AStudyofFailureDevelopmentinThickThermal …17147/...Abstract Thermal barrier coatings (TBC) are...

A Study of Failure Development in Thick ThermalBarrier Coatings

Karin CarlssonLITH-IEI-TEK--07/00236--SE

ExamensarbeteInstitutionen för ekonomisk och industriell utveckling

ExamensarbeteLITH-IEI-TEK--07/00236--SE

A Study of Failure Development in Thick ThermalBarrier Coatings

Karin Carlsson

Handledare: Håkan BrodinSIEMENS Industrial Turbomachinery AB

Examinator: Sten JohanssonIEI, Linköping University

Linköping, 6 December, 2007

Avdelning, InstitutionDivision, Department

Division of Engineering MaterialsDepartment of Management and EngineeringLinköpings universitetSE-581 83 Linköping, Sweden

DatumDate

2007-12-06

SpråkLanguage

� Svenska/Swedish� Engelska/English

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�

RapporttypReport category

� Licentiatavhandling� Examensarbete� C-uppsats� D-uppsats� Övrig rapport�

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URL för elektronisk versionhttp://www.ikp.liu.se/kmt/

http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-10397

ISBN—

ISRNLITH-IEI-TEK--07/00236--SE

Serietitel och serienummerTitle of series, numbering

ISSN—

TitelTitle

A Study of Failure Development in Thick Thermal Barrier CoatingsStudie av skadeutvecklingen hos tjockt termiskt barriärskikt

FörfattareAuthor

Karin Carlsson

SammanfattningAbstract

Thermal barrier coatings (TBC) are used for reduction of component temperaturesin gas turbines. The service temperature for turbines can be as high as 1100oCand the components are exposed to thermal cycling and gases that will causethe component to oxidize and corrode. The coatings are designed to protect thesubstrate material from this, but eventually it will lead to failure of the TBC. Itis important to have knowledge about when this failure is expected, since it isdetrimental for the gas turbine.

The scope of this thesis has been to see if an existing life model for thinTBC also is valid for thick TBC. In order to do so, a thermal cycling fatigue test,a tensile test and finite element calculation have been performed. The thermalcycling fatigue test and finite element calculation were done to find correlationsbetween the damage due to thermal cycling, the number of thermal cycles andthe energy release rate. The tensile test was preformed to find the amountaccumulated strain until damage.

The thermal cycling lead to failure of the TBC at the bond coat/top coatinterface. The measurment of damage, porosity and thickness of thermally grownoxide were unsatisfying due to problems with the specimen preparation. However,a tendency for the damage development were seen. The finite element calculationsgave values for the energy release rate the stress intensity factors in mode I andmode II that can be used in the life model. The tensile test showed that thefailure mechanism is dependent of the coating thickness and it gave a rough valueof the maximum strain acceptable.

NyckelordKeywords Thick Thermal Barrier Coatings, Thermal Cycling Fatigue, Tenile Test with

Acoustic Emission, Failure Development

AbstractThermal barrier coatings (TBC) are used for reduction of component tempera-tures in gas turbines. The service temperature for turbines can be as high as1100oC and the components are exposed to thermal cycling and gases that willcause the component to oxidize and corrode. The coatings are designed to protectthe substrate material from this, but eventually it will lead to failure of the TBC.It is important to have knowledge about when this failure is expected, since it isdetrimental for the gas turbine.

The scope of this thesis has been to see if an existing life model for thin TBCalso is valid for thick TBC. In order to do so, a thermal cycling fatigue test, a ten-sile test and finite element calculation have been performed. The thermal cyclingfatigue test and finite element calculation were done to find correlations betweenthe damage due to thermal cycling, the number of thermal cycles and the energyrelease rate. The tensile test was preformed to find the amount accumulated strainuntil damage.

The thermal cycling lead to failure of the TBC at the bond coat/top coat in-terface. The measurment of damage, porosity and thickness of thermally grownoxide were unsatisfying due to problems with the specimen preparation. However,a tendency for the damage development were seen. The finite element calculationsgave values for the energy release rate the stress intensity factors in mode I andmode II that can be used in the life model. The tensile test showed that the failuremechanism is dependent of the coating thickness and it gave a rough value of themaximum strain acceptable.

v

Preface

This thesis has been done for SIEMENS Industrial Turbomachinery AB in Fin-spång and is the final part of my education to achieve a Master’s degree in Sciencein Mechanical Engineering at Linköping University. The project was carried outduring the summer and fall of 2007 at Linköping University and at SIEMENS.

I would like to thank my supervisors, professor Sten Johansson at Linköping Uni-versity and Håkan Brodin at SIEMENS. I would also like thank Annethe Billeniusand Bo Skoog for assistance in the laboratory, Xin-Hai Li and Johan Moverare atSIEMENS for assistance with test equipment and all the others at the departmentof engineering materials for good advices and Friday-fika. I would also like to givea special thanks to my Jonas for giving me support at all times.

Linköping, 2007-11-12

Karin Carlsson

vii

Contents

1 Introduction 31.1 SIEMENS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2 Gas Turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Problem Description . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Theory 92.1 Superalloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2 Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.1 TBC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.3 Coating Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3.1 Zirconia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.3.2 MCrAlY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.4 Coating Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.4.1 Plasma Spraying . . . . . . . . . . . . . . . . . . . . . . . . 14

2.5 Oxidation and Corrosion . . . . . . . . . . . . . . . . . . . . . . . . 142.5.1 Pilling-Bedworth Ratio . . . . . . . . . . . . . . . . . . . . 15

2.6 Failure Mechanism of TBC . . . . . . . . . . . . . . . . . . . . . . 16

3 Experiment 193.1 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.1 Thermal Cycling Fatigue . . . . . . . . . . . . . . . . . . . 193.1.2 Tensile Test with Acoustic Emission . . . . . . . . . . . . . 21

3.2 Specimen Preparation . . . . . . . . . . . . . . . . . . . . . . . . . 213.2.1 Traditional Specimen Preparation . . . . . . . . . . . . . . 223.2.2 Ion Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.3 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.3.1 SEM and LOM . . . . . . . . . . . . . . . . . . . . . . . . . 233.3.2 Graphics Processing . . . . . . . . . . . . . . . . . . . . . . 243.3.3 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4 Finite Element Calculations . . . . . . . . . . . . . . . . . . . . . . 26

4 Results and Discussion 294.1 Thermal Cycling Fatigue . . . . . . . . . . . . . . . . . . . . . . . . 29

4.1.1 Specimen Preparation . . . . . . . . . . . . . . . . . . . . . 304.1.2 Evaluation of Failure Mechanism . . . . . . . . . . . . . . . 31

ix

x Contents

4.1.3 Evaluation of Porosity . . . . . . . . . . . . . . . . . . . . . 314.1.4 Crack Evaluation . . . . . . . . . . . . . . . . . . . . . . . . 334.1.5 Evaluation of Thermally Grown Oxides . . . . . . . . . . . 334.1.6 Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.2 Finite Element Calculations . . . . . . . . . . . . . . . . . . . . . . 374.3 Tensile Test with Acoustic Emission . . . . . . . . . . . . . . . . . 37

4.3.1 Failure Mechanisms . . . . . . . . . . . . . . . . . . . . . . 38

5 Conclusions and Future Work 415.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Bibliography 43

A Appendix 45A.1 Crack Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45A.2 Finite Element Calculations . . . . . . . . . . . . . . . . . . . . . . 48A.3 Tensile Test with Acoustic Emission . . . . . . . . . . . . . . . . . 51A.4 Chemical Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

List of Figures1.1 Industrial gas turbine, SGT800. Copyright SIEMENS Industrial

Turbomachinery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.2 A Brayton cycle which consist of a gas compressor, a combustion

chamber and a turbine. It is run in an open cycle. . . . . . . . . . 51.3 Temperature/entropy and pressure/volume diagrams for an ideal

Brayton cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 The thermal gradient in a TBC system. . . . . . . . . . . . . . . . 102.2 A duplex TBC system. In thick TBC the top coat is between 1-1,5

mm. The thermally grown oxide is less than 10 µm and the bondcoat is approximatly 0,15 mm. . . . . . . . . . . . . . . . . . . . . 11

3.1 Thermal cycling fatigue. The specimen-table shifts from the furnaceto air-cooling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2 The cooling part of a recorded thermal cycle. . . . . . . . . . . . . 203.3 The specimen used at thermal cycling fatigue test. . . . . . . . . . 203.4 The tensile test. The arrows points at the microphones which is

taped to the specimen holder. Attached to the specimen is a devicemeasuring the strain. . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.5 The tensile test specimen. . . . . . . . . . . . . . . . . . . . . . . . 223.6 Ion etching. An ion beam makes a cut in the specimen. . . . . . . 233.7 A picture used for evaluation of the porosity. Measurements were

made in three different areas, high, middle and low. . . . . . . . . 253.8 The crack length was measured as the horizontal ”image” of the

real crack. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.9 The displacement of the nodes in the circle is used for calculation

of a relation of KI and KII . . . . . . . . . . . . . . . . . . . . . . . 273.10 The right picture shows the modeled undulation and the left picture

shows the same part during loading. . . . . . . . . . . . . . . . . . 27

4.1 A cut done in the ceramic layer by ion etching. . . . . . . . . . . . 314.2 The as-coated specimen after ion etching. The ceramic layer is at

the bottom and the substrate at the top. The bond coat appears tohave two shades; the darker one is cut once more than the brighterone. The cut was done from the ceramic down to the substrate. . . 32

4.3 The specimen has been cycled 300 times. The crack is an exampleof a mixed failure. At number 1 the crack is propagating in theceramic and at 2 in the thermally grown oxide. . . . . . . . . . . . 32

4.4 A crack or a pull-out in a specimen cycled 300 times. . . . . . . . . 344.5 The results from all three crack measurment. The measurments are

represented by the dots and the line is a trendline. . . . . . . . . . 344.6 There is no oxide between the bond coat and top coat. . . . . . . . 354.7 The results from the measurements of the thermally grown oxide.

The line is a trendline based on the measurements diplayed as thedots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2 Contents

4.8 The results from the mesurements of the thermally grown oxide.The two lines represent the average and median values. . . . . . . 36

4.9 The stress intensity factor in mode I and II for a model with 6 µmthick thermally grown oxide. . . . . . . . . . . . . . . . . . . . . . 38

4.10 The failure of a tensile specimen with 1000 µm thick coating. Thereare both transverse and interface cracks. . . . . . . . . . . . . . . . 39

4.11 Spallation of the coating. Both transverse and interface cracks arevisible. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

A.1 Measurement 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45A.2 Measurement 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46A.3 Measurement 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46A.4 The three different measurments. . . . . . . . . . . . . . . . . . . . 47A.5 Energy release rate for different oxide thicknesses. . . . . . . . . . 48A.6 The stress intensity factor in mode I for different oxide thicknesses. 48A.7 The stress intensity factor in mode II for different oxide thicknesses. 49A.8 The energy release rate for different oxide thicknesses. . . . . . . . 49A.9 The stress intensity factor in mode I for different oxide thicknesses. 50A.10 The stress intensity factor in mode II for different oxide thicknesses. 50A.11 500 µm test 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51A.12 500 µm test 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51A.13 1000 µm test 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52A.14 1000 µm test 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52A.15 1500 µm test 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53A.16 1500 µm test 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53A.17 K0. Analysis started in the top coat. . . . . . . . . . . . . . . . . . 54A.18 The result from the line scan. Spectrum (x) represent the point

where the measurement was done. . . . . . . . . . . . . . . . . . . 54A.19 K300. Analysis started at the top of the bond coat. . . . . . . . . . 55A.20 The result from the line scan. Spectrum (x) represent the point

where the measurement was done. . . . . . . . . . . . . . . . . . . 55A.21 K450. Analysis started at the top of the bond coat. . . . . . . . . . 56A.22 The result from the line scan. Spectrum (x) represent the point

where the measurement was done. . . . . . . . . . . . . . . . . . . 56A.23 A internal bond coat oxide from K450. . . . . . . . . . . . . . . . . 57A.24 The results from the line scan. The different graphs represent dif-

ferent materials oxygen, silicon, chromium, cobalt, nickel, yttriumand aluminum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

List of Tables3.1 The procedure used for specimen preparation. . . . . . . . . . . . . 22

4.1 The measured porosity. High, middle and bottom indicates whereon the ceramic the porosity is measured. . . . . . . . . . . . . . . . 33

4.2 The visual inspection of the specimen during tensile test. . . . . . 39

Chapter 1

Introduction

This chapter gives a brief introduction to SIEMENS Industrial Turbomachineryand some basic theory of gas turbines. A problem description is also given.

1.1 SIEMENSSIEMENS Industrial Turbomachinery AB in Finspång produces steam and gasturbines, and engines for compressors and pumps in the oil and gas industry. Thehistory of the plant in Finspång goes back to 1631 when Louis De Geer boughtFinspong Bruk. The company De Geer made canons until 1911 and in 1913 thecompany was sold to STAL who started to produce steam turbines. In the 1940thSTAL developed three jet engines on commission for the Swedish Air Force, butthe Air Force chose a foreign engine. One of the jet engines was then developedinto a stationary gas turbine that generated 10 MW. It was launched 1955. Duringthe years, the plant has had different names. In the 1950th it became Stal-Lavalafter merging with Gustav De Laval’s De Laval Ångturbin AB, who was the firstto construct turbines in Sweden. Today it is a part of SIEMENS.Four types of gas turbines are produced in Finspång; SGT-500, SGT-600, SGT-700 and SGT-800, SGT-500 being the smallest and SGT-800 the largest. Theyare in the range of 15 to 50 MW.

1.2 Gas TurbinesGas turbines can be divided into two groups; airborne and land-based. The air-borne turbines are used for airplane propulsion and a common application for theland-based turbines are the electric power generation. In the base-load electricpower generation, which has been dominated by coal and nuclear power plants,gas turbines are being installed more and more. This can be explained by thegas turbines higher efficiency, lower capital cost, shorter installation time, betteremission characteristics and the abundance of natural gas supplies [1]. Gas tur-bines are also used for peaking power plants. An industrial gas turbine is depicted

3

4 Introduction

Figure 1.1. Industrial gas turbine, SGT800. Copyright SIEMENS Industrial Turboma-chinery.

in figure 1.1. Thermodynamically a gas turbine can be explained by the Braytoncycle, also known as the Joule cycle. The cycle consists of three components:

• A gas compressor (the compressor)

• A mixing chamber (the combustion chamber)

• An expander (the turbine)

Air is drawn in to the air inlet (1). When the air reaches the compressor (2)the temperature and the pressure are raised. The high-pressure air then proceedsto the combustor (3) where the air is mixed with fuel and ignited, resulting inhigh-temperature gases. The gases flow through the turbine (4) where it expandsto atmospheric pressure, producing mechanical energy. This energy is used togive the generator shaft (5) a torque and to drive the compressor. The exhaust isdischarged through the exhaust outlet (6). Either the exhaust gas goes throughexhaust treatments (run in an open cycle) or it is used to heat water in a steamturbine (combined cycle process). Figure 1.2 shows a simplified model of theBrayton cycle. The thermodynamic states for the gas are shown in figure 1.3. Themedium is considered to be an ideal gas. Numbers 1-4 in figure 1.2 corresponds tothe same numbers in figure 1.3. Between 1-2 and 3-4 the gas is compressed withno change in entropy (isentropic compression). Between 2-3 and 4-1 the pressureis kept constant, but between 2-3 there is an addition of heat while between 4-1there is a rejection of heat. The thermal efficiency can be written as follows

ηth,Brayton = wnetqin

= 1− qoutqin

= 1− cp(T4 − T1)cp(T3 − T2)

(1.1)

where w is work per unit mass, q is heat transfer per mass unit, cp is constantpressure specific heat and T is temperature. To increase the thermal efficiencyin a gas turbine, the pressure ratio and/or specific heat ratio of the working fluidcan be increased. But an increase in pressure means an increase in energy spent

1.2 Gas Turbines 5

Figure 1.2. A Brayton cycle which consist of a gas compressor, a combustion chamberand a turbine. It is run in an open cycle.

Figure 1.3. Temperature/entropy and pressure/volume diagrams for an ideal Braytoncycle.

6 Introduction

on compression and that is a loss unwanted in the power generation. So thereforean increase of the turbine inlet temperature, T3, is the better solution. But itis limited by the material in the turbine and combustor. Development of newmaterials for the turbine has increased the turbine inlet temperature from 540◦Cin the 1940s to 1425◦C today [1]. The materials that make this possible aresuperalloys and coatings.

1.3 Problem DescriptionAn industrial gas turbine is designed to be in service for about 120 000 hours(∼13 years). It can run on maximum load for months without stop and theinspection intervals are long, at least 10 000 hours. This requires reliable lifemodels.There is a thermal insulating and corrosion/oxidation resistant coating, calledthermal barrier coating (TBC), deposited onto hot parts of gas turbines. The lifemodel for this coating is based upon Paris’ law, a relation between crack growthrate and data for the fracture mechanism

da

dN= C(∆K)n (1.2)

where a is the crack length, N number of cycles, K the stress intensity factor (inmode I) and C and n are constants depending on temperature, material, environ-ment, frequency and stress ratio. The relation has to be modified to suit the lifemodel. A parameter describing the amount of damage in the coating is used in-stead of the crack length since this allows for a better coupling between modellingresults and experimental data. Due to the nature of the TBC, crack growth in thesystem does not only occur in mode I, but also in mode II, mostly a mixture of thetwo. The interface where failure often occur has an undulated shape. At the ridgesmode I is dominating, at the valleys mode II and in between there is a mixtureof the two. When the crack is growing in mode I the crack surfaces move directlyapart from one another and in mode II the surfaces slides over each other in anin-plane shear mode. Mode II results in friction leading to more energy neededto open the crack than in mode I. Using this assumption, the parameter for thestress intensity factor can be changed to a parameter δ describing the mode and aparameter ∆G which is the energy release rate describing the energy released atcrack opening. The model needs to be calibrated with input data obtained fromexperiments and finite element calculations. The existing life model for TBC ismodified for a thin coating, ∼300 µm. Nowadays are also thicker coatings, 1-1,5mm, used. SIEMENS are therefore interested to find out if their existing life modelcould work also for thick TBC. This means that new input data has to be acquiredbut also that a study of the failure mechanism has to be done. In a thin TBCfailure often occur near the coating interface and therefore the model is based onthis type of cracking. If the cracking in a thicker coating would behave differently,a new life model might be needed.The goal of this thesis has been to study the failure mechanism of thick TBC andfind the input data needed, which are:

1.3 Problem Description 7

• A correlation between total damage of the coating, due to thermal fatigue,and the number of cycles.

• A correlation between total damage and energy release rate.

• The amount accumulated strain until damage of the coating.

The correlation between damage and number of cycles, together with the study offailure mechanisms, was obtained by a thermal cycling fatigue test. The correlationbetween damage and energy release rate was modeled in a finite element program.The strain limit was obtained through a tensile test with acoustic emission.

Chapter 2

Theory

This chapter covers the theory of the thermal barrier coatings and also gives abackground to why coatings are used.

2.1 SuperalloysSuperalloys are materials that can be used at service temperatures over 540◦C.Some can be used at temperatures as high as 80% of their melting temperature [2].They are nickel, iron-nickel or cobalt based alloys with large amounts of alloyingelements added in order to create a combination of high strength and resistance tocreep and corrosion at elevated temperatures. In general, the melting temperaturefor superalloys is in the same range as the melting temperature of steel. For gasturbines, nickel-based alloys are the most suitable due to its excellent mechanicalproperties at high temperatures. The high temperature strength in superalloys isbased on a stable face-centered cubic matrix that is combined with either precipi-tation hardening or solid-solution hardening. Generally, the superalloys consist ofan austenitic (γ-phase) matrix and several secondary phases. The most common ofsecondary phases are metallic carbides and γ’, which is the ordered face-centeredcubic-phase in iron-nickel and nickel-based alloys. Solid-solution hardened superal-loys withstand higher temperature better than precipitation hardened superalloys.Their mechanical properties are more stable while the properties changes for pre-cipitation hardened superalloys. This is because at high temperature precipitateswill grow at dislocations and the carbides will coarsen. In this thesis a wroughtsolid-solution hardened nickel-base superalloy has been studied.

2.2 CoatingsThe superalloys are optimized for load-carrying capability, with less concern forenvironmental resistance. Therefore coatings are used, to protect superalloys fromenvironmental attacks such as oxidation and/or corrosion. Coatings are tailoredfor their specific application. For example, a coating with aluminum is desired

9

10 Theory

if the material is subjected to oxygen, and a coating with chromium if subjectedto a corrosive environment. The coating can either be a diffusion coating wherethe surface of the superalloy component has been enriched with elements thatare protective against corrosion and oxidation (chemical vapour deposition typeprocesses), or an overlay coating where a layer is deposited onto the component(physical deposition processes). In this thesis an overlay coating produced byplasma spraying has been studied.

2.2.1 TBCAs mentioned earlier, an increase in gas turbine inlet temperature is desired inorder to increase the thermal efficency. To be able to do so, a thermal insulatingcoating is needed since the superalloys has reached their upper thermal limit. Thiscoating together with an oxidation/corrosion resistant coating is called a thermalbarrier coating (TBC) system. It is deposited on the hot-path components of gasturbines, such as combustionliners turbines vanes and blades. TBC systems canalso increase the efficiency without increasing the inlet temperature by decreasingthe need of air cooling. If both the air cooling and the inlet temperatures are thesame, it will increase the reliability of the component, while the decrease in temper-ature will increase the creep resistance and the resistance to thermal/mechanicalfatigue of the load carrying component. The TBC cannot compensate the aircooling completely. In order to produce a high temperature gradient, to reducethe temperature of the metal substrate, rear-side cooling is required. Figure 2.1shows a typical temperature gradient in a TBC system. A 0.6-0.7 mm ceramiclayer can reduce the substrate temperature with more than 200oC [3].

Figure 2.1. The thermal gradient in a TBC system.

Composition

The most common TBC system is the duplex system. It consists of two layers; oneouter ceramic layer that insulates and one intermediate metallic layer that protects

2.2 Coatings 11

the substrate from oxidation and corrosion. The metallic layer also provides anadhesion surface for the ceramic layer. The ceramic coat is called the top coat andthe metallic is called the bond coat. There are also multilayered TBC systems,where there is an erosion resistant layer, a corrosion-oxidation resistant layer, athermal stress control layer and a diffusion resistant layer [4]. A schematic view ofa duplex TBC system is illustrated in figure 2.2 and is the kind used in this thesis.In the interface between top coat and bond coat one can find the thermally grownoxide.

Figure 2.2. A duplex TBC system. In thick TBC the top coat is between 1-1,5 mm.The thermally grown oxide is less than 10 µm and the bond coat is approximatly 0,15mm.

The bond coat is sprayed onto the substrate, most commonly, with plasma spray-ing, explained more in section 2.4.1. It creates a surface with good adhesionagainst the top coat. Two other important purposes are to level out the differ-ences in thermal expansion between the substrate and top coat and to act as analuminum-reservoir. Aluminum will react with oxygen and create the thermallygrown oxide, which protects the substrate from high temperature corrosion andoxidation. The bond coat is usually made of MCrAlY, see section 2.3.2, and isapproximately 50 to 125 µm [5]. Ideally the BC should be free from pores sincethe pores will transport oxygen, leading to formation of oxides which can be detri-mental.

The purpose of the top coat is to act as a thermal insulator. It is often madeof partly yttrium stabilized zirconia (PSZ). Zirconia is suited for TBC because ofits low thermal conductivity (∼1,5-2 W/m2K) and it has a coefficient of thermalexpansion (6-8·10−6/K) to be compared to the substrates (∼13-14·10−6/K). Thetop coat is allowed to have a certain porosity since it decreases the heat conduc-tivity but also results in a reduction of stiffness. The thickness of the top coat alsoaffects the reduction of heat, for example, using a thick TBC instead of thin onescould reduce the amount of cooling air by 60% [3]. The thickness can vary from100 to 2000 µm.

12 Theory

The thermally grown oxide is formed during high temperature exposure. It mainlyconsist of alumina, Al2O3. Oxygen diffuses through the permeable top coat andreacts with the aluminum, which has the highest affinity to oxygen, in the bondcoat and forms the alumina. Oxygen transport through aluminum is very slowand therefore provides an oxidation protection for the metal substrate. The thick-ness of the thermally grown oxide is connected to the life time of the TBC systemand is about 0,5 to 10 µm, depending on thermal exposure (time and temperature).

The substrate will sustain the structural load.

2.3 Coating Materials

The materials in the TBC must have a high melting point, no phase transformationbetween room temperature and operations temperature, low thermal conductivity,chemical inertness, a thermal expansion that match the thermal expansion of themetallic substrate, good adherence to the metallic substrate and low sinteringrate of the porous microstructure [4]. Rare earth elements, Ti, Zr, Hf, Al and Siare elements whose oxides suits TBC applications, but the most widely used andstudied material is ZrO2. It performs very well in high temperature applications,such as gas turbines. A typical material for the bond coat is MCrAlY.

2.3.1 Zirconia

Zirconia, ZrO2, is a ceramic. Ceramics are built-up by ionic bonding between an-ions and cations. Anions are generally larger than cations. Therefore the ceramiccan be seen as a close-packed structure of anions with cations in the interstitialsites. But for zirconia, the cation (Zr+) is larger than the anion (O−), so here itis the cation that forms the close-packed structure with anions in the interstitialsites. Zirconia has a high diffusion coefficient for oxygen in the cubic phase, whileoxygen vacancies are introduced to compensate for the dopants lower valency [2].Therefore, it is important with an oxidation resistant coating underneath the topcoat. Zirconia is an allotropic material. Allotropy (pure materials) and polymor-phism (alloys) materials are materials that have more than one crystal structure.At room temperature the stable structure for zirconia is monoclinic. At highertemperatures, more symmetric structures are stable. At 1240◦C the monoclinictransform into the tetragonal structure that is stable up to 2370◦C, where it trans-forms to cubic. It remains cubic until it melts at 2680◦C [6]. Under high pressurezirconia can form an orthorhombic structure. The reason why the ceramic under-goes transformation is that temperature or pressure on the material can changethe interatomic distance and the atom vibration, and the initial structure mayno longer be the most stabile one, leading to transformation to a more stablestructure.

2.4 Coating Deposition 13

Stabilized Zirconia

A change in structure means a change of the crystal structure’s size. This leads to achange in volume, which is detrimental for components of pure zirconia. They tendto fail when the temperature is dropped and the tetragonal structure transformsinto monoclinic, which means an increase in volume. To prevent this, dopants,such as yttria (Y2O3), are added. They stabilize the cubic phase, even at roomtemperature. Yttria stabilized zirconia (YSZ) contain 7wt% yttria, which is theoptimum amount [5]. Other dopants are MgO and CaO.

Crack Shielding

When a load is applied and a crack is growing, the metastable tetragonal phaseat the tip of the crack will transform into monoclinic. The increase in volume willlead to a compressive stress at the crack tip and thereby inhibit crack growth.Additional load is then required for further crack growth. This only applies forstabilized zirconia, not pure zirconia. However, if too much yttria is added thecubic structure becomes too stable to undergo a tranformation.

2.3.2 MCrAlYM stands for nickel and cobolt (and iron if applied on steel) or a combinationof the two. Chromium and aluminum are added for creating protective oxides,Cr2O3 and Al2O3. Yttrium is improving the adhesion of the thermally grownoxide [7]. NiCrAl undergoes a phase transformation at 1000oC. If cooled, β + γwill transform to α + γ’. Cobalt is added to stabilize this transformation. 20 to26wt% cobalt will also improve the ductility [5]. In a NiCrAl alloy system is theγ phase a face-centered cubic Ni structure with occasional Al and Cr atoms. γ’ isa face-centered cubic structure with Al or Cr as corner atoms and Ni as cube facesatoms. The β phase, NiAl, is a body-centered cubic (bcc) structure with Ni ascorner atoms and Al as center atom. The α phase is α -Cu which is body-centeredcubic. The microstructure of the MCrAlY consists of a β phase in a γ matrix. Theβ/γ ratio decides the resistance against oxidation and corrosion, phase stabilityand the resistance against cracking [5].

2.4 Coating DepositionThere are two common ways to deposit the overlay coatings of the TBC sys-tem: plasma spraying and electron beam physical vapor deposition (EB-PVD). Inplasma spraying a molten powder is sprayed upon the component. EB-PVD isdone by vaporizing a raw material with a high-energy electron beam. The vaporis then deposited onto the component. The two processes results in two differentstructures. The sprayed coating will have a structure consisting of molten particlesin the shape of pancakes, pores (the amount depends on the powder size), trans-verse microcracks and, depending on spray technique, a thermally grown oxide.EB-PVD coatings have a columnar microsturcture. It has a finer structure with

14 Theory

no porosity, but it has higher heat conductivity due to its columnar structure. It isalso more costly than plasma spraying. In this thesis air plasma sprayed specimensare used.

2.4.1 Plasma SprayingThe top coat and bond coat are deposited on the substrate by plasma spraying.A prealloyed powder is injected into a high-temperature plasma gas stream in aplasma-spray gun. The molten powder is then deposited with high velocity ontothe substrate. The powder particles solidify on the substrate surface, creating acoating. When the droplets hit the substrate surface they form ”splats”, leading toporosity parallel to the surface. Plasma spraying is performed with temperatures ashigh as 10 000 - 20 000◦C. It can be done in vacuum (vacuum-plasma spraying) orin an inert gas of low-pressure (low-pressure-plasma spraying) to avoid the powderto react with oxygen. There is also air-plasma spraying, which is the method usedfor both the top and bond coat of the TBC in larger parts of the industrial gasturbines. Spraying in vacuum or inert gas would seem to be a better choice butthe components are too big to be placed in a vacuum chamber or similar. The sizeof the powder used is around 40 µm. A larger powder will have problem meltingand a powder finer than 10 µm will not be able to penetrate the plasma and willtherefore also have problem melting. A fine powder gives a more dense coatingthan a coarser powder, since the pores between the ”splats” are relatively smallfor the finer powder. On the other hand, a fine powder does not give the coarsersurface that is desired for good adhesion between bond coat and top coat. So inorder to get a dense coating with an adherent surface, a fine powder is used tobuild up the coating. Then a coarser powder is sprayed upon the finer powder tocreate the undulating surface. The size of the powder also decides if there is achemical or a mechanical bonding between the top and bond coat. A finer powdermeans a chemical bonding while a coarser gives the undulating surface which givesthe mechanical bonding.

2.5 Oxidation and CorrosionDuring operation the gas turbine will be exposed to gases causing both corrosionand oxidation. These phenomena can be divided into three groups:

• Type II Hot Corrosion.

• Type I Hot Corrosion.

• Oxidation.

The hot corrosion is a result of accelerated oxidation at 680-1050◦C [2]. Thealloy, or in this case the coating, will be covered with thin films of salts, typicallyalkali and alkaline earth sulfates, which will react with the material. Generally,Type I hot corrosion occurs above the melting point of these salts and type II hotcorrosion below. Corrosion has two stages; the initiation stage where there is a

2.5 Oxidation and Corrosion 15

breakdown of the protective oxide and the propagation stage where the salts haveaccess to unprotected metal and corrosion will occur at a high rate. As mentionedearlier, chromium is most effective in giving a resistance to hot corrosion sinceCr2O3 has a faster growing rate than Al2O3, meaning that it will reform fasterin case of a breakdown of the oxide scale due to hot corrosion. In the case ofcorrosion of the top coat, molten sulfates will react with the stabilizer. MgO isthe most reactive one and Y2O3 the least one, hence Y2O3 being more commonlyused. Reaction between the sulfates and the stabilizer leads to a ceramic moreprone to phase transformation leading to premature failure of the top coat whenthermally cycled.Above 900◦C, oxidation will occur. The top coat will not react with the oxygen,but it has a high diffusion coefficient for oxygen (Zr in cubic phase) leading tooxidation of the bond coat. Islands of oxide will grow and form an oxide filmbetween the top coat and bond coat (the thermally grown oxide). Also internaloxidation of the bond coat will take place. The oxide film will continue to grow byoxygen and/or metal ion diffusion. In the MCrAlY coat, alumina will grow withoxygen diffusion through the oxide, while nickel, chromium and cobalt will diffusethrough the alumina to react with oxygen. Some metals react more easily withoxygen than other. This depends on the free energy for oxide formation. The largerthe negative free energy is, the easier it is for a metal to form an oxide. Aluminumreacts more easily than nickel, chromium and cobalt. The characteristics of theoxide growth can be explained by the Pilling-Bedworth Ratio.

2.5.1 Pilling-Bedworth RatioThe Pilling-Bedworth ratio (PBR) describes how much of a metal that has beenoxidized. (PBR = volume of oxide formed / volume of metal consumed.) If theratio is less than one, PBR < 1, then the oxide takes up less volume than the metal,from which it is formed. This gives a coating with porosity and the oxidation willcontinue rapidly. When the ratio is between one and two, 1 6 PBR 6 2, thevolume of the oxide and the metal is almost the same. This gives an adherent andnon-porous oxide. The oxide will work as a protecting film, for example aluminumand titanium. And when the ratio is greater than two, PBR > 2, the oxide willtake up a large share of the volume. This can lead to the oxide flaking off andnon-oxidized material will be exposed. In the TBC a ratio between one and twois wanted in order to have a protective coating. The mathematical expressionfor oxide growth is also connected to the PBR. For a porous oxide (PBR<1) theequation for oxide growth can be written as:

y = kt (2.1)

where y is the thickness of the oxide, t time, k a constants depending on thetemperature, environment and the composition. A non-porous (PBR>2) oxidegives the equation

y =√kt (2.2)

And a thin-oxide film (1 6 PBR 6 2)

y = k ln ct+ 1 (2.3)

16 Theory

where c is a constant.

2.6 Failure Mechanism of TBCThe gas turbine hot components will be exposed to high temperatures, ther-mal/mechanical cyclic loading and an environment causing both corrosion andoxidation. This will eventually lead to failure of the TBC. In the case of airplasma sprayed TBC, that means spallation of the top coat [5]. These failures willoccur if either the stress increases, the material strength decreases, or a combina-tion of the two. The stress will increase due to mismatch in thermal expansion,temperature gradient and growth of the thermally grown oxide. The material canloose strength due to sintering at high temperatures, propagation of cracks anddepletion of aluminum which will lead to growth of more brittle oxides.The metallic and ceramic layers have a mismatch between their coefficients ofthermal expansion. This mismatch will cause compressive residual stresses uponcooling. According to A.G. Evans et al. [7] these stresses are as big as 3-6 MPa.The stresses are redistributed around imperfections leading to both compressiveand tensile stresses. They will be detrimental when exceeding the yield strength.During isothermal service these stresses will decrease as a result of relaxation andcreep. There is a temperature gradient trough the TBC system, which will leadto different thermal expansions, causing internal stresses. As in the case above,these stresses will decrease at high temperature exposure. Thick TBC systemshave a steeper temperature gradient than thin ones, and are therefore more sus-ceptible for this phenomenon [8]. The growth of the thermally grown oxide inducescompressive stresses, generally less than 1 GPa [7]. This oxide is, as mentionedearlier, mostly made up by alumina. But due to aluminum depletion in the bondcoat, other oxides called spinels are formed. These oxides are larger than the alu-mina and will therefore contribute to even higher compressive stresses. Since theyare more brittle than alumina, the strength of the coating decreases. The majorreasons for aluminum depletion are the formation of thermally grown oxides anddiffusion into the substrate material. At high temperatures (above 1200◦C) theceramic may sinter. It means a decrease of microcracks and pores, changing thematerials stiffness leading to a change of fracture properties. There is an amountof microcracks in the ceramic top coat in the as-coated material. These cracksare beneficial since they reduce both the stiffness and the thermal conductivity.During service the cracks will propagate and formation of new cracks will be theresult. This will with time lead to failure and delamination of the coating. Thenew cracks will start from locations where the stresses are large enough to ex-ceed the yield strength, for example at imperfections or due to thermal expansionmismatch. There are three types of failures

• White failure. This fracture occurs in the ceramic. It is called white failurewhile the ceramic fracture surface appears white.

• Black failure. Which is a failure occurring in the thermally grown oxide.The α-alumina gives the fracture surface a black appearance.

2.6 Failure Mechanism of TBC 17

• Mixed failure, which is a mixture of black and white fractures.

According to D. Renusch and M. Schütze, who have studied thin TBC failure withacoustic emission, cracking predominantly occurs during cooling. They did notdetect any cracking during heating and they mean that this shows that it is thethermal expansion mismatch stress, developed between the ceramic and metalliclayers, that is the driving force for cracking [9].

Chapter 3

Experiment

In this chapter the experiments done will be presented and explained. Some ad-ditional background theory will also be covered.

3.1 TestingTwo tests were performed, a thermal cycling fatigue test (TCF) and a tensile testwith acoustic emission. The TCF test was done to find a relationship between thenumber of thermal cycles and the damage due to these cycles and also to see werefracture will occur. The tensile test was performed to find out how much strainthat can be applied until crack initiation and failure.

3.1.1 Thermal Cycling FatigueThe thermal cycling fatigue, TCF, test was done in a thermal fatigue furnace, seefigure 3.1, at SIEMENS Industrial Turbomachinery in Finspång, Sweden. The

Figure 3.1. Thermal cycling fatigue. The specimen-table shifts from the furnace toair-cooling.

19

20 Experiment

specimens were placed on the specimen-table together with three thermocouples.The thermocouples logged the temperature inside a dummy specimen during thewhole cycle. Figure 3.2 gives an example from the temperature log. The specimen-

Figure 3.2. The cooling part of a recorded thermal cycle.

table was kept inside the furnace for 60 minutes. The furnace was adjusted to amaximum temperature of 1100oC. After that the specimen-table was set underair-cooling, to a minimum temperature of 100oC for 11 minutes. During the aircooling, a dummy-table was positioned in the furnace to minimize the heat-loss.Consequently one thermal cycle is 60 minutes of heating and 11 minutes of cooling,giving a cooling rate of approximately 1.5oC/s. The test done was done for 50,100, 150, 200, 300 and 450 (total damage) cycles. It was carried out by placingall the specimens in the furnace at the same time and then removing them one byone as they reached their predetermined number of cycles. The specimen used hadthe dimensions 30 x 50 x 5 mm, where the TBC system was 1,5 mm, see figure3.3. They were coated by using air plasma spraying, at the same time as a realgas turbine.

Figure 3.3. The specimen used at thermal cycling fatigue test.

3.2 Specimen Preparation 21

3.1.2 Tensile Test with Acoustic Emission

A tensile test with acoustic emission was preformed at Linköping University inorder to find the maximum applied strain until fracture. The failure mechanismwas also studied by visual inspection of the specimen during and after the test.All specimens were loaded until fracture. The test was performed with acousticemission. Two microphones attached to the specimen holder, see figure 3.4, wereused to ”hear” when damage was introduced in the material when loaded in ten-sion. When a crack is growing or is initiated, there is a release of energy whichwill generate an elastic wave. The wave will propagate through the material tothe surface. There the microphones will detect this wave and transform it to anelectrical signal that can be processed in a computer. The program used, Mistras,collected data on how many times there where energy releases (hits) and how bigthis release was (energy). It also recorded the strain, time and load. The tensile

Figure 3.4. The tensile test. The arrows points at the microphones which is taped tothe specimen holder. Attached to the specimen is a device measuring the strain.

specimens were coated with air plasma spraying. Three different top coat thick-nesses; 500, 1000 and 1500 µm, were used in order to see the effect of thicknesson the result. 500-1500 µm represent the upper range of TBC thicknesses used inSIEMENS gas turbines. See figure 3.5 for the dimensions of the specimens.

3.2 Specimen Preparation

The preparation of the test specimens for microscopic investigation was carriedout in two different ways; traditional specimen preparation and ion etching.

22 Experiment

Figure 3.5. The tensile test specimen.

3.2.1 Traditional Specimen Preparation

The specimens were first impregnated with a low-viscosity epoxy resin in vacuum.The epoxy cured under ambient conditions. This was done in order to distinguishoriginal pores and cracks due to manufactoring from the ones produced later dur-ing specimen preparation. The test specimen was subsequently sectioned in adiamond-precision cutter and then cold mounted in epoxy. The reason for coldmounting, instead of using hot mounting, is that the specimen is sensitive to theadditional stresses that are associated with the heating. The specimens were thenground and polished according to the procedure described in table 3.1, in order toreach the true structure. The idea is first to grind the specimen enough so that allmaterial affected by the cutting is removed. Next step is to grind or polish with afiner grain size than the last step, to remove the affected material from previousstep. This continues until the true structure is revealed.

Step Force Speed Lubricant Plate Medium TimeGrinding 20 N 300 rpm Water MD-Pian #200 - until planeDiamond polishing 20 N 150 rpm Green MD-Plan 45 µm 15 minDiamond polishing 20 N 150 rpm Green MD-Plan 15 µm 15 minDiamond polishing 20 N 150 rpm Green MD-Plan 9 µm 15 minDiamond polishing 20 N 150 rpm Green MD-Plan 6 µm 15 minDiamond polishing 15 N 150 rpm Green MD-Dur 3 µm 15 minOxide polishing 5 N 300 rpm Water MD-Chem OP-A 20 s

Table 3.1. The procedure used for specimen preparation.

3.3 Evaluation 23

3.2.2 Ion EtchingDuring ion etching, the specimen is bombarded with ions in order to etch or makea cut. In this case a cut with an ion beam was desired. When making a cut,a shield with a polished sharp edge is placed over the specimen, see figure 3.6,in order to make an accurate sharp cut. An advantage with ion etching over forexample diamond cutting is that the cut area is free from mechanical deformations,meaning that no further specimen preparation is needed. The specimen for theion etching machine used may not be bigger than 12 x 7 x 1,4 mm (for a 90o-stub).It was cut in a diamond-precision cutter and then ground to the right thickness.Since the specimen only could be 1,4 mm thick and the top coat alone is 1,5 mm,grinding was done on both the substrate and the top coat.

Figure 3.6. Ion etching. An ion beam makes a cut in the specimen.

3.3 EvaluationIn order to measure and characterize porosity, cracks and oxides, studies weredone on a microscopic level. Light optic microscope (LOM) and scanning electronmicroscope (SEM) was used together with graphics processing programs.

3.3.1 SEM and LOMThe scanning electron microscope (FEGSEM Hitachi SU70) can provide pictureswith a magnification as high as x1.000.000, but only the range of x50 to x1000 wereused here. The lower magnification was used when studying the whole ceramictop coat, to see differences within the coat, and the higher magnification was usedwhen studying the thermally grown oxides and the crack path. In a SEM electronsare accelerated by an electron gun and brought together into a beam. The beam isthen focused by a system of electromagnetic lenses. When the beam hit the spec-imen the electron interacts with the near-surface region and new electrons fromthe material are emitted, so called secondary electrons. A detector detects thesecondary electrons and generates a signal to form an image. A scanning systemmoves the electron beam over the specimen surface building up an image point by

24 Experiment

point. Characteristic X-ray spectrums are also emitted from the specimen, whichis used for chemical analyses. There are also detectors for backscattered electrons(BSE). This is all done in vacuum. The specimens used in SEM have to be con-ductive, and since the ceramic top coat and the epoxy mounting are not, they hadto be coated with a thin layer of carbon. Two thin carbon rods are connected toeach other by their sharp edges in a low pressure atmosphere of argon. A volt-age is applied to the carbon sticks to produce an arc emitting carbon atoms thatwill be deposited onto the specimen in a thin layer. The SEM was used to takemicrographs to be used in the evaluation (cracks, porosity and oxides) but alsoto examine the material composition, by so called EDS-line scan and mapping, inorder to study the oxide composition and diffusion phenomena.

The light optic microscope was also used to take micrographs of the specimensto be used in the evaluation of cracks and porosity. The range of magnificationin the LOM was x5 to x100, where x20 and x40 were the magnifications usedfor investigating the coating. A LOM uses lenses and ocular to enlarge a pictureof an object. The object can either be lit from below or from above to improvethe picture. In this case the object is lit from above. A camera, connected to acomputer, is attached to the microscope to ”photograph” the object.

One might find it unnecessary to use both SEM and LOM, but there are reasonsfor using both. One is that SEM produces pictures that appear to be more threedimensional, making it easier to see cracks. Another is that the LOM pictures waseasier to export to an image processing program called MicroGOP (Contex VisionMicroGOP2000/S), which can be used for crack investigation.

3.3.2 Graphics ProcessingThree programs were used for graphic processing; Adobe Photoshop CS2, KappaImage Base and MicroGOP. Adobe Photoshop CS2 was used to stitch picturestogether. The crack evaluation was done from six SEM-pictures, with the mag-nification x200, stitched together to make one ”panoramic” picture. One singlepicture might not display the whole crack, while a ”panoramic” picture more likelywill. Pictures of the top coat were also stitched together to evaluate possible differ-ences through out the ceramic, for example differences in porosity, but also wherecracks appear. Kappa Image Base was used when taking picture in the LOM butalso to make measurements of the thermally grown oxide and the cracks. KappaImage Base has a function that allows one to measure for example distances. Themeasurements are gathered in a text file that can be exported to Microsoft OfficeExcel for compilation. MicroGOP is a program used both for image processingand analysis. It has a program module allowing the user to write scripts that auto-matically or semi automatically analyzes pictures. Marek Jan Chalupnik wrote aprogram described in his thesis, for evaluation of defects within thin TBC [10]. Theidea in this thesis was to use this program for the evaluation of thermally grownoxide thickness, porosity, and crack length. Using a program like that would give ameasuring technique that is repeatable, in other words would give the same result

3.3 Evaluation 25

for the same picture at different measurings. But unfortunately it did not workdue to problems with the coding of the program. The porosity level was howeverdetermined in the MicroGOP by using a function in the program which measuresthe percentage area of a certain shade, in this case the pores.

3.3.3 MeasurementsThe porosity, crack length and thickness of the thermally grown oxide were mea-sured. Percentage porosity was as explained above measured in the MicroGOP-program. In figure 3.7 there is an example of a image used for measurement. Theimage is in fact three, some times four, images stitched together in Adobe Pho-toshop CS2, hence the narrow and tall picture. Crack lengths for the thermally

Figure 3.7. A picture used for evaluation of the porosity. Measurements were made inthree different areas, high, middle and low.

cycled specimens were measured in Kappa Image Base. Two assumptions weremade:

• The non-cycled (as-coated) specimen has no damage.

• Cracks are interface and/or interface near cracks (see section 4.1.2).

leading to no measurement of the as-coated specimen and only consideration ofthe interface/interface near cracks. The length of the cracks was measured as thehorizontal ”image” of the real crack, which may follow the undulating interface,see figure 3.8. It is not really the crack length that is sought but the amount ofdamage, which can be calculated as

D = total crack lengththe total examined length

(3.1)

26 Experiment

Figure 3.8. The crack length was measured as the horizontal ”image” of the real crack.

The damage can be defined as zero for the as-coated specimen and one whenspalling is started [9]. In this thesis, the damage is defined as zero for the as-coated specimen and one when total spallation has occurred. The measured cracklengths were compiled in Microsoft Office Excel for calculation of damage and tofind the relation between number of cycles and amount of damage.The thermally grown oxide thickness was also measured in Kappa Image Base.Every specimen was represented by six pictures taken in the SEM with the mag-nification x1000. In every picture was, when possible, five measurements made.

3.4 Finite Element CalculationsThe program Trinitas, a finite element program developed at Linköping University,was used to calculate the energy release rate, G, and stress intensity factors formode I and II, KI and KII , for different crack lengths. A preexisting model, madebe M. Jinnestrand for his PhD-thesis, for a thin TBC system was modified tocorrespond to a thick TBC. The model uses the displacement at the crack surface,see figure 3.9, to calculate a relation between KI and KII . This relation togetherwith G is used to calculate KI and KII . The top coat/bond coat rough interface ismodeled as a sinusoidal curve, but with only two wavelengths to simplify the model,see figure 3.10. A preexisting crack is placed on top of the wave, propagating alongthe interface until it reaches the valley of the undulation/wave. The wavelengthis 140 µm, giving a maximum crack length of 70 µm. The amplitude of thesine curve is 10 respectively 15 µm giving a total height of 20 respective 30 µm.The calculations were done for 20 different crack lengths, ranging from 4 µm to66 µm, giving a damage ranging from 6% to 94%. The damage is calculated asthe crack length divided by the maximum crack length. For every crack length,three different thermally grown oxide thicknesses (4, 6 and 8 µm) and two differentaverage bond coat surface roughness (Ra) values were used in order to see howthey influence the life of the TBC system. The TBC system is assumed to bestress free at high temperatures. The model represents a TBC system at 1000◦C

3.4 Finite Element Calculations 27

which is cooled down to 0◦C. The cooling causes stresses in the material, due tothe thermal expansion mismatch and growth of oxides. Since the cool-down timeto low temperature is short, it is assumed that an elastic analysis is sufficient forretrieval of the fracture mechanical data.

Figure 3.9. The displacement of the nodes in the circle is used for calculation of arelation of KI and KII .

Figure 3.10. The right picture shows the modeled undulation and the left picture showsthe same part during loading.

Chapter 4

Results and Discussion

The results from the experiments will be presented in this chapter together witha discussion around them.

4.1 Thermal Cycling FatigueThe thermal cycling of the specimen was carried out successfully. The test shouldideally be preformed continuously, all cycles are the same, but to be able to re-move the specimens after the right number of cycles the TCF-machine stoppedat a preset number. That lead to a longer cooling part of some cycles with roomtemperature as minimum temperature. This should however not have any effecton the results.Six specimens were cycled; 50, 100, 150, 200, 300 cycles and one that was cy-cled until total spallation, approximately 450 cycles. The different specimen willhereafter be referred to as

• K0 or as-coated - the uncycled specimen

• K50 - specimen cycled 50 times





• K450 - specimen cycled until total spallation

The TCF test was done in order to simulate the thermal cycles in a gas turbine,but the test differs from real gas turbines in some ways. For example, the furnacein a TCF test does not contain the same gas composition as the turbine, whichwill give a lower number of cycles in reality if, for example, corrosion influencesthe top coat adherence. Another difference is that in the TCF test the substrate is

29

30 Results and Discussion

heated to the same temperature as the TBC system, while in a gas turbine thereis rear-side cooling of the substrate leading to a thermal gradient. The stressesdue to different thermal expansions caused by the thermal gradient will in otherwords not occur in the TCF test. In TBC systems, the thermal gradient is one ofthe causes of failure, especially in thick TBC systems which have a larger gradient.However, during air cooling in the TCF test a thermal gradient will occur, but thisone will be reversed to the one in the gas turbine and it will only occur during alimited amount of time. This gradient will probably not influence the results andtherefore will the fractures due to the thermal cycling not derive from a thermalgradient, only from the thermal expansion mismatch between ceramic and metalliclayers, and the thermally grown oxide.

4.1.1 Specimen PreparationThere were some problems with the specimen preparation leading to uncertaintiesin the microstructure analyses. Firstly, the low-viscosity epoxy did not penetrateall the way through the top coat when impregnated. A reason for this can be thatthe viscosity of the epoxy was not low enough. The result may also been better if ahigher pressure had been applied onto the specimen when the epoxy impregnatedit. Not having epoxy all the way through the ceramic coat led to problems indistinguishing the real cracks caused by the thermal cycling from the one causedby specimen preparation. There were also problems in reaching the true structure.The grinding and polishing created a lot of pull-outs in the ceramic. The pull-outsled to problems with getting a scratch-free specimen (the metallic part) but fore-most it meant a too high measured porosity level in the ceramic. Different variantsof the specimen preparation used, table 3.1, were tested. For example, the timewas varied and an additional step with 1 µm diamond paste was tested. Another,completely different method could have been tested to possible get better results.For example using SiC grit papers as done in [11]. This option was however notchosen since the epoxy impregnation did not work anyway. Instead more timewas given to the ion etching method. The specimen prepared with the traditionalspecimen preparation was however used in the present analysis.

The ion etching method was supposed to be used for all the seven specimensbut was only used for the as-coated specimen since it was too difficult to get goodresults. It was used to produce a specimen that could be used as a reference spec-imen for the traditional prepared specimen for how the true structure should looklike. A cut was done some millimeters into the specimen. However, the etchingmachine only managed to cut to a depth of approximately 15 µm, see figure 4.1,which here was not enough to study the structure. Therefore new tests were pre-formed to cut just at the edge of the specimen. With this method the ion etchingmachine managed to cut all the way through the specimen. The idea was to cutsome microns off at a time until the true structure was reached. Every cut tookabout four hours to do. Unfortunately, the ion gun in the etching machine wasshort-circuited every now and then. Material that is evaporated during etching isdeposited onto other surfaces in the machine, for example on the ion gun, leading

4.1 Thermal Cycling Fatigue 31

Figure 4.1. A cut done in the ceramic layer by ion etching.

to the short-circuit. A conclusion was drawn that the machine was not suitablefor cutting the material in question. To be able to make a cut, an ion gun with amore focused beam is needed, in other words a beam with higher intensity. Theresult obtain can be seen in figure 4.2. The ion etching machine has managed tocut through the whole way, but the cuts have been done on a surface still affectedby the previous sectioning.

4.1.2 Evaluation of Failure MechanismBy studying K300 and K450 in SEM and LOM a conclusion was drawn that thedamage due to the thermal cycling had occurred at or near the top coat/bondcoat interface. K450 fracture surface showed a mixture between black and whitefailures, in other words a mixed failure. K300 had larger cracks along the interfaceand in the interface near area. The overall impression was that the cracks startedfrom the ridge of the undulation and either followed the thermally grown oxide(black failure) or propagated into the ceramic (white failure), see figure 4.3. Asmentioned earlier, the stresses induced by the thermal expansion mismatch andthe thermally grown oxide are likely the driving forces for failure. This togetherwith the failure mechanism agrees with literature that says that these two causeswill lead to damage at or near the interface [5, 7, 8, 12, 13].

4.1.3 Evaluation of PorosityAccording to the manufacturer of the ceramic coating, the porosity of the as-coated specimen should be about 20%. The mean value of the measured porosityfor K0 was 34%. That means 14 percentage points higher than expected. Itcan be explained by the unsatisfying specimen preparation. Measurements forall specimens are presented in table 4.1. The measurements were done on three


Figure 4.2. The as-coated specimen after ion etching. The ceramic layer is at thebottom and the substrate at the top. The bond coat appears to have two shades; thedarker one is cut once more than the brighter one. The cut was done from the ceramicdown to the substrate.

Figure 4.3. The specimen has been cycled 300 times. The crack is an example of amixed failure. At number 1 the crack is propagating in the ceramic and at 2 in thethermally grown oxide.


positions of each picture; at the top, the middle and the bottom of the ceramiccoat. There were differences in the measurement of the porosity for the differentplaces, but not large enough to make any conclusions from it. Furthermore, therewas no tendency that for example the bottom always had higher porosity thanthe top. Neither could any conclusions be made whether sintering had occurredor not as a result of the uncertain measurements.

Specimen Porosity Top Middle BottomK0 34% 26% 31% 42%K50 27% 23% 26% 31%K100 28% 25% 27% 32%K150 28% 30% 27% 28%K200 35% 39% 30% 35%K300 30% 31% 28% 32%

Table 4.1. The measured porosity. High, middle and bottom indicates where on theceramic the porosity is measured.

4.1.4 Crack EvaluationDue to the large amount of pull-outs it was often difficult to distinguish a crackfrom a pull-out in the crack evaluation, for example see figure 4.4. One can seethat the area marked could have been a crack as well as a pull-out. The measuringwas done manually, hence there was no repeatability which is desired. Becauseof these two reasons, there is a high uncertainty around the results acquired.Three measurements were performed and the results are displayed in figure 4.5 andappendix A. The large distribution of the results shows that an accurate measurentis not possible to acquire. However, a tendency for the failure development can beseen. It appears that the failure development is exponential. In a thin TBC thereis first a rapid crack initiation and propagation followed by a plateau where thepropagation is slowed down, thereafter is yet again a rapid crack growth leadingto failure [12]. This indicates that thin and thick TBC have different damagedevelopments.

4.1.5 Evaluation of Thermally Grown OxidesThe thickness of the thermally grown oxide has been measured for all the specimensexcept K0. K0 should have a thin layer of oxide due to the air plasma spraying,but no oxide could be detected. In figure 4.6 one can se that there is no contactbetween bond coat and top coat, as it should have been, with or without oxide.The gap between bond coat and top coat can not be used for measuring thepossible thickness of the oxide since parts of the bond and top coat as well mayhave been pulled out. Instead, a thickness of 1 µm has been assumed after [12].The measurements on the remaining specimens were preformed on parts of thespecimen where the ceramic part was intact. If the ceramic layer is missing, thereis a big possibility that parts of the oxide is missing too. The results are presented


Figure 4.4. A crack or a pull-out in a specimen cycled 300 times.

Figure 4.5. The results from all three crack measurment. The measurments are repre-sented by the dots and the line is a trendline.


in figure 4.7 and figure 4.8. The first part of the thermally grown oxide-curveshould represent the time when the oxide mainly consist of alumina. Alumina hasa Pilling-Bedworth ratio of 1.28 [5] which gives a parabolic oxide growth - time(here cycles) curve. As a result of aluminum depletion a mixed oxide (aluminium,chromium, cobalt and nickel) called spinels may form. This oxide have highergrowth rate than alumina, leading to a curve that is logarithmic. This is howevernot how the obtained curves look like and the most likely reason for this is yet againthe specimen preparation. Chemical analyses of K300 and K450, see appendix A.4shows that the bond coats internal oxides consist of both aluminum and spinelsand a line analysis of the bond coat confirms that the bond coat is depleted ofaluminum in K300 and K450. However, the thermally grown oxide layer of K300and K450 only consists of alumina. This together with the fact that the internaloxides are thicker than the thermally grown oxide, also indicates that parts ofthe thermally grown oxide has been pulled out during preparation meaning andthat the measured thicknesses are not valid. The missing oxide results in a largemarginal of errors especially for K300 and K450 since spinels are more brittle andwill therefore more easily be pulled out. The true thickness of the thermally grownoxide when spallation occurred, could not be determined, but a rough estimation ofthe thickness, based on the measurements and the fact that the spinels are missing,is 6 µm. This estimated thickness is, compared to thin TBC [12], the same for thecorresponding amount of cycles. When failing, thin TBC has a thermally grownoxide thickness of approximately 7 µm.

Figure 4.6. There is no oxide between the bond coat and top coat.

4.1.6 DiffusionChemical analyses have been done in form of mapping and line scans (see ap-pendix A.4 for some of the analyses). These analyses gave suggestions to how the


Figure 4.7. The results from the measurements of the thermally grown oxide. The lineis a trendline based on the measurements diplayed as the dots.

Figure 4.8. The results from the mesurements of the thermally grown oxide. The twolines represent the average and median values.

4.2 Finite Element Calculations 37

diffusion may have occurred and information about the chemical composition ofthe oxides. The thermally grown oxide grows by both oxygen and metal diffusion.The alumina growth occurs by diffusion though the top coat and later the oxideitself while the growth mechanism of the spinels is metal diffusion through thealumina. The analyses gave that the oxides of K0, K50, K100 and K200 are madeup by alumina, while the oxide in K300 and K450 both consist of alumina andspinel. A line scan of an internal bond coat oxide revealed that the oxide closestto the bond coat is alumina then comes the spinels, as expected. The aluminadepletion has foremost occurred by formation of alumina. A line scan of the bondcoat and the substrate closest to the coat gave no indication of aluminum diffusioninto the substrate. But to be certain, more analyses are required.

4.2 Finite Element CalculationsAll numerical results are compiled in graphs in appendix A.2.The cracks due to the thermal cycling fatigue propagated in the interface (topcoat/bond coat) or close to interface. The finite element model is based on a crackpropagating purely in the interface. The thermally grown oxide in the modelhas an even thickness. In the experiments done, the thickness varied along theinterface, but in general the thickness was the same. Therefore the finite elementmodel is reasonable compared to the fracture mechanism observed.The results show that there is an increase in energy release rate with an increasein thickness of the thermally grown oxide. A thicker thermally grown oxide meansmore stresses within the material that will be released during crack opening. Thegreatest energy release rate occurs, for both bond coat surface roughnesses, around30% damage. According to the model used this occurs at a slope of the undulation(50% defines the valley and 0 and 100% the ridge). With an increase of Ra-valuethere is an increase in energy release rate. The fracture is a mixed-mode fracture.Neither KI nor KII has a value of zero during the failure, there is a mixture ofthe two at most of the time. However, the results show that KI is dominating atthe ridge and the slope near the ridge while KII is dominating at the slope nearthe valley and at the valley, see figure 4.9. Like the energy release rate, the stressintensity factors maximum value is larger for the rougher surface. The calculationfor thick TBC gave no difference from thin TBC other than that the values areslightly lower, for example, at 37% damage and 8 µm thick thermally grown oxideand the same surface roughness, the energy release rate for thick TBC is 43 N/mand for thin TBC 47 N/m. This was somewhat expected since all the parameterswere the same for the two different calculations except the thickness.

4.3 Tensile Test with Acoustic EmissionIn order to study the failure due to applied tensile strain, two tests were preformedfor each thickness. The test was ended when the coatings failed by spallation.The two thicker coatings had a different path of failure than the thinner one.However, all energy-strain and hits-strain graphs had the same appearance, see


Figure 4.9. The stress intensity factor in mode I and II for a model with 6 µm thickthermally grown oxide.

appendix A.3. First there is an increase of energy released (crack initiations andpropagation) in the material until it reaches a maximum around a strain of 0.4%.Thereafter the energy releases will decrease until there are sudden peaks. Thesepeaks appear at different strain depending on the thickness and indicates formationof cracks and delaminations. For 500 µm these peaks first appered around a strainof 1.6+%, for 1000 µm around 1.2% and for 1500 µm around 1%.

4.3.1 Failure MechanismsAs said above, the different thicknesses displayed different kind of failure mecha-nisms, at least on a macro level. Two major type of cracks occurred; transversecracks and interface crack, often starting at the edge, see figure 4.10 and 4.11. The500 µm coating are considered to be thin TBC and 1000 and 1500 µm thick ones.The thin one first displayed cracks along the bond/top coat interface. Visually itappeared as the cracks started at the edge of the specimen. Thereafter, transversecracking occurred in the top coat. Looking at the 500 µm specimen in a stereomicroscope one can see that there is a lot of small transverse cracks. Only a fewof them have propagated through the whole ceramic layer. The two thicker TBCsystems first displayed the transverse cracks and then cracks along the interface.The 1000 µm specimen had both larger transverse cracks that had propagatedthrough the top coat, but also a larger amount of smaller transverse cracks. Thelarger cracks together with the interface cracks lead to spallation. The 1500 µmspecimen only had the larger transverse cracks and there were only as few as fouron each side. In table 4.2 a visual observation of the specimen is compiled.An interpretation of the path of failure, based on the energy-strain and energy-hits graph together with the observed failure mechanism, is that there is first arather rapid initiation and propagation of micro cracks until about 0.4% strain.The micro cracks lower the ceramics stiffness, making it more compliant with themetal substrate, hence the decrease of acoustic emission. However, there is still apropagation of the cracks that eventually will lead to macro cracks and spallation.

4.3 Tensile Test with Acoustic Emission 39

The growth of the micro cracks and delamination can be seen as acoustic emissionpeaks at the end of the graphs. To validate this interpretation an investigationof the microstructure at different strains is needed. Until more tests and analyseshave been preformed a maximum strain at 0,4% has to be used.

Figure 4.10. The failure of a tensile specimen with 1000 µm thick coating. There areboth transverse and interface cracks.

Specimen Strain % Observation500 µm 0.8 No visible failure.

1.3 Interface crack at the edge.1.7 Transverse cracks.

1000 µm 0.9 Transverse cracks followed by an edge crack.1.2 There is a lot of visible cracks.1.3-1.4 Transverse cracks together with interface

cracks leads to partial spallation.1500 µm 0.4 Transverse cracks.

0.65 First edge crack.0.8-0.9 Transverse cracks are all the way through

the top coat.over 1 There are massive delamination.

Table 4.2. The visual inspection of the specimen during tensile test.


Figure 4.11. Spallation of the coating. Both transverse and interface cracks are visible.

Chapter 5

Conclusions and FutureWork

This chapter will cover the conclusion drawn from discussion of the results andsuggestions for future work.

5.1 ConclusionsThrough thermal cycling fatigue test a correlation between coating damage andnumber of cycles was received. However, due to difficulties with the specimenpreparation, the results can only be used in studying the tendency by which thefailure occurs. From microscopic investigations, a conclusion could be made thatthe failure occurred at the interface between the top coat and the bond coat, butalso in the ceramic near the interface, a mixed failure. The growth rate of thethermally grown oxide could not be determined, again due to unsatisfying spec-imen preparation, but EDS-analyses showed that depletion of aluminum in thebond coat between 200 and 300 cycles lead to formation of spinel. The finite ele-ment calculations produced results for the energy release rate and stress intensityfactors in mode I and II for different number of cycles. The results implied thatan increase of oxide thickness gives higher energy release rates and stress intensityfactors. An increase in surface roughness also increased the values. The results aresimilar to the ones for thin TBC. Concerning the life model and the data neededfor calibration, the results from the finite element calculations can be used but theresults from the thermal cycling fatigue test cannot be trusted, due to the highmargin of errors.

Visual inspection of the tensile test showed that thin (500 µm) and thick TBC(1000 and 1500 µm) had different failure developments on a macro level. The thinTBC first displayed interface cracks then larger transverse cracks while the thickerones first experienced transverse cracks and then interface cracks. Studying thespecimen in stereo microscope showed that a decrease of coating thickness resulted

41

42 Conclusions and Future Work

in an increase in number of transverse cracks. However, in the thicker coatingsthese cracks were larger, often propagated all the way through the top coat.All the three coating thicknesses resulted in similar acoustic emission energyreleases-strain graphs, as well as hits-strain graphs. This indicates that the failureon a micro level may be the same for all specimens.The amount of strain accommodated until damage of the coating is 0.4%.

5.2 Future WorkIn order to achieve more accurate and reliable results in future experiments a bet-ter procedure for specimen preparation is needed. For example, using an ion etchwith a more focused beam than the one used here, would give specimens with outpull-outs. However, the disadvantage of using the ion etch is that the specimenhave to be very small, here 12 x 7 x 1.4 µm, leading to a limited site of interestfor studies.The specimen preparation procedure used here might be sufficient if impregnatingthe specimen with an epoxy with viscosity low enough to allow penetratation ofthe whole ceramic.

Since the specimen preparation of thick TBC has appeared to be difficult a non-destructive test method might be a good alternative. D. Renusch and M. Schütze [9]have successfully used acoustic emission to measure the damage during cyclic oxi-dation. In future thermal cycle fatigue test it would be useful to adopt this method.

In other studies [8] it has been shown that thick TBC will fail differently to thinTBC due to the larger thermal gradient. This gradient was not produced in thetest done, therefore would a burner rig test be interesting. In this kind of test theceramic is exposed to a burning flame while the rear side of the substrate is aircooled, leading to a thermal gradient. The test is a thermal cycling fatigue test.So either will the test confirm that spallation occurs in the interface or it will givenew information.

To be able to learn more from the tensile test done with acoustic emission, micro-scopic analyses of the specimens loaded to different levels of strain should be done.By thermally cycling specimens and then do tensile tests, the effect of cycling canbe investigated.

Bibliography

[1] Robert H. Turner Yunus A. Cengel. Fundamentals of Thermal-Fluid Science.2001.

[2] ASM International Handbook Committee J.R. Davis. Heat-Resistant Mate-rials. 1997.

[3] D. Stöver H.B. Guo, R. Vaßen. Thermophysical properties and thermal cy-cling behavior of plasma sprayed thick thermal barrier coatings. Surface &Coatings Technology, 2005.

[4] R. Vassen. D. Stoever X.Q. Cao. Ceramic materials for thermal barrier coat-ings. Journal of European Ceramic Society, 2004.

[5] Sudhangshu Bose. High Temperature Coatings. 2007.

[6] W. David Kingery Yet-Ming Chiang, Dunbar Birnie III. Physical Ceramics,Prinicples for Ceramics Science and Engineering. 2007.

[7] J.W. Hutchinson G.H. Meier F.S. Pettit A.G. Evans, D.R. Mumm. Mech-anisms controlling the durability of thermal barrier coatings. Progress inMaterials Science, 2001.

[8] Robert A. Miller Dongming Zhu. Investigation of thermal high cycle and lowcycle fatigue mechanism of thick thermal barrier coatings. Materials Scienceand Engineering, 1998.

[9] M. Schütze D. Renusch. Measuring and modeling the tbc damage kinetics byusing acoustic emission analysis. Surface & Coatings Technology, 2007.

[10] Marek Jan Chalupnik. Automatic evaluation of defects within thermal bar-rier coatings using the image analysis system microgop. Master’s thesis,Linköpings universitet, 2004.

[11] Kelly Matthew. Metallographic techniques for evaluation of thermal barriercoatings produced by electron beam -physical vapor deposition. MaterialsCharacterization, 2007.

[12] Håkan Brodin. Failure of thermal barrier coatings under thermal and me-chanical fatigue loading. PhD thesis, Linköpings universitet, 2004.

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44 Bibliography

[13] E.H. Jordan M. Gell K.W. Schlichting, N.P. Padture. Failure modes inplasma-sprayed thermal barrier coatings. Materials Science and Engineering,2003.

Appendix A

Appendix

A.1 Crack EvaluationThe results from the crack evaluation are presented in figure A.1, A.2, A.3 andA.4.

Figure A.1. Measurement 1.

45

46 Appendix



A.1 Crack Evaluation 47

Figure A.4. The three different measurments.

48 Appendix

A.2 Finite Element CalculationsThe results from the finite element calculation are presented in figure A.5, A.6,A.7, A.8, A.9 and A.10. The first three graphs are for the model with a sine curvewith the amplitude 20 µm and the other three are for a model with the amplitude30 µm.

Figure A.5. Energy release rate for different oxide thicknesses.

Figure A.6. The stress intensity factor in mode I for different oxide thicknesses.

A.2 Finite Element Calculations 49

Figure A.7. The stress intensity factor in mode II for different oxide thicknesses.

Figure A.8. The energy release rate for different oxide thicknesses.

50 Appendix

Figure A.9. The stress intensity factor in mode I for different oxide thicknesses.

Figure A.10. The stress intensity factor in mode II for different oxide thicknesses.

A.3 Tensile Test with Acoustic Emission 51

A.3 Tensile Test with Acoustic EmissionThe results from the tensile test are presented in figure A.11, A.12, A.13, A.14,A.15 and A.16. Every figure include three graphs; energy released vs. strain,number of hits vs. strain and load vs. strain.

Figure A.11. 500 µm test 1.


52 Appendix



A.3 Tensile Test with Acoustic Emission 53



54 Appendix

A.4 Chemical AnalysesFigure A.17, A.18, A.19, A.20, A.21 and A.22 are line scans. Figure A.23 andA.24 are mappings.

Figure A.17. K0. Analysis started in the top coat.

Figure A.18. The result from the line scan. Spectrum (x) represent the point wherethe measurement was done.

A.4 Chemical Analyses 55

Figure A.19. K300. Analysis started at the top of the bond coat.


56 Appendix

Figure A.21. K450. Analysis started at the top of the bond coat.


A.4 Chemical Analyses 57

Figure A.23. A internal bond coat oxide from K450.

Figure A.24. The results from the line scan. The different graphs represent differentmaterials oxygen, silicon, chromium, cobalt, nickel, yttrium and aluminum.

58 Appendix

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