Applied Mechanics and Materials Vols. 260-261 (2013) pp 438-442 Online available since 2012/Dec/13 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.260-261.438
Thermal and Mechanical Characteristics of Thermal Barrier Coatings in Cyclic Thermal Fatigue Systems Kang-Hyeon Lee1,a, Sang-Won Myoung1,b, Min-Sik Kim1,c, Seoung-Soo Lee1,d, Eun-Hee Kim1,e, Yeon-Gil Jung1,f and Ungyu Paik2,g 1
School of Nano & Advanced Materials Engineering, Changwon National University, Changwon, Kyungnam 641-773, Republic of Korea 2
Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea a
[email protected],
[email protected],
[email protected], e f
[email protected],
[email protected],
[email protected], g
[email protected]
d
Keywords: Thermal barrier coating, Thermal fatigue system, Air Plasma Spray, Thermal property, Mechanical property.
Abstract. In this study, the relationship between microstructural evolution and mechanical properties of thermal barrier coatings (TBCs) has been investigated through different thermal fatigue systems, electric thermal fatigue (ETF) and flame thermal fatigue (FTF), including the thermal stability through the interface between the bond and top coats. The TBC system with the thicknesses of 300 µm in both the top and bond coats was prepared with METCO 204 NS and AMDRY 962, respectively, with the air plasma spray (APS) system using 9MB gun. To observe the oxidation resistance and thermal stability of TBC, the thermal exposure tests were performed with both thermal fatigue tests at a surface temperature of 850 °C with a temperature difference of 200 °C between the surface and bottom of sample, for 12,000 EOH in designed apparatuses. The hardness values are slightly increased due to the densification of top coat with increasing the thermal exposure time in both thermal fatigue tests. The influence of thermal fatigue condition on the microstructural evolution and interfacial stability of TBC is discussed. Introduction The operating temperature of gas turbines is increasing to enhance the energy efficiency, which requires a new cooling system and an increase of thermal barrier coating (TBC) thickness to protect a metallic substrate in thermomechanical environments [1,2]. The advantages of TBC include a potential increase in the efficiency and power density, and a decrease in continuance cost [3]. Zirconia-based ceramics are the most important coating materials for applications because of their low thermal conductivity, relatively high thermal expansion and super mechanical properties [1,3]. The future TBC systems will be more aggressively designed for the thermal protection of hot section components, thus allowing meaningful increase in engine operating temperatures, fuel efficiency and engine reliability. The development of next generation advanced TBCs will greatly rely on the better understanding of the coating behavior and failure mode under the high temperature, high thermal gradient cyclic conditions [2,4]. In this study, the thermal fatigue behavior of TBCs under electric thermal fatigue (ETF) and flame thermal fatigue (FTF) furnace conditions has been investigated. The comparison of the ETF and FTF tests on the microstructural evolution and mechanical property are discussed based on the experimental results and observations. Experimental procedure A nickel–based superalloy (Nimonic 80A, ThyssenKrupp VDM, Germany) was used as a substrate. The air plasma spray (APS) process was used to prepare Ni-Cr-Al-Y alloy bond coat (AMDRY 962, Sulzer Metco Holding AG, Switzerland) onto the substrate. The thickness of the bond coat was about All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 203.246.18.18-15/12/12,05:24:38)
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300 µm. The top layer was formed using zirconia (ZrO2) containing 8 wt% of yttria (Y2O3) (METCO 204 NS, Sultzer Metco Holding AG, Switzerland), which was air-plasma-sprayed onto the bond coat with same thickness. The coating process was carried out using a recommended gun (Metco 9MB, Sulzer Metco Holding AG, Switzerland). The spraying parameters are shown in Table 1 for the bond and top coats. These parameters were selected from conditions indicated by the manufacturer of the powders trying to guarantee the best deposition characteristics. The thermal exposure tests were performed at 850 °C with dwell time of 1h and 5 minutes until 12,000 EOH for the ETF and FTF tests, respectively. The thermal fatigue apparatus is specially designed—one side of the sample is thermally exposed and another side is cooled by air, and the temperature difference between the exposed and cooled sides is about 200 °C. In this study, equivalent operating hour (EOH) was used to evaluate the real circumstances in the gas turbine performance [5]. The cross-sectional microstructures of TBCs were observed using a scanning electron microscope (SEM, JEOL Model JSM-5610, Japan) in backscattered electron imaging mode. The thickness of thermally grown oxide (TGO) layer formed through the interface between the bond and top coats was measured using SEM, after thermal exposure tests. The hardness values of each sectional plane were measured from indentation experiments using a Vickers indenter (HV-114, Mitutoyo Corp., Japan) for loads of 10 N, with a Vickers tip. The hardness value was determined from equations proposed by Lawn [6]. Table 1. Coating parameters for preparing the bond and top coats Parameters Feedstock species Feed rate Gun to working distance Step distance Pass 1st and 2nd gas (Ar/H2) Current and voltage 1) Standard Cubic Feet per Hour
Bond coat
Top coat
AMDRY 962 50 g/min 150 mm 10 mm 16 times 96/27 SCFH1) 500A/70V
METCO 204 NS 45 g/min 110 mm 5 mm 18 times 96/23 SCFH 500A/70V
Results and discussion Cross-sectional micrographs of as-prepared top coat are shown in Fig.1. The top coat is shown a typical microstructure prepared by APS, and the thickness is well controlled. The top coat contains horizontal “splat” boundaries/cracks and pores, without any delamination or cracking through the interface between bond and top coat.
Fig. 1 Cross-sectional microstructure of as-prepared top coat. Overall TBC system is inserted inside figure.
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The microstructures of the top coat after thermal fatigue tests with different apparatuses (ETF and FTF) and thermal exposure time are shown in Fig. 2. After thermal exposure for 2,100 EOH, FTF sample maintains the initial microstructure (Fig. 2(B)), but ETF sample (Fig. 2(A)) is densified by decreasing splat boundary, because total thermal exposure time of ETF is longer than FTF. For thermal fatigue tests until 12,000 EOH, the horizontal defects disappear as a result of the re-sintering via diffusion and grain growth across splat boundaries as well as the removal of the planar microcracks. Also, the splat boundaries and cracks are coalesced, resulting in the relatively thicker and longer interlamellar cracks.
Fig. 2 Cross-sectional micrographs of top coats after thermal exposure with different conditions: (A) ETF 2,100 EOH, (B) FTF 2,100 EOH, (C) ETF 12,000 EOH, and (D) FTF 12,000 EOH. The microstructures of TBCs through the interface before and after thermal fatigue tests are shown in Fig. 3. The interface between the top and bond coats in the as-prepared TBC is clear (Fig. 3(A)). After the thermal fatigue tests, the TGO layer is formed through the interface between the top and bond coats. Shape of the TGO layer appears irregularly between top and bond coats. The thickness of TGO layer after thermal fatigue tests is shown in Fig. 4. The thickness is increased by increasing the thermal exposure time. Even though the thickness does not show significant difference for 2,100 EOH according to test methods, ETF sample shows a thicker TGO layer than FTF sample after 12,000 EOH.
Fig. 3 Cross-sectional micrographs of interfaces between the top and bond coats with before and after thermal fatigue tests: (A) as-prepared, (B) ETF 2,100 EOH, (C) FTF 2,100 EOH, (D) ETF 12,000 EOH, and (F) FTF 12,000 EOH
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Fig. 4 Thickness variation of TGO layer with thermal fatigue condition. The hardness values of the top coat before and after thermal fatigue tests with different apparatuses (ETF and FTF) were measured and the results are shown in Fig. 5. The hardness values are slightly increased with increasing the thermal exposure time, independent of test method. It may be due to the change in the microstructure through re-sintering during thermal fatigue tests.
Fig. 5 Hardness values of top coats before and after thermal fatigue test. Conclusions Microstructural evolution and mechanical properties have been investigated with the thermal fatigue apparatus and thermal exposure time. In as-prepared sample, the splat boundaries and pores are well developed randomly. With increasing thermal exposure time, the horizontal defects are decreased and relatively larger horizontal defects are developed, as a result of the densification of top coat via diffusion and grain growth across splat boundaries as well as the removal of the planar microcracks. The TGO layer is not fully developed in all samples and the thickness of TGO layer by FTF tests is thinner than that by EFT tests. In results of hardness, the values tend to increase upon increasing the exposure time, due to densification by re-sintering phenomenon during thermal fatigue tests. Taken together all results, ETF gives more significant thermal degradation to TBC system than FTF, because the actual operating hours (AOH) of ETF is longer than FTF.
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Acknowledgement This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (2012-0009452) and 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 (2011T100200224). Corresponding Author Yeon-Gil Jung,
[email protected], Tel: +82-55-213-3712, Fax: +82-55-262-6486 References [1] S.Y. Lee, J.Y. Kwon, T.W. Kang, Y.G. Jung, and U. Paik, Effects of thickness on thermal and mechanical properties of air plasma sprayed thermal barrier coatings. Mater. Sci. Forum. 658 (2010), 372-375. [2] D. Zhu, S.R. Choi, R.A. Miller, Development and thermal fatigue testing of ceramic thermal barrier coatings. Surf. Coat. Technol. 188-189 (2004), 146-152. [3] D. Zhu, R.A. Miller, Investigation of thermal high cycle and low cycle fatigue mechanisms of thick thermal barrier coating. Mater. Sci. Eng. A. 245 (1998), 212-223. [4] J.Y. Kwon, J.H. Kim, S.Y. Lee, Y.G. Jung, H. Cho, D.K. Yi, and U. Paik, Microstructural evolution and residual stresses of air plasma sprayed thermal barrier coatings under thermal exposure. Surf. Rev. Lett. 17 (2010), 1-7. [5] P.H. Lee, S.Y. Lee, J.Y. Kwon, S.W. Myoung, J.H. Lee, Y.G. Jung, H. Cho, and U. Paik, Thermal cycling behavior and interfacial stability in thick thermal barrier coatings. Surf. Coat. Technol. 205 (2010), 1250-1255. [6] B.R. Lawn, Fracture of Brittle Solids, Cambridge University Press, Cambridge, UK (1993) p.259-260.
Energy, Environment and Sustainable Development 10.4028/www.scientific.net/AMM.260-261
Thermal and Mechanical Characteristics of Thermal Barrier Coatings in Cyclic Thermal Fatigue Systems 10.4028/www.scientific.net/AMM.260-261.438
