Thermal stability of double-ceramic-layer thermal barrier coatings with various coating thickness

Materials Science and Engineering A 433 (2006) 1–7

Thermal stability of double-ceramic-layer thermal barrier coatings with various coating thickness Hui Dai a,b , Xinghua Zhong a,b , Jiayan Li a,b , Yanfei Zhang a,b , Jian Meng a , Xueqiang Cao a,∗ a

Key Lab of Rare Earth Chemistry & Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China b Graduate School of The Chinese Academy of Sciences, Beijing 100049, China Received 23 January 2006; received in revised form 12 April 2006; accepted 21 April 2006

Abstract Double-ceramic-layer (DCL) coatings with various thickness ratios composed of YSZ (6–8 wt.% Y2 O3 + ZrO2 ) and lanthanum zirconate (LZ, La2 Zr2 O7 ) were produced by the atmospheric plasma spraying. Chemical stability of LZ in contact with YSZ in DCL coatings was investigated by calcining powder blends at different temperatures. No obvious reaction was observed when the calcination temperature was lower than 1250 ◦ C, implying that LZ and YSZ had good chemical applicability for producing DCL coating. The thermal cycling test indicate that the cycling lives of the DCL coatings are strongly dependent on the thickness ratio of LZ and YSZ, and the coatings with YSZ thickness between 150 and 200 ␮m have even longer lives than the single-layer YSZ coating. When the YSZ layer is thinner than 100 ␮m, the DCL coatings failed in the LZ layer close to the interface of YSZ layer and LZ layer. For the coatings with the YSZ thickness above 150 ␮m, the failure mainly occurs at the interface of the YSZ layer and the bond coat. © 2006 Published by Elsevier B.V. Keywords: Thermal cycling; Plasma spraying; Thermal barrier coatings

1. Introduction During the last decade, research efforts were devoted to the development and manufacturing of ceramic TBCs on turbine parts because the traditional turbine materials have reached the limits of their temperature capabilities. TBCs have been widely used in hot-section metal components in gas turbines either to increase the inlet temperature with a consequent improvement of the efficiency or to reduce the requirements for the cooling air [1–3]. The typical TBC used in gas turbines consists of a bond coat produced by the vacuum or low pressure plasma-sprayed MCrAlY (M = Ni, Co) and a top coat of yttria partially stabilized zirconia made by the atmospheric plasma spraying or electron beam-physical vapor deposition (EB-PVD) [4,5]. A major disadvantage of YSZ is the limited operation temperature (1200 ◦ C) for the long-term application. At higher temperatures, the t phase transforms into the t-phase and c-phase. During cooling the t-phase will further transform into the m-phase, giving rise to

∗

Corresponding author. Tel.: +86 431 5262285; fax: +86 431 5262285. E-mail address: [email protected] (X. Cao).

0921-5093/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.msea.2006.04.075

the formation of microcracks in the coating [6,7]. Furthermore, due to the sintering of the coating at elevated temperatures, the porosity of the coating is reduced combined with the increase of Young’s modulus and tensile stress, which will lead to a reduced life under thermal cycling load. To overcome the disadvantages of YSZ, the search for candidate materials that can withstand higher gas-inlet temperature has been intensified in the past. Since the physical properties such as lower thermal conductivity than YSZ and high thermal stability up to its melting point, LZ was proposed as a promising material [6,8]. However, the low thermal expansion coefficient of LZ leads to high thermal stress between the LZ coating and the metallic bond coat, resulting in a short thermal cycling life [9]. The multilayer and graded structures have been produced to overcome this shortcoming [10,11]. As reported by the authors, the thermal cycling lives of multilayer coatings are two or three times longer than those of the single ceramic layer coatings [11]. However, the effect of the coating thickness ratio of LZ and YSZ of the DCL coating on the thermal cycling life has not been investigated. In the present work, the thermal stability of DCL coatings with various coating thickness ratios of LZ and YSZ were examined and the failure mechanisms were studied.

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Fig. 1. Spray-dried powder for plasma spraying.

Fig. 2. Thermal cycling procedure for DCL coatings. TSurf and TSub are for the surface and substrate temperatures, respectively.

2. Experimental The main chemicals used in this work were La2 O3 (99.99%, Chenghai Chemicals of Guangdong), ZrO2 (99.5%, Chenghai Chemicals of Guangdong), and YSZ (Sulzer Metco 204 NS). The LZ powder for plasma spraying was synthesized by the solid-state reaction followed by spray drying as described in detail in a previous paper [12]. This powder has a good flowability, a high density, and a particle size between 50 and 100 ␮m. The morphology of LZ powder used in this work is shown in Fig. 1. Sulzer Metco Vacuum Plasma Spray Unit with a F4 gun was used to deposit a 120 ␮m NiCoCrAlY bond coat (Ni 192-8 powder by Praxair Surface Technologies Inc.) on disk shaped Ni-base surperalloy substrates (supported by Beijing University of Aeronautics and Astronautics). The disk-shaped substrate has a bevelled edge to minimize the effect of stresses originated at the free edge of the specimen. The diameter and thickness of the substrate are 30 and 3 mm, respectively. The ceramic coatings with various thickness were produced by the atmospheric plasma spraying using Praxair-Tafa 55002000 Plasma-Spray Unit with a SG-100 gun. During the preparation of the samples for thermal cycling, steel substrates were also coated simultaneously. These coatings were used for the characterization of the spraying condition. The plasma spraying parameters are listed in Table 1. The thermal cycling was performed with a gas burner test facility operated with coal gas and oxygen. The substrate was cooled by the compressed air from the back. The surface temperature was measured with a pyrometer and that of the substrate

Table 1 Plasma spraying parameters for DCL coatings Current (A) Voltage (V) Coating distance (mm) Plasma gas (Ar/He, SLM/min) Powder feeding gas (Ar, SLM/min)

900 29 80 33/14 5.2

was measured by a thermocouple located at the center of the substrate. During thermal cycling, the temperatures of the surface and substrate are TSurf = 1250 ± 30 ◦ C and TSub = 965 ± 15◦ C, respectively. When 5% area of the ceramic coating was lost, the cycling was manually stopped and the cycling number was then the life of the coating. The cycling procedure is shown in Fig. 2. Powder blend of 50 mol% LZ and 50 mol% YSZ was mixed in the de-ionized water followed by ball-milling for 24 h using zirconia-balls. The slurry was then dried and subjected to heattreatment in a furnace in air at 1250 or 1400 ◦ C for different time. The X-ray powder diffraction patterns were collected at room temperature using Rigaku D/Max 2500 diffractometers with graphite monochromators (Cu K␣ radiation, 2θ angle range from 10◦ to 90◦ , step 0.02◦ ). The microstructures of the coatings were analyzed using a XL30 ESEM FEG scanning electron microscope. The cross-sectional samples were embedded in a transparent cold-setting epoxy and polished with diamond pastes down to 1 ␮m. 3. Results and discussion For DCL coatings, the YSZ layers firstly deposited on the bond coat have thickness of 50, 100, 150 and 200 ␮m. The LZ layers with thickness of 250, 200, 150 and 100 ␮m were then coated on top of the YSZ layer, and the total ceramic coating thickness was about 300 ␮m. For comparison, the single ceramic layer coatings of both YSZ and LZ were also produced. Fig. 3 shows the microstructures of the cross-sections of DCL coatings with various coating thickness before thermal cycling. All coatings have a porous microstructure, which is popular for the plasma-sprayed coatings. The porosity levels of YSZ and LZ coatings are similar even though the former has a higher melting point than the latter (2700 ◦ C for YSZ and 2300 ◦ C for LZ) [8]. It is noted that the weakest location in typical YSZ-TBCs is the interface of the top-coat and bond coat, where the crack or spallation usually occurs because of either the thermal and elastic mismatch between the top-coat and bond coat or the

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Fig. 3. Microstructures of the cross-sections of DLC coatings: (A) LZ/YSZ 250/50 ␮m, (B) LZ/YSZ 200/100 ␮m, (C) LZ/YSZ 150/150 ␮m, (D) LZ/YSZ 100/200 ␮m, (E) YSZ 310 ␮m, central part of the coating before thermal cycling.

growth stresses due to the formation of thermally-grown oxide (TGO) [13]. Moreover, at high temperatures, the chemical reaction between the top-coat and the TGO will dramatically reduce the performance of TBCs [14]. In the DCL coatings, the interface of LZ and YSZ is another weak location except that of YSZ and bond-coat. Chemical stability of LZ in contact with YSZ in DCL coatings was investigated by calcining powder blend at different temperatures. To maximize the interface area between the two ceramics, sub-micron powder blends of LZ and YSZ are used to evaluate the chemical stability of LZ in contact with YSZ. This simulates severe reaction condition, because in actual TBCs only a planar interface exits between the two ceramic layers. The chemical stability is studied at 1250 ◦ C, which is the target service temperature of next generation TBCs, and the reaction temperature of 1400 ◦ C represents a very severe testing condition. Fig. 4 shows the XRD patterns for the powder blend of 50 mol% LZ and 50 mol% YSZ: as-prepared, heat-treated at 1400 ◦ C for 12, 36 and 72 h. For comparison, the XRD patterns of original powders are also shown. Their lattice parameters are listed in Table 2. For blend with 12 h heat-treatment, the ˚ which is almost 0.02 A ˚ lattice parameter of LZ is 10.7955 A, smaller than that of the LZ starting powder. After heat-treatment

Fig. 4. X-ray diffraction patterns of 50 mol% LZ powder and 50 mol% YSZ powder blends with different heat-treatment at 1400 ◦ C.

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Table 2 Lattice parameters of 50 mol% LZ powder and 50 mol% YSZ powder blends with different heat treatments Annealing time at. 1250 ◦ C (h)

Lattice ˚ parameter (A)

Annealing time at 1400 ◦ C (h)

Lattice ˚ parameter (A)

24 72

10.8101 ± 0.0003 10.8101 ± 0.0006

12 36 72

Starting blend

10.8101 ± 0.0007

10.7955 ± 0.0004 10.7733 ± 0.0004 10.7734 ± 0.0003

Fig. 7. Thermal cycling lives of LZ/YSZ DCL coatings as a function of the thickness of YSZ layer. Dashed line for view guide.

Fig. 5. X-ray diffraction patterns of 50 mol% LZ powder and 50 mol% YSZ powder blends with different heat-treatment at 1250 ◦ C.

at 1400 ◦ C for 36 h, the XRD peaks of LZ also shift towards the lower d-value, indicating that another solid solution phase with high ZrO2 concentration appears. This is the solid solution of LZ and ZrO2 with pyrochlore structure. The La2 O3 –ZrO2 phase diagram shows a considerable solubility range for LZ from 0.87La2 O3 ·2ZrO2 to 1.15La2 O3 ·2ZrO2 whereby the crystal structure remains unchanged [9]. Fig. 5 shows the XRD patterns for the powder blend 50 mol% LZ and 50 mol% YSZ: as-prepared, heat-treated at 1250 ◦ C for 24 and 72 h. No obvious changes were observed after differ-

ent heat treatments, implying that no reaction between the LZ and YSZ powder take places. As a result, reaction between the LZ and YSZ would not be expected since the temperature of the ceramic layer interface (below 1150 ◦ C) is much lower than 1250 ◦ C during the operation, indicating that LZ and YSZ have good chemical applicability for producing DCL coating. The surfaces of single-layer LZ coatings before and after thermal cycling are shown in Fig. 6. Almost 1/6 of the LZ coating spalls off the substrate after only a few cycles. Thermal cycling lives of DCL coatings with various thickness ratios are shown in Fig. 7. The cycling number is plotted as a function of the thickness of YSZ. Except the coatings with YSZ thickness of 50 and 150 ␮m, each point in Fig. 7 was obtained by testing two specimens. Fig. 8 shows the microstructures of the crosssections of DCL coatings after thermal cycling. In the typical YSZ coating, the additional stress associated with the growth of the TGO is the main factor for the crack growth [15]. The micrograph of YSZ coating after thermal cycling is shown in Fig. 8D, the black scale between the bond coat and YSZ is TGO layer with thickness of about 4 ␮m.

Fig. 6. Surface photograghs of LZ coating before (A) and after (B) thermal cycling.

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As shown in Fig. 5, the cycling life of the DCL coating is strongly dependent on the thickness of YSZ. The life of the DCL coating with YSZ thickness smaller than 100 ␮m is short. Also, as shown in Fig. 8A, the coating with YSZ thickness of 100 ␮m fails in the LZ layer close to the interface of YSZ and LZ. With the increase of the thickness of YSZ layer from 100 to 150 ␮m, the thermal cycling lives of the DCL coatings also increased, implying that the YSZ interlayer has reduced the severe stress condition. When the thickness of YSZ layer is 150 ␮m, the DCL coating has even longer thermal cycling life than the single-layer YSZ coating. Compared to the DCL coating with YSZ thickness of 150 ␮m, the DCL coating with YSZ thickness of 200 ␮m shows a slight decrease of lifetime. For all the coatings with YSZ thickness larger than 150 ␮m, the cracks often occur at the interface of the YSZ layer and the bond coat (Fig. 8B–D). To rationalize these results, the stress state in TBCs is considered. The stress distribution of TBCs has strong influence on the performance indicators of the coatings, such as spallation and delamination resistance, fatigue life, bonding strength, etc. [16]. Generally, these stresses are the results of: (1) rapid contraction of the sprayed splats during plasma spray (i.e. quenching stress), (2) the mismatch of thermal expansions between the ceramic layer and the bond coat (i.e. thermal stress), (3) growth stress (i.e. bond coat oxidation) [17]. The quenching stress can be released or avoided by preheating the substrate during plasma spraying. The interfacial thermal stress is the main factor of crack initiation and extension. Moreover, if the stress state exceeds the adhesive or cohesive bonding forces of the coating, delamination and spallation may occur [18]. For the single-layer LZ coating, the interfacial thermal stress is much higher than that of the singlelayer YSZ coating because of the low thermal expansion coefficient. Moreover, the fracture toughness of LZ (1.2 MPa m1/2 ) is

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evidently lower than that of YSZ (2.4 MPa m1/2 ), implying that the crack initiation and growth will occur even with lower stress levels. This is why the single-layer LZ coating has a thermal cycling life of only a few cycles. YSZ has large thermal expansion coefficient and extremely high fracture toughness, but the high thermal conductivity and phase transition below 1200 ◦ C are the intrinsic shortcomings of YSZ, which restrict its longterm application at temperatures higher than 1200 ◦ C. Based on the discussion above, the DCL structure was developed, which could protect YSZ against high temperatures and minimize the thermal mismatch at the interface of the ceramic and the bond coat. The coating with YSZ thickness of 100 ␮m has much shorter thermal cycling life than those with YSZ thickness larger than 100 ␮m and even more interesting is the crack occurring in the LZ layer close to the interface of LZ layer and YSZ layer. As far as we know, there are some reasons that may lead to this kind of failure as described above. The Initial imperfection at the interface of ceramic layers may be one critical factor in determining the failure mechanism. However, as can be seen in Fig. 3B, the ceramic layer interface of the DCL coating with YSZ thickness of 100 ␮m was quite perfect. Moreover, it was reported in Ref. [11] that the thermal expansion mismatch between the ceramic layers would lead to the coating crack at the interface. As discussed above, the failure of DCL coatings with YSZ thickness larger than 100 ␮m mainly occurs at the interface of the YSZ layer and the bond coat. In additional, Fig. 9 shows the microstructure of the ceramic layer interface of DCL coating with YSZ thickness of 150 ␮m before and after thermal cycling, and the corresponding element maps of La, Zr and Y by EDS. After thermal cycling, the interface part of the DCL coating is still perfect and adheres to each other. No significant difference is observed between the microstructure of the interface before and

Fig. 8. Microstructures of the cross-sections of DLC coatings: (A) LZ/YSZ 200/100 ␮m, (B) LZ/YSZ 150/150 ␮m, (C) LZ/YSZ 100/200 ␮m, (D) YSZ 310 ␮m, rim part of the coating after thermal cycling.

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Fig. 9. EDS map scanning of interface of LZ layer and YSZ layer before (A) and after (B) thermal cycling.

after thermal cycling. As shown in EDS maps, a strip of YSZ was found in the LZ layer close to the interface, which may ascribe to the penetration of YSZ through the interconnected pores in the coating. Those observations indicate that thermal expansion mismatch between LZ and YSZ is not the main factor for the DCL coating failure. Based on the results in our experiments, a plausible explanation is put forward as follows. The introduction of YSZ coating as the interlayer between the LZ coating and the bond coat may contribute to the improvement of thermal cycling performance, because: the first, the thermal stress can be reduced, and the second the driving force for crack extension and the stress intensity factor can be reduced. On the other hand, it is believed that the reduction of thermal stress and stress intensity factor depend strongly on the thickness of the interlayer [19]. The thin YSZ contributes only negligibly to the change of the severe stress state in the LZ layer. Therefore, the cracking reason for the coating with YSZ thickness of 100 ␮m in the LZ layer could be that the stress between LZ and bond coat due to thermal expansion mismatch is not effectively relieved. From Fig. 5, it is evident that the lives of DCL coatings have obviously increased when the thickness of YSZ layer is larger than 150 ␮m, implying that the severe stress state in the LZ layer has been reduced to a low level. For the DCL coatings with YSZ thickness larger than 150 ␮m, due to the thermal protection of the LZ layer, the surface temperature of the YSZ layer was reduced. The calculation of the surface temperature is based on the thermal conductivity of LZ (1.56 W m−1 K−1 , relative density ∼97%, layer thickness) and YSZ (2.5 W m−1 K−1 , relative density ∼99%, layer thickness). The steady state heat conduction obeys the following equation: J = −λ

dT dl

(1)

where J is the heat flux, λ the thermal conductivity, T the temperature and l is the thickness of the coating. During thermal cycling, the mean temperatures of the surface and substrate of DLC coatings are TSurf = 1250 ◦ C and TSub = 970 ◦ C. For the DCL coating with YSZ thickness of 150 ␮m, the surface temperature of the

YSZ layer is below 1080 ◦ C, which is about 90 ◦ C lower than its phase transformation temperature and about 200 ◦ C lower than its sintering temperature (coating). When it comes to the DCL coating with YSZ thickness of 200 ␮m, the surface temperature (1125 ◦ C) increases because of the reducing thickness of LZ layer, indicating that the advantage of LZ layer to protect YSZ against higher temperatures are reduced. This may be the reason of the slightly decreasing lifetime of the DCL coating with YSZ thickness of 200 ␮m. For the DCL coatings with YSZ thickness larger than 150 ␮m, an additional failure mechanism seems to be dominant. The major factors that contribute to the stress growth in TBCs are: (1) the oxidation of bond coat results in the growth of Al2 O3 scale between the bond coat and the ceramic top coat [20,21], (2) phase transformation of YSZ [6,7], (3) the sintering of the ceramic coating combined with the increase of Young’s modulus [22]. As discussed above, due to the thermal protection of the LZ layer, the temperature of the YSZ layer is reduced, and the growth stress by the bond coat oxidation becomes the main failure factor for the DCL coatings with YSZ thickness larger than 150 ␮m. The successive chipping of the YSZ layer along the bond coat interface can be obviously observed as shown in Fig. 8B–D.

4. Summary The DCL coatings of LZ and YSZ with various thickness ratios were produced by the atmospheric plasma spraying. The cycling lives of the DCL coatings depend strongly on the thickness of YSZ. When its thickness is between 150 and 200 ␮m, the DCL coating has a longer thermal cycling life than the singlelayer YSZ coating, indicating that DCL structure is an efficient way to use the advantages and overcome the disadvantages of different coating materials, giving promise to the application of the TBCs at temperatures higher than 1250 ◦ C. Since no single material that has been studied so far satisfies all the requirements for high temperature TBCs, the DCL coating may be an important development direction.

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Acknowledgements The authors thank Dr. Q.S. Wang (Beijing Institute of Technology) for the invaluable assistance during plasma spraying, and great thanks also to Dr. R. Vassen (Forschungszentrum Juelich GmbH, Deutschland) for the vacuum plasma spraying of the bond coat and the support of YSZ powder. This work was financially supported by NSFC-20471058. References [1] F. Cernusci, P. Bianchi, M. Leoni, P. Scardi, J. Therm. Spray Technol. 8 (1) (1999) 102. [2] J.T. DeMasi-Marcin, D.K. Gupta, Surf. Coat. Technol. 68/69 (1994) 1. [3] J. Wigren, L. Pejryd, in: C. Coddet (Ed.), Proceedings of the 15th International Thermal Spray Conference on Thermal Spray Meeting the Challenges of the 21st Century, France, ASM International, Materials Park, OH, USA, 1998, p. 1531. [4] W.A. Nelson, R.M. Orenstein, J. Therm. Spray Technol. 6 (2) (1997) 176. [5] D. St¨over, C. Funke, Mater. Proc. Technol. 92/93 (1999) 195. [6] R. Vaßen, F. Tietz, G. Kerkhoff, D. St¨over, in: J., lecomte-Beckers, F., Schuber, P.J., Ennis, (Eds.), Proceedings of the 6th Li´ege Conference on Materials for advanced power engineering, vol. 3, Forschungszentrum J¨ulich GmbH, J¨ulich, Germany, p. 1627. [7] J. Thornton, Majumdar, in: A. Ohmori (Ed.), Proceedings of International Thermal Spray Conference on Thermal Spraying-Current Status and Future

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