixed oxides. In order to
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investigate the effect of temperature value on the behavior of TBCs, some isothermal oxidations tests at various temperatures were conducted. As shown in Table 4, this
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experiment consisted of 7 isothermal tests at 700, 800, 900, 950, 1000, 1040, and 1070℃
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each on 3 samples with different time exposures (12, 24, and 48 h).
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ACCEPTED MANUSCRIPT Table 4. Coding of samples, temperature and time conditions of the samples under the isothermal oxidation with variable temperatures
700
T-B
800
T-C
900
T-D
950
T-E
1000
T-F
1040
T-H
1070
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Time Exposure (hours) 12 24 48 12 24 48 12 24 48 12 24 48 12 24 48 12 24 48 12 24 48
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T-A
Sample Code T-A-1 T-A-2 T-A-3 T-B-1 T-B-2 T-B-3 T-C-1 T-C-2 T-C-3 T-D-1 T-D-2 T-D-3 T-E-1 T-E-2 T-E-3 T-F-1 T-F-2 T-F-3 T-G-1 T-G-2 T-G-3
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Temperatur e(°C)
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Series
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2.3. Isothermal Oxidation of BC without TC In order to understand the effect of TC layer on the BC oxidation behavior, 5 BC coated
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samples underwent isothermal loading at 1070℃ with various time exposures according to Table 5 It should be noted that to compare the results, it was considered that the conditions of
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this test were absolutely same as the conditions in the previous study, wherein the oxidation of samples with TC was investigated. Table 5. Coding of the samples with BC coating under isothermal oxidation test Sample Code A- BC-0 A- BC-1 A- BC-2 A- BC-3 A- BC-4 A- BC-5
Time exposure (hour) 0 12 24 48 72 120
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ACCEPTED MANUSCRIPT 2.4. Metallographic Preparation of the Samples To observe the oxidation behavior of samples and to have a semi-quantitative analysis, they were studied by a Philips XL30 scanning electron microscope (SEM) and a TE-SCAN MIRA3 field scanning electron microscope (FE-SEM).
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Prior to the cross-sectional cut, all the samples were cold mounted in an epoxy resin in order to prevent the potential damages to the coating and then cut by a diamond saw. SiC
Results
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3.
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diamond suspension of 1 μm was utilized for polishing.
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papers with grades of 120, 220, 400, 800, 1200 and 2000 were employed for grinding and
3.1. The Effect of Temperature
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Based on the introduction of this article, the isothermal tests at various temperatures and different exposure times were conducted with the aim of investigating the effect of
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temperature on the formation and growth of TGO and mixed oxides. Figure 1 shows the
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SEM images of the interface of the samples A-3 to F-3 (heat treated at 700, 800, 900, 950,
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1000, 1040, and 1070℃ with a time exposure of 48 h) alongside the aluminum map.
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Figure 1. SEM images: samples under isothermal loading with different temperatures
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As shown in Figure 1, it is obvious that in samples T-A-3 and T-B-3, which were under temperatures of 700 and 800℃, there is no indication of oxide formation in the TC/BC
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interface. In addition, the formation of oxide layers within the bond coat layer is not
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continued and its quantity is very low. Clearly, the formation of the TGO layer in the interface of the samples T-A-1, T-A-2, T-B-1 and T-B-2 does not occur due to the fewer
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hours of exposure to high temperature. In samples T-C-3 to T-F-3, the formation and growth of the TGO layer in the interface and within the BC are observable in a way that by
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increasing the temperature and time exposure, its quantity and continuity increase. The
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temperature of 900℃ (T-C series) is the point at which TGO starts the formation process. Thus, to have an insight of formation trend and effect of exposure time, the tree samples were
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evaluated by FE-SEM. Figure 2 presents the growth trend of the oxide layer in samples T-C-
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1, T-C-2, and T-C-3.
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Figure 2. The formation of TGO oxide in samples T-C-1, T-C-2, and T-C-3 with 12, 24 and 48 hours of exposing to a temperature of 900 C, respectively.
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In samples T-C-1 and T-C-2, which were exposed to a 900℃ temperature for 12 and 24 h, respectively, a discrete and thin layer thinner than 1 μm is formed while after 48 h a
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continuous layer with a thickness of approximately 1 μm can be observed. Accordingly, it
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can be argued that the energy required for the formation of the oxide layer will be feasible at 900℃ or higher temperatures.
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To study TGO thickening trend, using a metallographic software (ImageJ [25]), TGO
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thickness was measured at 7 different locations of samples interface and the mean values were reported as some graphs (Figure 3). Results are reported for three different time
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exposures of 12, 24, and 48 h for the samples that TGO is formed in their interfaces. As can be noted, almost for all samples, by increasing the exposure time the TGO thickness has
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increased in a way that the rate of TGO growth has decreased by the passage of time. Another interpretation of this graph is the dependency of TGO to time. For instance, the TGO thickness of T-E-3 that was exposed to a heating of 1000℃ for 48 h is higher than that of the T-F-2 sample that was exposed to a heating of 1040℃ for 24 h, suggesting the importance of the effect of time exposure in BC oxidation. The same occurred for samples T-D-2 and T-E1, T-E-2 and T-F-1 and T-H-1. It seems that the temperatures 900℃ and 1040℃ are critical such that as the temperature exceeds 900℃, the effect of temperature on the thickness of 10
ACCEPTED MANUSCRIPT TGO becomes more significant. On the other hand, when the value of temperature is higher than 1040℃, the effect of time exposure is more highlighted. It is of note that for sample T-C1, a series of discrete TGO was formed and distinguishing the TGO as a layer was not easy.
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Thus, the thickness of this sample was not reported.
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Figure 3. The average thickness of TGO under different temperatures
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To investigate the formation of mixed oxides, semi-quantitative chemical analyses using the SEM were performed. The formation of mixed oxides was not observed until sample T-F2. According to Figure 4, the formation of mixed oxides in sample T-F-2 is partially
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observable (the discrete lighter layer just above the TGO) while in sample T-F-3 the
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formation of CSN oxides is widely observable.
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Figure 4. Samples T-F-2 and T-F-3 and the formation of the mixed oxides in them
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Figure 5 shows an SEM image of BC/TC interface of sample T-F-3 (heat-treated at 1070℃ for 48 h). An energy-dispersive X-ray spectroscopy (EDS) analysis is provided in
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Table 6 at pointed areas for this sample.
Figure 5. SEM image of BC/TC interface for sample T-F-3 Table 6. Semi-quantitative analysis of different regions
A B C
Al (W%) 43.49 17.21 6.26
Ni (W%) 0.99 0.50 0.50
Cr (W%) 1.44 33.96 48.90
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Zr (W%) 3.15 3.30 3.09
Y (W%) 1.30 0.62 0.39
O (W%) 49.62 44.42 40.86
ACCEPTED MANUSCRIPT As can be seen from Figure 5, based on chemical analysis provided in Table 6, a uniform thick layer of TGO is formed in region A. Mixed oxides are formed in the B and C areas such that (Cr, Al)2O3 and Cr2O4 are formed mostly in B and C regions, respectively. It is noteworthy that the formation of nickel oxides was quite limited in this sample hence in
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some regions it is presented with the approximate weight percentage of 5. Therefore, it can be concluded that the least required temperature for providing the
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needed activation energy for the formation of mixed oxides can be considered equal to
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1040℃ and the least required maintenance time at this temperature for their formation can be considered as 24 h. Furthermore, through the investigations conducted on the samples
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exposed to a temperature of 1070℃, the formation of mixed oxides from sample T-H-2 (24 h
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under a constant temperature of 1070℃) was observable. Hence, it can be concluded that the time exposure required for CSN formation at 1070℃ is higher than that in 12 h (in the
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presence of TC). Accordingly, the time parameter for the formation of mixed oxides (at least
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in temperatures lower than 1070℃) is identified as one of the determining factors. 3.2. TC absence effect on BC oxidation
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The prepared heat-treated samples were investigated using an SEM device to observe
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TGO growth trend.
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(a)
(d)
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(c)
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(e) (f) Figure 6. The formation of the TGO layer and CSN oxides in the samples with bond coat coating in different hours of being exposed to the maximum temperature (1070 °C) a)0 hour b)12 hours c)24 hours d)48 hours e)72 hours f)120 hours
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Figure 6 (a)-6 (f) show the growth process of the oxide layers. Figure 6 (a) demonstrates the sample without the thermal loading applied, indicating the lack of oxide
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layer formation. Figure 6 (b) clearly shows the formation of various oxides (CSNs and TGO) such that mixed oxides are placed above the TGO, making it difficult to distinguish the TGO as a discrete layer. The formed layer has the maximum amount of aluminum oxide near the BC. As it gets closer to a free surface, the aluminum value decreases and the values for mixed oxides (Ni and Cr) undergo an increase. The noteworthy point here is the presence of cracks mostly perpendicular to the BC/oxides interface and the separation of the oxide from the BC, 14
ACCEPTED MANUSCRIPT indicating the fragility of the oxide due to the lack of TC bonding. By an increase in the exposure time, it becomes easier to observe the TGO as a stand-alone layer, making it distinguishable from mixed oxides with a specific line. The conditions for sample A-BC-2 almost resemble those of sample A-BC-1, while in sample A-BC-3 (with 48 h of heat-aging under a temperature of 1070℃), the TGO layer is stand-alone and a specific line separates it
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from mixed oxides. In the next two samples, with 72 and 120 h of heat aging, respectively,
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the thickness of mixed oxide layer remained almost constant while the thickness of the TGO
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layer increased. In these samples, there were not too many perpendicular cracks, probably due to the formation of a new oxide in the present empty spaces provided by said cracks. It
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should be noted that the black area is resin epoxy and the white area in the top of coating on Figure 6(e) is Aurum – a coating with Au to provide a higher quality. In this region, the Au
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is separated so it appears in white color.
Just like the procedure in Section 3.1, TGO thickness was measured at 7 different
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locations and the mean values and deviations were reported in Figure 7. The graph of Figure 7 presents the growth trend of TGO for samples with and without TC. Data for the state with
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TC is from our previous study [5].
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Figure 7. The thickness variations of the TGO in two states; with and without TC, under the isothermal loading with a temperature of 1070 C at different times.
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According to Figure 7, by comparing the growth rate of TGO in the two states, with and without TC, it is obvious that the growth rate of this layer is much lower in the latter state in
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comparison to the former. The most plausible reason for this difference could be related to
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the adhesion of the TGO to its substrate (BC) in the two states. In the next sections, this point of view will be investigated and discussed.
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3.2.1 TGO Adhesion to BC
It is clear that the prerequisite for thickening the TGO is the diffusion of Al through the
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TGO layer and Al diffusion is possible if there is a connection between the oxide layer and bond coat. When TGO is not bonded by a TC, after a short period of time, the lateral growth of the oxide layer will allow buckling weakens the TGO’s adhesion to BC, removing the possibility of the diffusion of the new A1 and the TGO thickening. This evidence may shed light on why the oxide layer is not as thick as the layer in a coating system comprised of TC. By comparing the SEM images of TGO in the states with and without TC (Figure 4 and 16
ACCEPTED MANUSCRIPT Figure 5 with Figure 6), it is clear that in the without TC state, TGO is quite fragile and discrete and contains many microcracks. To elucidate the matter, Figure 6 presents samples
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A-BC-1 and A-BC-2.
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(a) (b) Figure 8. The separation and weakening of the viscosity of oxide layer on BC, (a) Sample A-BC1; (b) Sample A-BC-5
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A universal spallation of the TGO layer can be seen in Figure 8. The reason of this
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spallation could refer to the amount of the stresses applied by BC to TGO due to the thermal loading. In order to study the effect of TC bonding on the amount of TGO stresses and then
4.
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TC states.
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its separation, a numerical simulation was conducted in section 4 in the two with and without
Numerical Model To explore the effect of TC on the applied stresses on TGO, a finite element analysis was
conducted. 4.1. Geometry, Loading and Boundary Conditions A model based on the SEM image of sample (e) in Figure 6 was extracted by a CAD software and converted to the finite element model. Then, according to Figure 9 (a), the 17
ACCEPTED MANUSCRIPT extracted SEM image geometry was assembled on a global 2D model that its dimensions are considered equal to the real samples. By doing so, there is no need to apply any boundary conditions to the sub-model extracted from SEM image and the boundary conditions will be the displacements applied by the global model, which provides more realistic results. The desired submodel was considered to be 6.5 mm away from the sample center (Figure 9 (a)).
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The thickness of the various layers was considered based on the tested samples, in a way that
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the thickness of topcoat and bond coat was considered to be 300 and 100 μm, respectively,
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and a substrate thickness of 3 mm was also factored in. The diameter of the sample was considered as 25 mm. As can be seen in Figure 9, mesh refinement was done to increase the
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mesh density in the area near interfaces. In order to investigate the mesh independence, the model with a number of various elements was studied and it was found that the results would
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be independent of mesh for 84849 and 54502 generalized plane strain quadratic elements for the states with TC models and without TC models, respectively.
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It is known that the isothermal loading leads to the growth in TGO layer; so, the applied loading leads to TGO growth in lateral and perpendicular directions. The value for the lateral
||
(1)
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and normal growth of TGO can be extracted from Equation (1) [26].
where is the strain rate of TGO in the thickness direction and || is the strain rate of TGO in the lateral direction. Here, is considered to be 10 according to the previous studies [27].
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ACCEPTED MANUSCRIPT (a)
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(b) (c) Figure 9. The geometry and configuration used in the modeling of the finite element; (a) schematics of the used geometry and the position of the investigated element (submodel), (b) the configuration of a sample with TC, (c) the configuration of a sample without TC
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In order to apply the lateral and horizontal growths with the ratio obtained by Equation
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(1), a local coordination system was set on the BC/TGO interface profile and the material
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properties were coordinated to be perpendicular and tangent to the interface. The initial thickness of TGO considered to be 1.5 μm and its growth rate is measured based on the
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experimental data shown in the graph of Figure 7. The results showed that TGO growth rate starts from 0 at 900℃ (based on experimental observations in this paper) and increases
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linearly to 5.269 × 10-6 [μm/s] at 1070℃. The model in a stress-free state at a temperature of
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25℃ was heated to 1070℃ within 120 sec, held at maximum temperature for 24 h, and cooled back for 120 sec to 25℃. To explore the effect of TC layer on the applied stresses
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from and a sample the bond coat to the TGO, the modeling for two following states was conducted; i.e., a sample without TC with TC. Figure 9 (b) and 9 (c) present the
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configuration employed for two samples with and without TC. The material properties applied in this simulation were chosen based on Table 7 [28,29]. Creep rate for TGO was extracted from Cho et al.’s study
[29] at 1200℃ and was
extrapolated to various temperatures using Arrhenius law and activation energy [28]. To assess various creep behaviors of the TGO, three different values for the creep strength of the TGO, called creep soft, creep hard, and creep ultra-hard, were considered. 19
ACCEPTED MANUSCRIPT Table 7. The specifications used in this modeling Substrate 184 145 0.3 0.3 12 × 10-6 16 × 10-6 -
BC 200 110 0.3 0.33 13.6 × 10-6 17.6 × 10-6 2.45 1.39 × 10-25 (at 1000°C)
TGO 400 325 0.23 0.25 8.0 × 10-6 9.3 × 10-6 2 (creep soft) 3.8947 × 10-24 (creep hard) 4.72 × 10-26 (creep ultra-hard) 4.7 × 10-28
TC 48 22 0.1 0.12 9.0 × 10-6 12.2 × 10-6 2 2.58 × 10-22
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Young's modulus (20 °C) [GPa] Young's modulus (1100 °C) [GPa] Poisson number (20 °C) Poisson number (1000 °C) CTE (20 °C) [K−1] CTE (1000 °C) [K−1] Creep exponent Creep pre-factor (1070 °C) [Pa-ns-1]
4.2. Numerical Results
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The evolution of applied vertical stresses to TGO (perpendicular to the surface) at two critical positions, peak, and valley (Figure 9 (c)) are calculated for the states with and
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without TC in three different creep behaviors of TGO materials. As shown in Figure 10, for the without TC case, creep behavior has a huge effect on the stress state variations in a way
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that for the creep soft case, the stresses are relaxed in the beginning while they are increased
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for the creep hard and creep ultra-hard cases in the same timeline and they are relaxed during the dwell time. For the with TC case, creep behavior has the same significant impact on the
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stress variations. In this case, stresses for creep soft are relaxed at the earliest stage of dwelling time and stresses are suppressed during all the three stages of various creep
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behavior. For the cases creep hard and creep ultra-hard, stresses become compressive after 5 h. Stresses for the creep soft in the without TC state are slightly larger than with TC state during all three steps (heating, hot times, and cooling) such that with the passage of time this difference decreases gradually. As can be seen from the diagram for all material behavior, vertical stresses in the without TC case are larger than the case with TC. Therefore, in without TC case TGO tends to separate due to large tensile stresses. In comparison, for the 20
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(creep hard and ultra-hard), which prevents TGO separation in these cases.
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Figure 10. Vertical stress evolution at the peak position for the states with and without TC and for creep soft, creep hard and creep ultra-hard behaviour
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Figure 11 illustrates the changes of vertical stresses at valley position (Figure 9) during
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loading. At this position, the behavior of stress evolution in without TC case almost resembles peak position. In with TC case, stresses are extremely suppressed during the
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heating and at the earliest stage of dwelling step, such that they become slightly compressive in the early step of hot times. With the passage of time, they start to increase gradually for
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both of creep hard and creep ultra-hard cases and become tensile after 5 h of dwelling time. Like the peak region, in this case, stresses are larger for the without TC case in creep hard and ultra-hard cases. For the "without TC - creep soft" case after 10 h of dwell time, the stress values become slightly smaller than the cases "with TC – creep hard" and "with TC – ultra".
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Figure 11. Vertical stress evolution at the peak position for the states with and without TC and for creep soft, creep hard and creep ultra-hard behaviour
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Figure 10 and Figure 11 demonstrate that existing top bonding leads to a decrease in the value of applied stresses to the TGO. In other words, in the state without the TC bigger stresses are applied to the TGO, which lead to the separation from BC/TGO interface. This
Summary and Conclusions
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separation intervenes the Al diffusion, and as a result, declines TGO growth rate.
In this study, the oxidation behavior of 21 APS-TBC samples was studied at a
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temperature range of 700-1070℃ with dwelling times of 12, 24, and 48 h. Moreover, by applying an isothermal temperature of 1070℃ to the 5 samples without TC for various heating times duration up to 120 h and comparing the results with the samples with TC, the effect of ceramic bonding on the TGO growth rate was experimentally explored. To explain the TGO growth behavior in the states with and without TC, a numerical simulation was conducted for these two states for three creep behaviors of TGO; i.e., creep soft, creep hard, and creep ultra-hard. 22
ACCEPTED MANUSCRIPT The main conclusions of the present work are outlined as follows: -
The minimum activation energy required for formation of TGO and mixed oxides is provided at 900℃ and 1040℃, respectively.
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The minimum time exposure required for formation of TGO and mixed oxides is
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more than 12 h. The growth rate for TGO without TC is lower than the state with TC.
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When there is no TC, the mixed oxides are formed at the top of the TGO as a thick
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continuous layer.
The main causes for the lower TGO growth rate in the state without TC is related to
Using a FEM model showed that in the state without TC the applied stresses to BC
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the effect of TC bonding absence and thus TGO buckling.
are greater than the state with TC.
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Acknowledgment
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Thanks to Dr. Martin Baeker for helpful advice on Numerical Modelling and to Hamid
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Razi for proofreading the manuscript.
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W.A. Nelson, R.M. Orenstein, TBC Experience in Land-Based Gas Turbines, J. Therm. Spray Technol. 6 (2000) 176–180. doi:10.1007/s11666-997-0009-5.
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K. Torkashvand, E. Poursaeidi, M. Mohammadi, Effect of TGO Thickness on the Thermal Barrier Coatings Life under Thermal Shock and Thermal Cycle Loading, Ceram. Int. (2018). doi:10.1016/j.ceramint.2018.02.140.
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L. Jin, G. Liu, P. Li, H. Zhou, C. Wang, G. Zhou, Adhesion strength and thermal shock properties of nanostructured 5La3TiYSZ , 8LaYSZ and 8CeYSZ coatings prepared by atmospheric plasma spraying, Ceram. Int. (2015) 1–8. doi:10.1016/j.ceramint.2015.06.027.
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L. Wang, Y. Wang, W.Q. Zhang, X.G. Sun, J.Q. He, Z.Y. Pan, C.H. Wang, Finite element simulation of stress distribution and development in 8YSZ and double-ceramic-layer La 2 Zr 2 O 7/8YSZ thermal barrier coatings during thermal shock, Appl. Surf. Sci. 258 (2012) 3540– 3551.
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A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Mechanisms controlling the durability of thermal barrier coatings, Prog. Mater. Sci. 46 (2001) 505–553. doi:10.1016/S0079-6425(00)00020-7.
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J. Toscano, R. Va, A. Gil, M. Subanovic, D. Naumenko, Parameters affecting TGO growth and adherence on M CrAlY-bond coats for TBC â€TM s, Surf. Coatings Technol. 201 (2006) 3906–3910. doi:10.1016/j.surfcoat.2006.07.247.
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W.R. Chen, X. Wu, B.R. Marple, P.C. Patnaik, Oxidation and crack nucleation/growth in an air-plasma-sprayed thermal barrier coating with NiCrAlY bond coat, Surf. Coatings Technol.
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M.-J. Pindera, J. Aboudi, S.M. Arnold, The effect of interface roughness and oxide film thickness on the inelastic response of thermal barrier coatings to thermal cycling, Mater. Sci. Eng. A. 284 (2000) 158–175.
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C.H. Lee, H.K. Kim, H.S. Choi, H.S. Ahn, Phase transformation and bond coat oxidation behavior of plasma-sprayed zirconia thermal barrier coating, Surf. Coatings Technol. 124 (2000) 1–12.
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J.A. Haynes, E.D. Rigney, M.K. Ferber, W.D. Porter, Oxidation and degradation of a plasmasprayed thermal barrier coating system, Surf. Coatings Technol. 86 (1996) 102–108.
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D. Renusch, M. Schorr, M. Schu, The role that bond coat depletion of aluminum has on the lifetime of APS-TBC under oxidizing conditions, Mater. Corros. 59. (2008) 547–555. doi:10.1002/maco.200804137.
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N. Czech, H. Fietzek, M. Juez-lorenzo, V. Kolarik, W. Stamm, Studies of the bond-coat oxidation and phase structure of TBCs, Surf. Coatings Technol. 113 (1999) 157–164.
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S. Bose, High temperature coatings, Butterworth-Heinemann, 2011.
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E.P. Busso, J. Lin, S. Sakurai, M. Nakayama, A mechanistic study of oxidation-induced degradation in a plasma-sprayed thermal barrier coating system.: Part I: model formulation, Acta Mater. 49 (2001) 1515–1528.
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M.J. Stiger, N.M. Yanar, F.S. Pettit, G.H. Meier, Mechanisms for the failure of electron beam physical vapor deposited thermal barrier coatings induced by high temperature oxidation, Elev. Temp. Coatings Sci. Technol. III. Warrendale Miner. Met. Mater. Soc. (1999) 51.
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J. Cho, C.M. Wang, H.M. Chan, J.M. Rickman, M.P. Harmer, Role of segregating dopants on the improved creep resistance of aluminum oxide, Acta Mater. 47 (1999) 4197–4207. doi:10.1016/S1359-6454(99)00278-5.
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ACCEPTED MANUSCRIPT Highlights Effect of temperature and time exposure on BC oxidation behaviour are evaluated.
Effect of ceramic bonding on BC oxidation behaviour is evaluated.
Critical oxidation temperatures are investigated.
By performing a FE simulation the effect of TC bonding is explained.
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Kaveh Torkashvand, Esmaeil Poursaeidi PII: DOI: Reference:
S0257-8972(18)30554-1 doi:10.1016/j.surfcoat.2018.05.069 SCT 23444
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
24 March 2018 26 May 2018 29 May 2018
Please cite this article as: Kaveh Torkashvand, Esmaeil Poursaeidi , Effect of temperature and ceramic bonding on BC oxidation behavior in plasma-sprayed thermal barrier coatings. Sct (2017), doi:10.1016/j.surfcoat.2018.05.069
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Effect of Temperature and Ceramic Bonding on BC Oxidation Behavior in Plasma-Sprayed Thermal Barrier Coatings Kaveh Torkashvanda, Esmaeil Poursaeidia,* a
Mechanical Engineering Department, Faculty of Engineering, University of Zanjan, Iran
Abstract
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This study was conducted to investigate formation and growth of thermally grown oxide
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(TGO) and mixed oxide clusters of Chromia, Spinel, and Nickel oxide (CSN) under the influence of temperature and the absence of top coat (TC). For this purpose, 21 plasma-
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sprayed thermal barrier coating (TBC) samples underwent isothermal loading at various
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temperatures with different time exposures and the effect of temperature and time exposure on the formation of TGO and CSN oxides were explored. It was concluded that the minimum
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temperature for formation of TGO and CSNs were 900 and 1040℃, respectively, and the minimum time exposure for CSNs to start the formation process was 12 h. Furthermore, an
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isothermal loading at 1070℃ was applied on 5 samples without TC to study the formation
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trend of TGO growth and found that the growth rate of TGO is lower when TC exists in the samples. In order to explain the behavior of TGO growth in this state, a finite element model
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was executed and the applied stresses to TGO from bond coat (BC) were calculated. Results show that the order of stresses in the state with TC is much lower than the state without TC,
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which consequently leads to TGO separation in the latter state and, as a result, a weaker adhesion to the BC and lower TGO growth rate. Keywords: Bond coat oxidation, Ceramic bonding effect, Temperature effect, isothermal oxidation,
*
Corresponding author. Tel.: +98 91 22133496; fax: +98 26 34467231; E-mail addresses: [email protected] (E. Poursaeidi)
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ACCEPTED MANUSCRIPT Thermal barrier coating
1.
Introduction Given the necessity of increasing the temperature of the gas entering the turbines to
enhance its efficiency, it is known that the thermal barrier coatings play a key role in protecting the components from hot gasses and thus prolonging their life [1–4]. A thermal
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barrier coating (TBC) system usually consists of four layers; ceramic top coat (TC), thermally
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grown oxide (TGO), metal bond coat (BC), and a superalloy substrate [5,6]. Meanwhile, in plasma sprayed TBCs, given the difference in the properties of the various layers, the
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complexity of the interfaces geometry and the formation and growth of various oxides, they
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are subjected to an intense and complicated loading [6–10]. As a TBC system is exposed to the high-temperature environment, the formation and growth of the TGO and other oxides
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will be inevitable [3,11]. Considering the literature, the separation of the thermal barrier coatings usually initiates within these oxides [12–14]. Stresses in the TBC system increase by
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increasing mixed oxides and TGO thickness [15,16].
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Temperature is one of the major contributors to the growth of oxides and the stress distribution in the system of thermal barrier coatings [15,17,18]. A few experiments have
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been conducted to investigate the effect of temperature on BC oxidation. For instance,
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Renusch et al. investigated the value of diffusion coefficient for bond coat aluminum diffusion by performing an isothermal oxidation in a temperature range of 950-1100℃ [19]. Czech et al. investigated TGO formation in the TC/TC interface of an EB-PVD TBC system by conducting an isothermal experiment at 900, 950, and 1000℃ [20]. In this regard, cooling plays a key role given the need to increase the temperature for enhancing the performance of high-temperature matters in turbines, on one hand, and for prolonging the life of components and their coating, on the other hand. Meanwhile, given the influence that temperature has on 2
ACCEPTED MANUSCRIPT the formation of oxides, knowing the critical temperatures is important in order to determine the optimum cooling temperature. Hence, in this project, through a detailed study, the effect of temperature and time exposure on the formation and growth of oxides is investigated. In the presence of the ceramic layer, TGO is a combination of two separate layers; one is
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in the vicinity of the bond coat with bigger grain sizes and the other is close to the TC with smaller grains [21–23]. Stiger et al. conducted an experiment to compare TGO growth with
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and without TC and concluded that in the presence of TC, TGO growth rate is faster [23].
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There have been a few studies on the how TGO layer grows without the presence of ceramic layer [21] and to the best of our knowledge there is no detailed study and explanation on the
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effect of ceramic coat on bond coat oxidation. Considering the effect of bond coat on the
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formation of TGO and other oxides, knowing more about the oxidation behavior of the bond coat is of utmost importance.
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In this study, the effect of temperature and time exposure on the formation and growth of
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TGO and other oxides was investigated by conducting an isothermal loading at various temperatures. Furthermore, by performing an isothermal loading with different time
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exposures, the behavior of bond coat oxidation without the presence of the ceramic top coat was studied. The formation and growth of TGO and mixed oxides including Chromium (Cr, Nickel oxide
NiO (CSN) were investigated and
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Al)2O3, Spinel Ni(Cr, Al)2O4, and
compared with the state of a complete TBC system (with TC); and the difference was discussed. In the next section, the procedure of experimental method including sample preparation isothermal tests is provided. After that, results are presented and discussed and at last a list of highlighted results are rendered.
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Experimental Procedure
2.1. Samples Preparation To prepare the required samples, disk shape samples with a diameter of 25 mm and a thickness of 3 mm were extracted from the root of a Ni-based superalloy turbine blade.
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The commercial NiCrA1Y powder with the specifications demonstrated in Table 1 was chosen as the bond coat coating. The thickness of this layer is considered as 100+20 μm. To
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coat the top coat layer with a thickness of 300+20 μm, we chose the ZrO2-8 wt.% Y2O3
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powder with the specifications shown in Table 1.
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Table 1. Specifications of materials used for various layers and the thickness of layers. Substrate GTD111 3 [mm] -
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Material Thickness Size of particles
Bond coat
Top coat
NiCrAlY 100 [micron] 60-100 [micron]
ZrO2–8wt.% Y2O3 300 [micron] 45-100 [micron]
The mass composition of powder for BC and TC is provided in Table 2.
Ni Balance
O2 0.02
Y 0.95
Hf 1.72
Fe 0.05
Si 0.45
Ti 0.05
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Al 0.08
Cr 21.02
Y 7.64
Zr Balance
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ZrO2 - 8wt. % Y2O3
Al 9.92
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NiCrAlY
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Table 2. The composition of top and bond coat layers
In order to apply the coating, after degreasing, the samples were grit-blasted by Al2O3 particles. Next, to apply the bond coat layer, they were placed on mechanical fixtures on a rotating table. Having the bond coat layer applied, the samples that were supposed to be under a loading without the presence of the ceramic layer were discarded and the rest of the samples were again placed on the fixtures and the top coat layer was implemented. The parameters involved in the coating process were set in accordance with Table 3 (same as our 4
ACCEPTED MANUSCRIPT previous studies). Table 3. The values of parameters used in coating [5,24].
2.2. Isothermal Oxidation at Various Temperatures
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Values BC (315-310), TC (330-333) BC (42), TC (45) 20 2 7
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Parameter Arc current [A] Voltage [V] Pressure of primary gas Ar [psi] Pressure of secondary gas H2 [psi] Feeding rate [g.min-1]
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When it comes to the discussion of high temperature, which is an accelerator parameter of oxidation phenomenon, it is necessary to investigate the impact of the value of temperature
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as an accelerator on the formation and growth speed of TGO and mixed oxides. In order to
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investigate the effect of temperature value on the behavior of TBCs, some isothermal oxidations tests at various temperatures were conducted. As shown in Table 4, this
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experiment consisted of 7 isothermal tests at 700, 800, 900, 950, 1000, 1040, and 1070℃
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each on 3 samples with different time exposures (12, 24, and 48 h).
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ACCEPTED MANUSCRIPT Table 4. Coding of samples, temperature and time conditions of the samples under the isothermal oxidation with variable temperatures
700
T-B
800
T-C
900
T-D
950
T-E
1000
T-F
1040
T-H
1070
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Time Exposure (hours) 12 24 48 12 24 48 12 24 48 12 24 48 12 24 48 12 24 48 12 24 48
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T-A
Sample Code T-A-1 T-A-2 T-A-3 T-B-1 T-B-2 T-B-3 T-C-1 T-C-2 T-C-3 T-D-1 T-D-2 T-D-3 T-E-1 T-E-2 T-E-3 T-F-1 T-F-2 T-F-3 T-G-1 T-G-2 T-G-3
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Temperatur e(°C)
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Series
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2.3. Isothermal Oxidation of BC without TC In order to understand the effect of TC layer on the BC oxidation behavior, 5 BC coated
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samples underwent isothermal loading at 1070℃ with various time exposures according to Table 5 It should be noted that to compare the results, it was considered that the conditions of
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this test were absolutely same as the conditions in the previous study, wherein the oxidation of samples with TC was investigated. Table 5. Coding of the samples with BC coating under isothermal oxidation test Sample Code A- BC-0 A- BC-1 A- BC-2 A- BC-3 A- BC-4 A- BC-5
Time exposure (hour) 0 12 24 48 72 120
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ACCEPTED MANUSCRIPT 2.4. Metallographic Preparation of the Samples To observe the oxidation behavior of samples and to have a semi-quantitative analysis, they were studied by a Philips XL30 scanning electron microscope (SEM) and a TE-SCAN MIRA3 field scanning electron microscope (FE-SEM).
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Prior to the cross-sectional cut, all the samples were cold mounted in an epoxy resin in order to prevent the potential damages to the coating and then cut by a diamond saw. SiC
Results
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3.
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diamond suspension of 1 μm was utilized for polishing.
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papers with grades of 120, 220, 400, 800, 1200 and 2000 were employed for grinding and
3.1. The Effect of Temperature
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Based on the introduction of this article, the isothermal tests at various temperatures and different exposure times were conducted with the aim of investigating the effect of
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temperature on the formation and growth of TGO and mixed oxides. Figure 1 shows the
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SEM images of the interface of the samples A-3 to F-3 (heat treated at 700, 800, 900, 950,
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1000, 1040, and 1070℃ with a time exposure of 48 h) alongside the aluminum map.
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Figure 1. SEM images: samples under isothermal loading with different temperatures
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As shown in Figure 1, it is obvious that in samples T-A-3 and T-B-3, which were under temperatures of 700 and 800℃, there is no indication of oxide formation in the TC/BC
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interface. In addition, the formation of oxide layers within the bond coat layer is not
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continued and its quantity is very low. Clearly, the formation of the TGO layer in the interface of the samples T-A-1, T-A-2, T-B-1 and T-B-2 does not occur due to the fewer
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hours of exposure to high temperature. In samples T-C-3 to T-F-3, the formation and growth of the TGO layer in the interface and within the BC are observable in a way that by
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increasing the temperature and time exposure, its quantity and continuity increase. The
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temperature of 900℃ (T-C series) is the point at which TGO starts the formation process. Thus, to have an insight of formation trend and effect of exposure time, the tree samples were
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evaluated by FE-SEM. Figure 2 presents the growth trend of the oxide layer in samples T-C-
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1, T-C-2, and T-C-3.
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Figure 2. The formation of TGO oxide in samples T-C-1, T-C-2, and T-C-3 with 12, 24 and 48 hours of exposing to a temperature of 900 C, respectively.
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In samples T-C-1 and T-C-2, which were exposed to a 900℃ temperature for 12 and 24 h, respectively, a discrete and thin layer thinner than 1 μm is formed while after 48 h a
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continuous layer with a thickness of approximately 1 μm can be observed. Accordingly, it
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can be argued that the energy required for the formation of the oxide layer will be feasible at 900℃ or higher temperatures.
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To study TGO thickening trend, using a metallographic software (ImageJ [25]), TGO
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thickness was measured at 7 different locations of samples interface and the mean values were reported as some graphs (Figure 3). Results are reported for three different time
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exposures of 12, 24, and 48 h for the samples that TGO is formed in their interfaces. As can be noted, almost for all samples, by increasing the exposure time the TGO thickness has
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increased in a way that the rate of TGO growth has decreased by the passage of time. Another interpretation of this graph is the dependency of TGO to time. For instance, the TGO thickness of T-E-3 that was exposed to a heating of 1000℃ for 48 h is higher than that of the T-F-2 sample that was exposed to a heating of 1040℃ for 24 h, suggesting the importance of the effect of time exposure in BC oxidation. The same occurred for samples T-D-2 and T-E1, T-E-2 and T-F-1 and T-H-1. It seems that the temperatures 900℃ and 1040℃ are critical such that as the temperature exceeds 900℃, the effect of temperature on the thickness of 10
ACCEPTED MANUSCRIPT TGO becomes more significant. On the other hand, when the value of temperature is higher than 1040℃, the effect of time exposure is more highlighted. It is of note that for sample T-C1, a series of discrete TGO was formed and distinguishing the TGO as a layer was not easy.
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Thus, the thickness of this sample was not reported.
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Figure 3. The average thickness of TGO under different temperatures
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To investigate the formation of mixed oxides, semi-quantitative chemical analyses using the SEM were performed. The formation of mixed oxides was not observed until sample T-F2. According to Figure 4, the formation of mixed oxides in sample T-F-2 is partially
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observable (the discrete lighter layer just above the TGO) while in sample T-F-3 the
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formation of CSN oxides is widely observable.
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Figure 4. Samples T-F-2 and T-F-3 and the formation of the mixed oxides in them
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Figure 5 shows an SEM image of BC/TC interface of sample T-F-3 (heat-treated at 1070℃ for 48 h). An energy-dispersive X-ray spectroscopy (EDS) analysis is provided in
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Table 6 at pointed areas for this sample.
Figure 5. SEM image of BC/TC interface for sample T-F-3 Table 6. Semi-quantitative analysis of different regions
A B C
Al (W%) 43.49 17.21 6.26
Ni (W%) 0.99 0.50 0.50
Cr (W%) 1.44 33.96 48.90
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Zr (W%) 3.15 3.30 3.09
Y (W%) 1.30 0.62 0.39
O (W%) 49.62 44.42 40.86
ACCEPTED MANUSCRIPT As can be seen from Figure 5, based on chemical analysis provided in Table 6, a uniform thick layer of TGO is formed in region A. Mixed oxides are formed in the B and C areas such that (Cr, Al)2O3 and Cr2O4 are formed mostly in B and C regions, respectively. It is noteworthy that the formation of nickel oxides was quite limited in this sample hence in
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some regions it is presented with the approximate weight percentage of 5. Therefore, it can be concluded that the least required temperature for providing the
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needed activation energy for the formation of mixed oxides can be considered equal to
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1040℃ and the least required maintenance time at this temperature for their formation can be considered as 24 h. Furthermore, through the investigations conducted on the samples
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exposed to a temperature of 1070℃, the formation of mixed oxides from sample T-H-2 (24 h
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under a constant temperature of 1070℃) was observable. Hence, it can be concluded that the time exposure required for CSN formation at 1070℃ is higher than that in 12 h (in the
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presence of TC). Accordingly, the time parameter for the formation of mixed oxides (at least
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in temperatures lower than 1070℃) is identified as one of the determining factors. 3.2. TC absence effect on BC oxidation
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The prepared heat-treated samples were investigated using an SEM device to observe
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TGO growth trend.
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(a)
(d)
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(c)
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(e) (f) Figure 6. The formation of the TGO layer and CSN oxides in the samples with bond coat coating in different hours of being exposed to the maximum temperature (1070 °C) a)0 hour b)12 hours c)24 hours d)48 hours e)72 hours f)120 hours
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Figure 6 (a)-6 (f) show the growth process of the oxide layers. Figure 6 (a) demonstrates the sample without the thermal loading applied, indicating the lack of oxide
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layer formation. Figure 6 (b) clearly shows the formation of various oxides (CSNs and TGO) such that mixed oxides are placed above the TGO, making it difficult to distinguish the TGO as a discrete layer. The formed layer has the maximum amount of aluminum oxide near the BC. As it gets closer to a free surface, the aluminum value decreases and the values for mixed oxides (Ni and Cr) undergo an increase. The noteworthy point here is the presence of cracks mostly perpendicular to the BC/oxides interface and the separation of the oxide from the BC, 14
ACCEPTED MANUSCRIPT indicating the fragility of the oxide due to the lack of TC bonding. By an increase in the exposure time, it becomes easier to observe the TGO as a stand-alone layer, making it distinguishable from mixed oxides with a specific line. The conditions for sample A-BC-2 almost resemble those of sample A-BC-1, while in sample A-BC-3 (with 48 h of heat-aging under a temperature of 1070℃), the TGO layer is stand-alone and a specific line separates it
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from mixed oxides. In the next two samples, with 72 and 120 h of heat aging, respectively,
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the thickness of mixed oxide layer remained almost constant while the thickness of the TGO
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layer increased. In these samples, there were not too many perpendicular cracks, probably due to the formation of a new oxide in the present empty spaces provided by said cracks. It
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should be noted that the black area is resin epoxy and the white area in the top of coating on Figure 6(e) is Aurum – a coating with Au to provide a higher quality. In this region, the Au
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is separated so it appears in white color.
Just like the procedure in Section 3.1, TGO thickness was measured at 7 different
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locations and the mean values and deviations were reported in Figure 7. The graph of Figure 7 presents the growth trend of TGO for samples with and without TC. Data for the state with
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TC is from our previous study [5].
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Figure 7. The thickness variations of the TGO in two states; with and without TC, under the isothermal loading with a temperature of 1070 C at different times.
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According to Figure 7, by comparing the growth rate of TGO in the two states, with and without TC, it is obvious that the growth rate of this layer is much lower in the latter state in
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comparison to the former. The most plausible reason for this difference could be related to
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the adhesion of the TGO to its substrate (BC) in the two states. In the next sections, this point of view will be investigated and discussed.
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3.2.1 TGO Adhesion to BC
It is clear that the prerequisite for thickening the TGO is the diffusion of Al through the
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TGO layer and Al diffusion is possible if there is a connection between the oxide layer and bond coat. When TGO is not bonded by a TC, after a short period of time, the lateral growth of the oxide layer will allow buckling weakens the TGO’s adhesion to BC, removing the possibility of the diffusion of the new A1 and the TGO thickening. This evidence may shed light on why the oxide layer is not as thick as the layer in a coating system comprised of TC. By comparing the SEM images of TGO in the states with and without TC (Figure 4 and 16
ACCEPTED MANUSCRIPT Figure 5 with Figure 6), it is clear that in the without TC state, TGO is quite fragile and discrete and contains many microcracks. To elucidate the matter, Figure 6 presents samples
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A-BC-1 and A-BC-2.
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(a) (b) Figure 8. The separation and weakening of the viscosity of oxide layer on BC, (a) Sample A-BC1; (b) Sample A-BC-5
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A universal spallation of the TGO layer can be seen in Figure 8. The reason of this
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spallation could refer to the amount of the stresses applied by BC to TGO due to the thermal loading. In order to study the effect of TC bonding on the amount of TGO stresses and then
4.
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TC states.
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its separation, a numerical simulation was conducted in section 4 in the two with and without
Numerical Model To explore the effect of TC on the applied stresses on TGO, a finite element analysis was
conducted. 4.1. Geometry, Loading and Boundary Conditions A model based on the SEM image of sample (e) in Figure 6 was extracted by a CAD software and converted to the finite element model. Then, according to Figure 9 (a), the 17
ACCEPTED MANUSCRIPT extracted SEM image geometry was assembled on a global 2D model that its dimensions are considered equal to the real samples. By doing so, there is no need to apply any boundary conditions to the sub-model extracted from SEM image and the boundary conditions will be the displacements applied by the global model, which provides more realistic results. The desired submodel was considered to be 6.5 mm away from the sample center (Figure 9 (a)).
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The thickness of the various layers was considered based on the tested samples, in a way that
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the thickness of topcoat and bond coat was considered to be 300 and 100 μm, respectively,
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and a substrate thickness of 3 mm was also factored in. The diameter of the sample was considered as 25 mm. As can be seen in Figure 9, mesh refinement was done to increase the
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mesh density in the area near interfaces. In order to investigate the mesh independence, the model with a number of various elements was studied and it was found that the results would
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be independent of mesh for 84849 and 54502 generalized plane strain quadratic elements for the states with TC models and without TC models, respectively.
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It is known that the isothermal loading leads to the growth in TGO layer; so, the applied loading leads to TGO growth in lateral and perpendicular directions. The value for the lateral
||
(1)
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and normal growth of TGO can be extracted from Equation (1) [26].
where is the strain rate of TGO in the thickness direction and || is the strain rate of TGO in the lateral direction. Here, is considered to be 10 according to the previous studies [27].
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(b) (c) Figure 9. The geometry and configuration used in the modeling of the finite element; (a) schematics of the used geometry and the position of the investigated element (submodel), (b) the configuration of a sample with TC, (c) the configuration of a sample without TC
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In order to apply the lateral and horizontal growths with the ratio obtained by Equation
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(1), a local coordination system was set on the BC/TGO interface profile and the material
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properties were coordinated to be perpendicular and tangent to the interface. The initial thickness of TGO considered to be 1.5 μm and its growth rate is measured based on the
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experimental data shown in the graph of Figure 7. The results showed that TGO growth rate starts from 0 at 900℃ (based on experimental observations in this paper) and increases
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linearly to 5.269 × 10-6 [μm/s] at 1070℃. The model in a stress-free state at a temperature of
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25℃ was heated to 1070℃ within 120 sec, held at maximum temperature for 24 h, and cooled back for 120 sec to 25℃. To explore the effect of TC layer on the applied stresses
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from and a sample the bond coat to the TGO, the modeling for two following states was conducted; i.e., a sample without TC with TC. Figure 9 (b) and 9 (c) present the
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configuration employed for two samples with and without TC. The material properties applied in this simulation were chosen based on Table 7 [28,29]. Creep rate for TGO was extracted from Cho et al.’s study
[29] at 1200℃ and was
extrapolated to various temperatures using Arrhenius law and activation energy [28]. To assess various creep behaviors of the TGO, three different values for the creep strength of the TGO, called creep soft, creep hard, and creep ultra-hard, were considered. 19
ACCEPTED MANUSCRIPT Table 7. The specifications used in this modeling Substrate 184 145 0.3 0.3 12 × 10-6 16 × 10-6 -
BC 200 110 0.3 0.33 13.6 × 10-6 17.6 × 10-6 2.45 1.39 × 10-25 (at 1000°C)
TGO 400 325 0.23 0.25 8.0 × 10-6 9.3 × 10-6 2 (creep soft) 3.8947 × 10-24 (creep hard) 4.72 × 10-26 (creep ultra-hard) 4.7 × 10-28
TC 48 22 0.1 0.12 9.0 × 10-6 12.2 × 10-6 2 2.58 × 10-22
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Young's modulus (20 °C) [GPa] Young's modulus (1100 °C) [GPa] Poisson number (20 °C) Poisson number (1000 °C) CTE (20 °C) [K−1] CTE (1000 °C) [K−1] Creep exponent Creep pre-factor (1070 °C) [Pa-ns-1]
4.2. Numerical Results
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The evolution of applied vertical stresses to TGO (perpendicular to the surface) at two critical positions, peak, and valley (Figure 9 (c)) are calculated for the states with and
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without TC in three different creep behaviors of TGO materials. As shown in Figure 10, for the without TC case, creep behavior has a huge effect on the stress state variations in a way
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that for the creep soft case, the stresses are relaxed in the beginning while they are increased
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for the creep hard and creep ultra-hard cases in the same timeline and they are relaxed during the dwell time. For the with TC case, creep behavior has the same significant impact on the
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stress variations. In this case, stresses for creep soft are relaxed at the earliest stage of dwelling time and stresses are suppressed during all the three stages of various creep
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behavior. For the cases creep hard and creep ultra-hard, stresses become compressive after 5 h. Stresses for the creep soft in the without TC state are slightly larger than with TC state during all three steps (heating, hot times, and cooling) such that with the passage of time this difference decreases gradually. As can be seen from the diagram for all material behavior, vertical stresses in the without TC case are larger than the case with TC. Therefore, in without TC case TGO tends to separate due to large tensile stresses. In comparison, for the 20
ACCEPTED MANUSCRIPT case with TC, these stresses either are very small (creep softly) or they are compressive
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(creep hard and ultra-hard), which prevents TGO separation in these cases.
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Figure 10. Vertical stress evolution at the peak position for the states with and without TC and for creep soft, creep hard and creep ultra-hard behaviour
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Figure 11 illustrates the changes of vertical stresses at valley position (Figure 9) during
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loading. At this position, the behavior of stress evolution in without TC case almost resembles peak position. In with TC case, stresses are extremely suppressed during the
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heating and at the earliest stage of dwelling step, such that they become slightly compressive in the early step of hot times. With the passage of time, they start to increase gradually for
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both of creep hard and creep ultra-hard cases and become tensile after 5 h of dwelling time. Like the peak region, in this case, stresses are larger for the without TC case in creep hard and ultra-hard cases. For the "without TC - creep soft" case after 10 h of dwell time, the stress values become slightly smaller than the cases "with TC – creep hard" and "with TC – ultra".
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Figure 11. Vertical stress evolution at the peak position for the states with and without TC and for creep soft, creep hard and creep ultra-hard behaviour
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Figure 10 and Figure 11 demonstrate that existing top bonding leads to a decrease in the value of applied stresses to the TGO. In other words, in the state without the TC bigger stresses are applied to the TGO, which lead to the separation from BC/TGO interface. This
Summary and Conclusions
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separation intervenes the Al diffusion, and as a result, declines TGO growth rate.
In this study, the oxidation behavior of 21 APS-TBC samples was studied at a
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temperature range of 700-1070℃ with dwelling times of 12, 24, and 48 h. Moreover, by applying an isothermal temperature of 1070℃ to the 5 samples without TC for various heating times duration up to 120 h and comparing the results with the samples with TC, the effect of ceramic bonding on the TGO growth rate was experimentally explored. To explain the TGO growth behavior in the states with and without TC, a numerical simulation was conducted for these two states for three creep behaviors of TGO; i.e., creep soft, creep hard, and creep ultra-hard. 22
ACCEPTED MANUSCRIPT The main conclusions of the present work are outlined as follows: -
The minimum activation energy required for formation of TGO and mixed oxides is provided at 900℃ and 1040℃, respectively.
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The minimum time exposure required for formation of TGO and mixed oxides is
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more than 12 h. The growth rate for TGO without TC is lower than the state with TC.
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When there is no TC, the mixed oxides are formed at the top of the TGO as a thick
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continuous layer.
The main causes for the lower TGO growth rate in the state without TC is related to
Using a FEM model showed that in the state without TC the applied stresses to BC
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the effect of TC bonding absence and thus TGO buckling.
are greater than the state with TC.
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Acknowledgment
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Thanks to Dr. Martin Baeker for helpful advice on Numerical Modelling and to Hamid
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Razi for proofreading the manuscript.
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ACCEPTED MANUSCRIPT Highlights Effect of temperature and time exposure on BC oxidation behaviour are evaluated.
Effect of ceramic bonding on BC oxidation behaviour is evaluated.
Critical oxidation temperatures are investigated.
By performing a FE simulation the effect of TC bonding is explained.
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