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Microstructural evolution of CoCrAlY bond coat on Ni-based superalloy DZ 125 at 1050 °C
Surface & Coatings Technology 205 (2011) 4374–4379
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Microstructural evolution of CoCrAlY bond coat on Ni-based superalloy DZ 125 at 1050 °C Tianquan Liang a,b,c, Hongbo Guo a,c,⁎, Hui Peng a,c, Shengkai Gong a,c a b c
School of Materials Science and Engineering, Beihang University, No. 37 Xueyuan Road, Beijing 100191, PR China College of Materials Science and Engineering, Guangxi University, No. 100 Daxue Road, Nanning 530004, PR China Beijing Key Laboratory for Advanced Functional Materials and Thin Film Technology, Beihang University, No. 37 Xueyuan Road, Beijing 100191, PR China
a r t i c l e
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Article history: Received 1 February 2011 Accepted in revised form 11 March 2011 Available online 28 March 2011 Keywords: CoCrAlY bond coat Thermal exposure Inter-diffusion Microstructural evolution
a b s t r a c t CoCrAlY alloy has been widely used as metallic protective coatings or the bond coats in thermal barrier coatings (TBCs) to protect the underlying superalloy from oxidation and hot-corrosion. In this paper, the TBC consisting of yttria stabilized zirconia (7YSZ) ceramic top coat and CoCrAlY bond coat was deposited onto directionally solidified nickel based superalloy DZ 125 by electron beam physical vapor deposition (EB-PVD). The microstructural evolution of the bond coat on this superalloy was investigated after thermal exposure for 100 h at 1050 °C. Due to a significant inward diffusion of Al, Co and Cr from the coating and outward diffusion of Ni, Hf, W and Ti from the substrate, the phase transformation from the Co-based Al-rich β-CoAl phase to the Al-deficient γ-CoNi solid solution phase occurred in the bond coat. Simultaneously, a large amount of Ni-based β-NiCoAl phase was present in the bond coat. In addition, the particles containing substrate strengthening elements Hf and/or W are abundant in the thermally grown oxides (TGO) and within the bond coat. The mechanism for the microstructural evolution is discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Thermal barrier coatings (TBCs) with a protective bond coat (BC) are widely applied onto the modern advanced turbine blades to enhance efficiency [1–5]. The bond coat usually belongs to diffusion coating or overlay coating [4]. These bond coats are good for oxidation resistance for the abundant Al to form a protective nature alumina scale at elevated temperature. They are also well against high-temperature hot-corrosion due to excess of 16.0 wt.% Cr, especially the co-existence of Al and Cr [4–6]. The overlay coating exhibits better resistance to oxidation and hotcorrosion [2,5,7], since the diffusion coating suffered from significant influence of substrate elements such as Ti, W, Mo and substantial carbides [2,4]. Recently, MCrAlY (M=Ni, Co or both) system is usually chosen as the overlay bond coat and coated on the superalloy components to provide protection from oxidation and hot-corrosion [6–8], such as CoCrAlY coating with high level of Al and Cr which could provide the necessary gas turbine service lifetime, especially for those operating under marine conditions [4,9]. The columnar sputtered Co-30Cr-12Al-0.5Y coatings are mainly composed of three phases: hexagonal close packed (hcp) ε-Co phase, face center cubic (fcc) γ phase of Co solid solution and a minority body center cubic (bcc) β-CoAl intermetallic phase [10]. Among these the ε ⁎ Corresponding author at: Beijing Key Laboratory for Advanced Functional Materials and Thin Film Technology, Beihang University, No. 37 Xueyuan Road, Beijing 100191, PR China. Tel.: +86 10 8231 7117; fax: +86 10 8233 8200. E-mail address: [email protected] (H. Guo). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.034
phase almost completely transformed to γ phase after annealing for 2 h at 1000 °C. The CoCrAlY coating exhibits excellent oxidation and hotcorrosion resistance owing to its high level of Al and Cr [2,11]. It has been suggested that the degradation of CoCrAlY coating is related to the depletion of Al and the decomposition of β-CoAl resulting from oxidation and inter-diffusion [2,5,12]. Studies [3,13,14] have shown that the microstructural change from Al-rich β phases to Al-deficient phases in the bond coat is harmful to its oxidation performance. The loss of aluminum develops less numerous protective spinel-type oxides and leads to spalling failure eventually. More about the oxidation behaviors, the scale morphologies and coating phases have been analyzed in previous work [2,15–18]. Also the outward diffusion of the substrate elements to the metallic coating accelerates the coating degradation [2,5,6,19]. Lih. et al. [20] illuminate prolonging the life of the CoCrAlY/ YSZ through aluminizing on CoCrAlY coating to form a more Al-rich β-CoAl phase as an Al reservoir. It is also reported by Peng [21] that the presence of reactive element Hf in the bond coat due to outward diffusion from the substrate improves the oxide scale adherence, whereas the excess of the reactive elements such as Y and Hf will cause accelerated thickening and chipping spallation of TGO [22,23]. It is reported by Maier et al. [24] that Ni-, Co- and Cr-oxides grown at the TGO/top coat interface tend to increase the stress and contribute to spalling failure of the top coat. However, the above researches have been mostly focused on the oxidation and hot-corrosion behaviors of Ni-based coatings (NiCoCrAlY, NiCrAlY coatings, NiPtAl, etc.) and the interdiffusion behavior between the Ni-based coatings and Ni-based superalloys. Also, there are a few
T. Liang et al. / Surface & Coatings Technology 205 (2011) 4374–4379 Table 1 Chemical composition of CoCrAlY bond coat in this present work. Elements
Al
Cr
Co
Y
Wt.% At.%
8.93 17.16
22.91 22.86
67.98 59.88
0.18 0.10
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the bond coat and the underlying superalloy has a very important effect on the nucleation, growth and adherence of thermally grown oxide scale and finally governs the thermal cyclic lifetime of TBC system. Furthermore, the performance of the coating is greatly influenced by the chemical compositions of the underlying superalloys. Considering this, it is necessary to carry out an investigation on the microstructural evolution of the CoCrAlY bond coat in a TBC system during thermal exposure. In the present work, the phase evolution in the EB-PVD CoCrAlY bond coat on directionally solidified Ni-based superalloy DZ 125 is investigated and the mechanism for the phase evolution of the bond coat is discussed in detail. 2. Experimental
Fig. 1. XRD patterns of CoCrAlY bond coatings under different conditions of as-deposited (a), after 4 h annealing in vacuum (b) and after 100 h oxidation at 1050 °C (c).
papers on oxidation and hot-corrosion behaviors of Co-based coatings (spraying, sputter, diffusion coatings etc.). Microstructural evolution of EB-PVD Co-based coatings resulting from interdiffusion between the Co-based coatings and Ni-based superalloys has seldom been concerned. Such microstructural evolution due to interdiffusion between
Directionally solidified Ni-based superalloy DZ 125 was used as the substrate material in this present work, with its nominal chemical composition of Ni-9Cr-10Co-9W-2Mo-5.2Al-0.9Ti-3.8Ta-1.5Hf-0.015B (mass fraction). The size of DZ125 rectangle specimens was approximately 20 mm×15 mm×3 mm and their surfaces were finely polished with 800-grit emery papers. Duplex thermal barrier coatings (TBCs) composed of CoCrAlY bond coat and 7 wt.% Y2O3 partially stabilized ZrO2 (7YSZ) ceramic top coat (TC) were deposited onto the substrate by electron beam physical vapor deposition (EB-PVD). The average thickness of BC and TC layers were ~55 μm and ~110 μm, respectively. The chemical composition of the CoCrAlY bond coat in this system is listed in Table 1. Before deposition of the top coat, the bond coated specimens were annealed in vacuum for 4 h at 1050 °C, and then surface strengthened by shot-peening [25,26]. Thermal exposure of the TBC coated specimens were carried out at 1050 °C in air furnace to investigate the phase evolution in the bond coat. After thermal exposure, the specimens were mounted in epoxy, sectioned and finely polished for cross-section observation.
Fig. 2. Cross-sectional micrographs of the as-deposited CoCrAlY bond coat (a), and the bond coat and its different zones after 4 h annealing in vacuum at 1050 °C (b) ~ (d).
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Table 2 Chemical compositions of β-CoAl and γ-Co phases in the CoCrAlY bond coat after 4 h annealing in vacuum at 1050 °C. Elements
Al Cr Co
β
γ
Wt.%
At.%
Wt.%
At.%
15.29 19.57 65.13
27.67 18.38 53.96
6.09 26.32 67.59
12.03 26.93 61.04
The phase components of the bond coat was identified by X-ray Diffraction (XRD, D/max 2200PC) using Cu Kα radiation (λ = 1.54056 Å). The cross-sectional morphologies of the bond coat were characterized by a CAMSCAN 3400 scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and back scattering electron (BSE) Fig. 4. Cross-sectional micrograph of the CoCrAlY bond coat after 100 h oxidation at 1050 °C.
detector. Their chemical compositions were determined by electron probe micro-analyzer (EPMA, JXA-8100). The phase microstructure in the CoCrAlY bond coat after evolution was studied by transmission electron microscopy (TEM, JEM-2100F).
3. Results and discussion 3.1. Microstructures of as-deposited bond coat The as-deposited CoCrAlY coating mainly consists of Co-based β-CoAl and γ-Co solid–solution (ss) phases, as identified by XRD analysis shown in Fig. 1. The β is a body-centered cubic (bcc) phase, and the γ is a facecentered cubic (fcc) phase. A small amount of σ-CoCr phase is also detected. The σ is characterized as a tetragonal lattice with parameters of a=8.81 Å and c=4.56 Å. The element Cr is solid dissolved in the phases of the bond coat. Since the minor σ phase disappears after 4 h annealing, the CoCrAlY bond coat can be considered as a two-phase structure. Fig. 2a shows the microstructures of the as-deposited CoCrAlY bond coat. The microstructures of the bond coat and its different zones after 4 h annealing in vacuum at 1050 °C, are shown in Fig. 2b–d, respectively. The dark area is identified as Co-based β-CoAl phase and the gray one is γ-Co phase (Fig. 2c). The columnar β and γ phases are periodically arranged. The β and γ in the as-deposited bond coat are slim and small in dimension (Fig. 2a). They are coarsening after 4 h annealing, especially near the top and the bottom of the bond coat (Fig. 2b). The diameter of the γ-Co phase near the top and the bottom is about 2 μm, and that is ~1 μm in the middle zone. The β-CoAl phase is ~2 μm in diameter. The chemical compositions of the β and γ phases in the as-deposited CoCrAlY coating are shown in Table 2. The Al concentration in the β-CoAl phase is about 15 wt.%, and that in the γ phase is only 6 wt.%. The Cr element is dissolved in the β and γ phases. The content of Co is nearly in the same level (about 67 wt.%) within the β-CoAl and γ-Co phases. The high concentration of Al in the β-CoAl phase helps to form a protective alumina scale during high-temperature exposure. Also, the γ phase Table 3 Chemical compositions of β-NiCoAl and γ-CoNi phases in the CoCrAlY bond coat after 100 h oxidation at 1050 °C. Elements
Fig. 3. TEM micrograph of CoCrAlY after 4 h annealing in vacuum at 1050 °C (a), and SADP of ordered B2-structured β-CoAl, B = [111] (b), SADP of fcc γ-Co (ss), B = [001] (c).
Al Ti Cr Co Ni W Ta
β
γ
Wt.%
At.%
Wt.%
At.%
12.61 0.52 8.52 30.49 47.37 0.48 –
23.73 0.56 8.33 26.28 40.98 0.13 –
3.44 0.22 18.69 41.96 32.97 2.29 0.42
7.16 0.26 20.19 40.00 31.55 0.70 0.13
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Fig. 6. TEM morphologies of BC/TGO interface, refractory element-rich particles and SADP of HfO2 on the surface of BC.
Fig. 5. TEM micrograph of CoCrAlY bond coat after 100 h oxidation at 1050 °C (a) and SADP of bcc β-NiCoAl, B = [111] (b), SADP of fcc γ-CoNi (ss), B = [112] (c).
contains higher level of Cr (26 wt.%) which contributes to enhance the oxidation and hot-corrosion resistance of the coating. The chemical compositions of the phases in the bond coat are desirable for hightemperature oxidation and hot-corrosion environment. 3.2. Microstructures of bond coat after heat treatment Fig. 3 shows the microstructure of CoCrAlY bond coat and its SADPs after 4 h annealing in vacuum at 1050 °C. The SADPs of β and γ phases are diffracted along the axis zones of [111] and [001], respectively. The columnar β-CoAl phase is about 1 μm in diameter and is identified as ordered B2 structure with a lattice parameter of a=0.291 nm, while the γ-Co phase is an fcc structure with a lattice parameter of a=0.355 nm. The lattice structures of these two phases are well consistent with that characterized by XRD (Fig. 1). Besides, many finer needle-like and platelike phases within the β phase are observed (Fig. 3a). The composition of
these precipitates is 11.6 Al, 20.9 Cr and 67.7 Co (in wt.%) determined by EDS. The concentrations of Cr and Co in these phases are similar to the average level of the matrix β phase, while that of Al is relatively lower than the average value as indicated in Table 2. These precipitates also appear inside the Al-rich β-NiAl matrix due to the limited solubility of Cr [2,27,28]. The cross-sectional micrograph of the CoCrAlY bond coat on superalloy DZ 125 after thermal exposure for 100 h at 1050 °C is shown in Fig. 4. There is an ~2 μm thick TGO at the surface of the bond coat. The morphology of the periodic arrangement of columnar β and γ phases no longer exists in the CoCrAlY coating. An ~ 18 μm thick Al-depleted zone is formed beneath the TGO. The XRD pattern of the CoCrAlY bond coat after thermal exposure for 100 h at 1050 °C indicates only γ phase (ss) and α-Al2O3 at the coating surface (Fig. 1). The island dark area is identified as β-NiCoAl phase, and the gray matrix is γ-CoNi phase (ss). Additionally, there are many Hf-rich oxides existing on the surface of the bond coat. The dark β phase formed in the inter-diffusion zone in the substrate also can be observed. The chemical compositions of β and γ phases in the bond coat after oxidation are listed in Table 3. The matrix γ phase contains about 42 wt.% Co and more than 33 wt.% Ni. And the concentrations of Al and Cr are about 3.5 wt.% and 18 wt.%, respectively. The chemical composition of β phase is 12 Al, 8.5 Cr, 30.5 Co and 47 Ni (in wt.%). Small amount of substrate elements such as Ti, W and Ta can be detected in the β and γ phases. The concentration of Co in these two phases of the bond coat decreases remarkably. In contrast to this, the concentration of Ni increases evidently. There is more than 65 wt.% of Co and no Ni in the bond coat before thermal exposure. The reduced amount of Co is more than 25 wt.%. Significant change of the chemical composition of these two phases is the increment of Ni. The content of Ni in the β phase is increased by 47 wt.% and that in the γ phase is increased by 33 wt.%. The residual β phase transforms into Ni-based β-NiCoAl, while the γ phase transforms into γ-CoNi solid solution. Another change of the chemical composition is the decrease of Cr content, especially in the β phase, the level of Cr is less than half of the as-deposition level.
Table 4 Chemical compositions of refractory-rich particles on the TGO/BC interface. Elements
O Al Cr Co Ni Hf W
Hf-rich oxides
W-rich particles
Wt.%
At.%
Wt.%
At.%
07.58 14.14 03.33 – 04.26 70.69 –
30.96 34.23 04.18 – 04.74 25.88 –
– 02.46 20.12 39.12 28.80 – 09.50
– 05.42 22.98 39.41 29.12 – 03.07
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An appropriate level of Hf in the bond coat contributes to enhance the oxide scale adherence by “pegs” mechanism [21–23,30,31]. However, an excessive level of Hf would accelerate the oxidation rate of the bond coat by “over-doping effect” [31–33]. Distribution of fine Hf-rich oxide particles in the bond coat is observed in Fig. 7. The chemical compositions of the particles are given in Table 5. The coarser particles at the grain boundaries are rich in Hf. The concentration of Hf in the zone B in Fig. 7a is as high as 68 wt.%. Since the grain boundaries are the short circuit diffusion path for those transferred elements at elevated temperatures, the substrate strengthening elements diffuse into the CoCrAlY bond coat along the columnar grain boundaries. Some particles are as fine as 10 nm in dimension. These fine particles are distributing inside the γ-CoNi solid solution phase (in Fig. 7b). Hf-rich particles could hardly be found in the β phase due to low solubility. 3.3. Microstructural evolution process
Fig. 7. TEM images of the bottom area of CoCrAlY bond coat after 100 h oxidation at 1050 °C (a) and the distributing Hf-rich particles (b).
Fig. 5a–c shows the TEM image and the SADPs of the β-NiCoAl phase and the γ-CoNi (ss) phase in the middle zone of CoCrAlY coating after thermal exposure for 100 h at 1050 °C, respectively. The β-NiCoAl phase in the bond coat shows bcc lattice structure (Fig. 5b) with the lattice constant of a = 0.291 nm. The lattice structure of γ-CoNi phase is fcc (Fig. 5c) with the lattice parameter of a = 0.355 nm. The microstructures of the TGO/BC interface and refractory element rich particles are shown in Fig. 6. The oxide scale is firmly bonded to the coating. There is no rumpling observed at the TGO/BC interface. Some large Hf-rich oxides are distributed in the surface layer of the bond coat. Hf diffuses into the TGO along the grain boundaries of the Al2O3 scale. Some (Hf, W)-rich particles are also observed in the bond coat as marked by the arrows. The white phase is Al2O3 and the HfO2 phase is identified as a monoclinic lattice structure by the SADP as shown in Fig. 6. The chemical compositions of the oxides and/or the (Hf, W)-rich particles in the bond coat are given in Table 4. The concentration of Hf in the oxides at the TGO/ BC interface is as high as 70 wt.% and the level of W in the particles distributed within the bond coat is 9.5 wt.%. These particles containing refractory elements usually exist or segregate at the grain boundaries [29].
Table 5 Chemical compositions of zones corresponding to Fig. 7. A
O Al Cr Co Ni Hf
B
C
Wt.%
At.%
Wt.%
At.%
Wt.%
At.%
– 02.61 22.25 38.32 36.82 –
– 05.37 23.75 36.08 34.80 –
05.43 – 05.79 09.31 11.02 68.45
28.75 – 09.43 13.39 15.92 32.51
03.55 02.02 17.71 29.81 27.30 19.62
12.92 04.36 19.82 29.44 27.06 06.40
To illustrate the microstructural evolution of CoCrAlY bond coat, ternary diagrams [34] for Al, Co, Cr and Ni are analyzed (Fig. 8). The composition the as-deposited CoCrAlY coating is located within the mix-zone of β-CoAl and γ-Co phases of Al-Cr-Co ternary diagram as marked in Fig. 8a. According to the Lever Law, the mass fractions of β-CoAl and γ-Co phases are calculated from this diagram. The proportion is 27.6% for the β phase and 72.4% for the γ-Co phase, respectively. The location of β-NiCoAl phase is given in Fig. 8b. In fact, it consists of NiAl and Ni3Al dual phases. The proportions of the phases are about 28% and 72%, respectively. The γ phase is a single and stable γ-CoNi solid solution after inter-diffusion shown in Fig. 8c. Depletion of Al in the β-CoAl phase and loss of Cr in the bond coat result in the degradation of the bond coat in oxidation and hot-corrosion resistance. The service lifetime of the bond coat mainly depends on the content of Al or the volume percentage of β phase in the coating. During high-temperature exposure at 1050 °C Al migrates to the surface of the metallic coating and forms thermally grown oxides (TGO). After longterm oxidation Al would be consumed. Depletion of Al causes a reduction of β-CoAl phase in BC since the Al element primarily comes from β phase as mentioned above. This is consistent with the known outward diffusion and preferential oxidation theory [17]. Another degradation way for the CoCrAlY coating is inward diffusion of Al into the substrate during thermal exposure. Simultaneously, Co and Cr elements are depleted owing to a severe inward diffusion. Also large amounts of Ni diffuse from the substrate to the bond coat and promote the phase transformation from Co-based β-CoAl and γ-Co (ss) to Ni-based β-NiCoAl and γ-CoNi (ss). Inward diffusion of Al to the substrate results in the formation of bcc β-NiCoAl phase in the inter-diffusion zone (IDZ) due to a reaction with the substrate elements [3]. The β phase is of low solubility for refractory elements such as Hf, W, Mo, and Ta. Significant outward diffusion of Ni from the substrate to the metallic coating would cause depletion of Ni in the IDZ, leading to a reduction of the γ′/γ matrix. This promotes precipitation of the substrate strengthening refractory elements from the new-forming β phase and the supersaturated γ solid solution. The refractory elements such as Hf and W could diffuse to the BC along the grain boundaries. They are dispersed in the bond coat either inside the grains or at the grain boundaries depending on their particle sizes. Severe interdiffusion between the bond coat and the underlying superalloy usually induces a series of phase transformation in the bond coat [35], resulting in the degradation of the bond coat. The degradation of the bond coat has an important impact on the lifetime of TBC system. As observed in the present work, severe inward diffusion of Al and Cr in the bond coat occurs, which tends to cause depletion of Al and Cr in the bond coat. As Al and Cr are the important oxidation-resistant elements in the bond coat, oxidation resistance of the bond coat is gradually degraded as a result of the depletion of Al and Cr in the bond coat. On the other hand, the elements in the superalloy such as Ni, Ta, Ti and W diffuse to the surface of
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bond coat during thermal exposure, which can cause accelerated thickening of TGO and weaken the oxide scale adherence [2]. It is well admitted that spallation failure of an EB-PVD TBC occurs in most cases by cracking within the TGO or along the interface between the TGO and bond coat. Therefore, it is just claimed by Schulz et al. [36] that the chemical composition of substrate alloy plays a decisive role in governing the TBC lifetime, and Hf-containing alloy improves adhesion of the TGO on the bond coat, therefore leading to a prolonged TBC lifetime [37]. 4. Conclusions The as-deposited EB-PVD CoCrAlY coating mainly consists of Co-based bcc β-CoAl, fcc γ-Co (Co-based solid solution) and minor tetragonal σ-CoCr phases. The columnar β and γ phases show periodic arrangements and evolve into Ni-based β-NiCoAl and γ-CoNi phases (ss) after 100 h oxidation and interdiffusion at 1050 °C, but remain the same lattice structures as those of the as-deposited. The phase evolutions are caused by oxidation of the bond coat and elements interdiffusion between the bond coat and the underlying superalloy DZ 125. An amount of refractory elements-rich phases is also observed in the bond coat. Acknowledgments This research is sponsored by National Natural Science Foundation of China (NSFC, no.50771009, no.50731001 and no.51071013) and National Basic Research Program (973 Program) of China under grant no. 2010CB631200. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] Fig. 8. Ternary diagrams of (a) Al-Cr-Co, (b) Al-Co-Ni and (c) Co-Cr-Ni (at.%) [34].
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Microstructural evolution of CoCrAlY bond coat on Ni-based superalloy DZ 125 at 1050 °C Tianquan Liang a,b,c, Hongbo Guo a,c,⁎, Hui Peng a,c, Shengkai Gong a,c a b c
School of Materials Science and Engineering, Beihang University, No. 37 Xueyuan Road, Beijing 100191, PR China College of Materials Science and Engineering, Guangxi University, No. 100 Daxue Road, Nanning 530004, PR China Beijing Key Laboratory for Advanced Functional Materials and Thin Film Technology, Beihang University, No. 37 Xueyuan Road, Beijing 100191, PR China
a r t i c l e
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Article history: Received 1 February 2011 Accepted in revised form 11 March 2011 Available online 28 March 2011 Keywords: CoCrAlY bond coat Thermal exposure Inter-diffusion Microstructural evolution
a b s t r a c t CoCrAlY alloy has been widely used as metallic protective coatings or the bond coats in thermal barrier coatings (TBCs) to protect the underlying superalloy from oxidation and hot-corrosion. In this paper, the TBC consisting of yttria stabilized zirconia (7YSZ) ceramic top coat and CoCrAlY bond coat was deposited onto directionally solidified nickel based superalloy DZ 125 by electron beam physical vapor deposition (EB-PVD). The microstructural evolution of the bond coat on this superalloy was investigated after thermal exposure for 100 h at 1050 °C. Due to a significant inward diffusion of Al, Co and Cr from the coating and outward diffusion of Ni, Hf, W and Ti from the substrate, the phase transformation from the Co-based Al-rich β-CoAl phase to the Al-deficient γ-CoNi solid solution phase occurred in the bond coat. Simultaneously, a large amount of Ni-based β-NiCoAl phase was present in the bond coat. In addition, the particles containing substrate strengthening elements Hf and/or W are abundant in the thermally grown oxides (TGO) and within the bond coat. The mechanism for the microstructural evolution is discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Thermal barrier coatings (TBCs) with a protective bond coat (BC) are widely applied onto the modern advanced turbine blades to enhance efficiency [1–5]. The bond coat usually belongs to diffusion coating or overlay coating [4]. These bond coats are good for oxidation resistance for the abundant Al to form a protective nature alumina scale at elevated temperature. They are also well against high-temperature hot-corrosion due to excess of 16.0 wt.% Cr, especially the co-existence of Al and Cr [4–6]. The overlay coating exhibits better resistance to oxidation and hotcorrosion [2,5,7], since the diffusion coating suffered from significant influence of substrate elements such as Ti, W, Mo and substantial carbides [2,4]. Recently, MCrAlY (M=Ni, Co or both) system is usually chosen as the overlay bond coat and coated on the superalloy components to provide protection from oxidation and hot-corrosion [6–8], such as CoCrAlY coating with high level of Al and Cr which could provide the necessary gas turbine service lifetime, especially for those operating under marine conditions [4,9]. The columnar sputtered Co-30Cr-12Al-0.5Y coatings are mainly composed of three phases: hexagonal close packed (hcp) ε-Co phase, face center cubic (fcc) γ phase of Co solid solution and a minority body center cubic (bcc) β-CoAl intermetallic phase [10]. Among these the ε ⁎ Corresponding author at: Beijing Key Laboratory for Advanced Functional Materials and Thin Film Technology, Beihang University, No. 37 Xueyuan Road, Beijing 100191, PR China. Tel.: +86 10 8231 7117; fax: +86 10 8233 8200. E-mail address: [email protected] (H. Guo). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.034
phase almost completely transformed to γ phase after annealing for 2 h at 1000 °C. The CoCrAlY coating exhibits excellent oxidation and hotcorrosion resistance owing to its high level of Al and Cr [2,11]. It has been suggested that the degradation of CoCrAlY coating is related to the depletion of Al and the decomposition of β-CoAl resulting from oxidation and inter-diffusion [2,5,12]. Studies [3,13,14] have shown that the microstructural change from Al-rich β phases to Al-deficient phases in the bond coat is harmful to its oxidation performance. The loss of aluminum develops less numerous protective spinel-type oxides and leads to spalling failure eventually. More about the oxidation behaviors, the scale morphologies and coating phases have been analyzed in previous work [2,15–18]. Also the outward diffusion of the substrate elements to the metallic coating accelerates the coating degradation [2,5,6,19]. Lih. et al. [20] illuminate prolonging the life of the CoCrAlY/ YSZ through aluminizing on CoCrAlY coating to form a more Al-rich β-CoAl phase as an Al reservoir. It is also reported by Peng [21] that the presence of reactive element Hf in the bond coat due to outward diffusion from the substrate improves the oxide scale adherence, whereas the excess of the reactive elements such as Y and Hf will cause accelerated thickening and chipping spallation of TGO [22,23]. It is reported by Maier et al. [24] that Ni-, Co- and Cr-oxides grown at the TGO/top coat interface tend to increase the stress and contribute to spalling failure of the top coat. However, the above researches have been mostly focused on the oxidation and hot-corrosion behaviors of Ni-based coatings (NiCoCrAlY, NiCrAlY coatings, NiPtAl, etc.) and the interdiffusion behavior between the Ni-based coatings and Ni-based superalloys. Also, there are a few
T. Liang et al. / Surface & Coatings Technology 205 (2011) 4374–4379 Table 1 Chemical composition of CoCrAlY bond coat in this present work. Elements
Al
Cr
Co
Y
Wt.% At.%
8.93 17.16
22.91 22.86
67.98 59.88
0.18 0.10
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the bond coat and the underlying superalloy has a very important effect on the nucleation, growth and adherence of thermally grown oxide scale and finally governs the thermal cyclic lifetime of TBC system. Furthermore, the performance of the coating is greatly influenced by the chemical compositions of the underlying superalloys. Considering this, it is necessary to carry out an investigation on the microstructural evolution of the CoCrAlY bond coat in a TBC system during thermal exposure. In the present work, the phase evolution in the EB-PVD CoCrAlY bond coat on directionally solidified Ni-based superalloy DZ 125 is investigated and the mechanism for the phase evolution of the bond coat is discussed in detail. 2. Experimental
Fig. 1. XRD patterns of CoCrAlY bond coatings under different conditions of as-deposited (a), after 4 h annealing in vacuum (b) and after 100 h oxidation at 1050 °C (c).
papers on oxidation and hot-corrosion behaviors of Co-based coatings (spraying, sputter, diffusion coatings etc.). Microstructural evolution of EB-PVD Co-based coatings resulting from interdiffusion between the Co-based coatings and Ni-based superalloys has seldom been concerned. Such microstructural evolution due to interdiffusion between
Directionally solidified Ni-based superalloy DZ 125 was used as the substrate material in this present work, with its nominal chemical composition of Ni-9Cr-10Co-9W-2Mo-5.2Al-0.9Ti-3.8Ta-1.5Hf-0.015B (mass fraction). The size of DZ125 rectangle specimens was approximately 20 mm×15 mm×3 mm and their surfaces were finely polished with 800-grit emery papers. Duplex thermal barrier coatings (TBCs) composed of CoCrAlY bond coat and 7 wt.% Y2O3 partially stabilized ZrO2 (7YSZ) ceramic top coat (TC) were deposited onto the substrate by electron beam physical vapor deposition (EB-PVD). The average thickness of BC and TC layers were ~55 μm and ~110 μm, respectively. The chemical composition of the CoCrAlY bond coat in this system is listed in Table 1. Before deposition of the top coat, the bond coated specimens were annealed in vacuum for 4 h at 1050 °C, and then surface strengthened by shot-peening [25,26]. Thermal exposure of the TBC coated specimens were carried out at 1050 °C in air furnace to investigate the phase evolution in the bond coat. After thermal exposure, the specimens were mounted in epoxy, sectioned and finely polished for cross-section observation.
Fig. 2. Cross-sectional micrographs of the as-deposited CoCrAlY bond coat (a), and the bond coat and its different zones after 4 h annealing in vacuum at 1050 °C (b) ~ (d).
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Table 2 Chemical compositions of β-CoAl and γ-Co phases in the CoCrAlY bond coat after 4 h annealing in vacuum at 1050 °C. Elements
Al Cr Co
β
γ
Wt.%
At.%
Wt.%
At.%
15.29 19.57 65.13
27.67 18.38 53.96
6.09 26.32 67.59
12.03 26.93 61.04
The phase components of the bond coat was identified by X-ray Diffraction (XRD, D/max 2200PC) using Cu Kα radiation (λ = 1.54056 Å). The cross-sectional morphologies of the bond coat were characterized by a CAMSCAN 3400 scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and back scattering electron (BSE) Fig. 4. Cross-sectional micrograph of the CoCrAlY bond coat after 100 h oxidation at 1050 °C.
detector. Their chemical compositions were determined by electron probe micro-analyzer (EPMA, JXA-8100). The phase microstructure in the CoCrAlY bond coat after evolution was studied by transmission electron microscopy (TEM, JEM-2100F).
3. Results and discussion 3.1. Microstructures of as-deposited bond coat The as-deposited CoCrAlY coating mainly consists of Co-based β-CoAl and γ-Co solid–solution (ss) phases, as identified by XRD analysis shown in Fig. 1. The β is a body-centered cubic (bcc) phase, and the γ is a facecentered cubic (fcc) phase. A small amount of σ-CoCr phase is also detected. The σ is characterized as a tetragonal lattice with parameters of a=8.81 Å and c=4.56 Å. The element Cr is solid dissolved in the phases of the bond coat. Since the minor σ phase disappears after 4 h annealing, the CoCrAlY bond coat can be considered as a two-phase structure. Fig. 2a shows the microstructures of the as-deposited CoCrAlY bond coat. The microstructures of the bond coat and its different zones after 4 h annealing in vacuum at 1050 °C, are shown in Fig. 2b–d, respectively. The dark area is identified as Co-based β-CoAl phase and the gray one is γ-Co phase (Fig. 2c). The columnar β and γ phases are periodically arranged. The β and γ in the as-deposited bond coat are slim and small in dimension (Fig. 2a). They are coarsening after 4 h annealing, especially near the top and the bottom of the bond coat (Fig. 2b). The diameter of the γ-Co phase near the top and the bottom is about 2 μm, and that is ~1 μm in the middle zone. The β-CoAl phase is ~2 μm in diameter. The chemical compositions of the β and γ phases in the as-deposited CoCrAlY coating are shown in Table 2. The Al concentration in the β-CoAl phase is about 15 wt.%, and that in the γ phase is only 6 wt.%. The Cr element is dissolved in the β and γ phases. The content of Co is nearly in the same level (about 67 wt.%) within the β-CoAl and γ-Co phases. The high concentration of Al in the β-CoAl phase helps to form a protective alumina scale during high-temperature exposure. Also, the γ phase Table 3 Chemical compositions of β-NiCoAl and γ-CoNi phases in the CoCrAlY bond coat after 100 h oxidation at 1050 °C. Elements
Fig. 3. TEM micrograph of CoCrAlY after 4 h annealing in vacuum at 1050 °C (a), and SADP of ordered B2-structured β-CoAl, B = [111] (b), SADP of fcc γ-Co (ss), B = [001] (c).
Al Ti Cr Co Ni W Ta
β
γ
Wt.%
At.%
Wt.%
At.%
12.61 0.52 8.52 30.49 47.37 0.48 –
23.73 0.56 8.33 26.28 40.98 0.13 –
3.44 0.22 18.69 41.96 32.97 2.29 0.42
7.16 0.26 20.19 40.00 31.55 0.70 0.13
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Fig. 6. TEM morphologies of BC/TGO interface, refractory element-rich particles and SADP of HfO2 on the surface of BC.
Fig. 5. TEM micrograph of CoCrAlY bond coat after 100 h oxidation at 1050 °C (a) and SADP of bcc β-NiCoAl, B = [111] (b), SADP of fcc γ-CoNi (ss), B = [112] (c).
contains higher level of Cr (26 wt.%) which contributes to enhance the oxidation and hot-corrosion resistance of the coating. The chemical compositions of the phases in the bond coat are desirable for hightemperature oxidation and hot-corrosion environment. 3.2. Microstructures of bond coat after heat treatment Fig. 3 shows the microstructure of CoCrAlY bond coat and its SADPs after 4 h annealing in vacuum at 1050 °C. The SADPs of β and γ phases are diffracted along the axis zones of [111] and [001], respectively. The columnar β-CoAl phase is about 1 μm in diameter and is identified as ordered B2 structure with a lattice parameter of a=0.291 nm, while the γ-Co phase is an fcc structure with a lattice parameter of a=0.355 nm. The lattice structures of these two phases are well consistent with that characterized by XRD (Fig. 1). Besides, many finer needle-like and platelike phases within the β phase are observed (Fig. 3a). The composition of
these precipitates is 11.6 Al, 20.9 Cr and 67.7 Co (in wt.%) determined by EDS. The concentrations of Cr and Co in these phases are similar to the average level of the matrix β phase, while that of Al is relatively lower than the average value as indicated in Table 2. These precipitates also appear inside the Al-rich β-NiAl matrix due to the limited solubility of Cr [2,27,28]. The cross-sectional micrograph of the CoCrAlY bond coat on superalloy DZ 125 after thermal exposure for 100 h at 1050 °C is shown in Fig. 4. There is an ~2 μm thick TGO at the surface of the bond coat. The morphology of the periodic arrangement of columnar β and γ phases no longer exists in the CoCrAlY coating. An ~ 18 μm thick Al-depleted zone is formed beneath the TGO. The XRD pattern of the CoCrAlY bond coat after thermal exposure for 100 h at 1050 °C indicates only γ phase (ss) and α-Al2O3 at the coating surface (Fig. 1). The island dark area is identified as β-NiCoAl phase, and the gray matrix is γ-CoNi phase (ss). Additionally, there are many Hf-rich oxides existing on the surface of the bond coat. The dark β phase formed in the inter-diffusion zone in the substrate also can be observed. The chemical compositions of β and γ phases in the bond coat after oxidation are listed in Table 3. The matrix γ phase contains about 42 wt.% Co and more than 33 wt.% Ni. And the concentrations of Al and Cr are about 3.5 wt.% and 18 wt.%, respectively. The chemical composition of β phase is 12 Al, 8.5 Cr, 30.5 Co and 47 Ni (in wt.%). Small amount of substrate elements such as Ti, W and Ta can be detected in the β and γ phases. The concentration of Co in these two phases of the bond coat decreases remarkably. In contrast to this, the concentration of Ni increases evidently. There is more than 65 wt.% of Co and no Ni in the bond coat before thermal exposure. The reduced amount of Co is more than 25 wt.%. Significant change of the chemical composition of these two phases is the increment of Ni. The content of Ni in the β phase is increased by 47 wt.% and that in the γ phase is increased by 33 wt.%. The residual β phase transforms into Ni-based β-NiCoAl, while the γ phase transforms into γ-CoNi solid solution. Another change of the chemical composition is the decrease of Cr content, especially in the β phase, the level of Cr is less than half of the as-deposition level.
Table 4 Chemical compositions of refractory-rich particles on the TGO/BC interface. Elements
O Al Cr Co Ni Hf W
Hf-rich oxides
W-rich particles
Wt.%
At.%
Wt.%
At.%
07.58 14.14 03.33 – 04.26 70.69 –
30.96 34.23 04.18 – 04.74 25.88 –
– 02.46 20.12 39.12 28.80 – 09.50
– 05.42 22.98 39.41 29.12 – 03.07
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An appropriate level of Hf in the bond coat contributes to enhance the oxide scale adherence by “pegs” mechanism [21–23,30,31]. However, an excessive level of Hf would accelerate the oxidation rate of the bond coat by “over-doping effect” [31–33]. Distribution of fine Hf-rich oxide particles in the bond coat is observed in Fig. 7. The chemical compositions of the particles are given in Table 5. The coarser particles at the grain boundaries are rich in Hf. The concentration of Hf in the zone B in Fig. 7a is as high as 68 wt.%. Since the grain boundaries are the short circuit diffusion path for those transferred elements at elevated temperatures, the substrate strengthening elements diffuse into the CoCrAlY bond coat along the columnar grain boundaries. Some particles are as fine as 10 nm in dimension. These fine particles are distributing inside the γ-CoNi solid solution phase (in Fig. 7b). Hf-rich particles could hardly be found in the β phase due to low solubility. 3.3. Microstructural evolution process
Fig. 7. TEM images of the bottom area of CoCrAlY bond coat after 100 h oxidation at 1050 °C (a) and the distributing Hf-rich particles (b).
Fig. 5a–c shows the TEM image and the SADPs of the β-NiCoAl phase and the γ-CoNi (ss) phase in the middle zone of CoCrAlY coating after thermal exposure for 100 h at 1050 °C, respectively. The β-NiCoAl phase in the bond coat shows bcc lattice structure (Fig. 5b) with the lattice constant of a = 0.291 nm. The lattice structure of γ-CoNi phase is fcc (Fig. 5c) with the lattice parameter of a = 0.355 nm. The microstructures of the TGO/BC interface and refractory element rich particles are shown in Fig. 6. The oxide scale is firmly bonded to the coating. There is no rumpling observed at the TGO/BC interface. Some large Hf-rich oxides are distributed in the surface layer of the bond coat. Hf diffuses into the TGO along the grain boundaries of the Al2O3 scale. Some (Hf, W)-rich particles are also observed in the bond coat as marked by the arrows. The white phase is Al2O3 and the HfO2 phase is identified as a monoclinic lattice structure by the SADP as shown in Fig. 6. The chemical compositions of the oxides and/or the (Hf, W)-rich particles in the bond coat are given in Table 4. The concentration of Hf in the oxides at the TGO/ BC interface is as high as 70 wt.% and the level of W in the particles distributed within the bond coat is 9.5 wt.%. These particles containing refractory elements usually exist or segregate at the grain boundaries [29].
Table 5 Chemical compositions of zones corresponding to Fig. 7. A
O Al Cr Co Ni Hf
B
C
Wt.%
At.%
Wt.%
At.%
Wt.%
At.%
– 02.61 22.25 38.32 36.82 –
– 05.37 23.75 36.08 34.80 –
05.43 – 05.79 09.31 11.02 68.45
28.75 – 09.43 13.39 15.92 32.51
03.55 02.02 17.71 29.81 27.30 19.62
12.92 04.36 19.82 29.44 27.06 06.40
To illustrate the microstructural evolution of CoCrAlY bond coat, ternary diagrams [34] for Al, Co, Cr and Ni are analyzed (Fig. 8). The composition the as-deposited CoCrAlY coating is located within the mix-zone of β-CoAl and γ-Co phases of Al-Cr-Co ternary diagram as marked in Fig. 8a. According to the Lever Law, the mass fractions of β-CoAl and γ-Co phases are calculated from this diagram. The proportion is 27.6% for the β phase and 72.4% for the γ-Co phase, respectively. The location of β-NiCoAl phase is given in Fig. 8b. In fact, it consists of NiAl and Ni3Al dual phases. The proportions of the phases are about 28% and 72%, respectively. The γ phase is a single and stable γ-CoNi solid solution after inter-diffusion shown in Fig. 8c. Depletion of Al in the β-CoAl phase and loss of Cr in the bond coat result in the degradation of the bond coat in oxidation and hot-corrosion resistance. The service lifetime of the bond coat mainly depends on the content of Al or the volume percentage of β phase in the coating. During high-temperature exposure at 1050 °C Al migrates to the surface of the metallic coating and forms thermally grown oxides (TGO). After longterm oxidation Al would be consumed. Depletion of Al causes a reduction of β-CoAl phase in BC since the Al element primarily comes from β phase as mentioned above. This is consistent with the known outward diffusion and preferential oxidation theory [17]. Another degradation way for the CoCrAlY coating is inward diffusion of Al into the substrate during thermal exposure. Simultaneously, Co and Cr elements are depleted owing to a severe inward diffusion. Also large amounts of Ni diffuse from the substrate to the bond coat and promote the phase transformation from Co-based β-CoAl and γ-Co (ss) to Ni-based β-NiCoAl and γ-CoNi (ss). Inward diffusion of Al to the substrate results in the formation of bcc β-NiCoAl phase in the inter-diffusion zone (IDZ) due to a reaction with the substrate elements [3]. The β phase is of low solubility for refractory elements such as Hf, W, Mo, and Ta. Significant outward diffusion of Ni from the substrate to the metallic coating would cause depletion of Ni in the IDZ, leading to a reduction of the γ′/γ matrix. This promotes precipitation of the substrate strengthening refractory elements from the new-forming β phase and the supersaturated γ solid solution. The refractory elements such as Hf and W could diffuse to the BC along the grain boundaries. They are dispersed in the bond coat either inside the grains or at the grain boundaries depending on their particle sizes. Severe interdiffusion between the bond coat and the underlying superalloy usually induces a series of phase transformation in the bond coat [35], resulting in the degradation of the bond coat. The degradation of the bond coat has an important impact on the lifetime of TBC system. As observed in the present work, severe inward diffusion of Al and Cr in the bond coat occurs, which tends to cause depletion of Al and Cr in the bond coat. As Al and Cr are the important oxidation-resistant elements in the bond coat, oxidation resistance of the bond coat is gradually degraded as a result of the depletion of Al and Cr in the bond coat. On the other hand, the elements in the superalloy such as Ni, Ta, Ti and W diffuse to the surface of
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bond coat during thermal exposure, which can cause accelerated thickening of TGO and weaken the oxide scale adherence [2]. It is well admitted that spallation failure of an EB-PVD TBC occurs in most cases by cracking within the TGO or along the interface between the TGO and bond coat. Therefore, it is just claimed by Schulz et al. [36] that the chemical composition of substrate alloy plays a decisive role in governing the TBC lifetime, and Hf-containing alloy improves adhesion of the TGO on the bond coat, therefore leading to a prolonged TBC lifetime [37]. 4. Conclusions The as-deposited EB-PVD CoCrAlY coating mainly consists of Co-based bcc β-CoAl, fcc γ-Co (Co-based solid solution) and minor tetragonal σ-CoCr phases. The columnar β and γ phases show periodic arrangements and evolve into Ni-based β-NiCoAl and γ-CoNi phases (ss) after 100 h oxidation and interdiffusion at 1050 °C, but remain the same lattice structures as those of the as-deposited. The phase evolutions are caused by oxidation of the bond coat and elements interdiffusion between the bond coat and the underlying superalloy DZ 125. An amount of refractory elements-rich phases is also observed in the bond coat. Acknowledgments This research is sponsored by National Natural Science Foundation of China (NSFC, no.50771009, no.50731001 and no.51071013) and National Basic Research Program (973 Program) of China under grant no. 2010CB631200. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] Fig. 8. Ternary diagrams of (a) Al-Cr-Co, (b) Al-Co-Ni and (c) Co-Cr-Ni (at.%) [34].
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