Sintering and Deformation Mechanisms in Plasma-Sprayed 7

School of Materials EngineeringThermal Barrier Coatings

Sintering and Deformation Mechanisms in Plasma-Sprayed 7 wt.% Y2O3-ZrO2 Thermal Barrier Coatings

Graeme DickinsonJohn Levin*

Christophe DeschaseauxRod Trice, Ph.D.

NSF CAREER Award (DMR 0134286)Purdue Research Foundation*Undergraduate support partly by REU DMR 0243830

October 24, 2003

School of Materials EngineeringPurdue University

School of Materials EngineeringThermal Barrier Coatings

Thermal Barrier Coatings

Ceramic top-coat(~200 µm)

Bondcoat (~100 µm)

Superalloy

Hot Gas Flow

A 200 µm thick YSZ topcoat provide a 200oC drop → engine efficiency increases by ~ 6 - 12%

R.A. Miller, Surf. Coat. Tech., 30, pp. 1-11 (1987)R. Stevens, “An Introduction to Zirconia”, Publication of Magnesium Elektron Inc.

Typical bondcoat materials: MCrAlY, PtAlTypical topcoat material: 7 wt% Y2O3-ZrO2 (YSZ) – focus of current research

Blade for a gas turbine engine

Introduction

www.kp.dlr.de/WB-WF/schaufel.jpg

School of Materials EngineeringThermal Barrier Coatings

Overview of Plasma Spray Process

www.swunited.com/plasma/pages/placap.htm

Introduction

School of Materials EngineeringThermal Barrier Coatings

Plasma-Sprayed YSZ Microstructure

• Lamellae are observed

• Interlamellar pores

• Intralamellar cracks

Introduction

BF TEM/Plan View Orientation

• Columnar grains

• Porosity of 15-20%

BF TEM/Cross-Sectional Orientation

School of Materials EngineeringThermal Barrier Coatings

Outline

• Introduction

• Experimental Procedure

• Results and Discussion-Room temperature mechanical behavior-Sintering behavior-Stress relaxation behavior

• Conclusions

Focus of Our Research: Link the microstructure, before and after high temperature exposure, to the thermal and mechanical properties of the YSZ coating off-of-the-substrate

School of Materials EngineeringThermal Barrier Coatings

Processing of Stand-Alone Coating SamplesExperimental Procedure

⇒Cut

Etching Al in HClTo remove alumina core

⇒

⇒ ⇒Plasma plume

YSZ Powder

AluminaRod

Melted Powder

Seven axisrobot

Turntable

Cool air

YSZ stand-alone coating

School of Materials EngineeringThermal Barrier Coatings

Plan View

Cross Section

Transmission Electron Microscopy Procedures

• Bright-field TEM analysis

• Cross-section and plan-view –

• Tripod polishing technique

• Gatan® ion-mill

• JEOL 2000FX

Experimental Procedure

School of Materials EngineeringThermal Barrier Coatings

Sample Characteristics

Diameter = 13 mmHeight = 12-15 mmThickness = 200-300 µmPorosity = 13-18%

Yttria rich, non-transformable t-ZrO2 coating

Overview of YSZ Stand-Alone Coatings

Cut-away view of tube

Overview of Stand-Alone Coating

School of Materials EngineeringThermal Barrier Coatings

In-Situ Observation of Crack Behavior in Plasma-Sprayed Coatings

Com

pres

sive

Stre

ss

Strain

Change in slopeon loading

Hysteresis

Room Temperature Mechanical Behavior

K.F. Wesling, D.F. Socie, and B. Beardsley, J. Am. Ceram. Soc., 77 [7] 1863-68 (1997).R.W. Trice, D.W. Prine, K.T. Faber, J. Am. Ceram. Soc., 83 [12] 3057-64 (2000).

-Significant cracking“heard” with acousticemission techniquesduring loading

-Two distinct slopesin stress-strain data

School of Materials EngineeringThermal Barrier Coatings

ESEM with Load Frame

Load FrameESEM

-Electro Scan 2020 ESEM

-Operated at 25 keV

-Working distance of 27 mm

Room Temperature Mechanical Behavior

School of Materials EngineeringThermal Barrier Coatings

YSZ Tube

Close-up of Load Frame

-Strain gages attached to sides of tube

-View outer surface of the coating

Room Temperature Mechanical Behavior

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Procedures Followed for Compression Testing of Tubes in ESEM

• A region was located which exhibited cracks oriented in various directions with respect to the applied stress.

• The sample was then incrementally stressed, with strain data andmicrographs taken at each stress increment (approximately 10 MPaper step)

Room Temperature Mechanical Behavior

School of Materials EngineeringThermal Barrier Coatings

0

20

40

60

80

100

120

140

0 500 1000 1500 2000 2500 3000

Stre

ss(M

Pa)

Strain, m/ m

30.3 GPa

62.1 GPa

Strain gages

Stress Direction

Horizontal Crack

VerticalCrack

Stress-Strain Results

-Distinct change in modulus with stress

Room Temperature Mechanical Behavior

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Effect of Stress on Intralamellar Cracks

0 MPa 129.1 MPa

20 µm 20 µm

Room Temperature Mechanical Behavior

Micrographs of top surface

- Cracks formed during cooling

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Vertical Crack 2 Horizontal Crack 3

Horizontal Crack 2

Horizontal Crack 1

20 µm

Vertical Crack 1

Intralamellar Cracks Investigated in Present Study

- Chose 5 cracks to investigate

- Measured change in crack width as a function of applied stress

Room Temperature Mechanical Behavior

School of Materials EngineeringThermal Barrier Coatings

Results of Crack Measurements

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 20 40 60 80 100 120 140

Cra

ck W

idth

(µm

)

Applied Stress (MPa)

Vertical Crack 1

Vertical Crack 2

Horizontal Crack 1Horizontal Crack 2

Horizontal Crack 3

-Vertically oriented cracks open, horizontallyoriented cracks close

-Seems plausible that theincrease in E with stressis due to partial closure ofhorizontally oriented cracks

Room Temperature Mechanical Behavior

School of Materials EngineeringThermal Barrier Coatings

Crack Nucleation and Propagation

33.3 MPa 58.3 MPa

83.3 MPa 129.1 MPa

• These images show a crack nucleating and propagating from an existing angled crack

• Significant crack propagation prior to failure

• Consistent with acoustic emission results on PS alumina

Trice, Prine, Faber, J. Am. Ceram. Soc., 83 [12] 3057-64 (2000)

Room Temperature Mechanical Behavior

School of Materials EngineeringThermal Barrier Coatings

Crack Initiation in Angled Micro-Cracks –Ashby and Hallam Results* on PMMA

OriginalCrack

Crack Close-up

*Ashby, M. F. and Hallam, S. D., Acta Metall., 34, pp. 497-510 (1986)

School of Materials EngineeringThermal Barrier Coatings

0

0.5

1

1.5

2

2.5

3

3.5

0 200 400 600 800 1000 1200

Ther

mal

Con

duct

ivity

, W/m

/K

Measurement Temperature, °C

1400oC/50-hr

As-Sprayed

1000oC/50-hr

1200oC/50-hr

Sintering Behavior of PS YSZ

-Closure of intralamellar cracks

-Full closure of intralamellar cracks

-Cont. sintering/some m-ZrO2

-Partial closure of interlamellar pores

Observed Microstructural Changes

Sintering Behavior

Trice, Su, Mawdsley, Faber, de Arellano-Lopez,

Wang, Porter, J. Mat. Sci. 37 235-65 (2002)

School of Materials EngineeringThermal Barrier Coatings

Dilatometry Measurements

• Linear resolution: 1 µm (~ 0.02% for a 15mm coating)• Dilatometer data are deconvoluted in order to isolate the shrinkage behavior of the YSZ coating.• Temperatures investigated: 800°C thru 1400°C, simple ramps and 10-hr isothermal tests

Top View

Sintering Behavior

School of Materials EngineeringThermal Barrier Coatings

Effect of Multiple Heating/Cooling Cycles

• Subtracted out the expansion of apparatus and coating

• Measurable shrinkage,detected at 950oC

0

500

1000

1500

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0 200 400 600 800 1000 1200 1400

Tem

pera

ture

, o C

Percent Linear S

hrinkage, %

Time, (minutes)

1st Run

2nd Run3rd Run

4th Run

Sintering Behavior

School of Materials EngineeringThermal Barrier Coatings

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0 100 200 300 400 500 600

Line

ar S

hrin

kage

, %

Time at Indicated Temperature, min

1400oC

1300oC

1200oC

1000oC800oC, 900oC

1100oC

Effect of 10-hr Isotherms on Linear Shrinkage of YSZ TubesSintering Behavior

School of Materials EngineeringThermal Barrier Coatings

Effect of Isothermal Temperature on Shrinkage Rate

-0.01

-0.008

-0.006

-0.004

-0.002

0

0 80 160 240 320 400 480 560

Line

ar S

hrin

kage

Rat

e, %

/ m

in

Time at Temperature, min

1100oC

1400oC

1300oC

1000oC

1200oC

Sintering Behavior

• Significant shrinkagestill occurring after 10-hrsAt 1300oC, 1400oC

School of Materials EngineeringThermal Barrier Coatings

Link Shrinkage Behavior with Microstructural ChangesAfter 10-hrs @ 800oC

• Narrow intralamellar microcracks observed

Cross-sectional view Plan view

After 10-hrs @ 900oC

Sintering Behavior

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Microstructural Changes with TemperatureAfter 10-hrs @ 1000oC

• No narrow intralamellar microcracks were observed.• Pores and wide intralamellar cracks remain.

Cross-sectional view Plan view

Sintering Behavior

School of Materials EngineeringThermal Barrier Coatings

Microstructural Changes with TemperatureAfter 10-hrs @ 1200oC

• Interlamellar lenticular pores are breaking down into channels of isolated globular pores by pinching between adjacent lamellae.

Cross-sectional view Cross-sectional view

Sintering Behavior

School of Materials EngineeringThermal Barrier Coatings

Stress Relaxation Experiments on Stand-Alone YSZ Coatings

Stress Relaxation

• Apply constant strain, monitor load• Test temps: 1000oC, 1050oC, 1100oC, 1200oC

School of Materials EngineeringThermal Barrier Coatings

Stress State in the YSZ During Heat-up

• Effective biaxial compressive stress applied withinthe plane of the coating

School of Materials EngineeringThermal Barrier Coatings

0

5

10

15

20

25

40 80 120 160 200

Stre

ss (M

Pa)

Time (min)

1000°C

1100°C

1200°C

1050°C

Original Stress Applied was 20 MPa

Stress Relaxation Behavior as a Function of TemperatureStress Relaxation

801200130110040010506001000

Time to 0 MPa,

min

Temp. oC

School of Materials EngineeringThermal Barrier Coatings

Stress Relaxation Behavior As a Function of Beginning Stress Level

Stress Relaxation

• Modeling behavior of thematerial

• TEM experiments

0

5

10

15

20

25

30

0 20 40 60 80 100 120

Stre

ss (M

Pa)

Time (min)

10 MPa Initial Stress

20 MPa Initial Stress

30 MPa Initial Stress

At 1000oC

School of Materials EngineeringThermal Barrier Coatings

Summary

• The stress-strain curve for the YSZ tube showed an increasing modulus with two distinct regions of differing modulus –linked to crack closure.

• Micrographs taken using an ESEM showed that cracks oriented parallel to the applied stress opened and cracks oriented perpendicularly to the applied stress closed.

School of Materials EngineeringThermal Barrier Coatings

Summary

• The shrinkage and shrinkage rate of PS YSZ increased with temperature and decreased with time. PS YSZ is still shrinking after 10 hours at 1300oC and 1400oC.

• Narrow intralamellar cracks are healing around 1000oC. All cracks have healed after 10 hours at 1000oC.

• Interlamellar pores were found to close at temperatures > 1000oC. This process seems to be triggered by pinching between two adjacent grains as they assume an equilibrium shape.