development and performance evaluations of hfo -si and ......tbc 2500ºf turbine tbc 2700 f (1482 c)...

National Aeronautics and Space Administration

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Development and Performance Evaluations of HfO2-Si and Rare Earth-Si Based Environmental Barrier Bond Coat Systems for

SiC/SiC Ceramic Matrix Composites

Dongming Zhu

Materials and Structures Division NASA John H. Glenn Research Center

Cleveland, Ohio 44135

41st International Conference on Metallurgical Coatings and Thin Films San Diego, California

May 2, 2014


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NASA EBC and CMC System Development − Emphasize temperature capability, performance and long-term durability • Highly loaded EBC-CMCs • 2700-3000°F (1482-1650°C) turbine and CMC combustor coatings • 2700°F (1482°C) EBC bond coat technology for supporting next generation

– Recession:


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Outline

• Environmental barrier coating (EBC) system development: needs and challenges

• Advanced bond coat development approaches, NASA HfO2-Si bond coat systems

– Focused on oxidation resistance, high temperature strength, toughness and creep properties

• Advanced Rare Earth – Silicon based 2700°F+ capable bond coat

developments – Development approaches – Oxidation resistance – Furnace and thermomecahnical durability

• Summary


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Use Temperature of Environmental Barrier Coatings Limited by Interface Reactions

— Significant interfacial pores and eutectic phases formation due to the water vapor attack and Si diffusion at 1300°C

— Heat flux condition further limit the use tempertatures

Si bond coat after 1350°C, 50 hr furnace test in air; 1” dia plasma sprayed EBC button specimen

Hot pressed BSAS+Si button specimen after 1350°C, 50 hr

furnace test in air

MulliteBSAS

Si

SEM images Interface reactions at 1300°C; total 200 hot hours BaO-Al2O3-SiO2 ternary phase diagram

Si bond coat

Interface Si bond coat melting of selected coating systems, under laser heat flux tests, 1” dia button specimen


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NASA EBC and CMC System Development

─ 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 high strength and toughness • Resistance to heat-flux, high pressure combustion environment, creep-fatigue loading

interactions • Bond coat cyclic oxidation resistance

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

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

─ EBC-CMC systems need advanced and affordable processing

• Using existing infrastructure and alternative coating production processing systems, including Plasma Spray, EB-PVD and Directed Vapor EB-PVD, and/or emerging Plasma Spray - Physical Vapor Deposition

• Affordable and safe, suitable for various engine components


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Degradation Mechanisms for Si Bond Coat — Silicon bond coat melts at 1410°C (melting point) — Fast oxidation rates (forming SiO2) and high volatility at high temperature — Low toughness at room temperature (0.8-0.9 MPa m1/2; Brittle to Ductile Transition

Temperature about 750°C) — Low strength and high creep rates at high temperatures, leading to coating

delamination — Interface reactions leading to low melting phases

• A more significant issue when sand deposit Calcium- Magnesium –Alumino-Siliacte (CMAS) is present

— Si and SiO2 volatility at high temperature (with and without moisture)

Brittle to Ductile transition in polycrystalline Si


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Design Requirements for 2700°F Bond Coat Systems

7

— High melting point and thermal stability — Develop slow growing, adherent protective scales

• High strength and low thermal expansion coefficient scales, and minimum element depletion in the bond coat due to the scale formation essential

— Provide oxidation and environment protection for SiC/SiC CMC substrate • Oxidation resistance in all operating temperature range, up to 1600°C, no pesting

— High creep strength and excellent fatigue resistance • High resistance to impact, erosion, and CMAS, and environment induced degradations

— Excellent bond strengths (important to provide strong bond for the EBC to the substrate!)

— Thermal expansion coefficient matching to the CMC substrate — Thermal chemical and thermal mechanical compatibility with EBC and CMC — Improved bond coat – CMC interface architecture and integration — Ensure low oxygen activity at the bond coat – CMC interfaces

• Preferably kinetics controlled and dynamic bond coat systems for durability


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Advanced High Temperature and 2700°F+ Bond Coat Development

̶ Development approach: • Advanced compositions ensuring high strength, high stability, high

toughness • Bond coat systems for prime reliant EBCs; capable of self-healing

High strength, high stability reinforced

composites: HfO2-Si and a series of Oxide-

Si systems

HfO2-Si based and minor alloyed systems for improved strength

and stability

Advanced 2700°F bond coat systems:

RE-Si based systems

Advanced 2700°F bond coat systems: RE-Si based Systems, grain boundary

engineering designs and/or composite systems -

HfO2-Si systems Advanced 2700°F+ Bond Coat systems

Other systems

Other systems


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HfO2-Si Bond Coats for Improved Temperature Capability, and High Temperature Strength

Hf-Si-O system

─ A relatively low cost bond coat system, and APS and EB-PVD processing capable ─ Excellent oxidation resistance, also ensuring low oxygen activities at the EBC-CMC interface ─ Upper use temperature 1400°C and can be up to 1482°C ─ SiO2-HfSiO4-HfO2 phase system at very high temperature ─ Thermal expansion coefficient ~ 5.5 x10-6 /K ─ Rare earth metal or other dopants added for improved stability


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HfO2-Si Bond Coats for Improved Temperature Capability, and High Temperature Strength

─ A relatively low cost bond coat system, and APS and EB-PVD processing capable ─ Excellent oxidation resistance, also ensuring low oxygen activities at the EBC-CMC interface ─ Upper use temperature 1400°C and can be up to 1482°C ─ SiO2-HfSiO4-HfO2 phase system at very high temperature ─ Thermal expansion coefficient ~ 5.5 x10-6 /K ─ Rare earth metal or other dopants added for improved stability

HfO2-Si and alloyed EBC bond coats using EB-PVD processing: achieving higher

temperature capability

Plasma sprayed HfO2-Si EBC bond coat


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Experimental: Mechanical Specimen Configurations

Specimen width = 4 mm

50 mm

40 mm

5mm

20 mm

— Flexural specimens with dimensions 4x5x50 mm, machined from hot-pressed air plasma spray (APS) HfO2-Si powders (billets size 75mmx50mmx10mm); test spans 20 and 40 mm • Using ASTM standards 1161 and 1211 • Si concentration range from 25 to 70wt% in the HfO2-Si systems

— The non-notched bar specimens used for strength, and creep testing — Single edge V-notched beam (SEVNB) specimens used for toughness tests — Test temperature range room temperature, 1200 up to 1500°C


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Experimental: Oxidation and Durability Tests

— Test specimens with dimensions 25 mm diameter disc specimens for oxidation, laser heat flux and furnace cyclic test (FCT) — Test specimens with dimensions 152x12.7 mm dog-bone, and 76x12.7mm for tensile creep rupture and fatigue tests • Tests were conducted including

o Thermogravimetric analysis (TGA) o FCT test o Laser + steam/CMAS water vapor cyclic test o Thermomechnical creep and fatigue


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Oxidation Resistance of HfO2-Si ─ TGA weight change measurements in flowing O2 ─ Parabolic oxidation kinetics generally observed ─ Solid-state reaction is also involved with the systems, and more complex behavior

at 1400 and 1500°C ─ Excellent oxidation resistance and improved oxidation resistance through APS

plasma spray powder processing optimization

0.0

1.0

2.0

3.0

4.0

5.0

0.00055 0.0006 0.00065 0.0007 0.00075 0.0008

lnK

p, m

g2/c

m4

1/T, K-1

Activation energy 101 kJ/mol

1200°C1300°C1400°C

R2=0.98258

1500°C

10218 Clad

10219 Clad II

10219 Clad Ia

- TGA weight change measurements at various temperatures - AE 10219 is first Generation HfO2-30wt%Si composite APS powders used in

NASA ERA liner component demonstrations - AE 10218 is HfO2-30wt%Si composite APS powders used in NASA ERA liner

component demonstrations. - AE 10219 Clad II is second Generation HfO2-30wt%Si composite APS powders

0

10

20

30

40

50

60

0 2 4 6 8 10 12

1200C1300C1400C1500C

Spec

ific w

eight

gain

, mg/

cm2

Time, hour1/2

AE 10219 Clad II

HfSiO4

HfO2

Si

Polished specimen microstructure after 1400°C test


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High Strength EBC and Bond Coat Composition Development

– Bond coats and bond coat constituents designed with high strength to achieve the ultimate coating durability, compared with EBCs’ strengths

– HfO2-Si based systems showed high strength and high toughness

0

50

100

150

200

250

300

350646-Specially toughened t' like HfO2648-EBC Bond Coat Constituent658-AE9932660-Y2Si2O7657-Zr-RE silicate669-Yb2Si2O7670-Rare Earth Disilicate681-HfO2-Si

0 200 400 600 800 1000 1200 1400 1600

Stre

ngth

, MPa

Temperature, °C

696-EBC Bond Coat Constituent

14


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High Toughness HfO2-Si Bond Coat Composition Development

– HfO2-Si Bond coats showed high toughness • Toughness >4-5 MPa m1/2 achieved • Emphasis on improving the lower temperature toughness • Annealing effects on improved lower temperature toughness being studied

15

0

1

2

3

4

5

0 200 400 600 800 1000 1200 1400 1600

As processed1300°C 20hr annealedSi

Frac

ture

toug

hnes

s, M

Pa m

1/2

Temperature, °C

May expect further increase from anealing

Strength drop due to creep strength decrease


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– The composites coatings have improved creep strength, and creep resistance at high temperatures

– Increased HfO2-HfSiO4 contents improve high temperature strength and creep resistance

16

Effects of Compositions on HfO2-Si Strength and Creep Rates

10-8

10-7

10-6

10-5

10-4

10-3

10-2

0 20 40 60 80 100

Creep rates at 1400°C, 30 MPa

HfO

2-Si

bon

d co

at c

reep

rate

s, 1/

s

Si content, wt%

0

20

40

60

80

100

120

0 20 40 60 80 100

Stre

ngth

, MPa

Si content, wt%

1400°C


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HfO2-Si/Ytterbium Silicate EBC System Furnace Cyclic Durability Test at 1500°C

– Coating processed using Triplex Pro plasma spray processing, not necessarily fully optimized

– Long-term furnace cyclic durability tested 1500°C for 500 hr in air – EBC with HfO2-Si bond coat adherent (no any spallation) after testing – Excellent oxidation resistance in protecting SiC/SiC – SiO2 loss in ytterbium silicate EBC (some area became ytterbia), and in the

HfO2-Si bond coat – Some HfO2 containing scales may be stable

HfO2 containing scales

17


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Advanced 2700°F+ Bond Coats (Beyond HfO2-Si)

18

̶ Development approach: • Advanced compositions ensuring high strength, high stability, high toughness • Bond coat systems for prime reliant EBCs; capable of self-healing

High strength, high stability reinforced

composites: HfO2-Si and a series of Oxide-

Si systems

HfO2-Si based and minor alloyed systems

systems

Advanced 2700°F bond coat systems:

RE-Si based systems

Advanced 2700°F bond coat systems: RE-Si

based Systems, grain boundary engineering

designs and/or composite systems -

Multicomponent RESi systems Studied Zr, Hf, Ta, N, Al Dopants

RESi systems HfO2-RE-Si systems HfO2-RE-Al-Si systems

HfO2-Si systems Advanced 2700°F+ Bond Coat systems


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– Ytterbium, Yttrium and Gadolinium – Silicon or Silicide systems

• Controlled silicon compositions and oxygen activities to achieve good thermal expansion match with SiC/SiC CMCs and EBCs, and high melting points and stability

• Focusing on multicomponent high temperature based systems to ensure high temperature capability, oxidation resistance and durability

• Emphasizing chemically and mechanically compatibility with SiC/SiC CMCs and various environmental barrier coatings, no free-standing silicon phases in composition designs

• Low temperature oxidation resistance and pesting issues are also addressed in the developments

2700°F+ Advanced EBC Bond Coat Developments: Rare Earth Silicon Systems and Effect of Dopants

19


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NASA Advanced 2700°F Silicide Based Bond Coats – System Processing for Various Component Applications

– Advanced systems developed with high Technology Readiness Levels (TRL) – Composition ranges studied mostly from 50 – 80 atomic% silicon

• PVD-CVD processing, for composition downselects - also helping potentially develop a low cost CVD or laser CVD approach

• Compositions initially downselected for selected EB-PVD and APS coating composition processing • Viable EB-PVD and APS systems downselected and tested; development new PVD-CVD approaches

YSi YbGdYSi GdYSi ZrSi+Y YbGdYSi GdYSi ZrSi+Y YbGdYSi GdYSi ZrSi+Ta YbGdYSi GdYSi ZrSi+Ta YbGdSi GdYSi-X HfSi + Si YbGdSi GdYSi-X

HfSi + YSi YbGdSi

HfSi+Ysi+Si YbGdSi YbSi YbGdSi

HfSi + YbSi YbSi

GdYbSi(Hf)

YYbGdSi(Hf) YbYSi YbHfSi YbHfSi

YbHfSi YbHfSi

YbHfSi YbSi

HfO2-Si; REHfSi

YSi+RESilicate YSi+Hf-RESilicate

9869 Hf-RESilicate

Used in ERA components as part of bond coat system

10157 Hf-RE-Al-Silicate

Used also in ERA components Used in ERA components as part of bond coat system

PVD-CVD EB-PVD APS Laser/CVD/PVD REHfSi


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Rare Earth Silicon Systems and Multi-Dopants for Stability

YbSix (no additional dopant) Exposed to 1100°C for 20 h

Undoped material: shows separation of

Si-rich/silica-rich phase

(Y,Hf)Six 1100°C for 20 h

When dopant included: The Si-rich/silica-rich phases converted to more stable HfO2 -

Hafnium silicate, and yttrium silicate containing phases

– Silicon-rich phase separations can limit high temperature stability – Further thermal stability and mechanical strength can be improved by:

• Composition controls (e.g. optimize silicon contents and addition of dopants) • Multi-dopant composition designs for reduced Si/SiO2 activity

21


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Oxidation Kinetics of RE-Si Based EBC Systems

0

100

200

300

400

500

600

0 100 200 300 400 500 600

NG4NG5NG6NG7NG8NG9NG2Sp

ecifi

c w

eigh

t cha

nge

w2 ,

mg2

/cm

4

Time, hours

YbGdYSi series

After 500 hr test of an EBC; some surface coating small area spallation after cooling down - Lower Si content, Yb-Gd-Y

system

– Thermogravimetric analysis (TGA) performed at 1500°C in flowing dry oxygen – Bond coat fully coated SiC/SiC (CVI) specimens


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Oxidation Resistance of Doped Rare Earth Silicide - Effect of Stoichiometry (oxidation vs. atomic percent Si)

– Thermogravimetric analysis (TGA) in dry O2 at 1500°C

– “Protective” scale of rare earth di-silicate formed (10-15 micrometers)

SiC

RESi(O)

RE2Si2O7-x

Multicomponent doped RE Silicide system after 100 hr exposure at 1500°C in O2

Transition region between Si-rich and silicate regions

40 �m

23


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Furnace Cycle Test Results of Selected RESi and ZrSi + Dopant Bond Coats

- Testing in Air at 1500°C, 1 hr cycles – Multi-component systems showed excellent furnace cyclic durability at 1500°C

24


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

– Demonstrated CMAS resistance of RESi at 1500°C, 100 hr

Area A

Area B


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- Selected EBC system processed by EB-PVD and plasma Spray: Doped RE Si (+Hf) Bond Coat + advanced multi-component EBC Top Coat on woven SiC/SiC CVI-SMI CMC

- Creep testing conducted with 15 ksi load and laser thermal gradient

26

Processing Advancements and Improvements for RE Si Bond Coats in EBC Systems

EBC System after 100 hr creep testing with 2700°F coating surface temperature and 2500°F CMC back temperature

RE(Hf) silicate EBC Top Coat

RESi Composite Bond Coat System: Striations indicate EB-PVD layers with compositional variations Excellent compatibility

Bond coat remains generally well-adhered to CMC substrate after the CMC failure, except some top bond coat composition segregation or processing defective regions


www.nasa.gov 27

Fatigue Tests of Advanced Bond Coats and EBC Systems

Tested, SA Tyrannohex with bond coat only

Tested, SA Tyrannohex with EBC system 188

• Uncoated CMCs, Bond coat/CMC and EBC/Bond Coat/CMC systems tested flexural fatigue tests with 15 Ksi loading

• Heating provided by steady-state laser • Strength and Fatigue cycles tested • Fatigue tests at 3 Hz, 2600-2700°F, stress ratio 0.05, surface tension-tension cycles

SiO2

Achieved long-term fatigue lives (near 500 hr) with EBC at 2700°F

Tested specimen cross-sections


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Summary • Advanced HfO2-Si and Rare Earth - Silicon based bond coat

compositions developed • The coatings showed excellent oxidation resistance and protection

for CMCs • HfO2-Si showed excellent strength, fracture toughness, its upper

use temperature may be limited to 1400°C due to higher silica activity, in particular in the CMAS environments

• The initial silicon content range of the Rare Earth-Silicon coatings was down-selected, multicomponent systems designed for further improved stability

• The rare earth – silicon based coatings showed 1500°C operating temperature viability and durability on SiC/SiC ceramic matrix composites

• The rare earth – silicon based coatings compositions will be down-selected; and further processing optimization planned


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Acknowledgements The work was supported by NASA Fundamental Aeronautics Programs, and

Aeronautical Science Project. The author is grateful to - Ralph Pawlik and Ron Phillips for their assistance in mechanical testing - Don Humphrey and Michael Cuy for assisting Thermogravimetric analysis

(TGA) and furnace oxidation tests


www.nasa.gov 30

0

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1480 1500 1520 1540 1560 1580 1600 1620

1220 1240 1260 1280 1300 1320 1340

Cycl

es to

failu

re

Interface temperature, K

Interface temperature, °C

, hr

Surface test temperature 1922°K (1649°C)

Use Temperature of Environmental Barrier Coatings Limited by Interface Reactions - Continued

— Various advanced EBC/mullite/mullite+BSAS/Si coating systems heat flux tested — Accelerated spallation under heat flux tests observed — Significantly reduced cyclic durability due to bond coat melting

Interface Si bond coat melting of selected coating systems, under laser heat flux tests, 1” dia button

specimen

Effect of interface temperatures on heat flux cyclic life


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X-ray Phase Studies of Hot Pressed HfO2-Si systems ─ X-ray phase showed HfO2-HfSiO4 some with silica phase near surface

Sample: HfO2-Si 1200°C, 50 hr Chemical Formula Compound Name Crystal System Ref. Code SemiQuant [%] Hf O2 Hafnium Oxide Monoclinic 04-006-7680 27 Hf ( Si O4 ) Hafnium Silicate Tetragonal 04-008-6813 7 Si Silicon Cubic 04-006-2527 66

Sample: HFO2-Si 1300°C, 100 hr Chemical Formula Compound Name Crystal System Ref. Code SemiQuant [%] Si Silicon Cubic 04-006-2527 46 Hf O2 Hafnium Oxide Monoclinic 04-006-7680 23 Hf ( Si O4 ) Hafnium Silicate Tetragonal 04-008-6813 15 Si O2 Silicon Oxide Tetragonal 04-012-1126 16 Sample: HfO2-Si 1400°C, 50 hr Chemical Formula Compound Name Crystal System Ref. Code SemiQuant [%] Si Silicon Cubic 04-006-2527 47 Hf O2 Hafnium Oxide Monoclinic 04-006-7680 20 Hf ( Si O4 ) Hafnium Silicate Tetragonal 04-008-6813 11 Si O2 Silicon Oxide Tetragonal 04-012-1126 22

HfSiO4

HfO2

Si

Polished specimen microstructure


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Advanced APS HfO2-Si Processing Optimizations – Design of Experiments (DoE) to help achieve improved density and phase

distributions – Processing performed using Triplex Pro, at Sulzer Metco

32

Spray parameters optimized


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0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 20000 40000 60000 80000 100000

AE 10218 -1200CAE 10218_1300CAE10218_1400CAE 10349_1400C

Total

cree

p str

ains

Time, s

2.465x10-4 1/s

4.155x10-6 1/s

1.98x10-7 1/s

2.58x10-8 1/s

30 MPa Creep

HfO2-Si Creep Behavior

33

Effects of Compositions on Strength and Creep Rates - Continued

– The composites coatings have high strength, and improved creep resistance at high temperatures

– Increased HfO2-HfSiO4 contents improve high temperature strength and creep resistance

- AE 10218 is HfO2-30wt%Si composite APS powder pressed specimens. - AE 10349 is HfO2-70wt%Si composite APS powders


www.nasa.gov 34

Creep Behavior of HfO2-Si Bond Coat – Stress Dependence – Creep rate stress dependence studied – Stress exponents determined to be 2.6

-19.0

-18.0

-17.0

-16.0

-15.0

-14.0

2.2 2.4 2.6 2.8 3 3.2 3.4 3.6

y = -24.513 + 2.6848x R= 0.99877

ln(c

reep

stra

in ra

te),

1/s

ln(�)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 500000 1000000 1500000 2000000

HfO2-30wt%Si_10 MPa

HfO2-30wt%Si_20 MPa

HfO2-30wt%Si_30 MPa

0 100 200 300 400 500

Tota

l stra

ins

Time, s

1.3852x10-8 1/s

7.6513x10-8 1/s

Time, hours

1.98x10-7 1/s 1300°C


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HfO2-Si EBC Durability Studies at Up to 1500°C

Cooling shower head jets

Test specimen

High temperature extensometer

Laser beam delivery optic system

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0

20

40

60

80

0 10 20 30 40 50 60 70

Strain, %

Stress, MPa

Tota

l cre

ep st

rain

, %

Stre

ss, M

Pa

Time, hr

Tsurface_EBC

=1371°C

TCMC-back

=1250°C

– APS and EB-PVD processing optimizations; long-term durability tested at 1400 – 1500°C

35

Tested 250h


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HfO2-Si Bond Coat 1400°C Thermal Gradient CMAS Tests

– High concentration SiO2 region may have lower CMAS resistance

36

CMAS region SiO2

development and performance evaluations of hfo -si and ......tbc 2500ºf turbine tbc 2700 f (1482 c)...

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