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(Pt or Pt–Au) composite bond coat and 8YSZ top coat on Ni-based superalloy
Applied Surface Science 286 (2013) 298–305
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Thermal barrier coatings with (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) composite bond coat and 8YSZ top coat on Ni-based superalloy Junqi Yao a , Yedong He a,∗ , Deren Wang a , Hui Peng b , Hongbo Guo b , Shengkai Gong b a b
Beijing Key Laboratory for Corrosion, Erosion and Surface Technology, University of Science and Technology Beijing, 100083 Beijing, China School of Materials Science and Engineering, Beihang University, 100191 Beijing, China
a r t i c l e
i n f o
Article history: Received 4 August 2013 Received in revised form 10 September 2013 Accepted 12 September 2013 Available online 20 September 2013 Keywords: Composite bond coat Noble metal TBC High-temperature oxidation resistance
a b s t r a c t Developing new bond coat has been acknowledged as an effective way to extend the service life of thermal barrier coating (TBC) during high temperature. In this study, novel thermal barrier coating system, which is composed with an (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) composite bond coat and a YSZ top coat on Ni-based superalloy, has been prepared by magnetron sputtering and EB-PVD, respectively. It is demonstrated, from the cyclic oxidation tests in air at 1100 ◦ C for 200 h, that the YSZ top coat and alloy substrate can be bonded together effectively by the (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) composite coating, showing excellent resistance to oxidation, cracking and buckling. These beneficial results can be attributed to the sealing effect of such composite coating, by which the alloy substrate can be protected from oxidation and the interdiffusion between the bond coat and alloy substrate can be avoided; and the toughening effect of noble metals and composite structure of bond coat, by which the micro-cracks propagation can be inhibited and the stress in bond coat can be relaxed. This ceramic/noble metal composite coating can be a considerable structure which would has great application prospect in the TBC. © 2013 Elsevier B.V. All rights reserved.
1. Introduction It has been widely acknowledged that thermal barrier coating (TBC) is still a hot research topic currently, which plays an important role in gas-turbine engines used for numerous industrial fields such as propulsion and power generation [1]. Thermal barrier coatings (TBCs) comprise thermally insulating materials having sufficient thickness and durability that they can sustain an appreciable temperature difference between the load bearing alloy and the coating surface [2]. Along with internal cooling of the underlying superalloy component, TBCs (100 m to 500 m in thickness) would provide major reductions in the surface temperature (100 ◦ C to 300 ◦ C) of the superalloy. This has enabled modern gas-turbine engines to operate at gas temperature well above the melting temperature of the superalloy (∼1300 ◦ C), thereby improving engine efficiency and performance [3]. The state-of-the-art TBC system is usually composed with a heat insulating top coat layer of Yttria-Stabilized Zirconia (YSZ) and a metallic bond coat layer on the superalloy substrate. The common bond coat is MCrAlY (M = Ni and/or Co) [4] or Pt-modified (Ni1−x Ptx Al) nickel aluminide [5], used to provide good adhesion, a strong oxidation protection layer for the superalloy substrate [6]. Also, the thermally grown oxide (TGO) often forms on the bond coat
∗ Corresponding author. Tel.: +86 10 62332715; fax: +86 10 62332715. E-mail addresses: [email protected], [email protected] (Y. He). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.09.075
during engine operation. It consists largely of ␣-Al2 O3 and results in a slow oxidation rate of the bond coat [7]. Nowadays, the growth of TGO has been considered as the most important phenomenon responsible for the spallation failure of TBCs. As we know, during the service of TBCs, TGO would be formed and grow gradually along with the oxidation of bond coat. As the TGO thickens, failure of EBPVD TBC results from one or more of the following mechanisms: the increased out-of-plane stresses in or near the bond coat; progressive TGO roughening caused by bond-coat cyclic creep; accelerated growth of embedded oxides due to the localized TGO cracking; cavity formation in the bond coat; and the large-scale buckling due to the inevitable thermal expansion mismatch between the TGO and bond coat [8]. It can be seen that the key to enhance the durability of current YSZ-based TBCs is the retention of strong bonding at the bond coat/TGO and TGO/top coat interfaces. To this end, it is necessary to create and maintain a strong initial bond, reduce the stresses and the accumulated strain energy that promotes cracking and buckling at the interfaces, and increase the stress tolerance and fracture toughness of bond coat [3]. Therefore, numerous attempts have been made in the components design, improved preparation methods of alloy bond coat and heat treatment in the current TBCs, expecting for forming the dense and adequate thin protective oxide scale by selective oxidation [9–12]. Nevertheless, it should be noted that these improvements are based on the alloy bond coat, but the interdiffusion between the bond coat and superalloy substrate cannot be avoided. Consequently, secondary reaction zone (SRZ)
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and detrimental topologically close-packed (TCP) phase could be formed in the superalloy inevitably, leading to significant reduction in mechanical properties of the superalloy [13,14]. Therefore, in TBC system, bond coat should not only comprise or form the protective metal oxide with sufficiently good adhesion, but also mitigate the thermal expansion mismatch between the bond coat, TGO and ceramic top coat or improve its toughness to relax more stresses. Recently, composite structure has been considered of growing interest as an effective way to improve the durability of materials, such as laminated composite and dispersion composite [15–19]. It is reasonable to propose that coatings with a composite structure should possess improved mechanical properties than that with a single phase structure [20–22]. Besides, noble metal has always been considered as an ideal barrier material because it has plenty of attractive properties that no other materials can be compared to, such as thermodynamic stability and low oxygen diffusion rate [23]. Therefore, a new-developed coating system with ceramic/noble metal composite structures has been investigated by our research group, such as (Al2 O3 –Y2 O3 )/Pt, (Al2 O3 –Y2 O3 )/Au and (Al2 O3 –Y2 O3 )/(Pt–Au) laminated composite coatings and Au nanoparticles doped ␣-Al2 O3 composite coating [24–27]. It has been demonstrated that this coating system exhibits excellent hightemperature oxidation, cracking and spallation resistance. And no interdiffusion between the coating and substrate takes place during high-temperature oxidation. In this work, (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) micro-laminated composite coatings have been developed as the bond coats of TBCs to replace the common alloy bond coat, while the EB-PVD YSZ has been fabricated as top coat. The beneficial effects of the novel composite bond coats on the high-temperature oxidation and spallation resistance of TBCs on Ni-based superalloy have been investigated and the mechanisms for such effects have been discussed.
time as the work pressure, sputtering power and substrate temperature were constant. Therefore, 9-layer (Al2 O3 –Y2 O3 )/Pt and (Al2 O3 –Y2 O3 )/(Pt–Au) micro-laminated composite coatings were prepared. And then, the bond coated samples were annealed in a tube type resistance furnace at 1000 ◦ C in air for 20 h to achieve a stresses-released and more stable microstructure. Finally, 8YSZ top coat was deposited onto the bond coat by EU-205 EB-PVD coater with the deposition rate of 3 m/min. During EB-PVD, the samples were fixed and mounted on the rotated mechanical arm with rotation speed of 14 r/min, then preheated to the pre-defined temperature (approximately 850 ◦ C), which are as same as Ref. [28]. For simplicity, the (Al2 O3 –Y2 O3 )/Pt laminated coating will be identified as AP coating and the relevant TBC identified as AP-TBC, while the (Al2 O3 –Y2 O3 )/(Pt–Au) laminated coating and relevant TBC are identified as APA coating and APA-TBC.
2. Experimental procedure
Phases in the coatings were characterized on the surfaces by X-ray diffraction (XRD, Cu K␣, PW 3710, Phillips, step wise of 0.02◦ , continuous scanning) in the 2 range of 20◦ –80◦ . The morphology and composition of the samples were characterized by high-resolution field emission scanning electron microscopy (FESEM, ZEISS SUPRA 55) with an energy-dispersive spectroscopy (EDS) system.
2.1. Preparation of the novel bond coats and TBCs The (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) micro-laminated composite coating was fabricated in an opposite-targets magnetron sputtering (MS) system (Model TUS-800MP, Technol Ltd. Co. Beijing) on Ni-based superalloy with ultra-precision polished surfaces (Ra = 0.03 m). Table 1 shows the composition of the Ni-based superalloy. The alloy substrates (15 mm × 10 mm × 2 mm) were fixed on a rotating specimen holder (the rotation speed is 20 rpm) to ensure that all the surfaces can be deposited uniformly. The base pressure of the chamber and the working pressure were chosen as 5.0 × 10−4 Pa and 1.0 Pa. The (Al2 O3 –Y2 O3 )/noble metal micro-laminated composite coatings were fabricated by layers. First, the bottom Y2 O3 doped Al2 O3 ceramic composite layer was deposited by radio frequency MS using a 2 wt% Y2 O3 doped ␣Al2 O3 ceramic target with the sputtering power of 70 W and substrates temperature of 120 ◦ C. Then, the Pt or Pt–Au layer was deposited by direct current MS using a pure Pt or Pt70 Au30 (wt%) alloy target (purity > 99.99%) with the sputtering power of 150 ◦ C and the same substrate temperature. The thickness of the laminated coating was mainly determined by the sputtering
Table 1 Composition (wt%) of the Ni-based superalloy substrate in this study. Composition (wt%) Ni
Al
Cr
Si
Mo
C
80.33
6.17
4.74
1.38
3.04
4.34
2.2. High-temperature cyclic oxidation test The high-temperature cyclic oxidation test was performed in a tube-type resistance furnace at 1100 ◦ C in air for 200 h. Quartz crucibles were used to accommodate different samples, respectively, and had been pre-heated to a constant weight. After a certain oxidation period of 10 h, samples were taken out and cooled to room temperature by natural cooling for 30 min. And then the weight gain (crucible with sample) and spallation (crucible without the sample) of samples were weighed by an electronic balance with an accuracy of 10−5 g. After that, the samples were put back to the furnace again for another cycle. The cyclic oxidation tests provided 20 times thermal cycles and the data weighed were divided by the surface area of corresponding samples to plot the kinetic curves as a function of time. 2.3. Characterization
3. Results 3.1. Microstructure and phase composition Fig. 1 shows the FE-SEM images of cross-section and surface morphologies of the AP and APA coating before annealing. It can be seen in Fig. 1a that nine alternate layers of Al2 O3 –Y2 O3 and Pt has been fabricated, and the total thickness is about 1.4 m. The micro-laminated coating is bonded well and no cracks or flaws are observed at the interfaces between layers. Besides, it is shown in Fig. 1a1 that a very smooth and compact surface with refined nanostructure of the micro-laminated coating was obtained. Also, the similar microstructure can be observed in the APA coating in Fig. 1b and b1 due to the same preparation method. Furthermore, it is demonstrated that each Al2 O3 –Y2 O3 layer has the average thickness of 200 nm, while each Pt or Pt–Au layer is 100 nm, approximately, as shown in the EDS line scans of the AP and APA micro-laminated coating in Fig. 1c and d. After the annealing of novel bond coat, the YSZ top coat has been fabricated by EB-PVD. Fig. 2 reveals the microstructure morphologies of two novel TBCs. It is shown in Fig. 2a and c that, the YSZ exhibits an average thickness of 100 m and typical dense columnar structure, which is corresponding to the surface SEM images in Fig. 2a1 and c1 . In addition, the detailed cross-section images
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Fig. 1. FE-SEM images of microstructure morphologies of the prepared coatings: AP coating (a) cross-section and (a1 ) surface; APA coating (b) cross-section and (b1 ) surface; EDS line scans (c) AP coating; (d) APA coating.
of the bond coats are shown in Fig. 2b and d. It is observed that, after annealing at 1000 ◦ C for 20 h, the thicknesses of Pt and Pt–Au nano-layers have been become nonuniform slightly. During the annealing, phase transformation of Al2 O3 layers could take place, which should give rise to the growth of internal stresses. However,
these stresses could be relaxed due to the excellent ductility and plasticity of noble metal nano-layers. Fig. 3 reveals the XRD patterns of the samples in different stages. The only diffraction peaks are observed in Fig. 3a for the main phase of substrate (JCPDS 65-6613). It is demonstrated in Fig. 3b
Fig. 2. FE-SEM images of microstructure morphologies of the prepared TBCs: AP-TBC (a) cross-section, (a1 ) surface; APA-TBC (b) cross-section, (b1 ) surface; detailed crosssection (c) AP-TBC; (d) APA-TBC.
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the excellent protection effects against the oxidation and thermal cyclic spallation have been achieved. Moreover, the weight gain and spallation of APA-TBC can be obtained as lower as 0.3125 mg/cm2 and 0.0825 mg/cm2 , respectively. This TBC exhibits relatively excellent oxidation resistance compared with the reported common TBC systems with alloy bond coat (YSZ/(Hf doped NiAl bond coat)/K3 Ni-based superalloy; YSZ/(Ni, Pt)Al bond coat/René N5 superalloy) [29,30]. 3.3. Morphologies of TBCs after cyclic oxidation
Fig. 3. XRD patterns of the samples in different stages: (a) blank; AP coating (b) before annealing; (c) after annealing; APA coating (d) before annealing; (e) after annealing; (f) APA-TBC.
and d that, noble metals in the sample can also be identified before annealing (Pt: JCPDS 65-2868, Au: JCPDS 65-8601), which indicate that the Al2 O3 layers are mainly amorphous after sputtering. But, it is shown in Fig. 3c and e that ␣-Al2 O3 phase (JCPDS 46-1212) is detected in the micro-laminated coating after annealing at 1000 ◦ C for 20 h, which implies that phase transformation has been taken place during annealing. Furthermore, it can be seen in Fig. 3f that only t-Zr0.92 Y0.08 O1.96 phase (JCPDS 48-0224) has been identified in the TBC bonded by (Al2 O3 –Y2 O3 )/noble metal micro-laminated coating, since the thickness of YSZ top coat is much greater than the penetration depth of X-ray diffraction (5–10 m on the condition).
Fig. 5 shows the morphologies of samples with different coatings after cyclic oxidation. It can be seen in Fig. 5a and b that, a loose and thick oxide scale is formed on the bare Ni-based superalloy during oxidation, which is mainly composed with ␣-Al2 O3 , NiAl2 O4 (JCPDS 10-0339) and (Al0.9 Cr0.1 )2 O3 (JCPDS 51-1394) (shown in Fig. 6). In this loose structure of oxide, rapid inward diffusion paths for oxygen could be provided, giving rise to the further oxidation of superalloy. Generally, under the low oxygen partial pressure, Al element should be oxidized preferentially comparing with other elements in the alloy [31]. When the Al content decreases, other elements such as Ni and Cr would be also oxidized subsequently. Thus, the degree of oxidation of the sample (weight gain) can be concluded qualitatively from the formed oxide phases. Fig. 7 reveals the EDS elements maps of the blank sample after cyclic oxidation. It can be seen that the oxides or spinels of Ni and Cr have also been formed due to the depletion of Al at the substrate/bond coat interfaces and the outward diffusion of Ni and Cr through the thin Al2 O3 layer. Thus, it can be concluded that the blank sample was oxidized seriously, which would bring out large weight gain, as shown in Fig. 4a. The mechanism for the spinel formation is also shown as follows. Since the Al element is selectively oxidized and consumed to a low content, Ni and Cr are also oxidized. During oxidation, the NiO and Cr2 O3 internal oxide islands are engulfed by Al2 O3 scale, and solid-state reactions ensue to form spinel and other mixed oxide [32,33]: NiO + Al2 O3 = NiAl2 O4
(1)
3.2. High-temperature cyclic oxidation behavior
0.9 Al2 O3 + 0.1 Cr2 O3 = (Al0.9 Cr0.1 )2 O3
(2)
Fig. 4 reveals the oxidation kinetic curves of different samples at 1100 ◦ C for 200 h. It is shown that high weight gain is achieved from the bare Ni-based superalloy, which implies that the substrate has been oxidized seriously; and the high spallation can be attributed to the excessive internal stresses between the formed scales and substrate. Comparatively, the weight gain and spallation of the two TBCs can be decreased significantly, which imply that
Comparatively, it can be observed in Fig. 5c and e, that the structures of two TBCs are maintained well, and no cracks or voids are observed after oxidation. The superalloy substrates and top coats are bonded together effectively by the two bond coats, without any cracking and buckling. There is also no interdiffusion occurred at the coating/substrate interface. The TGO cannot be identified obviously, which can be attributed to the same composition
Fig. 4. Oxidation kinetic curves of different samples at 1100 ◦ C for 200 h: (a) weight gain versus time; (b) spallation versus time.
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Fig. 5. FE-SEM images of microstructure morphologies of samples with different coatings after cyclic oxidation at 1100 ◦ C for 200 h: blank sample (a) surface; (b) cross-section; AP-TBC coating (c) cross-section; (d) detailed cross-section; APA-TBC coating (e) cross-section; (f) detailed cross-section.
(␣-Al2 O3 ) to the bond coat and very small thickness due to the good oxidation resistance of the two TBCs. This is also corresponding to the low weight gain in Fig. 4a. It is worth mentioning that the two underneath Pt nano-layers have been transformed into Pt nanoparticles which are distributed in the ␣-Al2 O3 layers after cyclic oxidation as shown in Fig. 5d. Besides, it can be seen in Fig. 5f that, all of the Pt–Au nano-layers have been transformed into Pt–Au nanoparticles which are distributed in the ␣-Al2 O3 layers completely and uniformly after cyclic oxidation. Consequently, the designed (Al2 O3 –Y2 O3 )/noble metal micro-laminated structure can be transformed slowly into a structure with dispersed noble metal particles during oxidation, from the lower noble metal layer to the upper layer. And this transformation can be promoted by the addition of Au element in Pt nano-layers. 4. Discussion It has been demonstrated from the experimental results that, the two TBCs exhibit excellent resistance to high-temperature oxidation, spallation, cracking and buckling. And the novel ceramic/noble metal composite bond coat has a great impact on service performance of the TBC on Ni-based superalloy under high temperature.
The mechanisms accounting for these beneficial effects are discussed from two aspects as follows. 4.1. Mechanism for the excellent high-temperature oxidation resistance of novel TBC In the designed bond coat with (Al2 O3 –Y2 O3 )/noble metal micro-laminated structure, the alloy substrate could be well sealed by continuous multi-layered Al2 O3 –Y2 O3 and noble metal. It is known that YAG nano-particles distributed in the ␣-Al2 O3 grain boundaries can refine the structure of coating and suppress the outward diffusion of alloy elements. Thus, the growth of TGO can be mainly determined by the inward diffusion of oxygen. In such YSZ-based TBC system, YSZ has been known as an oxygen ion conductor, while the ␣-Al2 O3 and noble metal (Pt or Au) possess the rather low oxygen diffusion coefficient [34–37]. Thus, the oxygen partial pressure can be decreased stepwise by the sealing effect of multi-sealed alternating Al2 O3 –Y2 O3 and noble metal nano-layers in the bond coat. In the practical application, the noble metal nanolayers are replaced by noble metal particles and platelets which distributed in the ␣-Al2 O3 layer after oxidation. Nevertheless, the ␣-Al2 O3 can still seal the substrate well, and suppress the inward
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4.2. Mechanism for the enhanced mechanical properties of novel TBC As we know, the protective effects of coating can be derived from not only the properties and composition of materials but also the integrality of its structure. Thus, the mechanical properties of coating certainly play an important role in the performance and service life of coating. It is vital to study the special mechanical properties of the proposed novel TBC structure. As analyzed by Evans [38], it can be used as the criterion for the failure of a coating that the elastic strain energy stored in the coating exceeds the fracture resistance, Gc : (1 − )
Fig. 6. XRD patterns of blank sample after cyclic oxidation at 1100 ◦ C for 200 h.
diffusion of oxygen to the superalloy substrate to a low degree. The selective oxidation of Al element in the superalloy substrate should take place to form ␣-Al2 O3 [31]. It can also have beneficial effect on decreasing the oxygen partial pressure, avoiding the further oxidation of substrate. Thus, only few Al2 O3 should be formed in the two TBCs after cyclic oxidation. Besides, the interdiffusion between the superalloy substrate and bond coat can be avoided, even though the micro-laminated structure of bond coat can transform into dispersed structure. Consequently, a dense and thin TGO nano-layer can be formed by selective oxidation and finally inhibit the further oxidation of substrate.
2h > Gc E
(3)
where , , h and E denote the Poisson ratio, stress, thickness and elastic modulus. Therefore, either decreasing the inner stresses or increasing the fracture toughness of coating can improve the durability and spallation resistance of the coating. In the (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) micro-laminated composite bond coat, the apparent thermal expansion coefficient (CTE) of bond coat can be increased by the addition of noble metal [39] compared to the single ␣-Al2 O3 , which is the main component of TGO. And then the thermal expansion mismatch at the bond coat/substrate interface and bond coat/YSZ top coat interface in the TBC system can be mitigated, giving rise to the lower stress [40]. As Au possesses the higher CTE (∼14.16 × 10−6 K−1 ) than Pt (∼9.0 × 10−6 K−1 ), the PtAu alloy can be more conducive to the decrease of thermal stress in the bond coat. Besides, the thermal stress caused by the unavoidable thermal expansion mismatch and the grown stress during cyclic oxidation can be relaxed by the plastic deformation of ductile noble metal nano-layers, exhibiting effective toughness. Thus, the structure of bond coat can be transformed during oxidation without damage, as shown in Fig. 8, showing excellent adhesion.
Fig. 7. EDS elements maps of the blank sample after cyclic oxidation.
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Fig. 8. The structure transformation schematic diagram of TBC bonded by (Al2 O3 –Y2 O3 )/noble metal micro-laminated coating during high-temperature oxidation.
Along with the structure transformation of bond coat during the cyclic oxidation, the Pt–Au nano-layers tend to transform into alloy nano-particles even more than Pt, which can be attributed to the higher plasticity, lower elastic modulus and metling point of PtAu alloy. And the toughening mechanisms become different after the structure transformation. As shown in Fig. 8, the structure transformation of bond coat begins from the lower layer to upper layer slowly, which implies that the stress in the lower layer is larger than that in the upper layer. This is correlated with the higher thermal expansion mismatch at the substrate/bond coat interface than the bond coat/top coat interface. In this stage, internal stresses can be relaxed and the strain energy can be absorbed by the doped noble metal particles and platelets due to their plastic deformation and crack bridging effect [41]. And based on the Al2 O3 –Y2 O3 phase diagram, YAG (Yttrium Aluminum Garnet phase) particles with nano-size must be formed in the Al2 O3 –Y2 O3 coating and segregated at the ␣-Al2 O3 grain boundaries, which can provide sufficient strength and toughness for the coating [42]. Thus, the micro-cracks propagation can be inhibited, which can avoid the formation of cracks in larger size. And excellent resistance to cracking, spallation and also buckling can be obtained in the designed novel bond coat, exhibiting effectively bonding strength at the substrate/bond coat interface and bond coat/top coat interface. Consequently, the durability of TBC can be improved and the service life can be extended. 5. Conclusions In conclusion, novel thermal barrier coatings bonded by (Al2 O3 –Y2 O3 )/noble metal (Pt or Pt–Au) composite coatings have been developed. The novel TBCs can improve the high-temperature oxidation of Ni-based superalloy and also exhibit excellent resistance to cracking, spallation and buckling, due to the sealing effect of (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) composite bond coat and the toughening effect of noble metals under thermal cycling. With the sealing effect, the oxygen diffusion to the alloy substrate and the interdiffusion between the bond coat and alloy substrate can be effectively suppressed to a low degree, leading to the significant improvement of the oxidation resistance. With the toughening effect of noble metals and composite structure of bond coat, the fracture toughness of bond coat can be enhanced, resulting in the effectively improvement of adhesion strength and resistance to cracking, spallation and buckling. This novel ceramic/noble metal composite coating can be a considerable structure which would has great application prospect in the TBC. Acknowledgments The authors thank the financial support from the Chinese National Nature Science Foundation (Grant no. 51071030).
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Thermal barrier coatings with (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) composite bond coat and 8YSZ top coat on Ni-based superalloy Junqi Yao a , Yedong He a,∗ , Deren Wang a , Hui Peng b , Hongbo Guo b , Shengkai Gong b a b
Beijing Key Laboratory for Corrosion, Erosion and Surface Technology, University of Science and Technology Beijing, 100083 Beijing, China School of Materials Science and Engineering, Beihang University, 100191 Beijing, China
a r t i c l e
i n f o
Article history: Received 4 August 2013 Received in revised form 10 September 2013 Accepted 12 September 2013 Available online 20 September 2013 Keywords: Composite bond coat Noble metal TBC High-temperature oxidation resistance
a b s t r a c t Developing new bond coat has been acknowledged as an effective way to extend the service life of thermal barrier coating (TBC) during high temperature. In this study, novel thermal barrier coating system, which is composed with an (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) composite bond coat and a YSZ top coat on Ni-based superalloy, has been prepared by magnetron sputtering and EB-PVD, respectively. It is demonstrated, from the cyclic oxidation tests in air at 1100 ◦ C for 200 h, that the YSZ top coat and alloy substrate can be bonded together effectively by the (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) composite coating, showing excellent resistance to oxidation, cracking and buckling. These beneficial results can be attributed to the sealing effect of such composite coating, by which the alloy substrate can be protected from oxidation and the interdiffusion between the bond coat and alloy substrate can be avoided; and the toughening effect of noble metals and composite structure of bond coat, by which the micro-cracks propagation can be inhibited and the stress in bond coat can be relaxed. This ceramic/noble metal composite coating can be a considerable structure which would has great application prospect in the TBC. © 2013 Elsevier B.V. All rights reserved.
1. Introduction It has been widely acknowledged that thermal barrier coating (TBC) is still a hot research topic currently, which plays an important role in gas-turbine engines used for numerous industrial fields such as propulsion and power generation [1]. Thermal barrier coatings (TBCs) comprise thermally insulating materials having sufficient thickness and durability that they can sustain an appreciable temperature difference between the load bearing alloy and the coating surface [2]. Along with internal cooling of the underlying superalloy component, TBCs (100 m to 500 m in thickness) would provide major reductions in the surface temperature (100 ◦ C to 300 ◦ C) of the superalloy. This has enabled modern gas-turbine engines to operate at gas temperature well above the melting temperature of the superalloy (∼1300 ◦ C), thereby improving engine efficiency and performance [3]. The state-of-the-art TBC system is usually composed with a heat insulating top coat layer of Yttria-Stabilized Zirconia (YSZ) and a metallic bond coat layer on the superalloy substrate. The common bond coat is MCrAlY (M = Ni and/or Co) [4] or Pt-modified (Ni1−x Ptx Al) nickel aluminide [5], used to provide good adhesion, a strong oxidation protection layer for the superalloy substrate [6]. Also, the thermally grown oxide (TGO) often forms on the bond coat
∗ Corresponding author. Tel.: +86 10 62332715; fax: +86 10 62332715. E-mail addresses: [email protected], [email protected] (Y. He). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.09.075
during engine operation. It consists largely of ␣-Al2 O3 and results in a slow oxidation rate of the bond coat [7]. Nowadays, the growth of TGO has been considered as the most important phenomenon responsible for the spallation failure of TBCs. As we know, during the service of TBCs, TGO would be formed and grow gradually along with the oxidation of bond coat. As the TGO thickens, failure of EBPVD TBC results from one or more of the following mechanisms: the increased out-of-plane stresses in or near the bond coat; progressive TGO roughening caused by bond-coat cyclic creep; accelerated growth of embedded oxides due to the localized TGO cracking; cavity formation in the bond coat; and the large-scale buckling due to the inevitable thermal expansion mismatch between the TGO and bond coat [8]. It can be seen that the key to enhance the durability of current YSZ-based TBCs is the retention of strong bonding at the bond coat/TGO and TGO/top coat interfaces. To this end, it is necessary to create and maintain a strong initial bond, reduce the stresses and the accumulated strain energy that promotes cracking and buckling at the interfaces, and increase the stress tolerance and fracture toughness of bond coat [3]. Therefore, numerous attempts have been made in the components design, improved preparation methods of alloy bond coat and heat treatment in the current TBCs, expecting for forming the dense and adequate thin protective oxide scale by selective oxidation [9–12]. Nevertheless, it should be noted that these improvements are based on the alloy bond coat, but the interdiffusion between the bond coat and superalloy substrate cannot be avoided. Consequently, secondary reaction zone (SRZ)
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and detrimental topologically close-packed (TCP) phase could be formed in the superalloy inevitably, leading to significant reduction in mechanical properties of the superalloy [13,14]. Therefore, in TBC system, bond coat should not only comprise or form the protective metal oxide with sufficiently good adhesion, but also mitigate the thermal expansion mismatch between the bond coat, TGO and ceramic top coat or improve its toughness to relax more stresses. Recently, composite structure has been considered of growing interest as an effective way to improve the durability of materials, such as laminated composite and dispersion composite [15–19]. It is reasonable to propose that coatings with a composite structure should possess improved mechanical properties than that with a single phase structure [20–22]. Besides, noble metal has always been considered as an ideal barrier material because it has plenty of attractive properties that no other materials can be compared to, such as thermodynamic stability and low oxygen diffusion rate [23]. Therefore, a new-developed coating system with ceramic/noble metal composite structures has been investigated by our research group, such as (Al2 O3 –Y2 O3 )/Pt, (Al2 O3 –Y2 O3 )/Au and (Al2 O3 –Y2 O3 )/(Pt–Au) laminated composite coatings and Au nanoparticles doped ␣-Al2 O3 composite coating [24–27]. It has been demonstrated that this coating system exhibits excellent hightemperature oxidation, cracking and spallation resistance. And no interdiffusion between the coating and substrate takes place during high-temperature oxidation. In this work, (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) micro-laminated composite coatings have been developed as the bond coats of TBCs to replace the common alloy bond coat, while the EB-PVD YSZ has been fabricated as top coat. The beneficial effects of the novel composite bond coats on the high-temperature oxidation and spallation resistance of TBCs on Ni-based superalloy have been investigated and the mechanisms for such effects have been discussed.
time as the work pressure, sputtering power and substrate temperature were constant. Therefore, 9-layer (Al2 O3 –Y2 O3 )/Pt and (Al2 O3 –Y2 O3 )/(Pt–Au) micro-laminated composite coatings were prepared. And then, the bond coated samples were annealed in a tube type resistance furnace at 1000 ◦ C in air for 20 h to achieve a stresses-released and more stable microstructure. Finally, 8YSZ top coat was deposited onto the bond coat by EU-205 EB-PVD coater with the deposition rate of 3 m/min. During EB-PVD, the samples were fixed and mounted on the rotated mechanical arm with rotation speed of 14 r/min, then preheated to the pre-defined temperature (approximately 850 ◦ C), which are as same as Ref. [28]. For simplicity, the (Al2 O3 –Y2 O3 )/Pt laminated coating will be identified as AP coating and the relevant TBC identified as AP-TBC, while the (Al2 O3 –Y2 O3 )/(Pt–Au) laminated coating and relevant TBC are identified as APA coating and APA-TBC.
2. Experimental procedure
Phases in the coatings were characterized on the surfaces by X-ray diffraction (XRD, Cu K␣, PW 3710, Phillips, step wise of 0.02◦ , continuous scanning) in the 2 range of 20◦ –80◦ . The morphology and composition of the samples were characterized by high-resolution field emission scanning electron microscopy (FESEM, ZEISS SUPRA 55) with an energy-dispersive spectroscopy (EDS) system.
2.1. Preparation of the novel bond coats and TBCs The (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) micro-laminated composite coating was fabricated in an opposite-targets magnetron sputtering (MS) system (Model TUS-800MP, Technol Ltd. Co. Beijing) on Ni-based superalloy with ultra-precision polished surfaces (Ra = 0.03 m). Table 1 shows the composition of the Ni-based superalloy. The alloy substrates (15 mm × 10 mm × 2 mm) were fixed on a rotating specimen holder (the rotation speed is 20 rpm) to ensure that all the surfaces can be deposited uniformly. The base pressure of the chamber and the working pressure were chosen as 5.0 × 10−4 Pa and 1.0 Pa. The (Al2 O3 –Y2 O3 )/noble metal micro-laminated composite coatings were fabricated by layers. First, the bottom Y2 O3 doped Al2 O3 ceramic composite layer was deposited by radio frequency MS using a 2 wt% Y2 O3 doped ␣Al2 O3 ceramic target with the sputtering power of 70 W and substrates temperature of 120 ◦ C. Then, the Pt or Pt–Au layer was deposited by direct current MS using a pure Pt or Pt70 Au30 (wt%) alloy target (purity > 99.99%) with the sputtering power of 150 ◦ C and the same substrate temperature. The thickness of the laminated coating was mainly determined by the sputtering
Table 1 Composition (wt%) of the Ni-based superalloy substrate in this study. Composition (wt%) Ni
Al
Cr
Si
Mo
C
80.33
6.17
4.74
1.38
3.04
4.34
2.2. High-temperature cyclic oxidation test The high-temperature cyclic oxidation test was performed in a tube-type resistance furnace at 1100 ◦ C in air for 200 h. Quartz crucibles were used to accommodate different samples, respectively, and had been pre-heated to a constant weight. After a certain oxidation period of 10 h, samples were taken out and cooled to room temperature by natural cooling for 30 min. And then the weight gain (crucible with sample) and spallation (crucible without the sample) of samples were weighed by an electronic balance with an accuracy of 10−5 g. After that, the samples were put back to the furnace again for another cycle. The cyclic oxidation tests provided 20 times thermal cycles and the data weighed were divided by the surface area of corresponding samples to plot the kinetic curves as a function of time. 2.3. Characterization
3. Results 3.1. Microstructure and phase composition Fig. 1 shows the FE-SEM images of cross-section and surface morphologies of the AP and APA coating before annealing. It can be seen in Fig. 1a that nine alternate layers of Al2 O3 –Y2 O3 and Pt has been fabricated, and the total thickness is about 1.4 m. The micro-laminated coating is bonded well and no cracks or flaws are observed at the interfaces between layers. Besides, it is shown in Fig. 1a1 that a very smooth and compact surface with refined nanostructure of the micro-laminated coating was obtained. Also, the similar microstructure can be observed in the APA coating in Fig. 1b and b1 due to the same preparation method. Furthermore, it is demonstrated that each Al2 O3 –Y2 O3 layer has the average thickness of 200 nm, while each Pt or Pt–Au layer is 100 nm, approximately, as shown in the EDS line scans of the AP and APA micro-laminated coating in Fig. 1c and d. After the annealing of novel bond coat, the YSZ top coat has been fabricated by EB-PVD. Fig. 2 reveals the microstructure morphologies of two novel TBCs. It is shown in Fig. 2a and c that, the YSZ exhibits an average thickness of 100 m and typical dense columnar structure, which is corresponding to the surface SEM images in Fig. 2a1 and c1 . In addition, the detailed cross-section images
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Fig. 1. FE-SEM images of microstructure morphologies of the prepared coatings: AP coating (a) cross-section and (a1 ) surface; APA coating (b) cross-section and (b1 ) surface; EDS line scans (c) AP coating; (d) APA coating.
of the bond coats are shown in Fig. 2b and d. It is observed that, after annealing at 1000 ◦ C for 20 h, the thicknesses of Pt and Pt–Au nano-layers have been become nonuniform slightly. During the annealing, phase transformation of Al2 O3 layers could take place, which should give rise to the growth of internal stresses. However,
these stresses could be relaxed due to the excellent ductility and plasticity of noble metal nano-layers. Fig. 3 reveals the XRD patterns of the samples in different stages. The only diffraction peaks are observed in Fig. 3a for the main phase of substrate (JCPDS 65-6613). It is demonstrated in Fig. 3b
Fig. 2. FE-SEM images of microstructure morphologies of the prepared TBCs: AP-TBC (a) cross-section, (a1 ) surface; APA-TBC (b) cross-section, (b1 ) surface; detailed crosssection (c) AP-TBC; (d) APA-TBC.
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the excellent protection effects against the oxidation and thermal cyclic spallation have been achieved. Moreover, the weight gain and spallation of APA-TBC can be obtained as lower as 0.3125 mg/cm2 and 0.0825 mg/cm2 , respectively. This TBC exhibits relatively excellent oxidation resistance compared with the reported common TBC systems with alloy bond coat (YSZ/(Hf doped NiAl bond coat)/K3 Ni-based superalloy; YSZ/(Ni, Pt)Al bond coat/René N5 superalloy) [29,30]. 3.3. Morphologies of TBCs after cyclic oxidation
Fig. 3. XRD patterns of the samples in different stages: (a) blank; AP coating (b) before annealing; (c) after annealing; APA coating (d) before annealing; (e) after annealing; (f) APA-TBC.
and d that, noble metals in the sample can also be identified before annealing (Pt: JCPDS 65-2868, Au: JCPDS 65-8601), which indicate that the Al2 O3 layers are mainly amorphous after sputtering. But, it is shown in Fig. 3c and e that ␣-Al2 O3 phase (JCPDS 46-1212) is detected in the micro-laminated coating after annealing at 1000 ◦ C for 20 h, which implies that phase transformation has been taken place during annealing. Furthermore, it can be seen in Fig. 3f that only t-Zr0.92 Y0.08 O1.96 phase (JCPDS 48-0224) has been identified in the TBC bonded by (Al2 O3 –Y2 O3 )/noble metal micro-laminated coating, since the thickness of YSZ top coat is much greater than the penetration depth of X-ray diffraction (5–10 m on the condition).
Fig. 5 shows the morphologies of samples with different coatings after cyclic oxidation. It can be seen in Fig. 5a and b that, a loose and thick oxide scale is formed on the bare Ni-based superalloy during oxidation, which is mainly composed with ␣-Al2 O3 , NiAl2 O4 (JCPDS 10-0339) and (Al0.9 Cr0.1 )2 O3 (JCPDS 51-1394) (shown in Fig. 6). In this loose structure of oxide, rapid inward diffusion paths for oxygen could be provided, giving rise to the further oxidation of superalloy. Generally, under the low oxygen partial pressure, Al element should be oxidized preferentially comparing with other elements in the alloy [31]. When the Al content decreases, other elements such as Ni and Cr would be also oxidized subsequently. Thus, the degree of oxidation of the sample (weight gain) can be concluded qualitatively from the formed oxide phases. Fig. 7 reveals the EDS elements maps of the blank sample after cyclic oxidation. It can be seen that the oxides or spinels of Ni and Cr have also been formed due to the depletion of Al at the substrate/bond coat interfaces and the outward diffusion of Ni and Cr through the thin Al2 O3 layer. Thus, it can be concluded that the blank sample was oxidized seriously, which would bring out large weight gain, as shown in Fig. 4a. The mechanism for the spinel formation is also shown as follows. Since the Al element is selectively oxidized and consumed to a low content, Ni and Cr are also oxidized. During oxidation, the NiO and Cr2 O3 internal oxide islands are engulfed by Al2 O3 scale, and solid-state reactions ensue to form spinel and other mixed oxide [32,33]: NiO + Al2 O3 = NiAl2 O4
(1)
3.2. High-temperature cyclic oxidation behavior
0.9 Al2 O3 + 0.1 Cr2 O3 = (Al0.9 Cr0.1 )2 O3
(2)
Fig. 4 reveals the oxidation kinetic curves of different samples at 1100 ◦ C for 200 h. It is shown that high weight gain is achieved from the bare Ni-based superalloy, which implies that the substrate has been oxidized seriously; and the high spallation can be attributed to the excessive internal stresses between the formed scales and substrate. Comparatively, the weight gain and spallation of the two TBCs can be decreased significantly, which imply that
Comparatively, it can be observed in Fig. 5c and e, that the structures of two TBCs are maintained well, and no cracks or voids are observed after oxidation. The superalloy substrates and top coats are bonded together effectively by the two bond coats, without any cracking and buckling. There is also no interdiffusion occurred at the coating/substrate interface. The TGO cannot be identified obviously, which can be attributed to the same composition
Fig. 4. Oxidation kinetic curves of different samples at 1100 ◦ C for 200 h: (a) weight gain versus time; (b) spallation versus time.
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Fig. 5. FE-SEM images of microstructure morphologies of samples with different coatings after cyclic oxidation at 1100 ◦ C for 200 h: blank sample (a) surface; (b) cross-section; AP-TBC coating (c) cross-section; (d) detailed cross-section; APA-TBC coating (e) cross-section; (f) detailed cross-section.
(␣-Al2 O3 ) to the bond coat and very small thickness due to the good oxidation resistance of the two TBCs. This is also corresponding to the low weight gain in Fig. 4a. It is worth mentioning that the two underneath Pt nano-layers have been transformed into Pt nanoparticles which are distributed in the ␣-Al2 O3 layers after cyclic oxidation as shown in Fig. 5d. Besides, it can be seen in Fig. 5f that, all of the Pt–Au nano-layers have been transformed into Pt–Au nanoparticles which are distributed in the ␣-Al2 O3 layers completely and uniformly after cyclic oxidation. Consequently, the designed (Al2 O3 –Y2 O3 )/noble metal micro-laminated structure can be transformed slowly into a structure with dispersed noble metal particles during oxidation, from the lower noble metal layer to the upper layer. And this transformation can be promoted by the addition of Au element in Pt nano-layers. 4. Discussion It has been demonstrated from the experimental results that, the two TBCs exhibit excellent resistance to high-temperature oxidation, spallation, cracking and buckling. And the novel ceramic/noble metal composite bond coat has a great impact on service performance of the TBC on Ni-based superalloy under high temperature.
The mechanisms accounting for these beneficial effects are discussed from two aspects as follows. 4.1. Mechanism for the excellent high-temperature oxidation resistance of novel TBC In the designed bond coat with (Al2 O3 –Y2 O3 )/noble metal micro-laminated structure, the alloy substrate could be well sealed by continuous multi-layered Al2 O3 –Y2 O3 and noble metal. It is known that YAG nano-particles distributed in the ␣-Al2 O3 grain boundaries can refine the structure of coating and suppress the outward diffusion of alloy elements. Thus, the growth of TGO can be mainly determined by the inward diffusion of oxygen. In such YSZ-based TBC system, YSZ has been known as an oxygen ion conductor, while the ␣-Al2 O3 and noble metal (Pt or Au) possess the rather low oxygen diffusion coefficient [34–37]. Thus, the oxygen partial pressure can be decreased stepwise by the sealing effect of multi-sealed alternating Al2 O3 –Y2 O3 and noble metal nano-layers in the bond coat. In the practical application, the noble metal nanolayers are replaced by noble metal particles and platelets which distributed in the ␣-Al2 O3 layer after oxidation. Nevertheless, the ␣-Al2 O3 can still seal the substrate well, and suppress the inward
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4.2. Mechanism for the enhanced mechanical properties of novel TBC As we know, the protective effects of coating can be derived from not only the properties and composition of materials but also the integrality of its structure. Thus, the mechanical properties of coating certainly play an important role in the performance and service life of coating. It is vital to study the special mechanical properties of the proposed novel TBC structure. As analyzed by Evans [38], it can be used as the criterion for the failure of a coating that the elastic strain energy stored in the coating exceeds the fracture resistance, Gc : (1 − )
Fig. 6. XRD patterns of blank sample after cyclic oxidation at 1100 ◦ C for 200 h.
diffusion of oxygen to the superalloy substrate to a low degree. The selective oxidation of Al element in the superalloy substrate should take place to form ␣-Al2 O3 [31]. It can also have beneficial effect on decreasing the oxygen partial pressure, avoiding the further oxidation of substrate. Thus, only few Al2 O3 should be formed in the two TBCs after cyclic oxidation. Besides, the interdiffusion between the superalloy substrate and bond coat can be avoided, even though the micro-laminated structure of bond coat can transform into dispersed structure. Consequently, a dense and thin TGO nano-layer can be formed by selective oxidation and finally inhibit the further oxidation of substrate.
2h > Gc E
(3)
where , , h and E denote the Poisson ratio, stress, thickness and elastic modulus. Therefore, either decreasing the inner stresses or increasing the fracture toughness of coating can improve the durability and spallation resistance of the coating. In the (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) micro-laminated composite bond coat, the apparent thermal expansion coefficient (CTE) of bond coat can be increased by the addition of noble metal [39] compared to the single ␣-Al2 O3 , which is the main component of TGO. And then the thermal expansion mismatch at the bond coat/substrate interface and bond coat/YSZ top coat interface in the TBC system can be mitigated, giving rise to the lower stress [40]. As Au possesses the higher CTE (∼14.16 × 10−6 K−1 ) than Pt (∼9.0 × 10−6 K−1 ), the PtAu alloy can be more conducive to the decrease of thermal stress in the bond coat. Besides, the thermal stress caused by the unavoidable thermal expansion mismatch and the grown stress during cyclic oxidation can be relaxed by the plastic deformation of ductile noble metal nano-layers, exhibiting effective toughness. Thus, the structure of bond coat can be transformed during oxidation without damage, as shown in Fig. 8, showing excellent adhesion.
Fig. 7. EDS elements maps of the blank sample after cyclic oxidation.
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Fig. 8. The structure transformation schematic diagram of TBC bonded by (Al2 O3 –Y2 O3 )/noble metal micro-laminated coating during high-temperature oxidation.
Along with the structure transformation of bond coat during the cyclic oxidation, the Pt–Au nano-layers tend to transform into alloy nano-particles even more than Pt, which can be attributed to the higher plasticity, lower elastic modulus and metling point of PtAu alloy. And the toughening mechanisms become different after the structure transformation. As shown in Fig. 8, the structure transformation of bond coat begins from the lower layer to upper layer slowly, which implies that the stress in the lower layer is larger than that in the upper layer. This is correlated with the higher thermal expansion mismatch at the substrate/bond coat interface than the bond coat/top coat interface. In this stage, internal stresses can be relaxed and the strain energy can be absorbed by the doped noble metal particles and platelets due to their plastic deformation and crack bridging effect [41]. And based on the Al2 O3 –Y2 O3 phase diagram, YAG (Yttrium Aluminum Garnet phase) particles with nano-size must be formed in the Al2 O3 –Y2 O3 coating and segregated at the ␣-Al2 O3 grain boundaries, which can provide sufficient strength and toughness for the coating [42]. Thus, the micro-cracks propagation can be inhibited, which can avoid the formation of cracks in larger size. And excellent resistance to cracking, spallation and also buckling can be obtained in the designed novel bond coat, exhibiting effectively bonding strength at the substrate/bond coat interface and bond coat/top coat interface. Consequently, the durability of TBC can be improved and the service life can be extended. 5. Conclusions In conclusion, novel thermal barrier coatings bonded by (Al2 O3 –Y2 O3 )/noble metal (Pt or Pt–Au) composite coatings have been developed. The novel TBCs can improve the high-temperature oxidation of Ni-based superalloy and also exhibit excellent resistance to cracking, spallation and buckling, due to the sealing effect of (Al2 O3 –Y2 O3 )/(Pt or Pt–Au) composite bond coat and the toughening effect of noble metals under thermal cycling. With the sealing effect, the oxygen diffusion to the alloy substrate and the interdiffusion between the bond coat and alloy substrate can be effectively suppressed to a low degree, leading to the significant improvement of the oxidation resistance. With the toughening effect of noble metals and composite structure of bond coat, the fracture toughness of bond coat can be enhanced, resulting in the effectively improvement of adhesion strength and resistance to cracking, spallation and buckling. This novel ceramic/noble metal composite coating can be a considerable structure which would has great application prospect in the TBC. Acknowledgments The authors thank the financial support from the Chinese National Nature Science Foundation (Grant no. 51071030).
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