Durability and CMAS Resistance of Advanced Environmental ......1500 C – Ytterbium silicate EBC –...

National Aeronautics and Space Administration

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Durability and CMAS Resistance of Advanced Environmental Barrier Coatings Systems for SiC/SiC Ceramic Matrix Composites

Dongming Zhu

Materials and Structures Division NASA John H. Glenn Research Center

Cleveland, Ohio 44135

Daytona Beach Conference, Florida January 25-30, 2015


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Acknowledgements The work was supported by NASA Fundamental Aeronautics Program. NASA - Air Force Collaborative

Program Venture 219 Project. Ron Phillips and Ralph Pawlik for assistance in mechanical testing Don Humphrey and Mike Cuy for their assistance in the TGA and furnace cyclic testing, respectively Ram Bhatt in the collaborative work for EBC-CMC integrations Louis Ghosn: FEM modeling Sue Puleo: Testing support NASA USRP Students Dan Miladinovich and Nadia Ahlborg, supported NASA CMAS Projects NASA Intern Students Matt Appleby, Brad Richards Terry McCue: SEM analysis Francisco Sola-Lopez: TEM Analysis ARFL: Oliver Easterday and Lynne M Pfledderer for helpful discussions and funding support for part of the

research work - The author is also grateful for helpful discussions: James Dicarlo, Janet Hurst, Bob Miller, James L.

Smialek, Bryan Harder, Narottam Basal, Valerie Weiner

Other Collaborators:

Sulzer Metco (US) - Mitch Dorfman; Chis Dambra Directed Vapor Technologies, International – Derek Hass and Balvinder Gogia Southwest Research Institute – Ronghua Wei General Electric Aviation, Rolls Royce, Pratt & Whitney, and Honeywell Engines Penn-State - Professor Doug Wolfe; UES – Dr. A. K. Rai


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Environmental Barrier Coating - CMAS Interaction Research Efforts

• Advanced EBC development – composition design and developments for improved CMAS resistance; thermomechanical-CMAS Interactions and durability – Zhu et al

• NASA-Air Force Venture and Viper Turbine Coating-CMAS Collaborative programs - Zhu, James Smialek, Robert A. Miller, Bryan Harder

• Formal NASA Intern Undergraduate Students – Nadia Ahlborg and Dan Miladinovich • Fundamental NASA in-house CMAS properties - Narottam Bansal and Valerie Weiner


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Outline

• Environmental barrier coating (EBC) development: the CMAS relevance

• Some generalized CMAS related failures

• CMAS degradation of environmental barrier coating (EBC) systems: rare

earth silicates – Ytterbium silicate and yttrium silicate EBCs – Some reactions, kinetics and mechanisms

• Advanced EBCs, HfO2- and Rare Earth - Silicon based 2700°F+ capable

bond coats • Summary


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NASA Environmental Barrier Coatings (EBCs) and Ceramic Matrix Composite (CMC) System Development

− Emphasize material temperature capability, performance and long-term durability- Highly loaded EBC-CMCs with temperature capability of 2700°F (1482°C)

• 2700-3000°F (1482-1650°C) turbine and CMC combustor coatings • 2700°F (1482°C) EBC bond coat technology for supporting next generation

– Recession: <5 mg/cm2 per 1000 h – Coating and component strength requirements: 15-30 ksi, or 100- 207 Mpa – Resistance to Calcium Magnesium Alumino-Silicate (CMAS)

2400°F (1316°C) Gen I and Gen II SiC/SiC CMCs

3000°F+ (1650°C+)

Gen I

Temperature Capability (T/EBC) surface

Gen II – Current commercialGen III

Gen. IV

Increase in T across T/EBC

Single Crystal Superalloy

Year

Ceramic Matrix Composite

Gen I

Temperature Capability (T/EBC) surface

Gen II – Current commercialGen III

Gen. IV

Increase in T across T/EBC

Single Crystal Superalloy

Year

Ceramic Matrix Composite

2700°F (1482 C)

2000°F (1093°C), PtAl and NiAl bond coats

Step increase in the material’s temperature capability

3000°F SiC/SiC CMC airfoil and combustor technologies

2700°F SiC/SiC thin turbine EBC systems for CMC

airfoils

2800ºF combustor TBC

2500ºF Turbine TBC 2700°F (1482°C) Gen III SiC/SiC CMCs


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EBC-CMAS Degradation is of Concern with Increasing Operating Temperatures

− Emphasize improving temperature capability, performance and long-term durability of ceramic turbine airfoils

• Increased gas inlet temperatures for net generation engines lead to significant CMAS -related coating durability issues – CMAS infiltration and reactions

Marcus P. Borom et al, Surf. Coat. Technol. 86-87, 1996

Current airfoil CMAS attack region - R. Darolia, International Materials Reviews, 2013


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Calcium Magnesium Alumino-Silicate (CMAS) Systems Used in Laboratory Tests

NASA modified CMAS ARFL PTI CMAS 02 (higher SiO2)

GE/Borom

Wellman

Kramer

Aygun

Smialek

Rai

Braue

− Synthetic CMAS compositions, in particular, NASA modified version (NASA CMAS), and the Air Force Powder Technology Incorporated PTI 02 CMAS currently being used

− Saudi Sands used for past turbine coating studies − CMAS SiO2 content typically ranging from 43-49 mole%; such as NASA’s CMAS (with NiO

and FeO)− Collaborations on-going with the Air Force; also planned DLR, ONEA etc on Volcanic Ash

Composition selections ARFL PTI 11717A 02 used at NASA for CMAS studies

Fully reacted As received

AFRL02 particle size distribution (34% Quartz, 30% Gypsum, 17% Aplite, 14% Dolomite, 5% Salt) Percentile Size (μm) 10 2.5 +/- 1.0 50 8.5 +/- 2.0 90 40.5 +/- 3.0

Fully reacted CMAS EDS


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CMAS Related Degradations in EBCs − CMAS effects

• Significantly reduce melting points of the EBCs and bond coats • Cause more severe degradations with thin airfoil EBCs • CMAS increase EBC diffusivities and permeability, thus less protective as an environmental

barrier • Reduced mechanical properties: such as strength and toughness reductions • Leads to grain boundary attack thus disintegrate EBCs • CMAS interactions with heat flux, thermal cycling, erosion and thermomechanical fatigue

Such as yttrium silicate

EBC and degradations

CMAS induced melting and failure

Coating surface Cross-section

50 m


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Phase diagrams showing yttrium di-silicate reactions with SiO2, NaO and Al2O3

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CMAS Related Degradations in EBCs - Continued − CMAS effects on EBC temperature capability

• Silicate reactions with NaO2 and Al2O3 silicate


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CMAS Related Degradations in EBCs − Fatigue – environmental interaction is of great concern

A 20 micrometer thick EBC bond coated Prepreg SiC/SiC CMC after 40 hr, 20 Ksi, stress ratio R=0.05 fatigue testing in air

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Load, N

Strain, %

Load

, N

Stra

in, %

Time, hours

R=0.05, 20 Ksi

EBC coated 1847-01-007 #5, fatigue tested at 3 Hz at 2400°F (1316°C)


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Environmental Barrier Coating Development Limitations and Requirements

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─ Current EBCs limited in their temperature capability, water vapor stability and long-term durability, especially for advanced high pressure, high bypass turbine engines

─ Advanced EBCs also require higher strength and toughness

• In particular, resistance to combined high-heat-flux, engine high pressure, combustion environment, creep-fatigue, loading interactions

─ EBCs need improved erosion, impact and calcium-magnesium-alumino-silicate

(CMAS) resistance and interface stability • Critical to reduce the EBC Si/SiO2 reactivity and their concentration tolerance

─ EBC-CMC systems need advanced processing for realizing complex coating compositions, architectures and thin turbine configurations for next generation high performance engines • Advanced high temperature processing of high stability cluster and nano-composites


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NASA EBC Systems

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NASA EBC Systems • HfO2 -RE2O3-SiO2/RE2Si2-xO7-2x environmental barrier systems

• Controlled silica content and transition element and rare earth dopants to improve EBC stability and toughness

• Develop HfO2-Si based + X (dopants) and more advanced rare earth composite compound composition systems for 2700°F+ long-term applications

• Develop prime-reliant composite EBC-CMC interfaces for fully integrated EBC-bond coat systems

• RE2O3-SiO2-Al2O3 Systems • Develop advanced NASA high toughness alternating layered systems • Advanced 1500°C bond coats


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Strength Results of Selected EBC and EBC Bond Coats - CMAS Reaction resulted in Strength Reduction in Silicates

Selected EBC systems – HfO2-RE-Si, along with co-doped rare earth silicates and rare earth alumino-

silicates , for optimized strength, stability and temperature capability – CMAS infiltrations can reduce the strength

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0

50

100

150

200

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350646-Specially alloy toughened t' like HfO2648-EBC Bond Coat Constituent658-EBC HfO2-Rare Earth-Alumino Silicate660-Y2Si2O7657-Zr-RE silicate669-Yb2Si2O7

659-AE9762681-HfO2-Si669-9762 with CMAS 1350C 50hr reacted693-HfO2-Si with CMAS 1350C 50hr reacted

0 200 400 600 800 1000 1200 1400 1600

Stre

ngth

, MPa

Temperature, °C

696-EBC Bond Coat Constituent

Yb2Si2O7 CMAS reacted tensile surface

Yb2Si2O7 CMAS reacted specimen fracture surface

Strength test data compared

EBCs

EBCs

HfO2-Si (Si-rich)

Yb2Si2O7


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Effect of CMAS Reaction on Toughness of HfO2-Si Bond Coat and Yb2Si2O7 EBC

– HfO2-Si bond coat and ytterbium di-silicate fracture toughness studied • HfO2-Si toughness >4-5 MPa m1/2 achieved at higher temperature • Annealing heat treatments at 1300°C improved lower temperature toughness • CMAS effect unclear due to the compounded effects of possible 1350°C CMAS reaction

degradation and annealing – Ytterbium silicate EBC toughness may also be reduced due to CMAS reactions

• More measurements are needed

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HfO2-Si illustrating notch distortion due to CMAS exposure at 1350°C for 50 hrs

Yb2Si2O7 notch after CMAS exposure at 1350°C for 50 hrs

0

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0 200 400 600 800 1000 1200 1400 1600

As processed HfO2-Si1300C 20hr annealed HfO2-SiCMAS 1350C 50hr Reacted HfO2-SiAs processed Yb2Si2O7CMAS 1350C 50hr Reacted Yb2Si2O7Si

Frac

ture

toug

hnes

s, M

Pa m

1/2

Temperature, °C

"Apparent Toughness Drop"due to strength decrease


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EBC CMAS Surface Reactions

– Ytterbium- and yttrium-disilicate silicates reactions and dissolutions in CAMS

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Ytterbium silicate surface CMAS melts: 50 hr 1300°C

Ytterbium silicate surface CMAS melts: 5 hr 1500°C

Yttrium silicate surface CMAS melts: 50 hr 1300°C

Yttrium silicate surface CMAS melts: 5 hr 1500°C


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EBC Reacted Apatite Phases under Long-Term Testing at 1500°C – Ytterbium silicate EBC

– Non stoichiometric characteristics of the CMAS – rare earth silicate reacted apatite phases

– Difference in partitioning of ytterbium vs. yttrium in apatite

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Composition in apatite (100 hr):


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EBC Reacted Apatite Phases under Long-Term Testing at 1500°C: Yttrium Silicate EBC

– Non stoichiometric characteristics of the CMAS – rare earth silicate reacted apatite phases

– Difference in partition of ytterbium vs. yttrium • Average AEO/RE2O3 ratio ~ 0.68 for ytterbium silicate – CMAS system • Average AEO/RE2O3 ratio ~ 0.22 for yttrium silicate – CMAS system

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Composition in apatite (100 hr):


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Stoichiometry of the Reacted Apatite Phases under Long-Term Testing at 1500°C

– Non stoichiometric characteristics of the CMAS – rare earth silicate reacted apatite phases – up to 200 hr testing

– Difference in partitioning of ytterbium vs. yttrium in apatite • Average AEO/RE2O3 ratio ~ 0.68 for ytterbium silicate – CMAS system • Average AEO/RE2O3 ratio ~ 0.22 for yttrium silicate – CMAS system

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AEO–RE2O3–SiO2 phase diagram

Ahlborg and Zhu, Surface & Coatings Technology 237 (2013) 79–87.

Ytterbium system

Yttrium system

From Zhu Irsee Presentation Pages 18-19: “NASA’s Advanced Environmental Barrier Coatings Development for SiC/SiC Ceramic Matrix Composites: Understanding Calcium Magnesium Alumino-Silicate (CMAS) Degradations and Resistance”, June 2014


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Effect of CMAS Reactions on Grain Boundary Phases

– CMAS and grain boundary phase has higher Al2O3 content (17-22 mole%)

• Eutectic region with high Al2O3 content ~1200°C melting point • Loss of SiO2 due to volatility

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tingggggggggggggggggggggggggggggggggggggggggggggggggggggggggggg ppppppppppppppppppppppppppppoiiintttttttttttttttttttttttttt

200 hr, 1500°C

NASA modified CMAS

Grain boundary final phase – low SiO2 and high Alumina


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Rare Earth Apatite Grain Growth

– Grain growth of apatite phase at 1500°C at various times

50 hr 150 hr 200 hr

50 hr 200 hr 150 hr

Ytterbium silicate system

Yttrium silicate system


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HfO2-Rare Earth Silicate Composite EBC Systems - Continued – Silica loss observed in the concentrated CMAS reacted regions

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Rare earth silicate - apatite phase rich phase region

HfO2 rich phase region

CMAS concentrated region, SiO2 content 20-30 mol% (SiO2 loss in the steam water vapor tests)

Coating surface

Cross-section (surface region)

Coating surface


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Oxidation kinetics vs Si content

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

60 65 70 75 80 85

RESi(O)RESi(O)RESi(O)RESi(O)RESi(O)RESi(O)RESi(O)RESi(O)RESi(O)RESi(O)RESi(O)RESi(O)

Kp,

mg2 /c

m4 -s

ec

Silicon composition, at%

22

– Thermogravimetric analysis (TGA) in dry O2 at 1500°C, tested up to 500 hr – “Protective” scale of rare earth di-silicate formed in oxidizing environments – Furnace cyclic test life also evaluated at 1500°C

SiC

RESi(O)

RE2Si2O7-x

40 m

High Stability Rare Earth Silicon Bond Coat with High Melting Point Coating Compositions: Designed with Improved Temperature capability

and CMAS Resistance


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High Stability Rare Earth Silicon Bond Coat with High Melting Point Coating Compositions: Designed with Improved Temperature capability

and CMAS Resistance - Continued – Thermogravimetric analysis (TGA) in dry O2 at 1500°C, tested up to 500 hr – “Protective” scale of rare earth di-silicate formed in oxidizing environments – Furnace cyclic or high heat flux test life evaluated at 1500°C up to 1000 hours with or without

CMAS

FCT life of RE-Si coatings An Yb-Gd2700°F EBC bond coat showed 500hr cyclic durability


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High Stability and CMAS Resistance Observed from the Rare Earth Silicon High Melting Point Coating Compositions

– Demonstrated CMAS resistance of NASA RE-Si System at 1500°C, 100 hr

– Silica-rich phase precipitation – Rare earth element leaching into

the melts (low concentration ~9 mol%) Area A

Area B

Surface side of the CMAS melts


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CMAS Reaction Kinetics in Bond Coats

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CMAS Partitioning on RE-Si bond coat, 1500°C, 100hr

RE incorporations

– SiO2 rich phase partitioning in the CMAS melts – Rare earth content leaching low even at 1500°C – More advanced compositions are being implemented for improved thermomechanical –

CMAS resistance


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Advanced EBC Compositions Improve the Resistance to CMAS

– Controlling CMAS wetting, viscosity, stability and melting points – Providing better EBC protections for CMCs in CMAS environments – EBC durability being validated under CMAS-mechanical loading

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400 hr, 69 Mpa creep rupture at EBC surface temperature 1400°C

202 hr, 69 MPa creep rupture at EBC surface temperature 1540°C; CMC failure


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Advanced EBC Compositions Improve the Resistance to CMAS - Continued

– Controlling CMAS wetting, viscosity, stability and melting points – Providing better EBC protections for CMCs in CMAS environments – EBC durability initially validated under long-term CMAS-mechanical loading

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400 hr, 69 Mpa creep rupture at EBC surface temperature 1400°C

202 hr, 69 MPa creep rupture at EBC surface temperature 1540°C; CMC failure


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Creep-Fatigue of EBCs-CMCs in Complex Heat Flux and Simulated Engine Environments

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10-7 10-5 0.001 0.1 10 1000

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stre

ss, M

Pa

Fatigue cycles

High temperature strength data

Fatigue or creep hot time, hours

EBC 2700F+; CMC temperature labeled (2550-2700F)

2550F2700F

• Long-term creep and fatigue used to validate EBCs at various loading levels • Demonstrated 2700°F EBC and bond coat capability in complex environments

SiO2

Advanced Bond Coat on CMC – intact after fatigue test with 15 ksi load and 2600-2700°F surface temperature for 460 hot hours

Advanced Bond Coat on CMC – intact after fatigue test with 15 ksi load and 2600-2700°F surface temp for 460 hot hours

SiO2

Fracture surface; 200+ hr at 2700°F+ creep rupture testing with CMAS; Advanced EBC protected CMCs

Stress-oxidation and stress-CMAS environmental testing


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Summary • CMAS degradation remains a challenge for emerging turbine engine

environmental barrier coating – SiC/SiC CMC component systems • CMAS leads to lower melting point of EBC and EBC bond coat

systems, and accelerated degradations • NASA advanced EBC compositions showed promise for CMAS

resistance at temperatures up to 1500°C+, and in combined with mechanical loading

• We have better understanding of CMAS interaction with rare earth silicates, and in controlling the compositions for CMAS resistance while maintaining high toughness

• We are developing better standardized CMAS testing, and working on CMAS induced life reductions, helping validate life modeling


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EBC-CMAS Degradation under Thermal Gradients − Effect of CMAS concentration on EBC-CMC system cyclic durability

• CMAS reacts with high SiO2 activity layer and reducing melting point • Low tough reaction layers such as apatite phases • Interactions with heat flux, thermal cycling, erosion and thermomechanical fatigue

EB-PVD ZrO2

HfO2-Yb2O3-Aluminosilicate Yb2Si2O7 Si

More severe degradation and delamination: Tsurface 1500°C Tinterface 1316°C

Durability and CMAS Resistance of Advanced Environmental ......1500 C – Ytterbium silicate EBC –...

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