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`Surface and Coatings Technology 133]134 2000 1]7
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`Current status of environmental barrier coatings for Si-Based
`ceramics
`
`K.N. LeeU
`Chemical Engineering Department, Cle¤eland State Uni¤ersity, Cle¤eland, 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
`(cid:14)
`.
`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. Q 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-
`con-based ceramics and composites for hot section
`structural components in gas turbines is their lack of
`environmental durability. Silicon-based ceramics ex-
`hibit excellent oxidation resistance in clean, dry oxygen,
`w x
`by forming a slow-growing, dense silica scale 1 . How-
`ever, the normally protective silica scale can be severely
`degraded by reacting with impurities, such as alkali
`w x
`w x
`salts 2 or water vapor 3 . Oxide coatings are a promis-
`ing approach to providing environmental protection for
`advanced heat engine components because oxides are
`in general more resistant than silicon-based ceramics
`w x
`to corrosive environments 4 .
`
`U Resident Researcher of NASA Glenn Research Centre MS106-1,
`21000 Brook Park Road, NASA Glenn Research Center, Cleveland,
`OH 44135, USA. Tel.: q1-216-433-5634; fax: q1-216-433-5544.
`(cid:14)
`.
`E-mail address: kang.n.lee@grc.nasa.gov K.N. Lee .
`
`There are several key issues that must be considered
`w
`x
`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-
`ments, as well as low oxygen permeability to limit the
`transport of oxygen. Secondly, the coating must possess
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`.
`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-
`patible with the substrate to avoid detrimental chemi-
`cal interaction.
`Early coatings work focused on protecting silicon-
`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 mulliterrefractory
`
`0257-8972r00r$ - see front matter Q 2000 Elsevier Science B.V. All rights reserved.
`(cid:14)
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`PII: S 0 2 5 7 - 8 9 7 2 0 0 0 0 8 8 9 - 6
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`K.N. Lee r Surface and Coatings Technology 133]134 2000 1]7
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`Fig. 1. Key issues in selecting coating materials.
`
`oxide coating systems. Solar Turbines initiated the de-
`velopment of plasma-sprayed refractory oxide coatings
`to protect SiC heat exchanger tubes from hot corrosion
`w x7 . Single layer mullite coatings showed the best dura-
`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
`(cid:14)
`.
`substrates. Oak Ridge National Laboratory ORNL
`developed an aqueous slurry process to apply refractory
`oxide coatings containing approximately 50]90 wt.%
`w x
`alumina 8 . Coatings containing mullite as the major
`phase performed the best, however, all coatings cracked
`
`and delaminated from the SiC during thermal expo-
`sure.
`The NASA Glenn group discovered that plasma-
`sprayed mullite contained a large amount of amor-
`phous mullite due to the rapid cooling rate during
`w x
`solidification 9 . The crystallization of amorphous mul-
`lite, accompanying a volumetric contraction, was identi-
`fied as the primary source of cracking in plasma-sprayed
`(cid:14)
`.
`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-
`maining adherent out to 1200 h at temperatures up to
`w
`x
`13008C in air 6,10 and out to 150 h at 10008C in a high
`w
`x
`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
`w x3 , shifting the focus of coatings research to protection
`
`Fig. 2. Evolution of mullite and mulliterrefractory oxide coating systems.
`
`.
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`Fig. 3. Comparison of first generation vs. second-generation plasma-sprayed mullite 48 h, 10008C, 2 h cycle, air .
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`K.N. Lee r Surface and Coatings Technology 133]134 2000 1]7
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`from water vapor. Mullite was found to lose silica
`through volatilization, leaving a porous alumina layer
`on the surface in simulated combustion environments
`(cid:14)8-mm-thick porous alumina after 50 h at 12308C, 6
`. w
`x (cid:14)
`atm., gas velocitys2000 cmrs 12 Fig. 4 . Therefore,
`.
`an environmental overlay coating was necessary when
`protection from water vapor was needed. NASA Glenn
`(cid:14)
`selected yttria-stabilized zirconia YSZ; ZrO ]8 wt.%
`2
`.
`Y O as the baseline overlay coating because of its
`2
`3
`proven performance as a thermal barrier coating in
`w
`x
`combustion environments 11 . Despite the large CTE
`of YSZ, the mulliterYSZ-coated SiC showed excellent
`w x
`adherence under thermal cycling in air 6 and sup-
`pressed silica volatilization in simulated combustion
`w
`x (cid:14)
`environments 13 Fig. 5 . The mulliterYSZ system,
`.
`however, developed water vapor-enhanced oxidation
`after approximately 100 h at 13008C. 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-
`w
`x
`(cid:14)
`.
`strate
`14
`Fig. 6 . There was evidence of silica
`(cid:14)
`.
`volatilization at the bottom of cracks Fig. 6 . Water
`vapor, the predominant oxidant in a H OrO environ-
`2
`2
`ment, is known to enhance the oxidation of SiC. The
`silica scale formed in high water vapor is porous, al-
`lowing the oxidation to propagate readily along the
`interface. The porous scale was attributed to the gener-
`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
`(cid:14) .
`system in water vapor include:
`i
`through-thickness
`(cid:14) .
`ii weak bonding of mullite
`cracking in the mullite;
`(cid:14)
`.
`onto silicon-based ceramics; and iii
`interface contami-
`w
`x
`nation 13 . Through-thickness-cracks provided a path
`
`(cid:14)
`Fig. 4. Mullite in high-pressure burner rig 50 h, 12308C, 6 atm.,
`equivalence ratios1.9, ¤ s 2000 cmrs .
`.
`gas
`
`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
`w
`x
`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
`w
`x
`diffusion couple study in our lab 15 . Thus, the mul-
`literSiC bond of as-sprayed coatings is mainly due to
`mechanical interlocking. Interfacial contamination can
`degrade coating durability by altering the physical and
`
`Fig. 5. Mullite and mulliterYSZ-coated SiC in high-pressure burner rig 12308C, 6 atm., equivalence ratios1.9, ¤ s 2000 cmrs .
`.
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`gas
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`K.N. Lee r Surface and Coatings Technology 133]134 2000 1]7
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`Fig. 6. Mullite and mulliterYSZ-coated SiC in cyclic water vapor furnace 200 h, 13008C, 2 h cycle, 90% H OrO , 1 atm. .
`.
`(cid:14)
`2
`2
`
`chemical properties of the silica scale, especially growth
`w
`x
`rate, viscosity and porosity 16 .
`The crack resistance of mullite coating can be im-
`proved by modifying the physical properties of the
`coating or by sealing the cracks with a crack-resistant
`w
`x
`overlay coating 13 . A new, mullite-based composite
`bond coat with substantially improved crack resistance
`(cid:14)
`and a crack-resistant, low silica activity top coat BSAS:
`.
`BaO-SrO-Al O -SiO were developed under the high
`2
`3
`2
`(cid:14)
`speed research-enabling propulsion materials HSR-
`.
`EPM program. Today these coatings are referred to as
`EBCs. Replacing the YSZ top coat in the mulliterYSZ
`system with BSAS delayed the onset of accelerated
`(cid:14)
`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
`
`.
`(cid:14)
`composite bond coat Fig. 8 , while still further im-
`provement was achieved in the composite bond
`coatrBSAS duplex coating system EPM EBC Fig. 9 .
`(cid:14)
`. (cid:14)
`.
`The improved water vapor durability with the compos-
`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 coatrSiC interface
`w
`x
`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 EBC-coated MI
`covered with a Si surface layer did not show acceler-
`ated oxidation out to 500 h at 13008C in 90% H OrO ,
`2
`2
`while a thick oxide scale developed on areas without a
`(cid:14)
`.
`Si surface layer in the same exposure Fig. 10 . Fig. 11
`
`Fig. 7. MulliterBSAS-coated SiC in cyclic water vapor furnace 200 h, 13008C, 2 h cycle, 90% H OrO , 1 atm. .
`.
`(cid:14)
`2
`2
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`K.N. Lee r Surface and Coatings Technology 133]134 2000 1]7
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`loss due to silica
`showed weight
`Uncoated MI
`volatilization, while EBC-coated MI showed fairly con-
`stant weight, demonstrating its effectiveness in prevent-
`ing the silica volatilization. Fig. 12 shows the cross-sec-
`tion of EPM EBC-coated MI after the HPBR expo-
`sure. Note the excellent durability regardless of the
`presence of Si surface layer in such a short exposure.
`
`3. Summary
`
`A new EBC system, siliconrmullite-based compos-
`iterBSAS, was developed under the HSR-EPM pro-
`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-
`vironments. A silicon bond layer further improved the
`EBC durability by providing stronger bonding of the
`
`Fig. 8. Modified mulliterYSZ-coated SiC in cyclic water vapor fur-
`nace 200 h, 13008C, 2 h cycle, 90% H OrO , 1 atm. .
`(cid:14)
`.
`2
`2
`
`shows the plot of weight change vs. time for uncoated
`and EPM EBC-coated MI exposed to HPBR at 12008C.
`
`Fig. 9. Modified mulliterBSAS-coated SiC in cyclic water vapor furnace 200 h, 13008C, 2 h cycle, 90% H OrO , 1 atm. .
`.
`(cid:14)
`2
`2
`
`Fig. 10. Modified mulliterBSAS-coated MI in cyclic water vapor furnace 500 h, 13008C, 2 h cycle, 90% H OrO , 1 atm. .
`.
`(cid:14)
`2
`2
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`K.N. Lee r Surface and Coatings Technology 133]134 2000 1]7
`)
`(
`
`Opila, N. Jacobson and N. Bansal of NASA Glenn
`Research Center, H. Wang, P. Meschter and C. Luthra
`of General Electric Corporate Research & Develop-
`ment Center, and H. Eaton and W. Allen of United
`Technology Research Center.
`
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`
`w x7
`
`Fig. 11. Weight change of coated and uncoated MI in high-pressure
`burner rig 12008C, 6 atm., equivalence ratios0.78, ¤ s 2000
`(cid:14)
`gas
`cmrs .
`.
`
`coating. Durability out to 1000 h in 2-h thermal cycling
`at 13008C in 90% H OrO and out to 200 h in a high
`2
`2
`pressure burner rig at 12008C has been demonstrated.
`
`Acknowledgements
`
`I am grateful to G.W. Leissler of DynacsrNASA
`Glenn for the preparation of plasma spray coatings and
`R.C. Robinson of DynacsrNASA Glenn for high pres-
`sure burner rig tests. I also would like to acknowledge
`HSR-EPM EBC Team Members: J. Smialek, K. Lee, E.
`
`Fig. 12. Modified mulliterBSAS-coated MI in high-pressure burner rig 12008C, 6 atm., equivalence ratios0.78, ¤ s 2000 cmrs .
`.
`(cid:14)
`gas
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`w x19
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