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National Aeronautics and Space Administration

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Phase Stability and Thermal Conductivity of Composite Environmental Barrier Coatings on SiC/SiC Ceramic

Matrix Composites

Advanced environmental barrier coatings are being developed to protect SiC/SiC

ceramic matrix composites in harsh combustion environments. The current coating

development emphasis has been placed on the significantly improved cyclic durability and

combustion environment stability in high-heat-flux and high velocity gas turbine engine

environments. Environmental barrier coating systems based on hafnia (HfO2) and

ytterbium silicate, HfO2-Si nano-composite bond coat systems have been processed and

their stability and thermal conductivity behavior have been evaluated in simulated turbine

environments. The incorporation of Silicon Carbide Nanotubes (SiCNT) into high stability

(HfO2) and/or HfO2-silicon composite bond coats, along with ZrO2, HfO2 and rare earth

silicate composite top coat systems, showed promise as excellent environmental barriers

to protect the SiC/SiC ceramic matrix composites.

https://ntrs.nasa.gov/search.jsp?R=20110008738 2020-07-11T18:12:51+00:00Z


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Phase Stability and Thermal Conductivity of Composite Environmental Barrier Coatings on

SiC/SiC Ceramic Matrix Composites

Samantha Benkel *1, 2 and Dongming Zhu 1

1 Durability and Protective Coatings BranchStructures and Materials Division

NASA Glenn Research CenterCleveland, Ohio 44135

2 Carnegie Mellon UniversityPittsburgh, Pennsylvania

The 35th International Conference on Advanced Ceramics & CompositesDaytona Beach, Florida

January 23-28, 2011

This work was supported by NASA Fundamental Aeronautics Program and * NASA LERCIP Internship Program


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Motivation• Environmental barrier coatings (EBCs) for light-weight SiC/SiC ceramic

matrix composite (CMC) components critical for advanced propulsion engines

• Higher temperature and higher strength capable coatings highly desirable

• Composite environmental barrier coatings have promise for improved temperature capability, stability and performance

– High temperature stability is still of concern– HfO2-Si and HfO2-SiCNT bond coats as a major emphasis– HfO2-Yb2Si2O7 and/or HfO2-Yb2SiO5 top coats

• Advanced coating systems were also tested

Underlying SiC/SiC CMC

Composite bond coats

Composite top CoatingIntermediate Coating


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Outline

• Synthesis of Silicon Carbide Nanotubes (SiCNT)– Process optimization and scale up– TGA stability study in air

• Furnace Testing of Hot-Processed HfO2-Si and HfO2-SiCNT Composite Systems– Phase stability at 1400-1450°C

• Thermal Conductivity of HfO2-Si and HfO2-SiCNT systems

• High Pressure Burner Rig (HPBR) Stability of EBC-CMCs

• Cyclic Durability of Advanced Coating-CMC systems


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Experimental• SiC nanotubes synthesized by Si reaction

with multiwall carbon nanotubes (CNTs)at 1400°C in Ar + H2 environments

• Materials/specimens (hot-press): 1”diameter x ~3 mm thick HfO2+Si andHfO2+SiCNT disc specimens or coatingHfO2+Si and HfO2+SiCNT specimens onCMCs

• Furnace stability testing in air byThermogravimetric Analysis (TGA)

• X-ray diffraction and SEMcharacterization

• 3500 W continuous wave (cw) CO2 laser(10.6 micron wavelength) used for hightemperature thermal conductivity testingand high heat flux cyclic durability testing

• High Pressure Burner Rig StabilityTesting

Laser test rig

High Pressure Burner Rig


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SiCNT Processing and Stability Testing• Optimized SiCNT processing with Si to Carbon nanotube (CNT) weight ratio

55:45 for environmental barrier coating applications• Obtained almost full β-SiC phase for the SiCNT based on X-ray diffraction• Retained excellent nanotube morphologies based on SEM characterizations• Scaled up the process with increased High quality SiCNT yields at 0.2 g/batch• Demonstrated SiCNT stability in air using TGA

Ar+5%H2 Ar+5%H2

0

20

40

60

80

100

0 20 40 60 80 100

SiCNTCNT

Wei

ght c

hang

e, %

Time, hours

800ºC, in air

SiCNT processing and TGA stability evaluation SEM and EDS characterizations of SiCNTs


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Laser Steady-State Heat Flux Thermal Conductivity Testing Approach

Schematic of the laser heat flux test principles

Pyrometer

Uniformly distributed laser beam through an integrated

lens

Pyrometer

Specimen fixture

Cooling air nozzle


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Furnace Testing of Hafnia-Silicon Systems

• Optimum mixture composition found between 25wt% and 50wt%Si

Temperature HfO2+25%Si HfO2+50%Si HfO2+75%Si

1350˚C

1400˚

1450˚C


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The HfO2+SiCNT Bond Coats Showed High Temperature Stability in Oxidizing Environments

• HfO2, SiCNT and HfO2 silicate are major phases after the 100hr testing

0

5000

1 104

1.5 104

2 104

2.5 104

10 20 30 40 50 60 70 80

HfO2-18wt%SiCNT after 100 hr test

β-SiC

HfO2

HfSiO4

0

5000

1 104

1.5 104

2 104

2.5 104

Rel

ative

Inte

nsity

Rel

ative

Inte

nsity

Diffraction angle, 2θ

Before Testing + 1400˚C/50hr + 1450˚C/50hr

SiC nanotube (SiCNT) and HfO2-based nano-composite coatings

HfO2-SiCNT composite coating

SiCNT


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The HfO2+Si Bond Coats Showed Significantly Improved Temperature Capability

• Higher stability observed even after 1450ºC testing

Cross-section, Hot-pressed HfO2-50wt%Si on CMC, 1350°C tested

0

1 104

2 104

3 104

4 104

5 104

6 104

10 20 30 40 50 60 70 80

HfO2-25wt%Si-1350°C



HfO2SiHfSiO4

-1 104

0

1 104

2 104

3 104

4 104

5 104

6 104

Rel

ativ

e In

tens

ity

Rel

ativ

e In

tens

ity

Diffraction angle, 2θ

1350°C 1400°C 1450°C

HfO2-HfO2+30Si bond coat on CMC,

1500°C tested for 200 hours


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High Pressure Burner Rig Recession Testing

• HfO2/ytterbium silicate/HfO2-Si composite bond coat tested at 200 m/s in high pressure burner rig and showed excellent stability

0.0001

0.001

0.01

0.1

1

10

0.0005 0.00055 0.0006 0.00065 0.0007 0.00075 0.0008

SiC/SIC CMCBSASBSAS (200m/s)HfO

2-1 (200 m/s)

Tyranohex SA SiC composite (200m/s)

Spec

ific

wei

ght c

hang

e, m

g/cm

2 -h

1/T, K-1

1300Temperature, °C

14001500 12001600

HfO2-(Y,Gd,Yb) 2O3+Yb2Si2O7/ Yb2Si2O7+20BSAS/ HfO2+30wt%Si (200m/s)

No detectable weight loss, after the 5 hr HPBR Testing

HfO2-(Y,Gd,Yb) 2O3+Yb2Si2O7/ Yb2Si2O7+20BSAS/ HfO2+30wt%Si on the test rig


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High Heat Flux Thermal Gradient Cyclic Durability Evaluation of Environmental Barrier Coating and Bond

Coat Systems• Laser high heat flux rig thermal gradient thermal cyclic testing at surface

temperature of 1482°C (2700°F)• Advanced bond coats tested at up to 1375°C

HfO2-ytterbium silicate coating with HfO2-Si bond coat, after

100hr testing at Tsurface1482°C/Tinterface 1375°C

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 20 40 60 80 100 120 140

8YSZ_mullite_mullite-BSAS_SiHfO2-YbS2_YbS2_BSAS_HfO2-SiEB-YSZGdYb_HfO2-YbS2_YbS2+BSAS_SiYbS+HfO2_YbS-BSAS_HfO2-Si-SiCNT

Ther

mal

con

duct

ivity

, W/m

-K

Time, hours

Tsurface 1482°C/Tinterface 1375°C





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Thermal Conductivity of HfO2-12.4wt%SiCNT

• Effect of nanotube fractions on thermal conductivity is being evaluated and modeled

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0

50

100

150

200

0 200 400 600 800 1000 1200 1400

kcera

qthru, w/cm2Ther

mal

con

duct

ivity

, W/m

-K

Hea

t flu

x, W

/cm

2

Surface Temperature, ºC


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Thermal Conductivity of HfO2-Si Systems• Thermal conductivity maintained stable after high temperature 100 hr

annealing

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0

50

100

150

200

0 200 400 600 800 1000 1200

kcera

qthruTher

mal

con

duct

ivity

, W/m

-K

Hea

t flu

x, W

/cm

2


HfO2+50Si on CMC 1450C pre-annealed

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0

50

100

150

200

0 200 400 600 800 1000 1200

kcera

qthruTher

mal

con

duct

ivity

, W/m

-K

Hea

t flu

x, W

/cm

2


HfO2+25Si on CMC 1450C pre-annealed

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

-50

0

50

100

150

200

0 200 400 600 800 1000 1200 1400

kcera

qthru, w/cm2Ther

mal

con

duct

ivity

, W/m

-K

Hea

t flu

x, W

/cm

2


HfO2+25Si free standing


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Summary

• Composite HfO2-Si and HfO2-SiCNT bond coats synthesized for high temperature environmental barrier coating applications

• The stability of HfO2-Si and HfO2-SiCNT bond coats demonstrated for stability up to 1450°C (1500°C for advanced processed coatings)

• Thermal conductivity of HfO2-Si and HfO2-SiCNT bond coats evaluated

• Multilayer EBC composite coatings showed combustion gas stability and thermal gradient durability in high pressure burner rig and laser high heat flux rig


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AcknowledgementsThe work was supported by NASA Fundamental

Aeronautics Program (FAP) Supersonics Project (Manager: Dale A. Hopkins) and NASA Integrated Systems Research Program (ISRP) Environmentally Responsible Aviation (ERA) Project (Manager: Janet Hurst).

The authors wish to thank Ms. Joyce A. Dever, Chief of Durability and Protective Coatings Branch, for her kind help and support in the research during the NASA Internship project.

Also Many Thanks to:• Bob Angus (Hot Press Operation)• Dr. Richard Rogers (X-ray Diffraction)• Terry McCue (SEM)• Don Humphrey, SiCNT TGA• Dennis Fox, EBC coating processing• Bob Pastel, High Pressure Burner Rig testing• NASA Lewis' Educational and Research Collaborative

Internship Project (LERCIP)

NASA Supersonics

NASA Subsonics/ERA

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