- Email: [email protected]
Characterization and oxidation behavior of NiCoCrAlY coating fabricated by electrophoretic deposition and vacuum heat treatment
Applied Surface Science 257 (2011) 4616–4620
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Characterization and oxidation behavior of NiCoCrAlY coating fabricated by electrophoretic deposition and vacuum heat treatment Zhiming Li, Shiqiang Qian ∗ , Wei Wang College of Material Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
a r t i c l e
i n f o
Article history: Received 26 September 2010 Received in revised form 14 December 2010 Accepted 14 December 2010 Available online 23 December 2010 Keywords: Oxidation NiCoCrAlY coating Nickel based superalloys Electrophoretic deposition Vacuum heat treatment
a b s t r a c t Electrophoretic deposition (EPD) was showed to be a feasible and convenient method to fabricate NiCoCrAlY coatings on nickel based supperalloys. The microstructure and composition of the NiCoCrAlY coatings after vacuum heat treatment were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDAX). Isothermal-oxidation test was performed at 1100 ◦ C in static air for 100 h. The results show that the major phases in electrophoretic deposited and vacuum heat treated NiCoCrAlY coating are ␥-Ni and ␥ -Ni3 Al phases, also there is an extremely small quantity of Al2 O3 in the coating. Composition fluctuations occur in the coating and a certain amount of titanium diffuse from the superalloy substrate to the top of the coating during vacuum heat treatment. The oxidation test results exhibit that the oxidation kinetics of this coating has two typical stages. The protective oxide layer is mainly formed in the initial linear growth stage and then the oxide layer hinders further oxidation of the coating in the subsequent parabolic growth stage. The coating can effectively protect the superalloy substrate from oxidation. A certain amount of rutile TiO2 is formed in the coating during oxidation and it is adverse to the oxidation resistance of the coating . Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction The development of nickel based superalloys has allowed a steady increase in the operating temperatures of gas turbine engines and then led to improved performance and efficiency of these engines over the past few decades [1]. However, now the operating temperatures of advanced gas turbines reach the limits that the superalloys can withstand. High-performance thermal barrier coatings (TBCs) have been seriously investigated to further upgrade the quality of the gas turbines. TBCs, which typically comprise metal bond coat and ceramic top coat, insulate high temperature engine components from the hot gas stream, and improve the performances of the engines [2]. As an important part of TBCs, the bond coat has been used to protect the superalloy substrate from oxidation at high temperature and alleviate the thermalexpansion mismatch stresses [3]. MCrAlY (M = Ni and/or Co) is one of the most commonly used bond coat material in TBCs [4]. The antioxidant mechanism of this bond coat is that the protective oxidation products such as Al2 O3 and Cr2 O3 are formed on the coating surface at high temperature and then the dense oxide layer isolates the metal parts from the oxidation environment. The addition of yttrium in the coating promotes the chemical combi-
∗ Corresponding author. Tel.: +86 21 67791203; fax: +86 21 67791201. E-mail addresses: [email protected] (Z. Li), [email protected] (S. Qian).
nation between oxidation layer and substrate, improving the cyclic oxidation resistance of the coating system [5]. A lot of techniques, such as air plasma spray (APS) [6], vacuum plasma spray (VPS) [7], solution-precursor plasma spray (SPPS) [8], electron-beam physical vapor deposition (EB-PVD) [9], high velocity oxygen fuel spray (HVOF) [10], magnetron sputtering [11], have been used to deposit MCrAlY bond coat on superalloys. However, almost all of these methods are costly. Electrophoretic deposition (EPD), a low cost technique, has been employed to deposit the bond coat for thermal barrier coating by Fayeulle and Jeandin [12]. In their study, the bond coat material was a NiCrAlY + Ta powder with a particle size ranging from 44 m and the electrophoretic coating possessed cellular structure. When applied to TBCs, the improvement of properties compared to those of conventional processes such as plasma spraying is significant. The advantages were the low cost and flexibility of the process coupled with the good properties. However, the suspending media in the study of Fayeulle and Jeandin was methanol 1 vol.% water, which has toxicity in humans. EPD is a proven technique for production of ceramic coatings [13–15]. However, the EPD of metal powder differs from that of ceramics. For one thing, the metal powder deposit is conductive, which makes the deposition behavior more difficult to control. For another, the density of metal is higher than that of ceramics and metal powder particles usually have large size, all these make it difficult to suspend metal particles. In the present work, the Ni, Co, Cr, Al and Y mixed particles were electrophoretic deposited in 2% water
0169-4332/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.097
Z. Li et al. / Applied Surface Science 257 (2011) 4616–4620 Table 1 Composition of the mixed metallic powders. Element
Co
Cr
Al
Y
Ni
Content (wt%)
20
20
9.5
0.8
Balance
and 98% ethanol solution on nickel based superalloy. The deposits were vacuum heat treated to be solutionized and dense, obtaining strong adhesion with the superalloy substrate. The microstructure and high temperature oxidation behavior of the NiCoCrAlY coating were investigated.
4617
used. Samples were placed in crucible and the crucible was placed into the hot zone of the furnace. The mass change of samples during the isothermal-oxidation tests was real-time detected by using an electronic analytical balance and a computer. The datacollection interval was 5 min in this study. X-ray diffraction (XRD) with Cu-K␣ radiation was used to study the phase composition of the oxidation products on the surface of oxidation tested coating. A JSM-6700F cold field emission scanning electron microscope (SEM) was employed to observe the morphology of the coating surface after oxidation test. 3. Results and discussion
2. Materials and methods
3.1. Characterization of electrophoretic deposited NiCoCrAlY coating after vacuum heat treatment
2.1. Electrophoretic deposition of NiCoCrAlY coatings Stable, electrophoretically-active baths of NiCoCrAlY were prepared by adding mixed Ni, Co, Cr, Al and Y powders with an average particle size of 5 m to 98% ethanol and 2% water solution. Composition of the metallic powders in the starting suspensions is shown in Table 1. The total concentration of the mixed powders was 30 g/l. In addition, aluminium chloride hexahydrate was used as the ionic activating agent with a concentration of 0.01 g/l. A small quantity of acetic acid was added into the suspensions and the initial pH value of the suspension was about 6.0. Sonication of fresh baths for 60–90 min at 50 kHz in a commercial ultrasonic cleaner resulted in baths which displayed a useful shelf life in excess of 10 h. GH3128 nickel-based superalloy was used as substrate. The composition of the substrate is given in Table 2. The substrate was prepared in the form of 15 × 15 × 2.5 mm3 specimens. The substrate surface was ground with 1000 grit SiC abrasive paper and cleaned thoroughly with acetone. The substrate specimen was connected to the cathode and anode material was graphite. For EPD, the two electrodes were fixed parallel in the suspension and a constant DC voltage was applied using a laboratory power supply. The distance between electrodes was 2.5 cm. Applied voltage and deposition time were 160 V and 2 min, respectively. The samples were withdrawn from the suspension with a speed of 10 mm/s after electrophoretic deposition, and then dried horizontally in air at room temperature for 24 h. Subsequently, the samples were heat-treated at 1200 ◦ C in vacuum for 1 h with heating rate of 6.5 ◦ C/min and furnace cooling. The real-time curves of the temperature and vacuum degree in the vacuum heat treatment process were shown in Fig. 1. The vacuum degree was affected by temperature. However, all the vacuum degree value at high temperature was limited in the range of 2.5 × 10−3 to 3.5 × 10−4 Pa. The X’ Pert Pro X-ray diffractometer (XRD) with Cu-K␣ radiation was used to investigate the structure of the vacuum heat treated coating. A HITACHI S-3400N Scanning Electron Microscope (SEM) with energy dispersive X-ray detector (EDX) system was employed to measure the coating composition; also the SEM was used to observe the cross-sectional morphology of the coating-substrate system.
Superficial SEM image and EDAX analysis of the electrophoretic deposited NiCoCrAlY coating after vacuum heat treatment are shown in Fig. 2. The major elements contained in the coating were Ni, Co, Cr, Al, Y, Ti and O. Although region A and B had the same elements, the content of each elements were different. In region A, nickel content was on the high side while the yttrium and aluminium content were on the low side. Correspondingly in region B, yttrium and aluminium were excessive while nickel was less. Although the EDAX analysis is semi-quantitative, the contrast of the element content was remarkable. Therefore, it can be concluded that there was some degree of composition fluctuations in the coating after vacuum heat treatment. It should be noted that a certain amount of Ti was existed in the coating. As element Ti was not contained in the metallic powders in the starting suspensions and there was no other source of element Ti in the whole process, it can be determined that the element Ti existed in the coating was diffused from the superalloy substrate in the vacuum heat treatment process. Moreover, when Hesnawi et al. [16] studied the isothermal oxidation behavior of EB-PVD NiCoCrAlY coating on nickel based superalloy, they demonstrated that the diffusion of the substrate element Ti to the surface of the coating deteriorates the adhesion of protective oxide to the coating and hence leads to spallation of the protective oxide layer. The element Al in the coating mainly came from the metal powders in the suspensions. Furthermore, another source was
2.2. Isothermal-oxidation testing Isothermal-oxidation test was performed at 1100 ◦ C in ambient atmosphere for 100 h. The test method was identical to that introduced in our previous study [11]. A perpendicular tube furnace with a temperature accuracy of ±0.5 ◦ C in the hot zone was
Fig. 1. Real-time curves of the temperature and vacuum degree in the vacuum heat treatment process.
Table 2 The nominal composition of the GH3128 superalloy substrate. Elements
Cr
W
Mo
Al
Fe
Ti
C
Ce
Zr
B
Ni
Content (wt%)
20.55
8.70
8.26
0.80
0.60
0.40
0.04
0.05
0.08
0.005
Balance
4618
Z. Li et al. / Applied Surface Science 257 (2011) 4616–4620
Fig. 2. Superficial SEM image and EDAX analysis of the electrophoretic deposited NiCoCrAlY coating after vacuum heat treatment.
the aluminium chloride hexahydrate which was used as ionic activating agent. During the electrophoretic deposition process, electrochemical reactions occurred at the two electrodes as current was passed. At the negative electrode, water was reduced [17]: 2H2 O + 2e− → H2 (g) + 2OH−
(1)
A concentration gradient of hydroxide ions was built up near the surface of negative electrode due to the electrochemical reaction. Moreover, aluminium chloride hexahydrate dissociated: AlCl3 ⇒ Al3+ + 2Cl−
(2)
OH− reacted with the Al3+ ions to form aluminium hydroxide: 3+
Al
−
+ 3OH ⇒ Al(OH)3
(3)
observed in our study, it might be ascribed to the different technique of the coating. Cross-sectional SEM image of the coating-substrate system is presented in Fig. 4. It can be seen that a dense coating with fine adhesion was produced. The thickness of the NiCoCrAlY coating in this study was about 35 m. In combination of the deposition time, electrophoretic deposition in this study showed high deposition rate as compared to many other techniques. 3.2. High temperature oxidation behavior The isothermal-oxidation kinetics of the NiCoCrAlY coating at 1100 ◦ C is illustrated in Fig. 5. The rings and solid lines
Aluminium hydroxide dehydrated to form alumina during drying and vacuum heat treatment: 2Al(OH)3 ⇒ Al2 O3 + 3H2 O
(4)
Actually, under these conditions the polymeric aluminium hydroxide acted as a binder to hold the particles and aid in particle adhesion. The adhesion of the deposit, therefore, no longer depended only on the London-van der Waals attractive forces between the primary particles [18]. In the coating, however, it should be noted that the content of Al2 O3 formed by this mechanism was extremely low as compared to the whole coating. The X-ray diffraction pattern of electrophoretic deposited NiCoCrAlY coating after vacuum heat treatment is presented in Fig. 3. It can be seen that ␥-Ni and ␥ -Ni3 Al phases were the major phases in the coating. In addition, there was an extremely small quantity of Al2 O3 which formation mechanism has been expounded above. Wang et al. [5] also found that ␥-Ni and ␥ -Ni3 Al phases are the major phases in the NiCoCrAlY coatings deposited by arc ion plating (AIP) on the Ni3 Al-base superalloy. However, -(Cr, Co) phase formed in their coatings during vacuum heat treatment was not
Fig. 3. XRD pattern of electrophoretic deposited NiCoCrAlY coating after vacuum heat treatment.
Z. Li et al. / Applied Surface Science 257 (2011) 4616–4620
Fig. 4. Cross-sectional SEM image of the coating-substrate system.
Fig. 6. XRD pattern of isothermal-oxidation tested specimen.
represent mass gain data points and fitting curves of these data points, respectively. At the beginning of the oxidation text, the weight gain increased linearly because the passive oxide layer was forming. After the brief period of linear growth, the weight gain grown basically in accordance with the parabolic rate law due to that the oxide layer hindered further oxidation of the coating. Thus the weight gain of the coating in the fist stage (W1 ) and the second stage (W2 ) can be expressed as: W1 = k1 t
(5)
2
(W2 ) = k2 t
(6)
where k1 and k2 are the weight gain rate constant of the fist and second stage of oxidation, respectively. Here the oxidation time t is in seconds, and the unit for the weight gain is milligram per square centimeter. Then the linear function was used to fit the data of the fist stage and the parabola function was used to fit the data of the second stage. The fitting results are as follows: k1 = 4.332 × 10−5 mg cm−2 s−1 −5
k2 = 1.691 × 10
2
mg cm
−4 −1
s
4619
(7) (8)
In the linear growth stage of oxidation, the weight gain increased 4.332 × 10−5 mg/cm2 within one second, and the total weight gain
Fig. 5. Isothermal-oxidation kinetics of the electrophoretic deposited and vacuum heat treated NiCoCrAlY coating at 1100 ◦ C.
reached to 0.42 mg/cm2 after the first stage of 170 min. In the parabolic growth stage, the weight gain rate was much lower than that of the linear growth stage, and the total weight gain was about 2.47 mg/cm2 after the whole oxidation test of 100 h. When compared with our previous study [11], the weight gain rate constant of the electrophoretic deposited and vacuum heat treated NiCoCrAlY coating in the parabolic growth stage (1.691 × 10−5 mg2 cm−4 s−1 ) is larger than that of the magnetron-sputtered NiCoCrAlY coating (1.434 × 10−5 mg2 cm−4 s−1 ). However, it is much smaller than that of the GH3128 superalloy (6.692 × 10−5 mg2 cm−4 s−1 ), which means that the oxidation rate of NiCoCrAlY coating in this study is much slower than that of the uncoated GH3128 superalloy and the NiCoCrAlY coating can effectively protect the superalloy substrate from oxidation. Fig. 6 gives the XRD pattern of the isothermal-oxidation tested NiCoCrAlY coating in this study. It can be seen that the main phase on the oxidation tested coating surface were Cr2 O3 , ␣-Al2 O3 , (Ni,Co)Cr2 O4 , and rutile TiO2 . Here, NiCr2 O4 and CoCr2 O4 were treated as one compound phase (Ni,Co)Cr2 O4 because they have nearly the same X-ray diffraction spectrum. During the oxidation process of NiCoCrAlY coating, Al was consumed by the formation of Al2 O3 at the coating surface as well as by interdiffusion at the interface of coating and substrate, producing aluminium depleted zones which increase with time. When the Al concentration reached a critical minimum value, Cr2 O3 or (Ni,Co)Cr2 O4 would be formed besides the alumina [19]. In this study, it should be noted that the content of rutile TiO2 reached a certain level in the oxidation tested coating. From the above characterization, the vacuum heat treated coating contained element Ti which diffused from the superalloy substrate in the vacuum heat treatment process. The element Ti in the coating surface oxidized at the early stage of the oxidation process. Furthermore, the element Ti in the superalloy substrate would diffuse to the coating surface during oxidation, and then reacted with the oxygen to form more rutile TiO2 . As reported by [16], the diffusion of element Ti makes against for the oxidation behavior of the coating. In order to upgrade the performance of this coating system, an efficient diffusion barrier between the coating and the superalloy substrate is promising, and it will also be our further study. Fig. 7 shows the superficial SEM images of isothermal-oxidation tested NiCoCrAlY coatings. It can be seen that the crystal size of the oxidation products were about 1.5 m. From the surface morphology and XRD pattern, whisker-like -Al2 O3 phase was not observed which means that the -␣ phase transformation completed after the oxidation test for 100 h. In this experimental study, the alumina particles formed from polymeric aluminium
4620
Z. Li et al. / Applied Surface Science 257 (2011) 4616–4620
Acknowledgements The authors gratefully acknowledge the financial supports from Special Foundation of the Shanghai Education Commission for Nano-Materials Research (0852nm01400) and Shanghai Leading Academic Discipline Project (J51402). References
Fig. 7. Superficial SEM image of the isothermal-oxidation tested NiCoCrAlY coating.
hydroxide probably served as the initial crystal nucleus of the oxidation products and facilitated the formation of ␣-Al2 O3 by accelerating the - to ␣-Al2 O3 phase transformation during oxidation process [20]. 4. Conclusions The structure of electrophoretic deposited and vacuum heat treated NiCoCrAlY coating was characterized, the isothermal-oxidation kinetics, the phase composition of oxidation products as well as the surface morphology of the oxidized coating were studied; the following conclusions can be drawn: (1) The electrophoretic deposited and vacuum heat treated NiCoCrAlY coating mainly comprise ␥-Ni and ␥ -Ni3 Al phases, also there is an extremely small quantity of Al2 O3 in the coating. Composition fluctuations occur in the coating and a certain amount of titanium diffuse from the superalloy substrate to the top of coating during vacuum heat treatment. (2) The oxidation kinetics of the electrophoretic deposited and vacuum heat treated NiCoCrAlY coating has two typical stages. The passive oxide layer is formed in the initial linear growth stage. Then the oxide layer hinder further oxidation of the coating in the subsequent parabolic growth stage. The coating can effectively protect the superalloy substrate from oxidation. A certain amount of rutile TiO2 is formed in the coating during oxidation and it is adverse to the oxidation resistance of the coating.
[1] J.H. Perepezko, The hotter the engine, the better, Science 326 (2009) 1068–1069. [2] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas-turbine engine applications, Science 296 (2002) 280–284. [3] R. Krishnamurthy, D.J. Srolovitz, Quantitative analysis of a new model for the sintering of columnar thermal barrier coatings, Mater. Sci. Eng. A 509 (2009) 46–56. [4] T. Patterson, A. Leon, B. Jayaraj, J. Liu, Y.H. Sohn, Thermal cyclic lifetime and oxidation behavior of air plasma sprayed CoNiCrAlY bond coats for thermal barrier coatings, Surf. Coat. Technol. 203 (2008) 437–441. [5] B. Wang, J. Gong, C. Sun, R.F. Huang, L.S. Wen, The behavior of MCrAlY coatings on Ni3 Al-base superalloy, Mater. Sci. Eng. A 357 (2003) 39–44. [6] A. Scrivani, G. Rizzi, C.C. Berndt, Enhanced thick thermal barrier coatings that exhibit varying porosity, Mater. Sci. Eng. A 476 (2008) 1–7. [7] U. Schulz, O. Bernardi, A. Ebach-Stahl, R. Vassen, D. Sebold, Improvement of EB-PVD thermal barrier coatings by treatments of a vacuum plasma-sprayed bond coat, Surf. Coat. Technol. 203 (2008) 160–170. [8] A. Jadhav, N.P. Padture, F. Wu, E.H. Jordan, M. Gell, Thick ceramic thermal barrier coatings with high durability deposited using solution-precursor plasma spray, Mater. Sci. Eng. A 405 (2005) 313–320. [9] T.R. Kakuda, A.M. Limarga, T.D. Bennett, D.R. Clarke, Evolution of thermal properties of EB-PVD 7YSZ thermal barrier coatings with thermal cycling, Acta Mater. 57 (2009) 2583–2591. [10] C.R.C. Lima, J.M. Guilemany, Adhesion improvements of thermal barrier coatings with HVOF thermally sprayed bond coats, Surf. Coat. Technol. 201 (2007) 4694–4701. [11] Z. Li, S. Qian, W. Wang, J. Liu, Microstructure and oxidation resistance of magnetron-sputtered nanocrystalline NiCoCrAlY coatings on nickel-based superalloy, J. Alloys Compd. 505 (2010) 675–679. [12] D. Fayeulle, M. Jeandin, Thermal barrier coatings with cellular electrophoretic bond coats, Mater. Manuf. Process. 5 (1990) 419–426. [13] K. König, S. Novak, A. Ivekovic, K. Rade, D. Meng, A.R. Boccaccini, S. Kobe, Fabrication of CNT-SiC/SiC composites by electrophoretic deposition, J. Eur. Ceram. Soc. 30 (2010) 1131–1137. [14] S. Radice, H. Dietsch, S. Mischler, J. Michler, Electrophoretic deposition of functionalized polystyrene particles for TiO2 multi-scale structured surfaces, Surf. Coat. Technol. 204 (2010) 1749–1754. [15] C.T. Kwok, P.K. Wong, F.T. Cheng, H.C. Man, Characterization and corrosion behavior of hydroxyapatite coatings on Ti6Al4V fabricated by electrophoretic deposition, Appl. Surf. Sci. 255 (2009) 6736–6744. [16] A. Hesnawi, H. Li, Z. Zhou, S. Gong, H. Xu, Isothermal oxidation behaviour of EB-PVD MCrAlY bond coat, Vacuum 81 (2007) 947–952. [17] E. de Beer, J. Duval, E.A. Meulenkamp, Electrophoretic deposition: a quantitative model for particle deposition and binder formation from alcohol-based suspensions, J. Colloid Interface Sci. 222 (2000) 117–124. [18] D.R. Brown, F.W. Salt, The mechanism of electrophoretic deposition, J. Appl. Chem. 15 (1965) 40–48. [19] U. Dragos, M. Gabriela, B. Waltraut, C. Ioan, Improvement of the oxidation behaviour of electron beam remelted MCrAlY coatings, Solid State Sci. 7 (2005) 459–464. [20] B.A. Pint, L.W. Hobbs, Limitations on the use of ion implantation for the study of the reactive element effect in beta-NiAl, J. Electrochem. Soc. 141 (1994) 2443–2453.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Characterization and oxidation behavior of NiCoCrAlY coating fabricated by electrophoretic deposition and vacuum heat treatment Zhiming Li, Shiqiang Qian ∗ , Wei Wang College of Material Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
a r t i c l e
i n f o
Article history: Received 26 September 2010 Received in revised form 14 December 2010 Accepted 14 December 2010 Available online 23 December 2010 Keywords: Oxidation NiCoCrAlY coating Nickel based superalloys Electrophoretic deposition Vacuum heat treatment
a b s t r a c t Electrophoretic deposition (EPD) was showed to be a feasible and convenient method to fabricate NiCoCrAlY coatings on nickel based supperalloys. The microstructure and composition of the NiCoCrAlY coatings after vacuum heat treatment were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDAX). Isothermal-oxidation test was performed at 1100 ◦ C in static air for 100 h. The results show that the major phases in electrophoretic deposited and vacuum heat treated NiCoCrAlY coating are ␥-Ni and ␥ -Ni3 Al phases, also there is an extremely small quantity of Al2 O3 in the coating. Composition fluctuations occur in the coating and a certain amount of titanium diffuse from the superalloy substrate to the top of the coating during vacuum heat treatment. The oxidation test results exhibit that the oxidation kinetics of this coating has two typical stages. The protective oxide layer is mainly formed in the initial linear growth stage and then the oxide layer hinders further oxidation of the coating in the subsequent parabolic growth stage. The coating can effectively protect the superalloy substrate from oxidation. A certain amount of rutile TiO2 is formed in the coating during oxidation and it is adverse to the oxidation resistance of the coating . Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction The development of nickel based superalloys has allowed a steady increase in the operating temperatures of gas turbine engines and then led to improved performance and efficiency of these engines over the past few decades [1]. However, now the operating temperatures of advanced gas turbines reach the limits that the superalloys can withstand. High-performance thermal barrier coatings (TBCs) have been seriously investigated to further upgrade the quality of the gas turbines. TBCs, which typically comprise metal bond coat and ceramic top coat, insulate high temperature engine components from the hot gas stream, and improve the performances of the engines [2]. As an important part of TBCs, the bond coat has been used to protect the superalloy substrate from oxidation at high temperature and alleviate the thermalexpansion mismatch stresses [3]. MCrAlY (M = Ni and/or Co) is one of the most commonly used bond coat material in TBCs [4]. The antioxidant mechanism of this bond coat is that the protective oxidation products such as Al2 O3 and Cr2 O3 are formed on the coating surface at high temperature and then the dense oxide layer isolates the metal parts from the oxidation environment. The addition of yttrium in the coating promotes the chemical combi-
∗ Corresponding author. Tel.: +86 21 67791203; fax: +86 21 67791201. E-mail addresses: [email protected] (Z. Li), [email protected] (S. Qian).
nation between oxidation layer and substrate, improving the cyclic oxidation resistance of the coating system [5]. A lot of techniques, such as air plasma spray (APS) [6], vacuum plasma spray (VPS) [7], solution-precursor plasma spray (SPPS) [8], electron-beam physical vapor deposition (EB-PVD) [9], high velocity oxygen fuel spray (HVOF) [10], magnetron sputtering [11], have been used to deposit MCrAlY bond coat on superalloys. However, almost all of these methods are costly. Electrophoretic deposition (EPD), a low cost technique, has been employed to deposit the bond coat for thermal barrier coating by Fayeulle and Jeandin [12]. In their study, the bond coat material was a NiCrAlY + Ta powder with a particle size ranging from 44 m and the electrophoretic coating possessed cellular structure. When applied to TBCs, the improvement of properties compared to those of conventional processes such as plasma spraying is significant. The advantages were the low cost and flexibility of the process coupled with the good properties. However, the suspending media in the study of Fayeulle and Jeandin was methanol 1 vol.% water, which has toxicity in humans. EPD is a proven technique for production of ceramic coatings [13–15]. However, the EPD of metal powder differs from that of ceramics. For one thing, the metal powder deposit is conductive, which makes the deposition behavior more difficult to control. For another, the density of metal is higher than that of ceramics and metal powder particles usually have large size, all these make it difficult to suspend metal particles. In the present work, the Ni, Co, Cr, Al and Y mixed particles were electrophoretic deposited in 2% water
0169-4332/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.097
Z. Li et al. / Applied Surface Science 257 (2011) 4616–4620 Table 1 Composition of the mixed metallic powders. Element
Co
Cr
Al
Y
Ni
Content (wt%)
20
20
9.5
0.8
Balance
and 98% ethanol solution on nickel based superalloy. The deposits were vacuum heat treated to be solutionized and dense, obtaining strong adhesion with the superalloy substrate. The microstructure and high temperature oxidation behavior of the NiCoCrAlY coating were investigated.
4617
used. Samples were placed in crucible and the crucible was placed into the hot zone of the furnace. The mass change of samples during the isothermal-oxidation tests was real-time detected by using an electronic analytical balance and a computer. The datacollection interval was 5 min in this study. X-ray diffraction (XRD) with Cu-K␣ radiation was used to study the phase composition of the oxidation products on the surface of oxidation tested coating. A JSM-6700F cold field emission scanning electron microscope (SEM) was employed to observe the morphology of the coating surface after oxidation test. 3. Results and discussion
2. Materials and methods
3.1. Characterization of electrophoretic deposited NiCoCrAlY coating after vacuum heat treatment
2.1. Electrophoretic deposition of NiCoCrAlY coatings Stable, electrophoretically-active baths of NiCoCrAlY were prepared by adding mixed Ni, Co, Cr, Al and Y powders with an average particle size of 5 m to 98% ethanol and 2% water solution. Composition of the metallic powders in the starting suspensions is shown in Table 1. The total concentration of the mixed powders was 30 g/l. In addition, aluminium chloride hexahydrate was used as the ionic activating agent with a concentration of 0.01 g/l. A small quantity of acetic acid was added into the suspensions and the initial pH value of the suspension was about 6.0. Sonication of fresh baths for 60–90 min at 50 kHz in a commercial ultrasonic cleaner resulted in baths which displayed a useful shelf life in excess of 10 h. GH3128 nickel-based superalloy was used as substrate. The composition of the substrate is given in Table 2. The substrate was prepared in the form of 15 × 15 × 2.5 mm3 specimens. The substrate surface was ground with 1000 grit SiC abrasive paper and cleaned thoroughly with acetone. The substrate specimen was connected to the cathode and anode material was graphite. For EPD, the two electrodes were fixed parallel in the suspension and a constant DC voltage was applied using a laboratory power supply. The distance between electrodes was 2.5 cm. Applied voltage and deposition time were 160 V and 2 min, respectively. The samples were withdrawn from the suspension with a speed of 10 mm/s after electrophoretic deposition, and then dried horizontally in air at room temperature for 24 h. Subsequently, the samples were heat-treated at 1200 ◦ C in vacuum for 1 h with heating rate of 6.5 ◦ C/min and furnace cooling. The real-time curves of the temperature and vacuum degree in the vacuum heat treatment process were shown in Fig. 1. The vacuum degree was affected by temperature. However, all the vacuum degree value at high temperature was limited in the range of 2.5 × 10−3 to 3.5 × 10−4 Pa. The X’ Pert Pro X-ray diffractometer (XRD) with Cu-K␣ radiation was used to investigate the structure of the vacuum heat treated coating. A HITACHI S-3400N Scanning Electron Microscope (SEM) with energy dispersive X-ray detector (EDX) system was employed to measure the coating composition; also the SEM was used to observe the cross-sectional morphology of the coating-substrate system.
Superficial SEM image and EDAX analysis of the electrophoretic deposited NiCoCrAlY coating after vacuum heat treatment are shown in Fig. 2. The major elements contained in the coating were Ni, Co, Cr, Al, Y, Ti and O. Although region A and B had the same elements, the content of each elements were different. In region A, nickel content was on the high side while the yttrium and aluminium content were on the low side. Correspondingly in region B, yttrium and aluminium were excessive while nickel was less. Although the EDAX analysis is semi-quantitative, the contrast of the element content was remarkable. Therefore, it can be concluded that there was some degree of composition fluctuations in the coating after vacuum heat treatment. It should be noted that a certain amount of Ti was existed in the coating. As element Ti was not contained in the metallic powders in the starting suspensions and there was no other source of element Ti in the whole process, it can be determined that the element Ti existed in the coating was diffused from the superalloy substrate in the vacuum heat treatment process. Moreover, when Hesnawi et al. [16] studied the isothermal oxidation behavior of EB-PVD NiCoCrAlY coating on nickel based superalloy, they demonstrated that the diffusion of the substrate element Ti to the surface of the coating deteriorates the adhesion of protective oxide to the coating and hence leads to spallation of the protective oxide layer. The element Al in the coating mainly came from the metal powders in the suspensions. Furthermore, another source was
2.2. Isothermal-oxidation testing Isothermal-oxidation test was performed at 1100 ◦ C in ambient atmosphere for 100 h. The test method was identical to that introduced in our previous study [11]. A perpendicular tube furnace with a temperature accuracy of ±0.5 ◦ C in the hot zone was
Fig. 1. Real-time curves of the temperature and vacuum degree in the vacuum heat treatment process.
Table 2 The nominal composition of the GH3128 superalloy substrate. Elements
Cr
W
Mo
Al
Fe
Ti
C
Ce
Zr
B
Ni
Content (wt%)
20.55
8.70
8.26
0.80
0.60
0.40
0.04
0.05
0.08
0.005
Balance
4618
Z. Li et al. / Applied Surface Science 257 (2011) 4616–4620
Fig. 2. Superficial SEM image and EDAX analysis of the electrophoretic deposited NiCoCrAlY coating after vacuum heat treatment.
the aluminium chloride hexahydrate which was used as ionic activating agent. During the electrophoretic deposition process, electrochemical reactions occurred at the two electrodes as current was passed. At the negative electrode, water was reduced [17]: 2H2 O + 2e− → H2 (g) + 2OH−
(1)
A concentration gradient of hydroxide ions was built up near the surface of negative electrode due to the electrochemical reaction. Moreover, aluminium chloride hexahydrate dissociated: AlCl3 ⇒ Al3+ + 2Cl−
(2)
OH− reacted with the Al3+ ions to form aluminium hydroxide: 3+
Al
−
+ 3OH ⇒ Al(OH)3
(3)
observed in our study, it might be ascribed to the different technique of the coating. Cross-sectional SEM image of the coating-substrate system is presented in Fig. 4. It can be seen that a dense coating with fine adhesion was produced. The thickness of the NiCoCrAlY coating in this study was about 35 m. In combination of the deposition time, electrophoretic deposition in this study showed high deposition rate as compared to many other techniques. 3.2. High temperature oxidation behavior The isothermal-oxidation kinetics of the NiCoCrAlY coating at 1100 ◦ C is illustrated in Fig. 5. The rings and solid lines
Aluminium hydroxide dehydrated to form alumina during drying and vacuum heat treatment: 2Al(OH)3 ⇒ Al2 O3 + 3H2 O
(4)
Actually, under these conditions the polymeric aluminium hydroxide acted as a binder to hold the particles and aid in particle adhesion. The adhesion of the deposit, therefore, no longer depended only on the London-van der Waals attractive forces between the primary particles [18]. In the coating, however, it should be noted that the content of Al2 O3 formed by this mechanism was extremely low as compared to the whole coating. The X-ray diffraction pattern of electrophoretic deposited NiCoCrAlY coating after vacuum heat treatment is presented in Fig. 3. It can be seen that ␥-Ni and ␥ -Ni3 Al phases were the major phases in the coating. In addition, there was an extremely small quantity of Al2 O3 which formation mechanism has been expounded above. Wang et al. [5] also found that ␥-Ni and ␥ -Ni3 Al phases are the major phases in the NiCoCrAlY coatings deposited by arc ion plating (AIP) on the Ni3 Al-base superalloy. However, -(Cr, Co) phase formed in their coatings during vacuum heat treatment was not
Fig. 3. XRD pattern of electrophoretic deposited NiCoCrAlY coating after vacuum heat treatment.
Z. Li et al. / Applied Surface Science 257 (2011) 4616–4620
Fig. 4. Cross-sectional SEM image of the coating-substrate system.
Fig. 6. XRD pattern of isothermal-oxidation tested specimen.
represent mass gain data points and fitting curves of these data points, respectively. At the beginning of the oxidation text, the weight gain increased linearly because the passive oxide layer was forming. After the brief period of linear growth, the weight gain grown basically in accordance with the parabolic rate law due to that the oxide layer hindered further oxidation of the coating. Thus the weight gain of the coating in the fist stage (W1 ) and the second stage (W2 ) can be expressed as: W1 = k1 t
(5)
2
(W2 ) = k2 t
(6)
where k1 and k2 are the weight gain rate constant of the fist and second stage of oxidation, respectively. Here the oxidation time t is in seconds, and the unit for the weight gain is milligram per square centimeter. Then the linear function was used to fit the data of the fist stage and the parabola function was used to fit the data of the second stage. The fitting results are as follows: k1 = 4.332 × 10−5 mg cm−2 s−1 −5
k2 = 1.691 × 10
2
mg cm
−4 −1
s
4619
(7) (8)
In the linear growth stage of oxidation, the weight gain increased 4.332 × 10−5 mg/cm2 within one second, and the total weight gain
Fig. 5. Isothermal-oxidation kinetics of the electrophoretic deposited and vacuum heat treated NiCoCrAlY coating at 1100 ◦ C.
reached to 0.42 mg/cm2 after the first stage of 170 min. In the parabolic growth stage, the weight gain rate was much lower than that of the linear growth stage, and the total weight gain was about 2.47 mg/cm2 after the whole oxidation test of 100 h. When compared with our previous study [11], the weight gain rate constant of the electrophoretic deposited and vacuum heat treated NiCoCrAlY coating in the parabolic growth stage (1.691 × 10−5 mg2 cm−4 s−1 ) is larger than that of the magnetron-sputtered NiCoCrAlY coating (1.434 × 10−5 mg2 cm−4 s−1 ). However, it is much smaller than that of the GH3128 superalloy (6.692 × 10−5 mg2 cm−4 s−1 ), which means that the oxidation rate of NiCoCrAlY coating in this study is much slower than that of the uncoated GH3128 superalloy and the NiCoCrAlY coating can effectively protect the superalloy substrate from oxidation. Fig. 6 gives the XRD pattern of the isothermal-oxidation tested NiCoCrAlY coating in this study. It can be seen that the main phase on the oxidation tested coating surface were Cr2 O3 , ␣-Al2 O3 , (Ni,Co)Cr2 O4 , and rutile TiO2 . Here, NiCr2 O4 and CoCr2 O4 were treated as one compound phase (Ni,Co)Cr2 O4 because they have nearly the same X-ray diffraction spectrum. During the oxidation process of NiCoCrAlY coating, Al was consumed by the formation of Al2 O3 at the coating surface as well as by interdiffusion at the interface of coating and substrate, producing aluminium depleted zones which increase with time. When the Al concentration reached a critical minimum value, Cr2 O3 or (Ni,Co)Cr2 O4 would be formed besides the alumina [19]. In this study, it should be noted that the content of rutile TiO2 reached a certain level in the oxidation tested coating. From the above characterization, the vacuum heat treated coating contained element Ti which diffused from the superalloy substrate in the vacuum heat treatment process. The element Ti in the coating surface oxidized at the early stage of the oxidation process. Furthermore, the element Ti in the superalloy substrate would diffuse to the coating surface during oxidation, and then reacted with the oxygen to form more rutile TiO2 . As reported by [16], the diffusion of element Ti makes against for the oxidation behavior of the coating. In order to upgrade the performance of this coating system, an efficient diffusion barrier between the coating and the superalloy substrate is promising, and it will also be our further study. Fig. 7 shows the superficial SEM images of isothermal-oxidation tested NiCoCrAlY coatings. It can be seen that the crystal size of the oxidation products were about 1.5 m. From the surface morphology and XRD pattern, whisker-like -Al2 O3 phase was not observed which means that the -␣ phase transformation completed after the oxidation test for 100 h. In this experimental study, the alumina particles formed from polymeric aluminium
4620
Z. Li et al. / Applied Surface Science 257 (2011) 4616–4620
Acknowledgements The authors gratefully acknowledge the financial supports from Special Foundation of the Shanghai Education Commission for Nano-Materials Research (0852nm01400) and Shanghai Leading Academic Discipline Project (J51402). References
Fig. 7. Superficial SEM image of the isothermal-oxidation tested NiCoCrAlY coating.
hydroxide probably served as the initial crystal nucleus of the oxidation products and facilitated the formation of ␣-Al2 O3 by accelerating the - to ␣-Al2 O3 phase transformation during oxidation process [20]. 4. Conclusions The structure of electrophoretic deposited and vacuum heat treated NiCoCrAlY coating was characterized, the isothermal-oxidation kinetics, the phase composition of oxidation products as well as the surface morphology of the oxidized coating were studied; the following conclusions can be drawn: (1) The electrophoretic deposited and vacuum heat treated NiCoCrAlY coating mainly comprise ␥-Ni and ␥ -Ni3 Al phases, also there is an extremely small quantity of Al2 O3 in the coating. Composition fluctuations occur in the coating and a certain amount of titanium diffuse from the superalloy substrate to the top of coating during vacuum heat treatment. (2) The oxidation kinetics of the electrophoretic deposited and vacuum heat treated NiCoCrAlY coating has two typical stages. The passive oxide layer is formed in the initial linear growth stage. Then the oxide layer hinder further oxidation of the coating in the subsequent parabolic growth stage. The coating can effectively protect the superalloy substrate from oxidation. A certain amount of rutile TiO2 is formed in the coating during oxidation and it is adverse to the oxidation resistance of the coating.
[1] J.H. Perepezko, The hotter the engine, the better, Science 326 (2009) 1068–1069. [2] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas-turbine engine applications, Science 296 (2002) 280–284. [3] R. Krishnamurthy, D.J. Srolovitz, Quantitative analysis of a new model for the sintering of columnar thermal barrier coatings, Mater. Sci. Eng. A 509 (2009) 46–56. [4] T. Patterson, A. Leon, B. Jayaraj, J. Liu, Y.H. Sohn, Thermal cyclic lifetime and oxidation behavior of air plasma sprayed CoNiCrAlY bond coats for thermal barrier coatings, Surf. Coat. Technol. 203 (2008) 437–441. [5] B. Wang, J. Gong, C. Sun, R.F. Huang, L.S. Wen, The behavior of MCrAlY coatings on Ni3 Al-base superalloy, Mater. Sci. Eng. A 357 (2003) 39–44. [6] A. Scrivani, G. Rizzi, C.C. Berndt, Enhanced thick thermal barrier coatings that exhibit varying porosity, Mater. Sci. Eng. A 476 (2008) 1–7. [7] U. Schulz, O. Bernardi, A. Ebach-Stahl, R. Vassen, D. Sebold, Improvement of EB-PVD thermal barrier coatings by treatments of a vacuum plasma-sprayed bond coat, Surf. Coat. Technol. 203 (2008) 160–170. [8] A. Jadhav, N.P. Padture, F. Wu, E.H. Jordan, M. Gell, Thick ceramic thermal barrier coatings with high durability deposited using solution-precursor plasma spray, Mater. Sci. Eng. A 405 (2005) 313–320. [9] T.R. Kakuda, A.M. Limarga, T.D. Bennett, D.R. Clarke, Evolution of thermal properties of EB-PVD 7YSZ thermal barrier coatings with thermal cycling, Acta Mater. 57 (2009) 2583–2591. [10] C.R.C. Lima, J.M. Guilemany, Adhesion improvements of thermal barrier coatings with HVOF thermally sprayed bond coats, Surf. Coat. Technol. 201 (2007) 4694–4701. [11] Z. Li, S. Qian, W. Wang, J. Liu, Microstructure and oxidation resistance of magnetron-sputtered nanocrystalline NiCoCrAlY coatings on nickel-based superalloy, J. Alloys Compd. 505 (2010) 675–679. [12] D. Fayeulle, M. Jeandin, Thermal barrier coatings with cellular electrophoretic bond coats, Mater. Manuf. Process. 5 (1990) 419–426. [13] K. König, S. Novak, A. Ivekovic, K. Rade, D. Meng, A.R. Boccaccini, S. Kobe, Fabrication of CNT-SiC/SiC composites by electrophoretic deposition, J. Eur. Ceram. Soc. 30 (2010) 1131–1137. [14] S. Radice, H. Dietsch, S. Mischler, J. Michler, Electrophoretic deposition of functionalized polystyrene particles for TiO2 multi-scale structured surfaces, Surf. Coat. Technol. 204 (2010) 1749–1754. [15] C.T. Kwok, P.K. Wong, F.T. Cheng, H.C. Man, Characterization and corrosion behavior of hydroxyapatite coatings on Ti6Al4V fabricated by electrophoretic deposition, Appl. Surf. Sci. 255 (2009) 6736–6744. [16] A. Hesnawi, H. Li, Z. Zhou, S. Gong, H. Xu, Isothermal oxidation behaviour of EB-PVD MCrAlY bond coat, Vacuum 81 (2007) 947–952. [17] E. de Beer, J. Duval, E.A. Meulenkamp, Electrophoretic deposition: a quantitative model for particle deposition and binder formation from alcohol-based suspensions, J. Colloid Interface Sci. 222 (2000) 117–124. [18] D.R. Brown, F.W. Salt, The mechanism of electrophoretic deposition, J. Appl. Chem. 15 (1965) 40–48. [19] U. Dragos, M. Gabriela, B. Waltraut, C. Ioan, Improvement of the oxidation behaviour of electron beam remelted MCrAlY coatings, Solid State Sci. 7 (2005) 459–464. [20] B.A. Pint, L.W. Hobbs, Limitations on the use of ion implantation for the study of the reactive element effect in beta-NiAl, J. Electrochem. Soc. 141 (1994) 2443–2453.