Development of EBCs with Enhanced Durability and Temperature Capability

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Development of EBCs with Enhanced Durability and
Temperature Capability
Kang Lee Cleveland State University NASA Glenn
Research Center Cleveland, OH
Environmental Barrier Coatings Workshop November
18-19, 2003 Gaylord Opryland Resort Convention
Center, Nashville, TN
This work is supported by NASA Ultra Efficient
Engine Technology (UEET) Program
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Co-workers
Dennis 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

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Objective
Develop advanced EBCs gt1000 hr life at 2700oF
(1482oC) EBC temp and 2400oF (1316oC) CMC temp
EBC
2700oF
SiC/SiC
DT300oF
2400oF
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Current EBCs
BSAS
Stability in H2O
Crack Resistance
Mullite or MulliteBSAS
Chem. Compatibility
Silicon
Adherence
SiC
xBaO(1-x)SrOAl2O32SiO2 3Al2O32SiO2

• Developed in the NASA HSR-EPM Program in joint
  research by NASA-GE-PW (2001 RD 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)

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Current EBC Issues - Recession
Projected BSAS Recession (1000 hr, 6 atm, vgas
24 m/sec)
1300oC 28 mm 1400oC 67 mm 1500oC 268 mm
Silica Volatility Model (Smialek et al)

• gas velocity
• P(H2O) water vapor pressure
• PTOTAL total pressure

  Volatility ?

• Assumptions
• Observed weight loss is due to silica loss only
• Si(OH)4 is the major volatile species

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Approach to Develop New EBC Systems
Temp goals
1482oC (2700oF)
New Top coat

• Identify new top coats
• - water vapor stability _at_ Tgt1482oC
• - chemical compatibility _at_ Tgt1400oC
• - mechanical compatibility

1400oC (2552oF)
Mullite or MulliteBSAS
Silicon
1316oC (2400oF)
MI SiC/SiC

• Water vapor TGA test
• - Water vapor stability of top coat _at_ 1500oC
• High steam thermal cycling test
• - Chemical Environmental durability _at_ 1300 -
  1400oC

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High Steam Thermal Cycling Rig (HSTC)

• HSTC Rig
• - simulate fuel lean environment
• (pH2O 0.9, Vgas 2.2 cm/sec)
• - environmental durability
• - chemical stability

Oxygen
Water
Lift
Quartz Wool
Cold Cycle
Furnace
Pt Wire
Hot Cycle
Sample
Alumina Tube

• Water vapor TGA
• - volatility

Exhaust
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Rare-Earth Silicate EBC Identified
Y Sc Yb Er
US PATENT PENDING
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Rare Earth Silicate CTE
Ogura et al. Touloukian et al,
Thermophysical Properties of Matter
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Mullite/Y2SiO5
1400oC- 46 hr, 1h cycles, HSTC
Glass

• Coating turned into Y-Al-Silicate bubbles

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Y2O3-Al2O3-SiO2 Phase Diagram
(silica)
(Y2Si2O7)
(mullite)
(Y2SiO5)
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Si/MulliteSAS/Sc Silicate
1380oC- 300 hr, 1h cycles, HSTC
Sc Silicate
Mullite SAS
A
B
Si
MI

• Through-thickness cracks in ceramic layers
• - CTE mismatch, phase instability?, etc.
• - relax stress

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Si/MulliteSAS/Sc Silicate
1380oC- 300 hr, 1h cycles, HSTC
Sc Silicate
Mullite SAS
Mullite SAS
Si
Si
Minimal oxidation
Enhanced oxidation at the crack tip -
glass formation
A
B
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Si/Mullite/Sc Silicate
1380oC- 300 hr, 1h cycles, HSTC
Sc Silicate
Mullite
Si
A
B
MI

• Through-thickness cracks in ceramic layers
• - more prone to cracking with mullite
  intermediate layer

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Si/Mullite/Sc Silicate
1380oC- 300 hr, 1h cycles
Mullite
Mullite
Si
Si
Minimal oxidation
Enhanced oxidation at the crack tip - no
glass formation
A
B
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CTE of Si, SiC and Si3N4
SiC
Si
Si3N4

• Lower CTE of Si3N4 causes bigger CTE mismatch

Touloukian et al, Thermophysical Properties of
Matter
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Si/MulliteSAS/Sc Silicate
1380oC- 200 hr, 1h cycles, HSTC
Sc Silicate
MulliteSAS
Si
A
AS 800

• Through-thickness cracks in all three layers

(Si bond coat in tension)
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Si/MulliteSAS/Sc Silicate
1380oC- 200 hr, 1h cycles, HSTC
Si
La2O depleted zone
AS 800
Cracks provide path for water vapor which can
rapidly oxidize Si bond coat
A
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Si/MulliteSAS/Sc Silicate
1380oC- 200 hr, 1h cycles, HSTC
Sc Silicate
MulliteSAS
Si
AS 800

• some cracks branch laterally
• - can cause coating delamination

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Conclusions

• 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)

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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 (MulliteBSAS vs. Mullite)
• Life goal
• Thermal cycling frequency
• - Aero or industrial gas turbines?