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`

`ELSEVIER
`
`Surface and Coatings Technology 133- 134 (2000) 1- 7
`
`SURIA&E
`&GI/Al/NliS
`IICHNOLDGY
`
`www.el sev ier. nl /1 ocate/surf coat
`
`Current status of environmental barrier coatings for Si-Based
`ceramics
`
`K.N. Lee*
`
`Chemical Engineering Department, Cleveland State University, Cleveland, OH, USA
`
`Abstract
`
`Silicon-based ceramics are the leading candidates for high temperature structural components in next generation gas turbine
`engines. One key drawback of silicon-based ceramics for such an application is volatilization of the protective silica scale in water
`vapor and the resulting rapid ceramic recession. Therefore, the realization of Si-based ceramics components in advanced gas
`turbine engines depends on the development of protection schemes from water vapor attack. Currently, plasma-sprayed external
`environmental barrier coatings (EBCs) are the most promising approach. In the late 1980s and early 1990s a wide range of
`refractory oxide materials were tested as coatings on Si-based ceramics to provide protection from hot corrosion. After the
`discovery of silica volatilization in water vapor in the early 1990s, the focus of EBC development research has been shifted
`towards the protection from water vapor attack. Experience learned form the earlier coating developmental effort provided the
`foundation upon which more complex and advanced EBC coatings have been developed. This paper will discuss the brief history
`and the current status of EBC development for Si-based ceramics with the main focus on water vapor protection. © 2000 Elsevier
`Science B.V. All rights reserved.
`
`Keywords: Environmental barrier coatings; Si-based ceramics; Mullite-based composite bond coat; BSAS
`
`1. Introduction
`
`One key barrier to the application of advanced sili(cid:173)
`con-based ceramics and composites for hot section
`structural components in gas turbines is their lack of
`environmental durability. Silicon-based ceramics ex(cid:173)
`hibit excellent oxidation resistance in clean, dry oxygen,
`by forming a slow-growing, dense silica scale [1]. How(cid:173)
`ever, the normally protective silica scale can be severely
`degraded by reacting with impurities, such as alkali
`salts [2] or water vapor [3]. Oxide coatings are a promis(cid:173)
`ing approach to providing environmental protection for
`advanced heat engine components because oxides are
`in general more resistant than silicon-based ceramics
`to corrosive environments [4].
`
`* Resident Researcher of NASA Glenn Research Centre MS106-l ,
`21000 Brook Park Road, NASA Glenn Research Center, Cleveland,
`OH 44135, USA. Tel.: + 1-216-433-5634; fax: + 1-216-433-5544.
`E-mail address: kang.n.lee@grc.nasa.gov (K.N. Lee).
`
`There are several key issues that must be considered
`in selecting coating materials [5,6]. Fig. 1 schematically
`shows the key issues. Firstly, the coating must possess
`the ability to resist reaction with aggressive environ(cid:173)
`ments, as well as low oxygen permeability to limit the
`transport of oxygen. Secondly, the coating must possess
`a coefficient of thermal expansion (CTE) close to that
`of the substrate material to prevent delamination or
`cracking due to CTE mis-match stress. Thirdly, the
`coating must maintain a stable phase under thermal
`exposure. Phase transformation typically accompanies
`a volumetric change, disrupting the integrity of the
`coating. Fourthly, the coating must be chemically com(cid:173)
`patible with the substrate to avoid detrimental chemi(cid:173)
`cal interaction.
`Early coatings work focused on protecting silicon(cid:173)
`based ceramics from molten salt corrosion. Mullite has
`attracted interest as a coating for Si-based ceramics
`mainly because of its close CTE match with SiC. Fig. 2
`shows the evolution of mullite and mullite/ refractory
`
`0257-8972/ 00/ $ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
`PU: SO 2 5 7 - 8 9 7 2 ( 0 0) 0 0 8 8 9 - 6
`
`3
`
`

`

`2
`
`KN. Lee/ Swface and Coatings Technology 133-134 (2000) 1- 7
`
`Environmental Durability
`
`,::::rrlri:' ~:~·~m~e;b;Ji<,
`
`Fig. 1. Key issues in selecting coating materials.
`
`oxide coating systems. Solar Turbines initiated the de(cid:173)
`velopment of plasma-sprayed refractory oxide coatings
`to protect SiC heat exchanger tubes from hot corrosion
`[7]. Single layer mullite coatings showed the best dura(cid:173)
`bility in thermal cycling and in hot corrosion tests.
`However, plasma-sprayed mullite coatings tended to
`develop cracks during thermal exposure. Corrosive
`species penetrated the cracks and attacked the SiC
`substrates. Oak Ridge National Laboratory (ORNL)
`developed an aqueous slurry process to apply refractory
`oxide coatings containing approximately 50-90 wt. %
`alumina [8]. Coatings containing mullite as the major
`phase performed the best, however, all coatings cracked
`
`and delaminated from the SiC during thermal expo(cid:173)
`sure.
`The NASA Glenn group discovered that plasma(cid:173)
`sprayed mullite contained a large amount of amor(cid:173)
`phous mullite due to the rapid cooling rate during
`solidification [9]. The crystallization of amorphous mul(cid:173)
`lite, accompanying a volumetric contraction, was identi(cid:173)
`fied as the primary source of cracking in plasma-sprayed
`mullite coatings (Fig. 3). Subsequently, a modified
`plasma spraying process was developed at NASA Glenn,
`which successfully eliminated most of the amorphous
`mullite
`from
`the coating. The second-generation
`plasma-sprayed mullite coating exhibited dramatically
`improved thermal shock resistance and durability, re(cid:173)
`maining adherent out to 1200 h at temperatures up to
`1300°C in air [6,10] and out to 150 h at 1000°C in a high
`pressure hot corrosion burner rig [11].
`By the mid 1990s, the volatilization of silica in water
`vapor and the resulting rapid recession of silicon-based
`ceramics emerged as a showstopper for the application
`of silicon-based ceramics in combustion environments
`[3], shifting the focus of coatings research to protection
`
`1985
`1990
`1995
`2000
`l~~~~l·~~~-1~~~~11
`
`• Solar Turbines 171
`(M. van Roode et al)
`- mullite
`- mu lli te/Al,0 3, Y20 3, YSZ
`- air plasma spray
`• ORNL (J. I. Federer) )8)
`- mullite (major phase),
`- mullite (maj or phase)
`/Al20 3 (major phase)
`- aqueous slurry
`
`• mullite showed the best
`performance
`• cracked under thennal
`exposure
`
`• NASA Glenn 19-17)
`(K. Lee et al)
`-mulli te
`- mullite/YSZ, cordi eri te
`- modified pl asma spray
`
`• Boston U. 1181
`(V. K. Sarin et al)
`- mu llite
`-CVD
`
`• ORNL )191
`(J . A. Haynes et a l)
`- mullite
`-CVD
`
`• enhanced crack resistance
`(crystalline mullite)
`• develop segmenta l cracks
`
`• excellent crack resistance
`( dense, crystalline)
`• low deposition rate
`
`• HSR-EPM
`(NASA-GE-PW)
`Current State-of-The Art
`- silicon / modified mullite / BSAS
`- modifi ed plasma spray
`
`Fig. 2. Evolution of mullite and mullite/refractory oxide coating systems.
`
`First generation mullite
`
`Second generation mullite
`
`Fig. 3. Comparison of first generation vs. second-generation plasma-sprayed mullite (48 h, 1000°C, 2 h cycle, air).
`
`i
`
`4
`
`

`

`K.N. Lee / Surface and Coatings Technology 133-134 (2000) 1- 7
`
`3
`
`from water vapor. Mullite was found to lose silica
`through volatilization, leaving a porous alumina layer
`on the surface in simulated combustion environments
`(8-µm-thick porous alumina after 50 h at 1230°C, 6
`atm., gas velocity= 2000 cm/ s) [12] (Fig. 4). Therefore,
`an environmental overlay coating was necessary when
`protection from water vapor was needed. NASA Glenn
`selected yttria-stabilized zirconia (YSZ; Zr0 2-8 wt. %
`Y20 3) as the baseline overlay coating because of its
`proven performance as a thermal barrier coating in
`combustion environments [11]. Despite the large CTE
`of YSZ, the mullite / YSZ-coated SiC showed excellent
`adherence under thermal cycling in air [6] and sup(cid:173)
`pressed silica volatilization in simulated combustion
`environments [13] (Fig. 5). The mullite/ YSZ system,
`however, developed water vapor-enhanced oxidation
`after approximately 100 h at 1300°C. The enhanced
`oxidation was initiated in areas where cracks in the
`mullite intersected the SiC interface, since water vapor
`transported through the cracks and attacked the sub(cid:173)
`strate [14] (Fig. 6). There was evidence of silica
`volatilization at the bottom of cracks (Fig. 6). Water
`vapor, the predominant oxidant in a H 20 / 0 2 environ(cid:173)
`ment, is known to enhance the oxidation of SiC. The
`silica scale formed in high water vapor is porous, al(cid:173)
`lowing the oxidation to propagate readily along the
`interface. The porous scale was attributed to the gener(cid:173)
`ation of gaseous silicon hydroxide species.
`
`2. Development of current state-of-the-art
`environmental barrier coatings (EBCs)
`
`Key durability issues in the mullite-based coating
`system in water vapor include: (i) through-thickness
`cracking in the mullite; (ii) weak bonding of mullite
`onto silicon-based ceramics; and (iii) interface contami(cid:173)
`nation [13]. Through-thickness-cracks provided a path
`
`Porous
`Alumina
`
`Mullite
`
`• •
`
`-1 10 µm
`
`Fig. 4. Mullite in high-pressure burner rig (50 h, 1230°C, 6 atm.,
`equivalence ratio = 1.9, vgas = 2000 cm/ s).
`
`for water vapor to oxidize the substrate, leading to the
`eventual failure of the system. YSZ overlay coating
`failed to seal the cracks in mullite since YSZ also
`cracked due to the large CTE mis-match between the
`two layers. It is believed that the development of
`through-thickness-cracks in mullite is due to stresses in
`the coating [13]. The presence of second phases, such
`as residual amorphous mullite and alumina, in the
`mullite coating and the resulting volumetric shrinkage
`and CTE mis-match are suggested to be the major
`sources for the stresses in the coating. Mullite does not
`form a strong chemical bond with SiC according to a
`diffusion couple study in our lab [15]. Thus, the mul(cid:173)
`lite / SiC bond of as-sprayed coatings is mainly due to
`mechanical interlocking. Interfacial contamination can
`degrade coating durability by altering the physical and
`
`Mullite/YSZ-Coated SiC
`
`Q,j
`
`---
`M e -0
`CJ
`......
`=ii e -1
`,_,
`=ii = -2
`ell -= u
`....
`-= =ii
`-~
`~
`
`-3
`
`-4
`
`-5
`
`0
`
`10
`
`20
`
`30
`
`40
`50
`60
`Time (h)
`
`70
`
`80
`
`90
`
`100
`
`Fig. 5. Mullite and mullite/ YSZ-coated SiC in high-pressure burner rig (1230°C, 6 atm., equivalence ratio = 1.9, vga s = 2000 cm/ s).
`
`5
`
`

`

`4
`
`KN. Lee / Surface and Coatings Technology I 33- 134 (2000) 1- 7
`
`Fig. 6. Mullite and mullite/ YSZ-coated SiC in cyclic water vapor furnace (200 h, 1300°C, 2 h cycle, 90% H 20 / 0 2 , 1 atm.).
`
`200h, l 300°C, 2h cycle
`
`chemical properties of the silica scale, especially growth
`rate, viscosity and porosity [16].
`The crack resistance of mullite coating can be im(cid:173)
`proved by modifying the physical properties of the
`coating or by sealing the cracks with a crack-resistant
`overlay coating [13). A new, mullite-based composite
`bond coat with substantially improved crack resistance
`and a crack-resistant, low silica activity top coat (BSAS:
`Ba0-Sr0-Al 20 3-Si0 2) were developed under the high
`speed research-enabling propulsion materials (HSR(cid:173)
`EPM) program. Today these coatings are referred to as
`EBCs. Replacing the YSZ top coat in the mullite/ YSZ
`system with BSAS delayed the onset of accelerated
`oxidation in water vapor by a factor of at least two (Fig.
`7). Further improvement in durability was achieved by
`replacing the mullite bond coat with the mullite-BSAS
`
`composite bond coat (Fig. 8), while still further im(cid:173)
`in
`the composite bond
`provement was achieved
`coat/ BSAS duplex coating system (EPM EBC) (Fig. 9).
`The improved water vapor durability with the compos(cid:173)
`ite bond coat and the BSAS top coat was attributed to
`the excellent crack resistance of these coatings.
`It has been shown that EBC durability can also be
`enhanced by improving the adherence through the
`modification of the mullite bond coat/SiC interface
`[17-19]. It was discovered in the HSR-EPM program
`that silicon is an excellent bond layer to improve the
`adherence of mullite onto SiC. EPM EEC-coated MI
`covered with a Si surface layer did not show acceler(cid:173)
`ated oxidation out to 500 hat 1300°C in 90% H 20 / 0 2 ,
`while a thick oxide scale developed on areas without a
`Si surface layer in the same exposure (Fig. 10). Fig. 11
`
`(two different areas of the same coupon)
`
`•
`
`., :>· .,.
`. ..
`..,:~
`., -:
`. .... . . ·.:
`' '-iL
`,
`-:..,. __ ;. :.,·~
`···".'· ..
`
`. ' . ..• . . - . . . . .. ~...
`
`,,4 ,
`
`··BS'AS ..
`. ·•
`'· . ., .
`.1.,,/
`.
`. .
`.
`.M ti
`"'
`,
`~ "Quite : . f •·· ·
`. ··
`' _Jo •. _.· -····-= . ···., · ... ~ . .,,,
`... -
`.-·· ...
`...
`... ...
`
`Fig. 7. Mullite/ BSAS-coated SiC in cyclic water vapor furnace (200 h, 1300°C, 2 h cycle, 90% H 20 / 0 2 , 1 atm.).
`
`,•
`
`~ .
`
`.~ .. ·.
`
`, .. '.: . . 1 ··· ,·00µm ·1
`. ,._•
`...
`
`6
`
`

`

`K.N. Lee / Surface and Coatings Technology 133-134 (2000) 1- 7
`
`5
`
`~ . I\
`
`.
`
`• '
`·" .
`- ~~
`- 1' Modlfl
`·
`• ¥.llllite :. • ·.
`
`Coating Side
`)
`
`(Modified Mullite)
`NSZ on RBSN CMC
`(50hr, /30(J'C, Air)
`
`Fig. 8. Modified mullite/ YSZ-coated SiC in cyclic water vapor fur(cid:173)
`nace (200 h, 1300°C, 2 h cycle, 90% H 20 / 0 2 , 1 atm.).
`
`shows the plot of weight change vs. time for uncoated
`and BPM BBC-coated MI exposed to HPBR at 1200°C.
`
`to silica
`loss due
`Uncoated MI showed weight
`volatilization, while BBC-coated MI showed fairly con(cid:173)
`stant weight, demonstrating its effectiveness in prevent(cid:173)
`ing the silica volatilization. Fig. 12 shows the cross-sec(cid:173)
`tion of BPM BBC-coated MI after the HPBR expo(cid:173)
`sure. Note the excellent durability regardless of the
`presence of Si surface layer in such a short exposure.
`
`3. Summary
`
`A new BBC system, silicon/ mullite-based compos(cid:173)
`ite / BSAS, was developed under the HSR-BPM pro(cid:173)
`gram. Mullite-based composite bond coat and BSAS
`top coat exhibited much improved crack resistance
`compared with mullite and YSZ, respectively, leading
`to dramatically enhanced durability in combustion en(cid:173)
`vironments. A silicon bond layer further improved the
`BBC durability by providing stronger bonding of the
`
`M~Jtified
`.Mullite
`
`. . .
`' •
`
`'· .
`
`. .
`
`. . .
`
`Fig. 9. Modified mullite/ BSAS-coated SiC in cyclic water vapor furnace (200 h, 1300°C, 2 h cycle, 90% H 20 / 0 2 , 1 atm.).
`
`~~)
`
`Modified 1
`Mullite
`""\ ,•· ·-"
`
`Fig. 10. Modified mullite/ BSAS-coated MI in cyclic water vapor furnace (500 h, 1300°C, 2 h cycle, 90% H 20 / 0 2 , 1 atm.).
`
`7
`
`

`

`20
`
`0)
`
`E
`
`Q)
`0)
`c: -20
`co
`.c
`U _40
`+.J
`S -60
`
`-80 ~--'----'----~ ............................. .....__,_,'-'-'---'----'----~---'----'---'----'----'....L...L-'-'-~....L..L~
`25
`0
`50
`75
`100 125 150 175 200
`Exposure Time (hrs)
`Fig. 11. Weight change of coated and uncoated MI in high-pressure
`burner rig (1200°C, 6 atm., equivalence ratio= 0.78, ugas = 2000
`cm/ s).
`
`coating. Durability out to 1000 h in 2-h thermal cycling
`at 1300°C in 90% H 20 / 0 2 and out to 200 h in a high
`pressure burner rig at 1200°C has been demonstrated.
`
`Acknowledgements
`
`I am grateful to G.W. Leissler of Dynacs/ NASA
`Glenn for the preparation of plasma spray coatings and
`R.C. Robinson of Dynacs/NASA Glenn for high pres(cid:173)
`sure burner rig tests. I also would like to acknowledge
`HSR-EPM EBC Team Members: J. Smialek, K. Lee, E.
`
`6
`
`K.N. Lee / Swface and Coatings Technology 133- 134 (2000) 1- 7
`
`Modified Mullite/BSAS
`
`Mullite/BSAS
`
`Opila, N. Jacobson and N. Bansal of NASA Glenn
`Research Center, H. Wang, P. Meschter and C. Luthra
`of General Electric Corporate Research & Develop(cid:173)
`ment Center, and H. Eaton and W. Allen of United
`Technology Research Center.
`
`References
`
`[1] N.S. Jacobson, Corrosion of silicon-based ceramics in combus(cid:173)
`tion environments, J. Am. Ceram. Soc. 76 (1) (1993) 3- 28.
`[2] N.S. Jacobson, J.L. Smialek, D.S. Fox, Molten salt corrosion of
`SiC and Si3N4, in: N.S. Cheremisinoff (Ed.), Handbook of
`Ceramics and Composites, 1, Marcel Dekker, New York, 1990,
`pp. 99-135.
`[3] E.J. Opila, R. Hann, Paralinear oxidation of CVD SiC in water
`vapor, J. Am. Ceram. Soc. 80 (1) (1997) 197- 205.
`[4] W.J. Lackey, D.P. Stinton, G.A. Cerny, AC. Schaffhauser, L.L.
`Fehrenbacher, Ceramic coatings for advanced heat engines -
`a review and projection, Adv. Ceram. Mater. 2 (1) (1987)
`24-30.
`[5] D.W. Graham, D.P. Stinton, Chemical vapor deposition of
`Ta 20 5 corrosion resistant coatings, in: J. Fairbanks (Ed.),
`Proceedings of the 1992 Coatings for Advanced Heat Engines
`Workshop, US Department of Energy, Washington, DC, 1992,
`pp. IV65- IV71.
`[6] K.N. Lee, R.A. Miller, Development and environmental dura(cid:173)
`bility of mullite and mullite/ YSZ dual layer coatings for SiC
`and Si 3 N4 ceramics, Surf. Coat. Technol. 86/ 87 (1996) 142- 148.
`J.R. Price, M. van Roode, C. Stala, Ceramic oxide-coated
`silicon carbide for high temperature corrosive environments,
`Key Eng. Maters. 72- 74 (1992) 71- 84.
`[8] J.I. Federer, Alumina base coatings for protection of SiC
`ceramics, J. Mater. Eng. 12 (1990) 141-149.
`[9] K.N. Lee, R.A. Miller, N.S. Jacobson, New generation of
`plasma-sprayed mullite coatings on silicon-carbide, J. Am.
`Ceram. Soc. 78 (3) (1995) 705-710.
`
`[7]
`
`'
`
`1
`
`•
`
`•
`-liiiil
`
`Fig. 12. Modified mullite/BSAS-coated MI in high-pressure burner rig (1200°C, 6 atm., equivalence ratio= 0.78, u8a, = 2000 cm/s).
`
`8
`
`

`

`KN. Lee/ Surface and Coatings Technology 133- 134 (2000) 1- 7
`
`7
`
`[10] K.N. Lee, R.A. Miller, Oxidation behavior of mullite-coated sic
`and SiC/ SiC composites under thermal cycling between room
`temperature and 1200- 1400°C, J. Am. Ceram. Soc. 79 (3)
`(1996) 620- 626.
`[11] K.N. Lee, N.S. Jacobson, R.A. Miller, Refractory oxide coatings
`on SiC ceramics, MRS Bull. 14 (10) (1994) 35- 38.
`[12] K.N. Lee, R.A. Miller, N.S. Jacobson, E.J. Opila, Environmen(cid:173)
`tal durability of mullite/ SiC and mullite/ YSZ coating/Sic
`Systems, in: J.B. Watchman (Ed.), Ceramic Engineering and
`Science Proceedings, The American Ceramic Society, Wester(cid:173)
`ville, OH, 1995, pp. 1037- 1044.
`[13] K.N. Lee, Key durability issues with mullite-based environmen(cid:173)
`tal barrier coatings for Si-based ceramics, International Gas
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`Indianapolis, IN, paper# T-443, (1999) 99- G.
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`9
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