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Electrophoretically deposited alumina as protective overlay for thermal barrier coatings against CMAS degradation
Surface & Coatings Technology 204 (2009) 797–801
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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
Electrophoretically deposited alumina as protective overlay for thermal barrier coatings against CMAS degradation P. Mohan ⁎, B. Yao, T. Patterson, Y.H. Sohn Advanced Materials Processing and Analysis Center (AMPAC), University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA
a r t i c l e
i n f o
Available online 2 October 2009 Keywords: Thermal barrier coating CMAS degradation Overlay barrier coating Electrophoretic deposition Environmental degradation
a b s t r a c t TBCs are increasingly susceptible to degradation by airborne CMAS deposits. In order to mitigate the CMAS attack, we fabricated a dense, crack-free alumina overlay for TBCs by electrophoretic deposition (EPD) technique. Overlay coatings of controlled thickness were successfully fabricated for YSZ TBCs, by EPD followed by controlled sintering at 1200 °C. YSZ with alumina overlay coatings were tested for CMAS attack at 1300 °C. Detailed examination of microstructural changes and phase evolution in CMAS tested specimens was performed by X-ray diffraction and electron microscopy. Dense alumina overlay produced by EPD was found to physically suppress the infiltration of CMAS. Furthermore, CMAS was found to crystallize into anorthite (CaAl2Si2O8) and MgAl2O4 spinel by chemically interacting with EPD α-Al2O3. A shift in CMAS glass composition to a crystallizable Al-rich glass composition promoted the formation of anorthite platelets (CaAl2Si2O8) and localized enrichment of Mg promoted the formation of MgAl2O4 spinel. EPD alumina overlay on commercial-production TBCs retained its adhesion and structural integrity after 20 cycles of 1hour furnace thermal cycle test at 1100 °C. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Thermal barrier coatings (TBCs) are widely employed to improve efficiency and performance of advanced gas turbine engines in aeropropulsion and power generation sectors. TBCs facilitate a quantum increase in the turbine entry temperature (TET) by providing thermal insulation to hot section components [1–3]. Extreme operating conditions entail TBCs to possess a complex multilayered structure, consisting of a ceramic topcoat (yttria stablized zirconia: YSZ — ZrO2 stabilized with 7 to 8 wt.% Y2O3) for thermal insulation, a thermally grown oxide (TGO) scale, predominantly alumina, a metallic bond coat that provides oxidation/corrosion resistance, and a superalloy substrate [2]. TBCs are increasingly susceptible to degradation by molten CMAS (calcium– magnesium alumino silicate) sand deposits, especially in aircraft engines that operate in a dust-laden environment, wherein ingestion of siliceous debris into engines has been commonly reported [4,5]. With an everincreasing demand for higher TET, degradation of TBCs by CMAS melt has received a greater attention in recent years. Airborne CMAS sand deposits were found to melt, adhere and infiltrate into TBCs above 1150 °C, and subsequently degrade the TBCs mechanically by repeated “freeze–thaw” cycle and, to a certain extent, direct chemical interaction with TBC constituents accelerated by the infiltration [4–7]. Earlier studies on ⁎ Corresponding author. Advanced Materials Processing and Analysis Center (AMPAC), University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA. Tel.: +1 321 948 9492. E-mail address: [email protected] (P. Mohan). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.09.055
thermomechanical damage by CMAS attack, explained that a sufficient stress to delaminate the coating could be introduced during infiltration of molten CMAS into porous ceramic topcoat followed by solidification upon cooling during thermal cycling [5]. In addition, thermochemical interactions of CMAS melt with TBC constituents such as dissolution of YSZ topcoat and the TGO have also been reported recently [6,7]. Promising approaches to mitigate CMAS attack of TBCs include (i) employing an impermeable surface coating for TBCs that can act as an inert barrier between CMAS deposit and TBC [8], (ii) utilizing a sacrificial overlay that can trap CMAS deposits through chemical interactions, which could potentially result in increased melting temperature and viscosity of CMAS melt [9] — a method examined in this study, (iii) surface sealing of YSZ topcoat [10] and (iv) modifying the YSZ topcoat chemistry [11–13]. TBCs with novel topcoat chemistry such as YSZ alloyed with Al2O3 and TiO2 as solid solution have been developed to mitigate CMAS degradation via crystallization of CMAS glass by increasing the Al content of CMAS [11]. Recently, TBCs with zirconate pyrochlores such as La2Zr2O7 [12] and Gd2Zr2O7 [13] topcoats received a great attention due to their promising resistance against CMAS attack. The objective of this study is to synthesize a dense crack-free overlay coating for TBCs, which can act as an impermeable barrier and/or a sacrificial layer against CMAS ingression. We attempted to fabricate dense alumina overlay coatings for TBCs by electrophoretic deposition (EPD) technique. EPD has been extensively developed as a technique to deposit wide range of materials, particularly ceramics, for various applications [14,15]. Among the various techniques available to fabricate overlay ceramic coatings of controlled thickness,
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P. Mohan et al. / Surface & Coatings Technology 204 (2009) 797–801
EPD is a versatile and “scale-up-ready” manufacturing technique that can produce crack-free overlay coatings of controlled thickness and porosity, uniform pore-distribution with improved microstructural homogeneity and superior adhesion. In EPD, charged particles in an appropriate colloidal suspension are typically drifted under the influence of a direct electric field towards an oppositely charged electrode, and deposit as a stable powder compact [14]. It should be noted that this process yields only a powder compact, and therefore EPD should be followed by a densification step. The temperature and time of the densification step depends on starting powder size and green density of the compact. Assessments regarding the protection against CMAS attack and thermal cyclic durability of EPD alumina overlay coatings on TBCs are documented.
focused ion beam (FIB) in-situ lift-out (INLO) using a FEI 200 TEM FIB (Hillsboro, OR, USA). With due consideration for real applications, durability of EPD alumina overlay on a 8YSZ/CoNiCrAlY/IN939 specimen was also examined through furnace thermal cyclic tests. TBC on IN939 superalloy substrate was composed of 300 µm thick APS YSZ topcoat (without surface grinding) and 200 µm thick APS CoNiCrAlY bond coat. A 30 µm thick alumina overlay was deposited on top of TBC by EPD for 3 min and densified at 1200 °C for 10 h. For furnace thermal cyclic test, each thermal cycle consisted of 10 min heat up to 1100 °C, 60 min hold at 1100 °C, followed by 10 min forced air quench. Adhesion and structural integrity of the EPD alumina overlay were examined after 20 thermal cycles by cross-sectional microstructural analysis.
2. Experimental details
3. Results
YSZ topcoat specimens of 300 µm thickness were prepared by air plasma spray (APS) technique, where 8YSZ powders with an average particle size of 106 µm (ZrO2—8 wt.% Y2O3, Praxair Inc, Danbury, CT, USA) were thermally sprayed on grit-blasted surfaces of graphite disks of 1 in. diameter. APS YSZ specimens were prepared with SulzerMetco F4MB plasma torch (Westbury, NY, USA) under a spraying current of 675 A with a primary gas Ar flow of 100 SCFH (standard cubic feet per hour), a secondary gas H2 flow of 20 SCFH and a carrier gas Ar flow of 8 SCFH. In order to fabricate a uniform alumina overlay using EPD, APS YSZ specimens were polished to a 600 grit finish and subsequently sputter-coated with a thin conductive Au–Pd layer (~10 nm) for applying a uniform electric field. For EPD process, a stable colloidal suspension of alumina powders (0.3 µm α-Al2O3 powder, Metlab Corporation, Niagara Falls, NY, USA) was prepared by dispersing alumina in an acetone–ethanol (Fisher Scientific, Pittsburgh, PA, USA) organic solvent mixture (volume ratio 1:1) at a solid loading concentration of 10 g/l with a dispersant additive of iodine at a concentration of 0.4 g/l. Au–Pd coated APS YSZ cathode and a graphite anode displaced by 1 cm in alumina colloidal suspension formed the EPD cell. During EPD, where 30 V was applied using a Protek 3032B DC power supply, positively charged alumina particles were drifted and deposited on the APS YSZ cathode for durations ranging up to 15 min. The EPD powder compact was dried in ambient atmosphere for 2 h and subjected to densification by sintering. Sintering was performed at 1200 °C for 10 h with a carefully controlled ramp up and ramp down rates of 2 °C/min. When this coating assembly was subjected to sintering, graphite substrate was burnt off yielding a freestanding modified-YSZ coating assembly consisting of 300 µm thick APS YSZ with a uniform crack-free dense EPD alumina overlay. YSZ with alumina overlay coating specimens in contact with a laboratory-synthesized CMAS deposit were isothermally oxidized in air at 1300 °C for 1 h. Based on the average composition of CMAS deposits [6], the laboratory-synthesized model CMAS used in this study had a chemical composition of 35CaO–10MgO–7Al2O3–48SiO2 (composition is in mole percent). This was prepared by mechanically milling the individual oxides of high purity in a SPEX shaker mill (SPEX CretPrep, Metuchen, NJ, USA) for 1 h at room temperature. The as-milled powders were subsequently spread at a concentration of 20 mg/cm2 for CMAS testing. X-ray diffraction studies were performed on CMAS tested specimens to examine the development of constituent phases using a Rigaku DMax B Diffractometer (Tokyo, Japan) with Cu-Kα radiation. Morphological and cross-sectional microstructural analyses were carried out to examine the CMAS interactions by using Hitachi S3500N scanning electron microscope (SEM) (Tokyo, Japan) equipped with X-ray energy dispersive spectrometer (XEDS). A Philips/FEI Tecnai F30 300KeV transmission electron microscope (TEM) (Hillsboro, OR, USA), equipped with high angle annular dark field (HAADF) and XEDS was also employed to examine the phase constituents of CMAS interacted specimen in detail. The sample preparation for TEM was performed by
3.1. Alumina overlay by electrophoretic deposition Alumina overlay coatings of thickness up to 100 µm were produced with deposition durations up to 15 min using the aforementioned EPD and densification process parameters. Cross-sectional backscattered electron micrograph shown in Fig. 1(a) reveals a continuous crack-free uniform alumina overlay. Cross-sectional secondary electron micrograph shown in Fig. 1(b) clearly illustrates the sintered microstructure of alumina, where the relative density of the sintered alumina overlay was estimated to be roughly 95% through image analysis. Typical surface morphology of alumina overlay is presented by the secondary electron micrograph in Fig. 1(c). Initial assessment of microstructure reveals excellent packing homogeneity associated with well-dispersed EPD suspension. 3.2. CMAS interactions with alumina overlay Chemical interactions of EPD alumina overlay with the CMAS melt, after exposure at 1300 °C for 1 h, are presented in Fig. 2. Surface morphology resulted from CMAS interaction with alumina as shown in Fig. 2(a) reveals crystallization of CMAS melt into platelets. Complete suppression of CMAS melt ingression is clearly demonstrated in the cross-sectional backscattered electron micrograph presented in Fig. 2(b). The phase constituents of the as-processed EPD alumina overlay and CMAS interacted alumina were examined by XRD as presented in Fig. 3, where α-Al2O3 as the as-processed overlay constituent and anorthite (CaAl2Si2O8) and MgAl2O4 spinel as constituent phases resulted from CMAS–alumina interaction were identified. Crystallization of CMAS melt via thermochemical interaction to anorthite and Mg-rich spinel was observed to completely suppress the CMAS ingression. Microstructural analysis by TEM as presented in Fig. 4, confirmed the compounds resulted from CMAS interaction with alumina overlay. The bright field TEM micrographs, Fig. 4(a) and (b) along with the respective selected area electron diffraction patterns, highlight the presence of anorthite CaAl2Si2O8 platelets and equiaxed MgAl2O4 spinel respectively. 3.3. Thermal cyclic test of TBC modified with alumina overlay Even though a dense alumina overlay produced by EPD that can be easily integrated into scale-up manufacturing, is promising in mitigating the CMAS attack, the thermal expansion mismatch between the EPD alumina and TBCs, and the possible deterioration of adhesion and structural integrity during engine run can dictate against applying such EPD overlay coatings. TBC (8YSZ/CoNiCrAlY/ IN939) modified with a 30 µm thick EPD alumina overlay without any smoothening of the APS YSZ topcoat (i.e., as plasma sprayed) revealed intact alumina overlay after 20 1-hour thermal cyclic test at 1100 °C as demonstrated by the cross-sectional backscattered electron micrograph shown in Fig. 5.
P. Mohan et al. / Surface & Coatings Technology 204 (2009) 797–801
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Fig. 2. Microstructural changes of EPD alumina overlay after interaction with CMAS deposit at 1300 °C for 1 h: (a) Secondary electron micrograph illustrating the evolution of platelet crystals from CMAS and (b) Cross-sectional backscattered electron micrograph demonstrating the complete suppression of CMAS melt ingression into the coating assembly.
Fig. 1. Microstructure of EPD alumina overlay on APS YSZ followed by sintering at 1200 °C for 10 h: (a) Cross-sectional backscattered electron micrograph showing the continuous crack-free alumina overlay on APS YSZ, (b) Cross-sectional secondary electron micrograph of the dense alumina overlay and (c) Secondary electron micrograph revealing the surface morphology of alumina overlay.
4. Discussion Acetone having a low viscosity (η = 0.3087 cP) can readily yield a stable suspension with sufficient solid loading concentration. However its low dielectric constant (20.7) limits the charge density on
Fig. 3. XRD patterns obtained from as-processed EPD alumina overlay and CMAS interacted EPD alumina. Primary phase constituents corresponding to the complete crystallization of CMAS due to the interaction are hexagonal anorthite (CaAl2Si2O8) and orthorhombic MgAl2O4 spinel.
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Fig. 5. Cross-sectional backscattered electron micrograph of a modified TBC (APS YSZ/ APS CoNiCrAlY) on IN939 superalloy substrate after 1-hour furnace thermal cyclic test at 1100 °C revealing the intact EPD alumina overlay after 20 cycles.
Fig. 4. Bright field TEM micrographs obtained from crystallized CMAS region resulted from thermochemical interaction with alumina highlighting (a) the presence of anorthite (CaAl2Si2O8) platelets and (b) MgAl2O4 spinel, as primary phase constituents along with the corresponding selected area electron diffraction patterns.
particles in a suspension, where addition of ethanol that has a higher dielectric constant (24.55) [14] helps achieve the desired stable suspension for EPD. Iodine as additive was used to generate a high proton density through chemical reactions. Thus, acetone–ethanol based suspension with I2 additive selected in this study, yielded a stable colloidal suspension of alumina for EPD processing. An applied voltage of 30 V/cm that was employed in this study was found to be sufficient to produce a thick and uniform deposit of Al2O3, given that the surface of the APS YSZ coatings, regardless of surface roughness, was coated with conductive Au–Pd film. After EPD, during drying, acetone having a high vapor pressure (24.6 kPa at 20 °C) tends to evaporate rapidly causing cracking due to capillary effect, whereas the presence of ethanol having lower vapor pressure (5.8 kPa at 20 °C), helps avoid cracking because of relatively slower drying. Alumina green compact with superior packing homogeneity and high packing density achieved by EPD could potentially lower the temperature and time required for sintering. Starting powder size can be also minimized to accelerate the sintering. In this study, a relative density of ~ 95% was achieved for the EPD alumina overlay after sintering at
1200 °C for 10 h. It is worth mentioning that use of smaller (e.g., nano) powders and innovative sintering techniques can help reduce this sintering temperature and time of sintering and avoid any degradation of TBCs and underlying metallic components during post-EPD processing. During CMAS exposure, due to the absence of interconnected pores/ cracks in dense alumina overlay, direct thermochemical interaction via dissolution of alumina by CMAS melt was found to dominate over the CMAS infiltration into the coating assembly. Crystallization of CMAS to anorthite (Tm: 1553 °C) and MgAl2O4 spinel (Tm: 2135 °C) during thermochemical interactions with alumina completely arrested the CMAS melt ingression into the coating assembly. Based on the available CaO–SiO2–Al2O3 ternary phase diagram, typical CMAS glass composition without considering the Mg content falls in pseudo-wollastonite field, whose glass composition is difficult to crystallize [11]. Enrichment of Al content in CMAS shifted this “difficult-to-crystallize” pseudo-wollastonite glass composition to a “crystallizable” Al-rich glass composition that falls in the anorthite field. Thus the dissolution of α-Al2O3 by CMAS resulted in crystallization of CMAS to anorthite platelets (CaAl2Si2O8). Concurrently, the localized enrichment of Mg content promoted the formation of MgAl2O4 spinel. Formation of these compounds with aforementioned high melting temperature could be beneficial in further suppression from both thermodynamic stability and kinetics aspects. Observation made from 1-hour furnace thermal cyclic test at 1100 °C for EPD alumina overlay on commercial-production TBC (APS YSZ/ CoNiCrAlY bond coat/IN939 superalloy) further demonstrated the promising capability of EPD alumina overlay. 5. Summary Dense crack-free alumina overlay coatings for TBCs were successfully fabricated by electrophoretic deposition followed by sintering at 1200 °C for 10 h. EPD alumina overlay was found to protect APS YSZ from CMAS attack. Attributed mechanisms include the suppression of CMAS melt infiltration and thermochemical interaction of α-Al2O3 overlay with CMAS melt that enriches the Al content in CMAS melt. A shift in CMAS glass composition to a crystallizable Al-rich glass composition promoted the formation of anorthite (CaAl2Si2O8) platelets and MgAl2O4 spinel, where these compounds have very high melting point. Thermal cyclic testing of TBCs on bond-coated superalloy substrate, modified with EPD alumina overlay of 30 µm thickness, further demonstrated the promising durability of EPD alumina overlay.
P. Mohan et al. / Surface & Coatings Technology 204 (2009) 797–801
References [1] R.A. Miller, J. Am. Ceram. Soc. 67 (8) (1984) 517. [2] N.P. Padture, M. Gell, E.H. Jordan, Science 296 (2002) 280. [3] A.G. Evans, D.R. Mumms, J.W. Hutchinson, G.H. Meier, F.S. Pettit, G.H. Meier, Prog. Mater. Sci. 46 (2001) 505. [4] M.P. Borom, C.A. Johnson, L.A. Peluso, Surf. Coat. Technol. 86–87 (1996) 116. [5] C. Mercer, S. Faulhaber, A.G. Evans, R. Darolia, Acta Mater. 53 (4) (2005) 1029. [6] S. Kramer, J. Yang, C.G. Levi, J. Am. Ceram. Soc. 89 (10) (2006) 3167. [7] P. Mohan, B. Yuan, T. Patterson, V.H. Desai, Y.H. Sohn, Mat. Sci. Forum 595–598 (2008) 207.
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[8] W.C. Hasz, M.P. Borom, and C.A. Johnson, U.S. Patent No. 5871820 (1999). [9] W.C. Hasz, C.A. Johnson and M.P. Borom, U.S. Patent No. 5660885 (1997). [10] C. Batista, A. Portinha, R.M. Ribeiro, V. Teixeira, C.R. Oliveira, Surf. Coat. Technol. 200 (24) (2006) 6783. [11] A. Aygun, A.L. Vasiliev, N.P. Padture, X. Ma, Acta Mater. 55 (2007) 6734. [12] R. Vassen, X. Cao, F. Tietz, D. Basu, D. Stover, J. Am. Ceram. Soc. 83 (8) (2000) 2023. [13] S. Kramer, J. Yang, C.G. Levi, J. Am. Ceram. Soc. 91 (2) (2008) 576. [14] L. Besra, M. Liu, Prog. Mater. Sci. 52 (2007) 1. [15] S.T. Aruna, K.S. Rajam, Mater. Chem. Phys. 111 (2008) 131.
Contents lists available at ScienceDirect
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
Electrophoretically deposited alumina as protective overlay for thermal barrier coatings against CMAS degradation P. Mohan ⁎, B. Yao, T. Patterson, Y.H. Sohn Advanced Materials Processing and Analysis Center (AMPAC), University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA
a r t i c l e
i n f o
Available online 2 October 2009 Keywords: Thermal barrier coating CMAS degradation Overlay barrier coating Electrophoretic deposition Environmental degradation
a b s t r a c t TBCs are increasingly susceptible to degradation by airborne CMAS deposits. In order to mitigate the CMAS attack, we fabricated a dense, crack-free alumina overlay for TBCs by electrophoretic deposition (EPD) technique. Overlay coatings of controlled thickness were successfully fabricated for YSZ TBCs, by EPD followed by controlled sintering at 1200 °C. YSZ with alumina overlay coatings were tested for CMAS attack at 1300 °C. Detailed examination of microstructural changes and phase evolution in CMAS tested specimens was performed by X-ray diffraction and electron microscopy. Dense alumina overlay produced by EPD was found to physically suppress the infiltration of CMAS. Furthermore, CMAS was found to crystallize into anorthite (CaAl2Si2O8) and MgAl2O4 spinel by chemically interacting with EPD α-Al2O3. A shift in CMAS glass composition to a crystallizable Al-rich glass composition promoted the formation of anorthite platelets (CaAl2Si2O8) and localized enrichment of Mg promoted the formation of MgAl2O4 spinel. EPD alumina overlay on commercial-production TBCs retained its adhesion and structural integrity after 20 cycles of 1hour furnace thermal cycle test at 1100 °C. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Thermal barrier coatings (TBCs) are widely employed to improve efficiency and performance of advanced gas turbine engines in aeropropulsion and power generation sectors. TBCs facilitate a quantum increase in the turbine entry temperature (TET) by providing thermal insulation to hot section components [1–3]. Extreme operating conditions entail TBCs to possess a complex multilayered structure, consisting of a ceramic topcoat (yttria stablized zirconia: YSZ — ZrO2 stabilized with 7 to 8 wt.% Y2O3) for thermal insulation, a thermally grown oxide (TGO) scale, predominantly alumina, a metallic bond coat that provides oxidation/corrosion resistance, and a superalloy substrate [2]. TBCs are increasingly susceptible to degradation by molten CMAS (calcium– magnesium alumino silicate) sand deposits, especially in aircraft engines that operate in a dust-laden environment, wherein ingestion of siliceous debris into engines has been commonly reported [4,5]. With an everincreasing demand for higher TET, degradation of TBCs by CMAS melt has received a greater attention in recent years. Airborne CMAS sand deposits were found to melt, adhere and infiltrate into TBCs above 1150 °C, and subsequently degrade the TBCs mechanically by repeated “freeze–thaw” cycle and, to a certain extent, direct chemical interaction with TBC constituents accelerated by the infiltration [4–7]. Earlier studies on ⁎ Corresponding author. Advanced Materials Processing and Analysis Center (AMPAC), University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA. Tel.: +1 321 948 9492. E-mail address: [email protected] (P. Mohan). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.09.055
thermomechanical damage by CMAS attack, explained that a sufficient stress to delaminate the coating could be introduced during infiltration of molten CMAS into porous ceramic topcoat followed by solidification upon cooling during thermal cycling [5]. In addition, thermochemical interactions of CMAS melt with TBC constituents such as dissolution of YSZ topcoat and the TGO have also been reported recently [6,7]. Promising approaches to mitigate CMAS attack of TBCs include (i) employing an impermeable surface coating for TBCs that can act as an inert barrier between CMAS deposit and TBC [8], (ii) utilizing a sacrificial overlay that can trap CMAS deposits through chemical interactions, which could potentially result in increased melting temperature and viscosity of CMAS melt [9] — a method examined in this study, (iii) surface sealing of YSZ topcoat [10] and (iv) modifying the YSZ topcoat chemistry [11–13]. TBCs with novel topcoat chemistry such as YSZ alloyed with Al2O3 and TiO2 as solid solution have been developed to mitigate CMAS degradation via crystallization of CMAS glass by increasing the Al content of CMAS [11]. Recently, TBCs with zirconate pyrochlores such as La2Zr2O7 [12] and Gd2Zr2O7 [13] topcoats received a great attention due to their promising resistance against CMAS attack. The objective of this study is to synthesize a dense crack-free overlay coating for TBCs, which can act as an impermeable barrier and/or a sacrificial layer against CMAS ingression. We attempted to fabricate dense alumina overlay coatings for TBCs by electrophoretic deposition (EPD) technique. EPD has been extensively developed as a technique to deposit wide range of materials, particularly ceramics, for various applications [14,15]. Among the various techniques available to fabricate overlay ceramic coatings of controlled thickness,
798
P. Mohan et al. / Surface & Coatings Technology 204 (2009) 797–801
EPD is a versatile and “scale-up-ready” manufacturing technique that can produce crack-free overlay coatings of controlled thickness and porosity, uniform pore-distribution with improved microstructural homogeneity and superior adhesion. In EPD, charged particles in an appropriate colloidal suspension are typically drifted under the influence of a direct electric field towards an oppositely charged electrode, and deposit as a stable powder compact [14]. It should be noted that this process yields only a powder compact, and therefore EPD should be followed by a densification step. The temperature and time of the densification step depends on starting powder size and green density of the compact. Assessments regarding the protection against CMAS attack and thermal cyclic durability of EPD alumina overlay coatings on TBCs are documented.
focused ion beam (FIB) in-situ lift-out (INLO) using a FEI 200 TEM FIB (Hillsboro, OR, USA). With due consideration for real applications, durability of EPD alumina overlay on a 8YSZ/CoNiCrAlY/IN939 specimen was also examined through furnace thermal cyclic tests. TBC on IN939 superalloy substrate was composed of 300 µm thick APS YSZ topcoat (without surface grinding) and 200 µm thick APS CoNiCrAlY bond coat. A 30 µm thick alumina overlay was deposited on top of TBC by EPD for 3 min and densified at 1200 °C for 10 h. For furnace thermal cyclic test, each thermal cycle consisted of 10 min heat up to 1100 °C, 60 min hold at 1100 °C, followed by 10 min forced air quench. Adhesion and structural integrity of the EPD alumina overlay were examined after 20 thermal cycles by cross-sectional microstructural analysis.
2. Experimental details
3. Results
YSZ topcoat specimens of 300 µm thickness were prepared by air plasma spray (APS) technique, where 8YSZ powders with an average particle size of 106 µm (ZrO2—8 wt.% Y2O3, Praxair Inc, Danbury, CT, USA) were thermally sprayed on grit-blasted surfaces of graphite disks of 1 in. diameter. APS YSZ specimens were prepared with SulzerMetco F4MB plasma torch (Westbury, NY, USA) under a spraying current of 675 A with a primary gas Ar flow of 100 SCFH (standard cubic feet per hour), a secondary gas H2 flow of 20 SCFH and a carrier gas Ar flow of 8 SCFH. In order to fabricate a uniform alumina overlay using EPD, APS YSZ specimens were polished to a 600 grit finish and subsequently sputter-coated with a thin conductive Au–Pd layer (~10 nm) for applying a uniform electric field. For EPD process, a stable colloidal suspension of alumina powders (0.3 µm α-Al2O3 powder, Metlab Corporation, Niagara Falls, NY, USA) was prepared by dispersing alumina in an acetone–ethanol (Fisher Scientific, Pittsburgh, PA, USA) organic solvent mixture (volume ratio 1:1) at a solid loading concentration of 10 g/l with a dispersant additive of iodine at a concentration of 0.4 g/l. Au–Pd coated APS YSZ cathode and a graphite anode displaced by 1 cm in alumina colloidal suspension formed the EPD cell. During EPD, where 30 V was applied using a Protek 3032B DC power supply, positively charged alumina particles were drifted and deposited on the APS YSZ cathode for durations ranging up to 15 min. The EPD powder compact was dried in ambient atmosphere for 2 h and subjected to densification by sintering. Sintering was performed at 1200 °C for 10 h with a carefully controlled ramp up and ramp down rates of 2 °C/min. When this coating assembly was subjected to sintering, graphite substrate was burnt off yielding a freestanding modified-YSZ coating assembly consisting of 300 µm thick APS YSZ with a uniform crack-free dense EPD alumina overlay. YSZ with alumina overlay coating specimens in contact with a laboratory-synthesized CMAS deposit were isothermally oxidized in air at 1300 °C for 1 h. Based on the average composition of CMAS deposits [6], the laboratory-synthesized model CMAS used in this study had a chemical composition of 35CaO–10MgO–7Al2O3–48SiO2 (composition is in mole percent). This was prepared by mechanically milling the individual oxides of high purity in a SPEX shaker mill (SPEX CretPrep, Metuchen, NJ, USA) for 1 h at room temperature. The as-milled powders were subsequently spread at a concentration of 20 mg/cm2 for CMAS testing. X-ray diffraction studies were performed on CMAS tested specimens to examine the development of constituent phases using a Rigaku DMax B Diffractometer (Tokyo, Japan) with Cu-Kα radiation. Morphological and cross-sectional microstructural analyses were carried out to examine the CMAS interactions by using Hitachi S3500N scanning electron microscope (SEM) (Tokyo, Japan) equipped with X-ray energy dispersive spectrometer (XEDS). A Philips/FEI Tecnai F30 300KeV transmission electron microscope (TEM) (Hillsboro, OR, USA), equipped with high angle annular dark field (HAADF) and XEDS was also employed to examine the phase constituents of CMAS interacted specimen in detail. The sample preparation for TEM was performed by
3.1. Alumina overlay by electrophoretic deposition Alumina overlay coatings of thickness up to 100 µm were produced with deposition durations up to 15 min using the aforementioned EPD and densification process parameters. Cross-sectional backscattered electron micrograph shown in Fig. 1(a) reveals a continuous crack-free uniform alumina overlay. Cross-sectional secondary electron micrograph shown in Fig. 1(b) clearly illustrates the sintered microstructure of alumina, where the relative density of the sintered alumina overlay was estimated to be roughly 95% through image analysis. Typical surface morphology of alumina overlay is presented by the secondary electron micrograph in Fig. 1(c). Initial assessment of microstructure reveals excellent packing homogeneity associated with well-dispersed EPD suspension. 3.2. CMAS interactions with alumina overlay Chemical interactions of EPD alumina overlay with the CMAS melt, after exposure at 1300 °C for 1 h, are presented in Fig. 2. Surface morphology resulted from CMAS interaction with alumina as shown in Fig. 2(a) reveals crystallization of CMAS melt into platelets. Complete suppression of CMAS melt ingression is clearly demonstrated in the cross-sectional backscattered electron micrograph presented in Fig. 2(b). The phase constituents of the as-processed EPD alumina overlay and CMAS interacted alumina were examined by XRD as presented in Fig. 3, where α-Al2O3 as the as-processed overlay constituent and anorthite (CaAl2Si2O8) and MgAl2O4 spinel as constituent phases resulted from CMAS–alumina interaction were identified. Crystallization of CMAS melt via thermochemical interaction to anorthite and Mg-rich spinel was observed to completely suppress the CMAS ingression. Microstructural analysis by TEM as presented in Fig. 4, confirmed the compounds resulted from CMAS interaction with alumina overlay. The bright field TEM micrographs, Fig. 4(a) and (b) along with the respective selected area electron diffraction patterns, highlight the presence of anorthite CaAl2Si2O8 platelets and equiaxed MgAl2O4 spinel respectively. 3.3. Thermal cyclic test of TBC modified with alumina overlay Even though a dense alumina overlay produced by EPD that can be easily integrated into scale-up manufacturing, is promising in mitigating the CMAS attack, the thermal expansion mismatch between the EPD alumina and TBCs, and the possible deterioration of adhesion and structural integrity during engine run can dictate against applying such EPD overlay coatings. TBC (8YSZ/CoNiCrAlY/ IN939) modified with a 30 µm thick EPD alumina overlay without any smoothening of the APS YSZ topcoat (i.e., as plasma sprayed) revealed intact alumina overlay after 20 1-hour thermal cyclic test at 1100 °C as demonstrated by the cross-sectional backscattered electron micrograph shown in Fig. 5.
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Fig. 2. Microstructural changes of EPD alumina overlay after interaction with CMAS deposit at 1300 °C for 1 h: (a) Secondary electron micrograph illustrating the evolution of platelet crystals from CMAS and (b) Cross-sectional backscattered electron micrograph demonstrating the complete suppression of CMAS melt ingression into the coating assembly.
Fig. 1. Microstructure of EPD alumina overlay on APS YSZ followed by sintering at 1200 °C for 10 h: (a) Cross-sectional backscattered electron micrograph showing the continuous crack-free alumina overlay on APS YSZ, (b) Cross-sectional secondary electron micrograph of the dense alumina overlay and (c) Secondary electron micrograph revealing the surface morphology of alumina overlay.
4. Discussion Acetone having a low viscosity (η = 0.3087 cP) can readily yield a stable suspension with sufficient solid loading concentration. However its low dielectric constant (20.7) limits the charge density on
Fig. 3. XRD patterns obtained from as-processed EPD alumina overlay and CMAS interacted EPD alumina. Primary phase constituents corresponding to the complete crystallization of CMAS due to the interaction are hexagonal anorthite (CaAl2Si2O8) and orthorhombic MgAl2O4 spinel.
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Fig. 5. Cross-sectional backscattered electron micrograph of a modified TBC (APS YSZ/ APS CoNiCrAlY) on IN939 superalloy substrate after 1-hour furnace thermal cyclic test at 1100 °C revealing the intact EPD alumina overlay after 20 cycles.
Fig. 4. Bright field TEM micrographs obtained from crystallized CMAS region resulted from thermochemical interaction with alumina highlighting (a) the presence of anorthite (CaAl2Si2O8) platelets and (b) MgAl2O4 spinel, as primary phase constituents along with the corresponding selected area electron diffraction patterns.
particles in a suspension, where addition of ethanol that has a higher dielectric constant (24.55) [14] helps achieve the desired stable suspension for EPD. Iodine as additive was used to generate a high proton density through chemical reactions. Thus, acetone–ethanol based suspension with I2 additive selected in this study, yielded a stable colloidal suspension of alumina for EPD processing. An applied voltage of 30 V/cm that was employed in this study was found to be sufficient to produce a thick and uniform deposit of Al2O3, given that the surface of the APS YSZ coatings, regardless of surface roughness, was coated with conductive Au–Pd film. After EPD, during drying, acetone having a high vapor pressure (24.6 kPa at 20 °C) tends to evaporate rapidly causing cracking due to capillary effect, whereas the presence of ethanol having lower vapor pressure (5.8 kPa at 20 °C), helps avoid cracking because of relatively slower drying. Alumina green compact with superior packing homogeneity and high packing density achieved by EPD could potentially lower the temperature and time required for sintering. Starting powder size can be also minimized to accelerate the sintering. In this study, a relative density of ~ 95% was achieved for the EPD alumina overlay after sintering at
1200 °C for 10 h. It is worth mentioning that use of smaller (e.g., nano) powders and innovative sintering techniques can help reduce this sintering temperature and time of sintering and avoid any degradation of TBCs and underlying metallic components during post-EPD processing. During CMAS exposure, due to the absence of interconnected pores/ cracks in dense alumina overlay, direct thermochemical interaction via dissolution of alumina by CMAS melt was found to dominate over the CMAS infiltration into the coating assembly. Crystallization of CMAS to anorthite (Tm: 1553 °C) and MgAl2O4 spinel (Tm: 2135 °C) during thermochemical interactions with alumina completely arrested the CMAS melt ingression into the coating assembly. Based on the available CaO–SiO2–Al2O3 ternary phase diagram, typical CMAS glass composition without considering the Mg content falls in pseudo-wollastonite field, whose glass composition is difficult to crystallize [11]. Enrichment of Al content in CMAS shifted this “difficult-to-crystallize” pseudo-wollastonite glass composition to a “crystallizable” Al-rich glass composition that falls in the anorthite field. Thus the dissolution of α-Al2O3 by CMAS resulted in crystallization of CMAS to anorthite platelets (CaAl2Si2O8). Concurrently, the localized enrichment of Mg content promoted the formation of MgAl2O4 spinel. Formation of these compounds with aforementioned high melting temperature could be beneficial in further suppression from both thermodynamic stability and kinetics aspects. Observation made from 1-hour furnace thermal cyclic test at 1100 °C for EPD alumina overlay on commercial-production TBC (APS YSZ/ CoNiCrAlY bond coat/IN939 superalloy) further demonstrated the promising capability of EPD alumina overlay. 5. Summary Dense crack-free alumina overlay coatings for TBCs were successfully fabricated by electrophoretic deposition followed by sintering at 1200 °C for 10 h. EPD alumina overlay was found to protect APS YSZ from CMAS attack. Attributed mechanisms include the suppression of CMAS melt infiltration and thermochemical interaction of α-Al2O3 overlay with CMAS melt that enriches the Al content in CMAS melt. A shift in CMAS glass composition to a crystallizable Al-rich glass composition promoted the formation of anorthite (CaAl2Si2O8) platelets and MgAl2O4 spinel, where these compounds have very high melting point. Thermal cyclic testing of TBCs on bond-coated superalloy substrate, modified with EPD alumina overlay of 30 µm thickness, further demonstrated the promising durability of EPD alumina overlay.
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