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

7 wt% Yttria-Stabilized Zirconia7 wt% Yttria-Stabilized Zirconia

Jonathan LevinJonathan LevinPurdue UniversityPurdue University

Advisor: Prof. TriceAdvisor: Prof. Trice

Background on MaterialBackground on Material

The material used in this experiment was 7 wt% The material used in this experiment was 7 wt% yttria-stabilized zirconia (YSZ).yttria-stabilized zirconia (YSZ).

Yttria-stabilized zirconia is a commonly used Yttria-stabilized zirconia is a commonly used thermal barrier coating (TBC) in applications thermal barrier coating (TBC) in applications such as coating turbine blades in gas engines.such as coating turbine blades in gas engines.

Thermally coating the turbine blades lets Thermally coating the turbine blades lets engines run hotter and therefore more fuel engines run hotter and therefore more fuel efficiently.efficiently.11

1. http://www.nist.gov.sigmaxi.Posters02/Dobbins-abst.html1. http://www.nist.gov.sigmaxi.Posters02/Dobbins-abst.html

Introduction

Thermally Coated Turbine BladeThermally Coated Turbine Blade

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

Introduction

Plasma SprayingPlasma Spraying

For this experiment, For this experiment, thin, stand-alone YSZ thin, stand-alone YSZ tubes with cylindrical tubes with cylindrical geometry were used.geometry were used.

These tubes were These tubes were created using the created using the method of plasma method of plasma spraying.spraying.

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

Introduction

Plasma Spray GunPlasma Spray GunIntroduction

MicrostructureMicrostructure

The molten YSZ droplets The molten YSZ droplets from the plasma gun from the plasma gun solidify as layered solidify as layered lamellae on the lamellae on the substrate.substrate.This method results in This method results in many defects in the many defects in the microstructure in the form microstructure in the form of microcracks and pores of microcracks and pores in and between the in and between the lamellae.lamellae.

Intralamellar Crack

Intralamellar Pore

Interlamellar Pore

Introduction

Background on ExperimentBackground on Experiment

Previous experiments have shown an increase in Previous experiments have shown an increase in modulus as the stress on the tube increases.modulus as the stress on the tube increases.

In this experiment, a In this experiment, a compressive stress was compressive stress was put on the tube to see if put on the tube to see if this increase in modulus this increase in modulus was due to cracks was due to cracks perpendicular to the perpendicular to the applied load closing in, applied load closing in, increasing the density of increasing the density of the material.the material.

Introduction

Background on ExperimentBackground on Experiment

How cracks parallel How cracks parallel to the applied stress to the applied stress open and propagate open and propagate was also examined was also examined in this experimentin this experiment

A compressive stress was used because it A compressive stress was used because it approximates the compressive thermal approximates the compressive thermal stresses experienced in service.stresses experienced in service.

Introduction

Surface MicrostructureSurface MicrostructureIntroduction

Sample PreparationSample Preparation

Cut

Dissolve Al in HClRemoving alumina core

Procedure

ESEM with Load FrameESEM with Load Frame

This experiment will be performed using an This experiment will be performed using an environmental scanning electron microscope environmental scanning electron microscope (ESEM) equipped with a load frame.(ESEM) equipped with a load frame.

Load FrameESEM

Procedure

Specimen in Load FrameSpecimen in Load Frame

YSZ Tube

Procedure

Strain GaugesStrain Gauges

Two strain gauges were Two strain gauges were attached to opposite sides of attached to opposite sides of the tube in the axial direction.the tube in the axial direction.

The tube was then placed The tube was then placed into the load frame and into the load frame and balanced by putting a small balanced by putting a small load on the tube and load on the tube and inserting thin metal shims on inserting thin metal shims on either side until the strains on either side until the strains on each side were as close to each side were as close to equal as possible.equal as possible.

Strain Gauge

Procedure

Experimental ProcedureExperimental Procedure

Once the sample was calibrated and the Load Once the sample was calibrated and the Load frame was inserted in to the ESEM, a region was frame was inserted in to the ESEM, a region was found which exhibited cracks oriented in various found which exhibited cracks oriented in various directions.directions.

The sample was then incrementally loaded, with The sample was then incrementally loaded, with strain data and micrographs taken at each stress strain data and micrographs taken at each stress increment.increment.

The behavior of the cracks was observed at The behavior of the cracks was observed at each increment.each increment.

Procedure

Stress-Strain CurveStress-Strain Curve

The stress-strain The stress-strain curve obtained from curve obtained from the compression test the compression test of the tube showed of the tube showed two distinct regions two distinct regions with differing with differing modulus.modulus.

The modulus also The modulus also increased, probably increased, probably due to cracks due to cracks closing.closing.

Results and Discussion

MicrographsMicrographs

0 MPa 129.1 MPa

Results and Discussion

Crack BehaviorCrack Behavior

The Micrographs The Micrographs show the vertical show the vertical cracks opening and cracks opening and the horizontal cracks the horizontal cracks closing.closing.

This graph shows This graph shows crack width vs. stress crack width vs. stress for five crack locations.for five crack locations.

Results and Discussion

Stress and Strain in SampleStress and Strain in Sample

zz E

z

F

A

Poisson’s Effect causes the tube to Poisson’s Effect causes the tube to have a positive strain in the have a positive strain in the directions perpendicular to the directions perpendicular to the direction of the applied stress. This direction of the applied stress. This is because as the tube is is because as the tube is compressed, it is resisting a compressed, it is resisting a change in volume by changing change in volume by changing shape instead.shape instead.

zx E

This causes vertical This causes vertical cracks to open, cracks to open, horizontal cracks to horizontal cracks to close, and both to close, and both to propagate.propagate.

Results and Discussion

Crack PropagationCrack Propagation

33.3 MPa 58.3 MPa

83.3 MPa 129.1 MPa

These images show a These images show a crack propagating crack propagating from an existing from an existing angled crack.angled crack.

This starts occurring This starts occurring around 58.3 which is around 58.3 which is about half of the about half of the failure stress. This failure stress. This agrees with previous agrees with previous work done on the work done on the failure of brittle solids.failure of brittle solids.22

2. Ashby, M. F. and Hallam, S. D. “The Failure of Brittle Solids Containging Small Cracks Under Compressive Stress States.” Acta Metall., 34, pp. 497-510 (1986)

Results and Discussion

ConclusionsConclusions

Thin YSZ tubes were tested in compression to Thin YSZ tubes were tested in compression to observe crack behavior in the material.observe crack behavior in the material.

The stress-strain curve for the YSZ tube showed The stress-strain curve for the YSZ tube showed and increasing modulus with two distinct regions and increasing modulus with two distinct regions of differing modulus.of differing modulus.

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

ConclusionsConclusions

One region exhibited a crack initiating from a One region exhibited a crack initiating from a angled crack and propagating roughly in the angled crack and propagating roughly in the direction of the applied stress. direction of the applied stress.

The crack initiated at roughly half of the failure The crack initiated at roughly half of the failure stress which agrees with previous work done on stress which agrees with previous work done on the failure of brittle solids.the failure of brittle solids.

AcknowledgementsAcknowledgements

I would like to thank the financial support of the I would like to thank the financial support of the National Science Foundation grant #DMR0134286. National Science Foundation grant #DMR0134286. I would also like to thank Prof. Rodney Trice for his I would also like to thank Prof. Rodney Trice for his support and guidance and Jan Eberle for his support and guidance and Jan Eberle for his technical support with the ESEM.technical support with the ESEM.

Questions?Questions?