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


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


Overview of Plasma Spray Process

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

Introduction


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


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


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


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


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


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


ESEM with Load Frame

Load FrameESEM

-Electro Scan 2020 ESEM

-Operated at 25 keV

-Working distance of 27 mm



YSZ Tube

Close-up of Load Frame

-Strain gages attached to sides of tube

-View outer surface of the coating



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)



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



Effect of Stress on Intralamellar Cracks

0 MPa 129.1 MPa

20 µm 20 µm


Micrographs of top surface

- Cracks formed during cooling


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



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



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)



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)


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)


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


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


-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


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


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

• Narrow intralamellar microcracks observed

Cross-sectional view Plan view

After 10-hrs @ 900oC

Sintering Behavior


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


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


Stress Relaxation Experiments on Stand-Alone YSZ Coatings

Stress Relaxation

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


Stress State in the YSZ During Heat-up

• Effective biaxial compressive stress applied withinthe plane of the coating


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


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



At 1000oC


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.


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.

Sintering and Deformation Mechanisms in Plasma-Sprayed 7 ...

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