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Effect of post heat treatment on thermal durability of thermal barrier coatings in thermal fatigue tests
Surface & Coatings Technology 215 (2013) 46–51
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effect of post heat treatment on thermal durability of thermal barrier coatings in thermal fatigue tests Sang-Won Myoung a, Seoung-Soo Lee a, Hyun-Seong Kim a, Min-Sik Kim a, Yeon-Gil Jung a,⁎, Sung-Il Jung b, Ta-Kwan Woo b, Ungyu Paik c,⁎⁎ a b c
School of Nano and Advanced Materials Engineering, Changwon National University, #9 Sarim-dong, Changwon, Gyeongnam 641-773, Republic of Korea Research and Development Center, Sung Il Co., Ltd. (SIM), #1587-4 Songjeong-dong, Gangseo-Gu, Busan 618-270, Republic of Korea Department of Energy Engineering, Hanyang University, Haengdang-dong, Sungdon-gu, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Available online 6 November 2012 Keywords: Thermal barrier coating Coating parameter Microstructure Post heat treatment Thermal durability
a b s t r a c t The effects of post heat treatment and coating parameters on the microstructural development of thermal barrier coatings (TBCs) have been investigated and the thermal durability related to the microstructural evolution has been evaluated through cyclic thermal exposure tests. TBC systems with thicknesses of 2000 and 200 μm in the top and bond coats, respectively, were prepared with the air–plasma spray system using a 9 MB gun with ZrO2–8 wt.% Y2O3 (METCO 204 C-NS) for the top coat and Ni-based metallic powder (AMDRY 962) for the bond coat. The post heating was performed at a temperature of 1000 °C for 3 h with a heating rate of 5 °C/min in flowing argon (Ar) gas at 200 ml/min. The thermal exposure tests were performed with a dwell time of 60 min for 874 cycles using a specially designed apparatus—one side of the sample was exposed to a high temperature of 1100 °C and the other side was air cooled to 950 °C. The post heat treatment is an efficient process in improving the thermal durability of thick TBCs, and the TBC with porous microstructure obtained by controlling coating parameters shows a better thermal durability than those with intermediate or dense microstructures. Results indicate that post heat treatment and microstructural control are important in proposing efficient processes to improve the lifetime performance of thick TBCs in high-temperature environments. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The hot-section stationary and rotating components of gas turbines are protected by thermal barrier coatings (TBCs). To improve the structural effectiveness and adhesion between the ceramic coating and metal substrate, a bond coat is usually employed as an interlayer [1–5]. The TBC system can provide a major reduction in the surface temperature of the metallic components of up to 300 °C when combined with the use of internal air cooling of the underlying metallic component. To enhance the energy efficiency in gas turbine systems, the operating temperature is increased to more than 1300 °C, which requires a new cooling system and an increase in TBC thickness and chemistry [6–8]. As the operating temperature has increased, the top coat thickness has gradually increased above 600 μm and reached 2000 μm, the increase in thickness in the top
⁎ Corresponding author. Tel.: +82 55 213 3712; fax: +82 55 262 6486. ⁎⁎ Corresponding author. Tel.: +82 22 220 0502; fax: +82 22 281 0502. E-mail addresses: [email protected] (Y.-G. Jung), [email protected] (U. Paik). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.08.078
coat reducing the surface temperature of cooled components in gas turbine engines at a rate of 4–9 °C per 25 μm [9–15]. Usually, TBC systems are deposited by air–plasma spray (APS) and electron beam physical vapor deposition (EB-PVD) processes. APS is the more commercial method, because of less expense and lower thermal conductivity than EB-PVD, even though there are many defects such as pores, microcracks, and unmelted particles, showing a lower strain tolerance than EB-PVD [16–19]. The microstructure of TBC systems in the APS is determined by the feedstock powder as well as the spraying conditions. In particular, the gun distance, feedstock feeding rate, step distance, and number of coating passes are important parameters for controlling the quality of TBCs [20–22]. In the present study, to improve the thermal durability, the microstructure of the top coat was designed using the APS coating system. The effects of the microstructural design and post heat treatment on the thermal durability of the newly designed TBCs were investigated through thermal exposure tests. Advantages of the post heat treatment in TBCs for enhancing the thermal durability are discussed and optimized coating parameters to prepare thick TBC are proposed, based on the microstructural evolution and mechanical properties.
S.-W. Myoung et al. / Surface & Coatings Technology 215 (2013) 46–51
47
2. Experimental procedure
3. Results and discussion
2.1. Materials and preparation
3.1. Microstructure
The TBCs used in this study were prepared using a commercial APS coating system (9MB, Sulzer Metco, Switzerland). All samples were disk shaped (25.4 mm diameter, 5.0 mm thick) with a Ni-based superalloy (Nimonic 263, a nominal composition of Ni–20Cr–20Co– 5.9Mo–0.5Al–2.1Ti–0.4Mn–0.3Si–0.06C, in wt.%, ThyssenKrupp VDM, Germany) as a substrate. Details of the substrate have been described elsewhere [23]. The substrate was sand-blasted using Al2O3 powder, and then the bond and top coats were deposited within 2 h using the APS system. The feedstock powders for the bond and top coats were AMDRY 962 (hereinafter 962; Sulzer Metco Holding AG, Switzerland, nominal composition of Ni–22Cr–10Al–1.0Y in wt.% and particle size of 56–106 μm) and METCO 204 C-NS (hereinafter C-NS; Sulzer Metco Holding AG, Switzerland, 8 wt.% Y2O3 doped in ZrO2, particle size of 45–125 μm) [24], respectively. The thicknesses of the bond and top coats were approximately d = 200 ± 50 and d = 2000 ± 100 μm, respectively. Three types of TBC sample were prepared with different spraying conditions such as gun distance, feedstock feeding rate, step distance, and coating passes. Detailed coating parameters for the APS process are shown in Table 1 with modifications to the manufacturer's specifications. The post heat treatment process was performed at a temperature of 1000 °C for 3 h with a heating rate of 5 °C/min in flowing argon (Ar) gas at 200 ml/min in a tube type furnace.
The cross-sectional microstructures of the as-prepared TBCs with different coating conditions are shown in Fig. 1. The bond and top coats of 200 ± 50 μm and 2000 ± 100 μm, respectively, are well developed by changing the number of coating passes. The developed microstructures are dependent on the gun distance and feeding rate, showing dense (Fig. 1(A)), intermediate (Fig. 1(B)), and porous (Fig. 1(C)) microstructures. When the powder feeding rate is decreased, the microstructure becomes dense. For the gun distance to an object, when the distance is shortened the microstructure becomes dense. However, the coating deposition efficiency is decreased with increasing distance and decreasing feeding rate, which needs more coating passes. The microstructure of the bond and top coats can be controlled by changing the coating parameters. The magnified microstructures of the top coat and the interfacial microstructures between the bond and top coats of the TBCs designed in this study are shown in Fig. 2. The TBCs prepared with different coating parameters show a sound condition without any cracking or delamination at the interface. The intrinsic defects, such as global pores, splat boundaries, and oxide materials, are uniformly dispersed in all of the as-prepared top coats. The relatively dense microstructure (Fig. 2(A)) contains smaller and relatively uniform “splat” boundaries/cracks, whereas the relatively porous microstructures (Fig. 2(B) and (C)) indicate thicker “splat” boundaries/cracks, bigger pores, and lots of unmelted particles. The microstructural development with coating parameters can be well verified through quantification based on image analysis. The results of the image analysis are summarized in Table 2. The highest porosity was obtained in the case of TBC-C showing about 28.9% porosity, and the porosities of the top coats in the as-prepared TBC-A and TBC-B were 16.2% and 23.8%, respectively. Fig. 3 shows the cross-sectional microstructures of the TBCs without and with the post heat treatment after the thermal exposure tests for 874 cycles. In this study, the equivalent operating hour (EOH) was adjusted to correspond to real chemical reactions occurring in gas turbine operations. The EOH, usually used in ALSTOM and ABB gas turbines, is a function of the operating hours, coefficient of correction, load rejection, equivalent hours for startup, trips, rapid load change, standby, fuel operation, etc. [2]. The maintenance/inspection interval units are performed based on the EOH rather than the actual running hours:
2.2. Characterization Cross-sectional microstructures of the TBC samples with three coating conditions were observed by scanning electron microscopy (SEM, JEOL Model JSM-5610, Japan). To observe the microstructure and to measure the mechanical properties, TBC samples were cold-mounted using a liquid epoxy resin, and then polished using silicon carbide paper and 3 μm and 1 μm diamond pastes, respectively. The thickness of the thermally grown oxide (TGO) layer formed at the interface between the bond and top coats was measured by SEM after thermal exposure. The hardness and toughness values on the sectional planes of each top coat were determined from indentation experiments using a microindenter (HM-122, Mitutoyo Corp., Japan) for loads of 10 and 50 N, respectively, with a Vickers tip. The hardness and toughness values were determined using equations proposed by Lawn [25]. Nanoindentation tests were carried out on the sectional planes of the bond and top coats to determine the elastic modulus using a nanoindenter (Nanoinstruments; MTS Systems Corp., Eden Prairie, USA) employing a Berkovich tip (radiusb 100 nm). All experiments were performed in air at room temperature and at least five runs were performed. An electric thermal fatigue apparatus was designed in order to evaluate the thermal durability of thick TBCs; the top surface of the sample can be thermally exposed to 1100 °C (maximum 1600 °C) and the bottom surface of the sample can be cooled to 950 °C (minimum 400 °C) by air cooling. Therefore, the specially designed apparatus can simulate the high-temperature environments of gas turbines. The holding and cooling times were 60 and 10 min, respectively, which were conducted for 874 cycles.
h i EOH ¼ AOH þ 20x ΣSi þ ΣLRi þ ΣTi þ ΣLCi xF:
ð1Þ
Here, EOH, AOH, ΣSi, ΣLRi, ΣTi, ΣLCi, and F represent the equivalent operating hours, actual operating hours (cyclic dwell times), coefficient of correction, load rejection, trip, rapid load change, and fuel factor (gas: 1.0), respectively. As described in Eq. (1), the EOH is more dependent on the number of start/stop than the cyclic dwell time. Usually, in the field, the EOH is calculated by multiplying the number of start/stop by twenty or twenty one. Therefore, in this study, we employed the same concept to calculate the EOH. A thermal exposure of 874 cycles corresponds to 18,000 EOH.
Table 1 Coating parameters for preparing the bond and top coats. Parameters
TBC-A
TBC-B
TBC-C
Feedstock species: top coat/bond coat Feed rate (g/min): top coat/bond coat Distance (mm): top coat/bond coat Gun speed (mm/s): top coat/bond coat Coating pass: top coat/bond coat
METCO 204 C-NS/AMDRY 962 45/60 150/150 500/500 250–300/14
METCO 204 C-NS/AMDRY 962 45/60 200/200 500/500 850–950/10
METCO 204 C-NS/AMDRY 962 60/60 200/300 500/500 360–460/20
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Fig. 1. Cross-sectional microstructures of TBCs with different coating conditions: (A) dense microstructure of TBC-A, (B) intermediate microstructure of TBC-B, and (C) porous microstructure of TBC-C.
The TBCs with relatively dense microstructures (Fig. 3(A-1) and (A-2)) without and with the post heat treatment were delaminated after approximately 30 cycles (650 EOH) and 150–530 cycles (3,200– 11,500 EOH), respectively. The TBCs with intermediate microstructures (Fig. 3(B-1) and (B-2)) without and with the post heat treatment were delaminated after 300–740 cycles (6,500–15,600 EOH) and approximately 850 cycles (17,800 EOH). The TBCs with relatively porous microstructure (Fig. 3(C-1) and (C-2)) survived without any cracking or delamination for the 874 cycles (18,000 EOH), independent of the post heat treatment, although oxidation behavior is extensively developed in the bond coat. The results of the image analysis of the surviving samples (C series) after thermal exposure are shown in Table 2. The total porosities without and with post heat treatment were reduced from 28.9% to 17.5% and 17.8%, respectively, owing to the densification of the top coat during thermal exposure. An interesting observation is that of the defect species contained in the microstructure. The length of the horizontal and vertical defects was not changed with a relatively small mean pore size after thermal exposure, independent of the post heat treatment. Usually, the dense top coat promotes heat transfer and thermal diffusion, and shows less strain tolerance than the porous
top coat. These results indicate that the TBCs after the post heat treatment have better thermal durability than those without the post heat treatment, when the same conditions are employed to prepare the top coat. The microstructures of surviving TBCs with the post heat treatment after thermal exposure tests for 874 cycles (18,000 EOH), such as the top coat, the TGO layers formed between the top and bond coats, the bond coat, and the interface between the bond coat and the substrate, are shown in Fig. 4. The thickness of the TGO layer was measured as 7.3 ± 0.4 μm (mean ± standard deviation); the TGO layer exhibits an irregular shape on both sides of the top and bond coats. After thermal exposure, cracking or delamination was not observed at the interface of the surviving TBCs. If the thickness of the TGO layer is greater than 10 μm, the interface between the TGO layer and the top coat normally starts to delaminate and shows a failure phenomenon [26–28]. The TBC sample after the post heat treatment showed similar oxidation phenomena in the interface and the bond coat as that without the post heat treatment. Normally, the integration of small horizontal cracks into the long and thick horizontal cracks increases during thermal exposure, as shown in
Fig. 2. Highly magnified microstructures at both the top coat and interface of TBCs with different coating conditions: (A) TBC-A, (B) TBC-B, and (C) TBC-C. Each number indicates the microstructures of the top coat and the interface between the top and bond coats, respectively.
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Table 2 Microstructural analysis of the TBCs prepared with different coating parameters before and after thermal exposure. Microstructural parameters
Total porosity from image analyzer (%) Fraction of horizontal defects in porosity (%) Fraction of vertical defects in porosity (%) Fraction of pores in porosity (%) Length of horizontal defects (μm) Length of vertical defects (μm) Mean pore size (μm)
As-prepared TBC
After thermal exposure test (C series)
TBC-A
TBC-B
TBC-C
Without post heat-treatment
With post heat-treatment
16.2 1.8 1.0 13.4 12.1 (2.1–57.7) 12.0 (2.5–35.9) 13.6
23.8 3.9 1.2 18.7 16.1 (2.1–61.9) 12.3 (2.7–37.5) 15.7
28.9 2.3 1.1 25.5 13.4 (3.3–44.6) 10.3 (2.9–26.9) 30.5
17.5 1.5 0.6 15.4 18.4 (3.3–45.0) 10.9 (4.5–26.3) 13.8
17.8 2.2 0.7 16.9 19.5 (3.1–71.4) 8.9 (2.3–10.4) 15.8
Fig. 4(B), because of the densification of the top coat and the thermal expansion mismatch. The thermal expansion mismatch between the ceramic coating and the metallic bond coating, and the high thermal stresses, are the other major factors leading to spallation of the coating [6,7].
3.2. Mechanical properties The hardness and toughness values measured on the sectional planes of each TBC prepared are shown in Figs. 5 and 6, respectively. In the case of as-prepared TBCs, the hardness values of dense (A-1), intermediate (B-1), and porous (C-1) microstructures were determined to be 3.0 ± 0.3, 2.5 ± 0.3, and 2.4 ± 0.3 GPa, respectively. After the post heat treatment, the hardness values of each microstructure were not significantly changed from those samples without the post heat treatment. After thermal exposure, the hardness values of the surviving samples are increased, independent of the post heat treatment, being 3.6 ± 0.3 and 3.5 ± 0.1 GPa for the TBCs without and with the post heat treatment, respectively. These results can be explained by assuming that the increase in the hardness value is due to densification of the top coat by the resintering effect during thermal exposure.
The toughness values of the as-prepared top coats without ((A-1), (B-1), and (C-1)) and with ((A-2), (B-2), and (C-2)) the post heat treatment were determined to be in the range of 0.4–0.5 and 0.5–0.7 MPa·m 0.5, respectively. The thermal exposure did not significantly increase the toughness values in the surviving TBCs, being 0.9 ± 0.3, and 0.8 ± 0.1 MPa·m 0.5 for the TBCs without and with the post heat treatment, respectively, which is in good agreement with the microstructural evolution. The toughness value on the section is particularly important because the primary location of the delamination failure in TBCs is at the interface of the top and bond coats. Because the TBCs are largely controlled by local or short-crack toughness, the indentation toughness is the most relevant toughness in the context of TBC failure, although the ultimate delamination failure of TBCs is a long-crack phenomenon [14,15]. The effects of microstructural evolution and thermal exposure on elastic modulus are shown in Fig. 7. In the case of as-prepared TBCs, the modulus values of the top and bond coats were determined to be in range of 52–59 and 103–115 GPa, respectively. The modulus variation in the top and bond coats was similar to those of hardness and toughness. After thermal exposure, the modulus values of TBCs without and with post heat treatment were slightly increased, owing to the densification of the top coat, and were determined to be 63 ± 6 and 65 ± 2 GPa, respectively. Also, the modulus values in
Fig. 3. Cross-sectional microstructures of TBCs after thermal exposure tests for 874 cycles (18,000 EOH): (A) TBC-A, (B) TBC-B, and (C) TBC-C. Each number indicates the TBCs without and with post heat treatment, respectively.
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Fig. 4. Highly magnified microstructures of surviving TBC with post heat treatment after thermal exposure tests for 874 cycles (18,000 EOH): (A) top coat, (B) interface between the top and bond coats, (C) bond coat, and (D) interface between the bond coat and the substrate.
TBCs with a thickness of 2000 μm were successfully prepared using the 9MB coating system by controlling the gun distance and feeding rate, showing dense, intermediate, and porous microstructures in the top coat. The intrinsic defects, such as global pores, splat boundaries, and oxide materials, are uniformly dispersed in all of the as-prepared top coats. After thermal exposures of 874 cycles
(18,000 EOH), the TBCs with relatively porous microstructure show a sound condition at the interface without any cracking or delamination, independent of the post heat treatment. In the case of delaminated samples, the TBCs with the post heat treatment represent better thermal durability than those without the post heat treatment. The measured thickness of the TGO layer was 7.3 ± 0.4 μm, showing an irregular shape on both sides of the top and bond coats. The hardness values before and after thermal exposure were determined to be in the range of 2.4–3.0 and 3.5–3.6 GPa, respectively, and the toughness values before and after thermal exposure were determined to be in the range of 0.4–0.7 and 0.8–0.9 MPa·m 0.5, respectively. The modulus values of the top and bond coats before thermal exposure were determined to be in the range of 52–59 and 103–115 GPa, respectively, which were slightly increased to
Fig. 5. Hardness values of TBCs before and after thermal exposure tests: (A) TBC-A, (B) TBC-B, and (C) TBC-C. Open and closed symbols (or each number) indicate TBCs without and with post heat treatment. Indentation for hardness measurement was conducted on sectional planes at 10 N.
Fig. 6. Toughness values of TBCs before and after thermal exposure tests: (A) TBC-A, (B) TBC-B, and (C) TBC-C. Open and closed symbols (or each number) indicate TBCs without and with post heat treatment. Indentation for toughness measurement was conducted on sectional planes at 50 N.
the bond coat were increased because of oxidation, being 137 ± 5 and 153 ± 2 GPa for the bond coats without and with the post heat treatment, respectively. The modulus values show good consistency between the microstructural evolution and other mechanical properties. 4. Conclusions
S.-W. Myoung et al. / Surface & Coatings Technology 215 (2013) 46–51
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Fundamental R&D Program for Core Technology of Materials funded by the Korean Ministry of Knowledge Economy (10041233). References
Fig. 7. Elastic modulus values of the top and bond coats after thermal exposure tests in TBC-C: (A) top coat and (B) bond coat. Each number indicates the top and bond coats without and with post heat treatment, respectively. The elastic modulus values of each as-prepared coat are indicated within the figure as a rectangular box.
63–65 GPa in the top coat and somewhat increased to 137–153 GPa in the bond coat. The hardness, toughness, and elastic modulus values well reflect the microstructural evolution, independent of the post heat treatment. The results indicate that control of the coating parameters can provide a microstructure for thermal stability in TBCs, and a relatively porous microstructure should be applied in thick TBCs. These observations allowed us to design various microstructures in TBCs and to propose an efficient TBC to protect the substrate in high-temperature environments. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (2011-0030802), by the Power Generation & Electricity Delivery of the Korean Institute of Energy Technology Evaluation and Planning (KETEP) grants funded by the Korean Ministry of Knowledge Economy (2009T100200025/2011T100200224), and by a grant from the
[1] H.B. Guo, R. Vaßen, D. Stöver, Surf. Coat. Technol. 192 (2005) 18. [2] P.H. Lee, S.Y. Lee, J.-Y. Kwon, S.W. Myoung, J.H. Lee, Y.G. Jung, H. Cho, U. Paik, Surf. Coat. Technol. 205 (2010) 1250. [3] A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Prog. Mater. Sci. 46 (2001) 505. [4] S.I. Jung, J.H. Kim, J.H. Lee, Y.G. Jung, U. Paik, K.S. Lee, Surf. Coat. Technol. 204 (2009) 802. [5] J.Y. Kwon, K.H. Dong, J.H. Lee, Y.G. Jung, U. Paik, Prog. Org. Coat. 61 (2008) 300. [6] S. Ahmaniemia, P. Vuoristo, T. Mäntylä, C. Gualco, A. Bonadei, R. Di Maggio, Surf. Coat. Technol. 190 (2005) 378. [7] S. Amagasa, K. Shimomura, M. Kadowaki, K. Takeishi, H. Kawai, S. Aoki, K. Aoyema, J. Eng. Gas Turbines Power 116 (1994) 597. [8] D. Schwingel, C. Persson, R. Taylor, T. Johanesson, J. Wigren, High Temp. High Press. 27–28 (1995) 273. [9] A.N. Khan, J. Lu, Surf. Coat. Technol. 166 (2003) 37. [10] http://www.gepower.com/prod_serv/products/tech_docs/en/downloads/ger3569g. pdf. [11] N.P. Padture, M. Gell, E.H. Jordan, Science 296 (2002) 280. [12] T. Bhatia, A. Ozturk, L. Xie, E.H. Jordan, B.M. Cetegen, M. Gell, X. Ma, N.P. Padture, J. Mater. Res. 17 (2002) 2363. [13] L. Xie, X. Ma, E.H. Jordan, N.P. Padture, T.D. Xiao, M. Gell, Mater. Sci. Eng. A 362 (2003) 204. [14] K.W. Schlichting, N.P. Padture, E.H. Jordan, M. Gell, Mater. Sci. Eng. A 342 (2003) 120. [15] A. Rabiei, A.G. Evans, Acta Mater. 48 (2000) 3963. [16] N.P. Padture, K.W. Schlichting, T. Bhatia, A. Ozturk, B. Cetegen, E.H. Jordan, M. Gell, S. Jiang, T.D. Xiao, P.R. Strutt, E. Garcia, P. Miranzo, M.I. Osendi, Acta Mater. 49 (2001) 2251. [17] S.W. Myoung, J.H. Kim, W.R. Lee, Y.G. Jung, K.S. Lee, U. Paik, Surf. Coat. Technol. 205 (2010) 1229. [18] G. Mauer, R. Vaßen, D. Stöver, Surf. Coat. Technol. 202 (2008) 4374. [19] K. Bobzin, F. Ernst, J. Zwick, K. Richardt, D. Sporer, R.J. Molz, in: B.R. Marple, M.M. Hyland, Y.-C. Lau, C.-J. Li, R.S. Lima, G. Montavon (Eds.), ASM International, Materials Park, OH, USA, 2007, pp. 723. [20] P. Roy, G. Bertrand, C. Coddet, Powder Technol. 157 (2005) 20. [21] D. Schwingel, R. Taylor, T. Haubold, J. Wigren, C. Gualco, Surf. Coat. Technol. 1–3 (1998) 99. [22] Y.C. Zhou, T. Hashida, Int. J. Solids Struct. 38 (2001) 4325. [23] http://www.sulzermetco.com/. [24] www.metallographic.com/Procedures/ThermalSprayCoatings.htm. [25] B.R. Lawn, Fracture of Brittle Solids, second ed. Cambridge University Press, Cambridge, 1993. [26] K. Ma, J.M. Schoenung, Surf. Coat. Technol. 205 (2011) 5178. [27] J.K. Kwon, J.H. Lee, Y.G. Jung, U. Paik, Surf. Coat. Technol. 201 (2006) 3483. [28] H.J. Jang, D.H. Park, Y.G. Jung, J.C. Jang, S.C. Choi, U. Paik, Surf. Coat. Technol. 200 (2006) 4355.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effect of post heat treatment on thermal durability of thermal barrier coatings in thermal fatigue tests Sang-Won Myoung a, Seoung-Soo Lee a, Hyun-Seong Kim a, Min-Sik Kim a, Yeon-Gil Jung a,⁎, Sung-Il Jung b, Ta-Kwan Woo b, Ungyu Paik c,⁎⁎ a b c
School of Nano and Advanced Materials Engineering, Changwon National University, #9 Sarim-dong, Changwon, Gyeongnam 641-773, Republic of Korea Research and Development Center, Sung Il Co., Ltd. (SIM), #1587-4 Songjeong-dong, Gangseo-Gu, Busan 618-270, Republic of Korea Department of Energy Engineering, Hanyang University, Haengdang-dong, Sungdon-gu, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Available online 6 November 2012 Keywords: Thermal barrier coating Coating parameter Microstructure Post heat treatment Thermal durability
a b s t r a c t The effects of post heat treatment and coating parameters on the microstructural development of thermal barrier coatings (TBCs) have been investigated and the thermal durability related to the microstructural evolution has been evaluated through cyclic thermal exposure tests. TBC systems with thicknesses of 2000 and 200 μm in the top and bond coats, respectively, were prepared with the air–plasma spray system using a 9 MB gun with ZrO2–8 wt.% Y2O3 (METCO 204 C-NS) for the top coat and Ni-based metallic powder (AMDRY 962) for the bond coat. The post heating was performed at a temperature of 1000 °C for 3 h with a heating rate of 5 °C/min in flowing argon (Ar) gas at 200 ml/min. The thermal exposure tests were performed with a dwell time of 60 min for 874 cycles using a specially designed apparatus—one side of the sample was exposed to a high temperature of 1100 °C and the other side was air cooled to 950 °C. The post heat treatment is an efficient process in improving the thermal durability of thick TBCs, and the TBC with porous microstructure obtained by controlling coating parameters shows a better thermal durability than those with intermediate or dense microstructures. Results indicate that post heat treatment and microstructural control are important in proposing efficient processes to improve the lifetime performance of thick TBCs in high-temperature environments. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The hot-section stationary and rotating components of gas turbines are protected by thermal barrier coatings (TBCs). To improve the structural effectiveness and adhesion between the ceramic coating and metal substrate, a bond coat is usually employed as an interlayer [1–5]. The TBC system can provide a major reduction in the surface temperature of the metallic components of up to 300 °C when combined with the use of internal air cooling of the underlying metallic component. To enhance the energy efficiency in gas turbine systems, the operating temperature is increased to more than 1300 °C, which requires a new cooling system and an increase in TBC thickness and chemistry [6–8]. As the operating temperature has increased, the top coat thickness has gradually increased above 600 μm and reached 2000 μm, the increase in thickness in the top
⁎ Corresponding author. Tel.: +82 55 213 3712; fax: +82 55 262 6486. ⁎⁎ Corresponding author. Tel.: +82 22 220 0502; fax: +82 22 281 0502. E-mail addresses: [email protected] (Y.-G. Jung), [email protected] (U. Paik). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.08.078
coat reducing the surface temperature of cooled components in gas turbine engines at a rate of 4–9 °C per 25 μm [9–15]. Usually, TBC systems are deposited by air–plasma spray (APS) and electron beam physical vapor deposition (EB-PVD) processes. APS is the more commercial method, because of less expense and lower thermal conductivity than EB-PVD, even though there are many defects such as pores, microcracks, and unmelted particles, showing a lower strain tolerance than EB-PVD [16–19]. The microstructure of TBC systems in the APS is determined by the feedstock powder as well as the spraying conditions. In particular, the gun distance, feedstock feeding rate, step distance, and number of coating passes are important parameters for controlling the quality of TBCs [20–22]. In the present study, to improve the thermal durability, the microstructure of the top coat was designed using the APS coating system. The effects of the microstructural design and post heat treatment on the thermal durability of the newly designed TBCs were investigated through thermal exposure tests. Advantages of the post heat treatment in TBCs for enhancing the thermal durability are discussed and optimized coating parameters to prepare thick TBC are proposed, based on the microstructural evolution and mechanical properties.
S.-W. Myoung et al. / Surface & Coatings Technology 215 (2013) 46–51
47
2. Experimental procedure
3. Results and discussion
2.1. Materials and preparation
3.1. Microstructure
The TBCs used in this study were prepared using a commercial APS coating system (9MB, Sulzer Metco, Switzerland). All samples were disk shaped (25.4 mm diameter, 5.0 mm thick) with a Ni-based superalloy (Nimonic 263, a nominal composition of Ni–20Cr–20Co– 5.9Mo–0.5Al–2.1Ti–0.4Mn–0.3Si–0.06C, in wt.%, ThyssenKrupp VDM, Germany) as a substrate. Details of the substrate have been described elsewhere [23]. The substrate was sand-blasted using Al2O3 powder, and then the bond and top coats were deposited within 2 h using the APS system. The feedstock powders for the bond and top coats were AMDRY 962 (hereinafter 962; Sulzer Metco Holding AG, Switzerland, nominal composition of Ni–22Cr–10Al–1.0Y in wt.% and particle size of 56–106 μm) and METCO 204 C-NS (hereinafter C-NS; Sulzer Metco Holding AG, Switzerland, 8 wt.% Y2O3 doped in ZrO2, particle size of 45–125 μm) [24], respectively. The thicknesses of the bond and top coats were approximately d = 200 ± 50 and d = 2000 ± 100 μm, respectively. Three types of TBC sample were prepared with different spraying conditions such as gun distance, feedstock feeding rate, step distance, and coating passes. Detailed coating parameters for the APS process are shown in Table 1 with modifications to the manufacturer's specifications. The post heat treatment process was performed at a temperature of 1000 °C for 3 h with a heating rate of 5 °C/min in flowing argon (Ar) gas at 200 ml/min in a tube type furnace.
The cross-sectional microstructures of the as-prepared TBCs with different coating conditions are shown in Fig. 1. The bond and top coats of 200 ± 50 μm and 2000 ± 100 μm, respectively, are well developed by changing the number of coating passes. The developed microstructures are dependent on the gun distance and feeding rate, showing dense (Fig. 1(A)), intermediate (Fig. 1(B)), and porous (Fig. 1(C)) microstructures. When the powder feeding rate is decreased, the microstructure becomes dense. For the gun distance to an object, when the distance is shortened the microstructure becomes dense. However, the coating deposition efficiency is decreased with increasing distance and decreasing feeding rate, which needs more coating passes. The microstructure of the bond and top coats can be controlled by changing the coating parameters. The magnified microstructures of the top coat and the interfacial microstructures between the bond and top coats of the TBCs designed in this study are shown in Fig. 2. The TBCs prepared with different coating parameters show a sound condition without any cracking or delamination at the interface. The intrinsic defects, such as global pores, splat boundaries, and oxide materials, are uniformly dispersed in all of the as-prepared top coats. The relatively dense microstructure (Fig. 2(A)) contains smaller and relatively uniform “splat” boundaries/cracks, whereas the relatively porous microstructures (Fig. 2(B) and (C)) indicate thicker “splat” boundaries/cracks, bigger pores, and lots of unmelted particles. The microstructural development with coating parameters can be well verified through quantification based on image analysis. The results of the image analysis are summarized in Table 2. The highest porosity was obtained in the case of TBC-C showing about 28.9% porosity, and the porosities of the top coats in the as-prepared TBC-A and TBC-B were 16.2% and 23.8%, respectively. Fig. 3 shows the cross-sectional microstructures of the TBCs without and with the post heat treatment after the thermal exposure tests for 874 cycles. In this study, the equivalent operating hour (EOH) was adjusted to correspond to real chemical reactions occurring in gas turbine operations. The EOH, usually used in ALSTOM and ABB gas turbines, is a function of the operating hours, coefficient of correction, load rejection, equivalent hours for startup, trips, rapid load change, standby, fuel operation, etc. [2]. The maintenance/inspection interval units are performed based on the EOH rather than the actual running hours:
2.2. Characterization Cross-sectional microstructures of the TBC samples with three coating conditions were observed by scanning electron microscopy (SEM, JEOL Model JSM-5610, Japan). To observe the microstructure and to measure the mechanical properties, TBC samples were cold-mounted using a liquid epoxy resin, and then polished using silicon carbide paper and 3 μm and 1 μm diamond pastes, respectively. The thickness of the thermally grown oxide (TGO) layer formed at the interface between the bond and top coats was measured by SEM after thermal exposure. The hardness and toughness values on the sectional planes of each top coat were determined from indentation experiments using a microindenter (HM-122, Mitutoyo Corp., Japan) for loads of 10 and 50 N, respectively, with a Vickers tip. The hardness and toughness values were determined using equations proposed by Lawn [25]. Nanoindentation tests were carried out on the sectional planes of the bond and top coats to determine the elastic modulus using a nanoindenter (Nanoinstruments; MTS Systems Corp., Eden Prairie, USA) employing a Berkovich tip (radiusb 100 nm). All experiments were performed in air at room temperature and at least five runs were performed. An electric thermal fatigue apparatus was designed in order to evaluate the thermal durability of thick TBCs; the top surface of the sample can be thermally exposed to 1100 °C (maximum 1600 °C) and the bottom surface of the sample can be cooled to 950 °C (minimum 400 °C) by air cooling. Therefore, the specially designed apparatus can simulate the high-temperature environments of gas turbines. The holding and cooling times were 60 and 10 min, respectively, which were conducted for 874 cycles.
h i EOH ¼ AOH þ 20x ΣSi þ ΣLRi þ ΣTi þ ΣLCi xF:
ð1Þ
Here, EOH, AOH, ΣSi, ΣLRi, ΣTi, ΣLCi, and F represent the equivalent operating hours, actual operating hours (cyclic dwell times), coefficient of correction, load rejection, trip, rapid load change, and fuel factor (gas: 1.0), respectively. As described in Eq. (1), the EOH is more dependent on the number of start/stop than the cyclic dwell time. Usually, in the field, the EOH is calculated by multiplying the number of start/stop by twenty or twenty one. Therefore, in this study, we employed the same concept to calculate the EOH. A thermal exposure of 874 cycles corresponds to 18,000 EOH.
Table 1 Coating parameters for preparing the bond and top coats. Parameters
TBC-A
TBC-B
TBC-C
Feedstock species: top coat/bond coat Feed rate (g/min): top coat/bond coat Distance (mm): top coat/bond coat Gun speed (mm/s): top coat/bond coat Coating pass: top coat/bond coat
METCO 204 C-NS/AMDRY 962 45/60 150/150 500/500 250–300/14
METCO 204 C-NS/AMDRY 962 45/60 200/200 500/500 850–950/10
METCO 204 C-NS/AMDRY 962 60/60 200/300 500/500 360–460/20
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Fig. 1. Cross-sectional microstructures of TBCs with different coating conditions: (A) dense microstructure of TBC-A, (B) intermediate microstructure of TBC-B, and (C) porous microstructure of TBC-C.
The TBCs with relatively dense microstructures (Fig. 3(A-1) and (A-2)) without and with the post heat treatment were delaminated after approximately 30 cycles (650 EOH) and 150–530 cycles (3,200– 11,500 EOH), respectively. The TBCs with intermediate microstructures (Fig. 3(B-1) and (B-2)) without and with the post heat treatment were delaminated after 300–740 cycles (6,500–15,600 EOH) and approximately 850 cycles (17,800 EOH). The TBCs with relatively porous microstructure (Fig. 3(C-1) and (C-2)) survived without any cracking or delamination for the 874 cycles (18,000 EOH), independent of the post heat treatment, although oxidation behavior is extensively developed in the bond coat. The results of the image analysis of the surviving samples (C series) after thermal exposure are shown in Table 2. The total porosities without and with post heat treatment were reduced from 28.9% to 17.5% and 17.8%, respectively, owing to the densification of the top coat during thermal exposure. An interesting observation is that of the defect species contained in the microstructure. The length of the horizontal and vertical defects was not changed with a relatively small mean pore size after thermal exposure, independent of the post heat treatment. Usually, the dense top coat promotes heat transfer and thermal diffusion, and shows less strain tolerance than the porous
top coat. These results indicate that the TBCs after the post heat treatment have better thermal durability than those without the post heat treatment, when the same conditions are employed to prepare the top coat. The microstructures of surviving TBCs with the post heat treatment after thermal exposure tests for 874 cycles (18,000 EOH), such as the top coat, the TGO layers formed between the top and bond coats, the bond coat, and the interface between the bond coat and the substrate, are shown in Fig. 4. The thickness of the TGO layer was measured as 7.3 ± 0.4 μm (mean ± standard deviation); the TGO layer exhibits an irregular shape on both sides of the top and bond coats. After thermal exposure, cracking or delamination was not observed at the interface of the surviving TBCs. If the thickness of the TGO layer is greater than 10 μm, the interface between the TGO layer and the top coat normally starts to delaminate and shows a failure phenomenon [26–28]. The TBC sample after the post heat treatment showed similar oxidation phenomena in the interface and the bond coat as that without the post heat treatment. Normally, the integration of small horizontal cracks into the long and thick horizontal cracks increases during thermal exposure, as shown in
Fig. 2. Highly magnified microstructures at both the top coat and interface of TBCs with different coating conditions: (A) TBC-A, (B) TBC-B, and (C) TBC-C. Each number indicates the microstructures of the top coat and the interface between the top and bond coats, respectively.
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Table 2 Microstructural analysis of the TBCs prepared with different coating parameters before and after thermal exposure. Microstructural parameters
Total porosity from image analyzer (%) Fraction of horizontal defects in porosity (%) Fraction of vertical defects in porosity (%) Fraction of pores in porosity (%) Length of horizontal defects (μm) Length of vertical defects (μm) Mean pore size (μm)
As-prepared TBC
After thermal exposure test (C series)
TBC-A
TBC-B
TBC-C
Without post heat-treatment
With post heat-treatment
16.2 1.8 1.0 13.4 12.1 (2.1–57.7) 12.0 (2.5–35.9) 13.6
23.8 3.9 1.2 18.7 16.1 (2.1–61.9) 12.3 (2.7–37.5) 15.7
28.9 2.3 1.1 25.5 13.4 (3.3–44.6) 10.3 (2.9–26.9) 30.5
17.5 1.5 0.6 15.4 18.4 (3.3–45.0) 10.9 (4.5–26.3) 13.8
17.8 2.2 0.7 16.9 19.5 (3.1–71.4) 8.9 (2.3–10.4) 15.8
Fig. 4(B), because of the densification of the top coat and the thermal expansion mismatch. The thermal expansion mismatch between the ceramic coating and the metallic bond coating, and the high thermal stresses, are the other major factors leading to spallation of the coating [6,7].
3.2. Mechanical properties The hardness and toughness values measured on the sectional planes of each TBC prepared are shown in Figs. 5 and 6, respectively. In the case of as-prepared TBCs, the hardness values of dense (A-1), intermediate (B-1), and porous (C-1) microstructures were determined to be 3.0 ± 0.3, 2.5 ± 0.3, and 2.4 ± 0.3 GPa, respectively. After the post heat treatment, the hardness values of each microstructure were not significantly changed from those samples without the post heat treatment. After thermal exposure, the hardness values of the surviving samples are increased, independent of the post heat treatment, being 3.6 ± 0.3 and 3.5 ± 0.1 GPa for the TBCs without and with the post heat treatment, respectively. These results can be explained by assuming that the increase in the hardness value is due to densification of the top coat by the resintering effect during thermal exposure.
The toughness values of the as-prepared top coats without ((A-1), (B-1), and (C-1)) and with ((A-2), (B-2), and (C-2)) the post heat treatment were determined to be in the range of 0.4–0.5 and 0.5–0.7 MPa·m 0.5, respectively. The thermal exposure did not significantly increase the toughness values in the surviving TBCs, being 0.9 ± 0.3, and 0.8 ± 0.1 MPa·m 0.5 for the TBCs without and with the post heat treatment, respectively, which is in good agreement with the microstructural evolution. The toughness value on the section is particularly important because the primary location of the delamination failure in TBCs is at the interface of the top and bond coats. Because the TBCs are largely controlled by local or short-crack toughness, the indentation toughness is the most relevant toughness in the context of TBC failure, although the ultimate delamination failure of TBCs is a long-crack phenomenon [14,15]. The effects of microstructural evolution and thermal exposure on elastic modulus are shown in Fig. 7. In the case of as-prepared TBCs, the modulus values of the top and bond coats were determined to be in range of 52–59 and 103–115 GPa, respectively. The modulus variation in the top and bond coats was similar to those of hardness and toughness. After thermal exposure, the modulus values of TBCs without and with post heat treatment were slightly increased, owing to the densification of the top coat, and were determined to be 63 ± 6 and 65 ± 2 GPa, respectively. Also, the modulus values in
Fig. 3. Cross-sectional microstructures of TBCs after thermal exposure tests for 874 cycles (18,000 EOH): (A) TBC-A, (B) TBC-B, and (C) TBC-C. Each number indicates the TBCs without and with post heat treatment, respectively.
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Fig. 4. Highly magnified microstructures of surviving TBC with post heat treatment after thermal exposure tests for 874 cycles (18,000 EOH): (A) top coat, (B) interface between the top and bond coats, (C) bond coat, and (D) interface between the bond coat and the substrate.
TBCs with a thickness of 2000 μm were successfully prepared using the 9MB coating system by controlling the gun distance and feeding rate, showing dense, intermediate, and porous microstructures in the top coat. The intrinsic defects, such as global pores, splat boundaries, and oxide materials, are uniformly dispersed in all of the as-prepared top coats. After thermal exposures of 874 cycles
(18,000 EOH), the TBCs with relatively porous microstructure show a sound condition at the interface without any cracking or delamination, independent of the post heat treatment. In the case of delaminated samples, the TBCs with the post heat treatment represent better thermal durability than those without the post heat treatment. The measured thickness of the TGO layer was 7.3 ± 0.4 μm, showing an irregular shape on both sides of the top and bond coats. The hardness values before and after thermal exposure were determined to be in the range of 2.4–3.0 and 3.5–3.6 GPa, respectively, and the toughness values before and after thermal exposure were determined to be in the range of 0.4–0.7 and 0.8–0.9 MPa·m 0.5, respectively. The modulus values of the top and bond coats before thermal exposure were determined to be in the range of 52–59 and 103–115 GPa, respectively, which were slightly increased to
Fig. 5. Hardness values of TBCs before and after thermal exposure tests: (A) TBC-A, (B) TBC-B, and (C) TBC-C. Open and closed symbols (or each number) indicate TBCs without and with post heat treatment. Indentation for hardness measurement was conducted on sectional planes at 10 N.
Fig. 6. Toughness values of TBCs before and after thermal exposure tests: (A) TBC-A, (B) TBC-B, and (C) TBC-C. Open and closed symbols (or each number) indicate TBCs without and with post heat treatment. Indentation for toughness measurement was conducted on sectional planes at 50 N.
the bond coat were increased because of oxidation, being 137 ± 5 and 153 ± 2 GPa for the bond coats without and with the post heat treatment, respectively. The modulus values show good consistency between the microstructural evolution and other mechanical properties. 4. Conclusions
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Fundamental R&D Program for Core Technology of Materials funded by the Korean Ministry of Knowledge Economy (10041233). References
Fig. 7. Elastic modulus values of the top and bond coats after thermal exposure tests in TBC-C: (A) top coat and (B) bond coat. Each number indicates the top and bond coats without and with post heat treatment, respectively. The elastic modulus values of each as-prepared coat are indicated within the figure as a rectangular box.
63–65 GPa in the top coat and somewhat increased to 137–153 GPa in the bond coat. The hardness, toughness, and elastic modulus values well reflect the microstructural evolution, independent of the post heat treatment. The results indicate that control of the coating parameters can provide a microstructure for thermal stability in TBCs, and a relatively porous microstructure should be applied in thick TBCs. These observations allowed us to design various microstructures in TBCs and to propose an efficient TBC to protect the substrate in high-temperature environments. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (2011-0030802), by the Power Generation & Electricity Delivery of the Korean Institute of Energy Technology Evaluation and Planning (KETEP) grants funded by the Korean Ministry of Knowledge Economy (2009T100200025/2011T100200224), and by a grant from the
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