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Development of EBCs with Enhanced Durability and Temperature Capability
Environmental Barrier Coatings WorkshopNovember 18-19, 2003
Gaylord Opryland Resort & Convention Center, Nashville, TN
Kang LeeCleveland State University
NASA Glenn Research CenterCleveland, OH
This work is supported by: NASA Ultra Efficient Engine Technology (UEET) Program
2
Co-workersDennis Fox - water vapor thermogravimetry
Craig Robinson - high pressure burner rig test
Narottam Bansal - fabrication of hot pressed EBC
Jeff Eldridge - phase stability/stress measurement
Dongming Zhu/Robert Miller - thermal conductivity
Outline• Objective • Approach
• Results (SiC/SiC, Si3N4)
• Conclusions
3
Develop advanced EBCs:
>1000 hr life at 2700oF (1482oC) EBC temp and 2400oF (1316oC) CMC temp
EBC
SiC/SiC
2700oF
2400oF
T=300oF
Objective
4
Silicon
Mullite** orMullite+BSAS
BSAS*
SiC
• Developed in the NASA HSR-EPM Program in joint research by NASA-GE-PW (2001 R&D 100 Award)• Successful 15,000h engine test in Solar Centaur 50s SiC/SiC combustor liners under the DOE-CSGT Program (scaled up by Pratt & Whitney)
Current EBCs
* xBaO(1-x)SrOAl2O32SiO2
** 3Al2O32SiO2
Stability in H2O
Chem. Compatibility
Adherence
Crack Resistance
5
Projected BSAS Recession(1000 hr, 6 atm, vgas = 24 m/sec)
Volatility
2/1
22
2/1
)(
)(
TOTALP
OHPv gas velocityP(H2O) : water vapor pressurePTOTAL : total pressure
Silica Volatility Model (Smialek et al)
Assumptions• Observed weight loss is due to silica loss only • Si(OH)4 is the major volatile species
1300oC : 28 m1400oC : 67 m1500oC : 268 m
Current EBC Issues - Recession
6
Approach to Develop New EBC Systems
• Water vapor TGA test - Water vapor stability of top coat @ 1500oC • High steam thermal cycling test - Chemical & Environmental durability @ 1300 - 1400oC
NewTop coat
MI SiC/SiC
Mullite** orMullite+BSAS
Silicon
Temp goals
• Identify new top coats - water vapor stability @ T>1482oC - chemical compatibility @ T>1400oC - mechanical compatibility
1482oC(2700oF)
1316oC(2400oF)
1400oC(2552oF)
7
• HSTC Rig - simulate fuel lean environment (pH2O = 0.9, Vgas = 2.2 cm/sec) - environmental durability - chemical stability
Oxygen Water
Exhaust
Sample
Pt Wire
Quartz Wool
Fu
rnac
eLift
Alumina Tube
Cold Cycle
Hot Cycle
High Steam Thermal Cycling Rig(HSTC)
• Water vapor TGA - volatility
8
Rare-Earth Silicate EBC Identified
1500C 50%H2O kinetics
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0 20 40 60 80 100
Time (h)
Spe
cific
wei
ght
(mg/
cm2)
Y2SiO5
Sc2SiO5
Yb2SiO5
Er2SiO5
BSAS
US PATENT PENDING
YScYbEr
9
Rare Earth Silicate CTE
Material CTE (10-6/oC)
Y2SiO5 5 ~ 6*
Er2SiO5 5 ~ 7
Yb2SiO5 3.5 ~ 4.5
Sc2SiO5 5 ~ 6
* Ogura et al.** Touloukian et al, “Thermophysical Properties of Matter”
Material CTE (10-6/oC)
SiC** 4.5 ~5.5
Si3N4** 3 ~ 4
Si** 3.5 ~ 4.5
BSAS (celsian)
Mullite
4 ~ 5
5 ~ 6
10
Glass
1400oC- 46 hr, 1h cycles, HSTC
Mullite/Y2SiO5
• Coating turned into Y-Al-Silicate bubbles
11
Y2O3-Al2O3-SiO2 Phase Diagram
(mullite)
(silica)
(Y2SiO5)
(Y2Si2O7)
12
1380oC- 300 hr, 1h cycles, HSTC
Si/Mullite+SAS/Sc Silicate
Sc Silicate
Mullite + SAS
Si
MI
• Through-thickness cracks in ceramic layers - CTE mismatch, phase instability?, etc. - relax stress
A B
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Mullite + SAS
Si
Sc Silicate
Mullite + SAS
Si
1380oC- 300 hr, 1h cycles, HSTC
Si/Mullite+SAS/Sc Silicate
Minimal oxidation Enhanced oxidation at the crack tip - glass formation
A: B:
14
1380oC- 300 hr, 1h cycles, HSTC
Si/Mullite/Sc Silicate
Sc Silicate
Mullite
Si
MI
• Through-thickness cracks in ceramic layers - more prone to cracking with mullite intermediate layer
A B
15
1380oC- 300 hr, 1h cycles
Si/Mullite/Sc Silicate
Mullite
Si
Mullite
Si
Minimal oxidation Enhanced oxidation at the crack tip - no glass formation
A: B:
16
CTE of Si, SiC and Si3N4
50 300 550 800 1050 1300 1550
Temperature (K)
-1
1
3
5
Co
effi
cien
t o
f T
her
mal
Exp
ansi
on
(10
-6/C
)SiC
Si3N4
Si
Touloukian et al, “Thermophysical Properties of Matter”
• Lower CTE of Si3N4 causes bigger CTE mismatch
17
1380oC- 200 hr, 1h cycles, HSTC
Si/Mullite+SAS/Sc Silicate
Sc Silicate
Mullite+SAS
Si
AS 800
• Through-thickness cracks in all three layers
(Si bond coat in tension)
A
18
1380oC- 200 hr, 1h cycles, HSTC
Si/Mullite+SAS/Sc Silicate
Si
AS 800
La2O depleted zone
Cracks provide path for water vapor whichcan rapidly oxidize Si bond coat
A:
19
1380oC- 200 hr, 1h cycles, HSTC
Si/Mullite+SAS/Sc Silicate
• some cracks branch laterally - can cause coating delamination
Sc Silicate
Mullite+SAS
Si
AS 800
20
• Rare earth silicates (Sc, Yb, Lu) are promising as 2700oF EBC top coat Low CTE (4 ~ 7 x10-6/oC) Superior water vapor stability compared to BSAS Superior chemical compatibility compared to BSAS Low thermal conductivity (equal to or better than YSZ)
• Issues with rare earth silicates Through-thickness cracks - What is the effect on long-term durability? Economy (high cost of Sc, Lu)
Conclusions
21
Conclusions (cont)• Si3N4 EBC issues
Cracks into Si bond coat can rapidly oxidize Si bond coat
- should not be as severe in infrequent cycling (industrial turbines)
Cracks branching laterally can cause delamiantion Adherence on as processed surface?
• Selection of EBC system depends on operating conditions EBC surface temperature (BSAS vs. RE silicates) Substrate temperature (Mullite+BSAS vs. Mullite) Life goal Thermal cycling frequency
- Aero or industrial gas turbines?
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