volcanic ash degradation on thermal barrier coatings and

Volcanic Ash Degradation on Thermal

Barrier Coatings and Preliminary

Fabrication of Protective Coatings

A thesis submitted to The University of Manchester for the degree of

Master of Philosophy

in the Faculty of Engineering and Physical Sciences

2014

Kuan-I Lee

School of Materials

LIST OF CONTENTS

Page 1

List of Contents

Abstract….. ...................................................................................... 4

Declaration ....................................................................................... 6

Copyright Statement ....................................................................... 7

Acknowledgement ........................................................................... 8

List of Figures................................................................................ 10

List of Tables ................................................................................. 13

Introduction ................................................................. 14 Chapter 1

Literature review ......................................................... 18 Chapter 2

2.1 Thermal Barrier Coatings ............................................................18

2.1.1 Single Crystal Nickel-Based Superalloy .......................................... 19

2.1.2 Top Coat .......................................................................................... 21

2.1.3 Bond Coat ....................................................................................... 27

2.1.4 Thermal Grown Oxide ..................................................................... 29

2.2 CMAS Degradation .......................................................................30

2.2.1 CMAS .............................................................................................. 30

2.2.2 Degradation Mechanisms of CMAS attack ...................................... 31

2.2.3 CMAS Penetration on Service Retired Engine Turbine ................... 33

2.3 Prevention of CMAS degradation ...............................................35

2.3.1 Protective Overlay ........................................................................... 36

2.3.2 Doping Elements ............................................................................. 37

2.3.3 New Top Coat Materials .................................................................. 39

Research Proposal ...................................................... 41 Chapter 3

3.1 Experimental Scope .....................................................................41

LIST OF CONTENTS

Page 2

3.2 Experimental Procedure ..............................................................42

Volcanic Ash and Its Penetration Mechanisms in Chapter 4

TBCs…….. ...................................................................................... 49

4.1 Study of Volcanic Ash ..................................................................49

4.1.1 Structure and Surface Morphology .................................................. 49

4.1.2 Melting Point of Volcanic Ash .......................................................... 51

4.2 Mechanisms of volcanic ash attack ...........................................54

4.2.1 Delamination Type of Degradation Mechanism on TBCs ................ 55

4.2.2 Penetration Type of Degradation Mechanism on TBCs ................... 58

4.3 Investigation of the Reaction between Volcanic Ash and YSZ

by DTA and XRD Analysis ....................................................................62

Characterization of Protective Coating Materials ..... 69 Chapter 5

5.1 Reaction and Permeability of Volcanic Ash in YSZ and Alumina

Plates .....................................................................................................69

5.2 Investigation of the reaction between volcanic ash and alumina

by DTA and XRD ....................................................................................71

Fabrication of Protective Coatings by Sol-Gel Based Chapter 6

Method…… .................................................................................... 76

6.1 YSZ protective coatings ..............................................................77

6.1.1 Precursor for YSZ coatings ............................................................. 77

6.1.2 Characterization of YSZ coatings .................................................... 79

6.2 Alumina protective coatings .......................................................82

6.2.1 Precursor for alumina coatings ........................................................ 82

6.2.2 Characterization of Alumina Coatings ............................................. 83

6.3 Preliminary Fabrication and Characterization of YSZ and

Alumina Protective Coatings on TBC Top Coats ...............................85

LIST OF CONTENTS

Page 3

6.3.1 Fabrication and Characterization of YSZ Protective Coatings on TBC

Top Coats .................................................................................................... 85

6.3.2 Fabrication and Characterization of Al2O3 protective coating on TBC

top coat ........................................................................................................ 86

Conclusions and Suggested Future Work ................ 89 Chapter 7

7.1 Conclusions ..................................................................................89

7.2 Suggested Future work ...............................................................91

References ..................................................................................... 94

ABSTRACT

Page 4

Abstract

Thermal barrier coatings (TBCs) made of yttria stabilized zirconia (YSZ) have

been applied to aero engines industry since 1970s. However, because of the

increasing operational temperature, TBCs are suffering from molten foreign

deposits known as calcium-magnesium-alumino-silicate (CMAS). Molten CMAS

sinters YSZ top coat and shortens the lifetime of TBCs. Alumina has been widely

proved to prevent CMAS from degradation and is the most common material

chosen to avert a CMAS attack in state-of-the-art TBC technology.

This study uses real volcanic ash to study the degradation process of TBCs

and potential mitigation mechanisms. The results show that, similar to CMAS,

volcanic ash severely penetrates the thickness and fills the columnar gaps of the

TBC top coat. It is also found that the yttria content of the YSZ top coat decreases

substantially with high temperature exposure to volcanic ash, which has a

detrimental effect on the phase stability of YSZ. In terms of mitigation, volcanic

ash reacts with alumina around 1310 °C, forming anorthite (CaAl2Si2O8),

magnetite (Fe3O4), and spinel (Al1.75Mg0.889Mn0.351O4) as reactive products, which,

according to the literature, have melting temperatures above that of the volcanic

ash studied and the typical values reported for CMAS. Since the new melting

temperatures are now above the typical surface operating temperature of turbine

components, the melting-induced penetration of volcanic ash and CMAS can be

considerably suppressed.

ABSTRACT

Page 5

For the protective coatings, both YSZ and Al2O3 could be made by using

sol-gel based spray coating. However, different grade of thermal expansion

between TBC layers and protective coating during heat treatment will break the

structure of protective coating which is needed to be conquered in the future.

DECLARATION

Page 6

Declaration

No portion of the work referred to in the thesis, ‘Volcanic Ash Degradation

on Thermal Barrier Coatings and Preliminary Fabrication of Protective

Coatings’, has been submitted in support of an application for another degree or

qualification of this or any other university or other institute of learning.

COPYRIGHT STATEMENT

Page 7

Copyright Statement

Copyright in text of this thesis rests with the author. Copies (by any process)

either in full, or of extracts, may be made only in accordance with instructions

given by the author and lodged in the John Rylands University Library of

Manchester. Details may be obtained from the Librarian. This page must form part

of any such copies made. Further copies (by any process) of copies made in

accordance with such instructions may not be made without the permission (in

writing) of the author.

The ownership of any intellectual property rights which may be described in

this thesis is vested in The University of Manchester, subject to any prior

agreement to the contrary, and may not be made available for use by third parties

without the written permission of the University, which will prescribe the terms and

conditions of any such agreement.

Further information on the conditions under which disclosures and exploitation

may take place is available from the Head of School of Materials, or the

Vice-President and Dean of the Faculty of Engineering and Physical Sciences.

ACKNOWLEGEDEMENT

Page 8

Acknowledgement

It would not have been able to finish my thesis without the guidance of my

committee members, help and support from the kind people around me.

First and foremost, I have to thank my supervisors, Prof. Ping Xiao, who has

supported me throughout my thesis with his suggestion and knowledge whenever

I was struggled. His support and encouragement is invaluable to me, not only in

academic field but also in life experience. Without his support and

encouragement, this thesis would not have been completed.

In addition, I would also like to show gratitude to my supervisor when I did the

internship in National Institute of Materials Science (NIMS), Japan, Dr. Rudder

Wu. His suggestion and support has been invaluable on both an academic and a

personal level, for which I am extremely grateful.

I am also grateful to my present colleagues Ying Chen, Xun Zhang, Fan Yang,

and Liberty Wu for helping with inspiration, technical support, and sharing their

knowledge with me. I would also like to thank technical staffs in both Materials

Science Centre and NIMS, Dr. Christopher Wilkins and Ms. Horu Gao for the use

of SEM, Mr. Andy Forrest for the use of DSC measurement, Dr. Tanaka Hidehiko

and Ms. Suzuta Keiko for the manipulation of TG/DTA machine, and Mr. Gary

Harrison for the use and discuss of XRD measurement. And special thanks go out

to Prof. Hiroshi Harada from NIMS for the invaluable advice and comments on my

research topic.

ACKNOWLEGEDEMENT

Page 9

Last but not least, I would like to thank all my family members especially my

parents for their unconditional support throughout my study. The patience and

understanding shown by my mother, father and sister during these two years, I

am greatly appreciated for their unlimited support of every aspect of my life.

LIST OF FIGURES

Page 10

List of Figures

Figure 2.1 Cross-sectional SEM image of thermal barrier coating system ......... 19

Figure 2.2 Coefficients of thermal expansion (CTEs) and thermal conductivities of

materials for the principal components in TBC systems [1,3,11] .................. 22

Figure 2.3 Phase diagram of YSZ [1,12] ............................................................ 22

Figure 2.4 Typical cross-sectional SEM microstructure of APS TBCs [19] ......... 24

Figure 2.5 Schematic image of air-plasma spray processing system ................ 24

Figure 2.6 Typical cross-sectional SEM microstructure of EB-PVD TBCs [19] .. 26

Figure 2.7 Schematic image of electron-beam physical vapor deposition system

..................................................................................................................... 26

Figure 2.8 Cross-sectional image of TBC top coat penetrated by CMAS [42].... 32

Figure 2.9 Schematic image of cross-sectional engine turbine blade [33] ......... 33

Figure 2.10 Cross-sectional image demonstrates the complete suppression of

CMAS melt ingression into the coating assembly [49] ................................. 37

Figure 2.11 (a) Cross-sectional image of APS top coat made by YSZ doping

alumina and titanium oxide interact with CMAS, and (b) its EDS mapping of Si

..................................................................................................................... 38

Figure 2.12 Mechanism of CMAS/Gd2Zr2O7 interaction. (a) In early stage,

CMAS/Gd2Zr2O7 react in TBCs columnar gaps, (b) columnar gaps are filled

up with reaction products and the columnar tips just receive small attack from

CMAS, (c) columnar tips is severely attacked by CMAS, (d) additional

crystalline forms during cooling [60]............................................................. 40

Figure 4.1 SEM image of volcanic ash ............................................................... 50

Figure 4.2 XRD pattern of volcanic ash ............................................................. 50

Figure 4.3 DTA curve for volcanic ash ............................................................... 52

Figure 4.4 (A) Surface image of volcanic ash and (B) its surface roughness after

heat treatment at 1150 °C for 30 mins ......................................................... 52



LIST OF FIGURES

Page 11





Figure 4.8 SEM image of the as-received TBC sample ..................................... 54

Figure 4.9 SEM image of TBC after 30 minutes heat treatment at 1250 °C ....... 54

Figure 4.10 Schematic image of the distribution of stress on YSZ columns [44] 57

Figure 4.11 Schematic image of A) short column cracks near the sintering

separation B) long delamination crack [44] .................................................. 57

Figure 4.12 The diagram of energy release rate and CMAS thickness [44] ....... 57

Figure 4.13 TBC sample under the heat treatment at 1250 °C for 15 minutes with

volcanic ash ................................................................................................. 58

Figure 4.14 EDX mapping of (A) Zirconium and (B) Silicon for the TBC sample

under the heat treatment at 1250 °C for 15 minutes with volcanic ash ........ 58

Figure 4.15 SEM image of volcanic ash attack on TBC after 3 hours heat

treatment at 1250 °C.................................................................................... 59

Figure 4.16 SEM image of YSZ top coat after 10 hours heat treatment with

volcanic ash at 1250 °C ............................................................................... 60

Figure 4.17 EDX point analysis of YSZ top coat after 10 hours heat treatment with

volcanic ash at 1250 °C ............................................................................... 61

Figure 4.18 Concentrations of Y2O3 after thermal exposure to volcanic ash at

1250 °C for 1, 3, 10 hours ............................................................................ 62

Figure 4.19 DTA curves of volcanic ash, Al2O3, and volcanic ash-Al2O3 mixture 63

Figure 4.20 XRD spectra of (a) volcanic ash-YSZ mixture samples and (b) pure

YSZ samples before and after heat treatment at 490 °C, 950 °C, 1160 °C, and

1350 °C ........................................................................................................ 64

Figure 4.21 Schematic diagram of monoclinic phase YSZ after 490°C thermal

exposure ...................................................................................................... 67

Figure 5.1 Volcanic ash attack on YSZ plates after thermal exposure at 1250 °C

for (a) 5 hours and (b) 20 hours ................................................................... 70

Figure 5.2 Volcanic ash attack on Al2O3 plates after thermal exposure at 1250 °C

LIST OF FIGURES

Page 12

for (a) 5 hours and (b) 20 hours ................................................................... 71

Figure 5.3 DTA curves of volcanic ash, Al2O3, and volcanic ash-Al2O3 mixture . 72

Figure 5.4 XRD spectra of volcanic ash-Al2O3 mixture before and after heat

treatment at 410 °C, 525 °C, 1060 °C, and 1310 °C .................................... 72

Figure 5.5 CaO-SiO2-Al2O3 ternary phase diagram [43] .................................... 75

Figure 6.1 Schematic image of the sol-gel based coating method ..................... 77

Figure 6.2 (A) Cross-sectional and (B) surface SEM image of YSZ coating made

by Sol-gel based dip coating for 1 time on stainless steel substrate ............ 81


by Sol-gel based dip coating for 5 times on stainless steel substrate .......... 81


by Sol-gel based spray coating for 6 times on stainless steel substrate ...... 81

Figure 6.5 Sol-gel based spray coating for 6 times on the stainless steel substrate

..................................................................................................................... 84

Figure 6.6 (a) Surface morphology and (b) cross-sectional image of YSZ

protective coatings ....................................................................................... 86

Figure 6.7 EDX mapping of YSZ protective coatings ......................................... 86

Figure 6.8 the (a) surface morphology and (b) cross-sectional image of Al2O3

protective coatings ....................................................................................... 87

Figure 6.9 EDX mapping of Al2O3 protective coatings........................................ 87

Figure 6.10 Schematic image of the damage mechanism of protective coatings

during thermal exposure .............................................................................. 88

Figure 7.1 schematic image of the new design of TBCs .................................... 92

Figure 7.2 Illustration of the YSZ/volcanic ash and Gd2Zr2O7/volcanic ash

interaction [41] ............................................................................................. 93

LIST OF TABLES

Page 13

List of Tables

Table 2.1 CMAS composition from service retired turbine blades ...................... 31

Table 2.2 The thickness of CMAS deposit on the blade and its correlated point

shown in Figure 2.9 [33] .............................................................................. 33

Table 4.1 Composition of the volcanic ash from Sakurajima volcano, Japan ..... 50

Table 4.2 Data of EDX point analysis ................................................................. 61

Table 4.3 Ratio of tetragonal phase YSZ to monoclinic phase YSZ for volcanic

ash-YSZ mixed sample and YSZ measured by using XRD Semi quantitative

measurement before and after thermal exposure at different temperatures 65

Table 4.4 The comparison of degradation mechanisms by volcanic ash from

Sakurajima, Eyjafjallajökull and artificial CMAS ........................................... 67

Table 6.1 Parameters of YSZ precursors ........................................................... 79

Table 6.2 Parameters of alumina precursors ..................................................... 83

Table 6.3 XRF results for alumina coating ......................................................... 84

Table 6.4 Composition of YSZ protective coating measured by EDX point analysis

..................................................................................................................... 86

Table 6.5 Composition of Al2O3 protective coating measured by EDX point

analysis ........................................................................................................ 88

CHAPTER 1 INTRODUCTION

Page 14

Chapter 1

Introduction

For turbine components located in the hot section of engines, their

fabrication procedure is highly demanding because they have to endure the flow

of hot gas which often leads to problems such as corrosion and oxidation.

Furthermore, turbine components also have to endure the impact of high velocity

objects such as sand particles. Due to the continuous improvement of

engineering skills, the engine could be improved in different ways such as the

turbine design, increasing the combustion temperature, and the use of materials.

Since reducing the emission of carbon dioxide has been one of the important

considerations of advanced engine design, and along with the increasing price of

fossil fuel, demand for the increase of fuel efficiency became one of the most

important issues for the future work of engine design. According to these motives,

increasing the operation temperature was an obvious trend over the past 50

years [1].

For the state-of-the-art engine, the operating temperature in the combustion

chamber is more than 1500 °C [2], which is much higher than the melting point of

nickel-based superalloy (1320 - 1450 °C [3]) [4], the material which is used in the

substrate material in engine turbine. Besides, compared with older generations of


Page 15

engines, the stress of modern engine became much higher owing to the higher

temperature, pressure and the faster speed of rotors when the engine is in

operation. But, because of the outstanding property such as the resistance

against creep, fatigue and oxidation, nickel-based superalloy is still the first

choice to be the substrate materials for today’s engine turbine [5,6].

The reason why modern engines can endure such a harsh operating

environment is because of the advanced casting skills. Larger single crystal

turbine blades and vanes could be made with complicated channels inside the

turbine blades which benefit the cooling process [1]. The use of thermal barrier

coatings (TBCs) is the other reason why nickel-based superalloy turbine blades

can operate under the extreme hot environment. The fundamental function of

thermal barrier coatings is to provide an overlay with low thermal conductivity and

mitigate the heat transfer from engine gas [6]. By using TBC, 100 – 300 °C has

been decreased from top layer of TBC system to the superalloy substrate [7].

Furthermore, according to the trend of nickel-based superalloy designing, creep

resistance becomes much better, but along with the sacrifice of oxidation

resistance [3]. Thus, thermal barrier coating becomes a necessary application to

the engine turbine as a thermal insulator for increasing operation temperature.

For thermal barrier coating systems, there are three layers, top coat,

thermally grown oxide (TGO) and bond coat, which covers the nickel-based

superalloy substrate. Unlike traditional failure mechanisms, with the increase of


Page 16

the operating temperature of engine turbine, there is a degradation mechanism of

hot section turbine materials, known as the calcium-magnesium-alumino-silicates

(CMAS) degradation. Molten CMAS attacks on the YSZ top coat layer is

becoming more apparent and requires the attention of materials scientists. When

the engine is in operation, airborne ashes, dusts and debris are ingested into the

engine, they melt, and form CMAS before being deposited on the surface of hot

area of turbine. Molten CMAS also penetrates into the YSZ top coat, and

shortens the lifetime of TBCs [1] which degradation mechanism will be explained

in the next chapter. Since the eruption of Eyjafjallajokull volcano in Iceland on

April 14th, 2010 and caused interruption of flights due to concern about the

possible damage from volcanic ash during aeroplane flying, the interaction

between volcanic ash and thermal barrier coatings has been paid attention by

materials scientists. The reaction between volcanic ash and TBCs could cause

failure of TBCs, and leading to risk of aero engine. The main components of

volcanic ash are Ca, Mg, Al, Si and Fe contained oxides, which have similarity to

the CMAS generated from atmospheric dust during high temperature combustion

in aero engines. Both CMAS and volcanic ash could react with TBCs and cause

failure of TBCs, which induced degradations of hot section in engine components.

The degradation mechanisms of volcanic ash are very similar to CMAS, which

also penetrates into the YSZ columnar gaps and pores, and lead the top coat

sintering. In this research, volcanic ash coming from Sakurajima volcano, Japan

is introduced into the research. Unlike artificial CMAS made from the mixture of


Page 17

CaO, MgO, Al2O3 and SiO2, the composition and phase structure in this volcanic

ash are much more complicated. And this different composition and phase

structure may lead different kind of degradation mechanisms.

The objectives of this study can be separated to four parts. 1) Study of

volcanic ash. The chemical composition, phase structure, surface morphology,

and melting temperature of volcanic ash will be analysed in this part. 2) Study of

degradation mechanisms of TBCs attacked by volcanic ash. 3) Investigate

coating material for protection from volcanic ash attack. 4) Making a protective

coating on the top of TBCs using sol-gel based method.

CHAPTER 2 LITERATURE REVIEW

Page 18

Chapter 2

Literature review

2.1 Thermal Barrier Coatings

For the state-of-the-art jet engine, in order to achieve better thrust output and

fuel efficiency, the operation temperature must be increased beyond the melting

temperature of nickel-based superalloy and come with severe oxidation and

creep in the hot section of turbine blade [1]. Thus, a protective overlay coating is

necessary in order to extend the service lifetime of turbine blades. Especially for

increasing the operation temperature of engine turbine, thermal barrier coating

technology plays a very important role in aircraft engineering.

Thermal barrier coating system is a coating system which is mainly for

reducing the heat transfer from the hot engine gas to the substrate of turbine

blades and also provides the function for mitigating creep and oxidation in engine

system. Thus, TBC system not only has to endure the extremely hot operation

environment, but also withstands the stress and thousands thermal cycles.

For the state-of-the-art TBCs, the structure is composed by three layers

which is shown in Figure 2.1; two ceramic layers on the upper layers and one

metal layer located in the bottom, which are top coat, thermal grown oxide (TGO),


Page 19

and bond coat coated on nickel-based superalloy substrate. For the top coat, it is

usually made from 7 to 8 wt.% yttria stabilized zirconia (YSZ) and the main

purpose of the top coat is reducing the heat transfer from engine gas to the

substrate material. Bond coat is covered on the substrate and provides oxygen

resistance and adhesion for top coat. Between top coat and bond coat, there is a

thin oxide layer called TGO. It can improve the oxidation resistance of TBC

system by hindering further oxidation [3,8,9].

Figure 2.1 Cross-sectional SEM image of thermal barrier coating system

2.1.1 Single Crystal Nickel-Based Superalloy

Nickel is the fifth abundant element in the world which has a melting

temperature about 1455 °C, face-cantered cubic structure. Compared with other

aero metals, its density (8907 kg/m3) is much higher than other aero metals such

as titanium and aluminium. Within the history of aero engineering, single crystal


Page 20

nickel-based superalloy is one of the most important inventions for engine

industry. The application of nickel-based superalloy can be tracked to World War

2, it has been used in aircraft engines and petroleum industries at that moment [5].

Today, the utilization of nickel-based superalloy has become much wider and it is

the main substrate material for state-of-the-art engine turbines.

For its composition, nickel-based superalloy is one of the most complex

alloys in the world. Such complexity comes from the need to satisfy properties

such as oxidation resistance, tensile strength, creep resistance, corrosion

resistance, and fatigue life. Thus, various levels of cobalt, chromium, aluminium,

and titanium and small amount of boron, zirconium, and hafnium are added into

the alloy. Furthermore, other common addition of alloy elements such as

molybdenum, tungsten, tantalum, and niobium are added to the nickel-based

superalloy as well [5]. In the later generation of superalloys, rhenium and

ruthenium are also added into the superalloy in order to improve its creep

resistance and phase stability [10].

For the phase of Ni-based superalloy, gamma phase (γ) and gamma prime

phase (γ’) are the most common phase. Gamma phase is made from FCC

structure which contains significant amount of elements such as chromium,

tungsten, rhenium, cobalt ruthenium and molybdenum. Gamma prime phase is a

precipitated intermetallic phase. For the case of nickel based superalloy, γ’ phase

is coherent with γ matrix and form FCC structure. The precipitate is enriched with


Page 21

elements such as titanium aluminium and tantalum [10]. Generally speaking,

added elements can be categorized as γ and γ’ phase formers. γ formers are the

elements which tend to segregate to the matrix γ and stabilized it. On the other

hand, γ’ formers are the elements segregates to γ’ precipitate and form an

ordered phase like Ni3Al or Ni3Ti.

For the trend of single crystal nickel-based superalloy designing, creep

resistance becomes much better in 4th and 5th generation superalloys compared

with former generation of nickel-based superalloys. However, the oxidation

resistance has been sacrificed and which also reveals the importance of thermal

barrier coating in aero engine system.

2.1.2 Top Coat

The top coat layer is a thermal insulating and strain tolerant ceramic coating

with the main purpose being minimizing heat transfer from the combusted gas to

the superalloy substrate. For the state-of-the-art TBCs, yttria stabilized zirconia

(YSZ) is generally used as the top coat material due to its excellent phase stability

and low thermal conductivity at elevated temperatures. YSZ provides a low

thermal conductivity (2 - 3 W/mK) with the minimum temperature sensitivity

among all known ceramics, and good thermal-mechanical compatibility with

substrate [11]. Compared with other ceramic materials, the high thermal

expansion of YSZ makes it close to the thermal expansion of nickel-based

superalloy substrate shown in Figure 2.2 [1,3,11], which may alleviate the stress


Page 22

accumulated from the thermal expansion mismatch. The porous and crack

structure can reduce the modulus and provide exceptional strain tolerance

because of the low elastic modulus. However, the porous structure allows the

rapid short-circuit diffusion of oxygen and results in the oxidation of the bond coat.

Figure 2.2 Coefficients of thermal expansion (CTEs) and thermal conductivities

of materials for the principal components in TBC systems [1,3,11]

Figure 2.3 Phase diagram of YSZ [1,12]


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The main difference between YSZ and pure zirconia lies in its phase

transformation, the phase diagram is shown in Figure 2.3. For pure zirconia, two

different phases are formed during thermal cycles. When the temperature is

located between room temperature and 1170 °C, zirconia adapts in monoclinic

phase. The phase transformation to tetragonal occurs when the temperature is in

the range between 1170 °C and 2370 °C, and the phase turns to cubic when the

temperature is between 2370 °C and 2680 °C (melting point) which means phase

transformation will happen when the engine is in operation [12]. However, when

the phase transformation happens, there is a 4 to 6% of volume change occurred,

which is extremely harmful to the lifetime of top coat. In order to avoid the

transformation, adding another element will stabilize the cubic phase of zirconia

and commonly yttrium, magnesium, calcium, and gadolinium et al. are added.

For aero-engines, zirconia with 7 to 8 wt.% of added Y2O3 to stabilize the

tetragonal phase is used. The stabilizing effect of yttria is manifested in Figure 2.3

[12]. Actually, the thermal conductivity of YSZ could be lower by adding more

Y2O3 but the lifetime of TBC would be sacrificed as well. This phenomenon is still

unexplained [11,13,14]. When the temperature is below 1050 °C, YSZ is

composed of cubic and monoclinic phase. When the temperature is higher than

1050 °C, monoclinic phase transforms into tetragonal phase. Modern techniques

for YSZ coating are high rate and non-equilibrium processes which lead to its

metastable tetragonal prime phase. Contributed by the tetragonal prime phase,

TBC provides longer lifetime because it does not transfer to monoclinic phase


Page 24

and becomes crack propagation free [15].

For the fabrication of YSZ top coat, the two major methods are air-plasma

spray (APS) and electron beam physical vapor deposition (EB-PVD).

Figure 2.4 Typical cross-sectional SEM microstructure of APS TBCs [19]

Figure 2.5 Schematic image of air-plasma spray processing system

Air-plasma spray:

Figure 2.4 is the microstructure of YSZ top coat made by APS method and

the method is illustrated in Figure 2.5. APS is a method which results in splats

with inter-lamella gaps parallel to the substrate, which has been widely applied in


Page 25

hot components since 1960s. In APS process, argon has been ionized to plasma

by electron arc and introduces YSZ powder into the plasma jet. YSZ stays in semi

plastic state and propelled to the substrate. The molten YSZ flattens and solidify

to form a YSZ coating [16,17]. Top coat which made by APS has a splat grain

morphology with many layers which are parallel to the substrate and provides 10 -

25% porosity which leads to low elastic modulus and low thermal conductivity.

Typically, thermal conductivity of APS YSZ top coat is about 0.8 - 1.1 W/mK

[11,18].

Electron Beam Physical Vapor Deposition:

The cross-sectional microstructure of EB-PVD YSZ top coat and its method

are illustrated in Figure 2.6 and Figure 2.7. EB-PVD method is a more expensive

method, which has been in use since 1980s. It produces a columnar structure

with inter-column interface across the entire coating. The evacuated deposition

chamber allows passage of electrons from the electron gun to the YSZ ingot. The

generated electron beam is accelerated to a high kinetic energy and directed

towards the evaporation material. The substrate is heated and placed into the

YSZ vapour and the vapour deposits onto the substrate with the rate 4 - 10 μm

per minute [20]. Oxygen is also being supplied into the chamber in a controlled

amount in order to preserve the stoichiometric composition [20]. Generally, the

feature of EB-PVD TBCs have columnar structure with typical coating thickness

of about 120 - 150 μm and column width of about 1 - 6 μm and there are lots of

http://en.wikipedia.org/wiki/Electron_gun


Page 26

holes and pores inside the YSZ columns. The thermal conductivity of EB-PVD

YSZ top coat is about 1.5~1.9 W/mK [16].

Figure 2.6 Typical cross-sectional SEM microstructure of EB-PVD TBCs [19]

Figure 2.7 Schematic image of electron-beam physical vapor deposition system

TBC top coat made by EB-PVD method provides better adhesion and strain

tolerance than that fabricated by APS method because the columnar structure


Page 27

mitigates the stress from thermal expansion mismatch [21]. Also, EB-PVD top

coat provides better in erosion resistance. Compared with the fracture which

happens through the splat boundaries on APS top coat, fracture only occurs in

the surface region of columnar EB-PVD top coat structure [19]. However,

compared with their thermal conductivity, APS top coat offers better thermal

insulation [22]. In general, EB-PVD top coat is still the more popular coating

technique for the jet engine turbine blades.

2.1.3 Bond Coat

For the state-of-the-art nickel-based superalloy, the formation of protective

alumina layer is difficult because the component of aluminium is not rich enough

inside of the superalloy. Thus, applying an aluminium rich layer on top of the

superalloy substrate is beneficial. For TBC industry, there are two major types of

bond coat, which are overlay bond coat (i.e. MCrAlX) and diffusion bond coat (i.e.

PtNiAl).

Overlay bond coat:

Overlay bond coat is made by low-pressure plasma spraying and it is made

from two phases, β-NiAl and γ+γ’ bond. For its composition, it could be written as

MCrAlX. The notation M represents Ni or the combination of Ni and Co, while X is

the reactive element such as yttrium and zirconium with concentration lower than

10 ppm and made to improve the adhesion of alumina scale by gettering the

sulphur [24,25]. Also, the compositions between the overlay bond coat and the


Page 28

superalloy substrate are quite different even though inter-diffusion happens when

the engine is in operation. Thus, the surface property such as creep resistance

and oxidation resistance would be considered as the design of overlay bond coat

material.

Diffusion bond coat:

In order to preferentially form an alumina protective TGO layer during engine

operation, a coating with high concentration of aluminium is necessary. For

diffusion bond coat, platinum is used in order to enhance the oxidation resistance

without the aluminizing process [26] and the average thickness is located in the

range between 30 to 80 μm which is much thinner compared with the overlay

bond coat. Also, a thin platinum layer enhances the activity of aluminium on the

surface which helps the formation of the protective alumina scale [1]. For the

state-of-the-art fabrication technique, there are two different kinds of bond coats

β-NiAl bond coat and γ+γ’ bond coat which are presented as follows.

For the β-NiAl bond coat, it is formed by depositing a layer of platinum on

superalloy and annealing in an aluminium rich environment. After annealing,

nickel will be diffused out of the superalloy and react with Pt and Al, and form

Pt/NiAl aluminide. Single β phase NiAl with platinum in solid solution can be

fabricated through this process [27]. On the other hand, γ+γ’ two-phase bond coat

has later been developed, which is typically made by electroplating a platinum

layer on the surface of superalloy substrate followed by a diffusion treatment [28].


Page 29

Compared with β-NiAl bond coat, γ+γ’ bond coat provides better compatibility with

typical superalloy substrate and is more durable against mechanical damage

because of its higher yield strength and creep resistance [29,30]. The fabrication

process of γ+γ’ bond coat has a lower cost because it doesn't need aluminizing.

2.1.4 Thermal Grown Oxide

Thermal grown oxide is the layer which is formed at high temperature due to

oxidation of bond coat and is used to protect against oxidation when the engine is

in operation and it is the main factor to control the lifetime of TBC. Since the

structure of YSZ top coat, i.e. oxygen conductive and porous, the oxygen can

easily penetrate into the bond coat layer when engine is in operation. Besides,

the high ionic diffusivity of oxygen in ZrO2 will also lead the oxidation of bond coat

layer. Thus, a protective thermal grown oxide plays an important role to protect

the substrate superalloy of engine turbine from environmental damage. For the

requirement of TGO layer, TGO layer needs to be stable during thermal cycling

and it also needs to have a good adhesion with both the bond coat and YSZ

layers. For the state-of-the-art TGO, a thin layer alumina which is grown from the

bond coat during the heat treatment can provide a good function to fit that

purpose. Generally, the protective alumina layer is based on α phase alumina

because it provides good stability at high temperature, good adhesion and low

oxygen diffusivity [23]. Generally, TGO layer develops large compressive stress

which is coming from the thermal shrinkage mismatch with the metallic substrate


Page 30

during the cooling stage of a thermal cycle. Also, the growth of TGO may also

induce stress, but it is much lower than the stress resulted from the thermal

mismatch [31]. For all the compressive stress TGO made, it is the main driving

force for the TBC degradation.

2.2 CMAS Degradation

Calcium-Magnesium-Alumino-Silicates (CMAS) penetration into TBCs is one

of the most serious problems for thermal barrier coatings. With further increasing

of the operation temperature, CMAS damages to YSZ top coat layer is becoming

more apparent and requires the attention of materials scientists.

2.2.1 CMAS

Actually, Calcium-Magnesium-Alumino-Silicates (CMAS) is produced from

siliceous mineral debris such as dust or volcanic ash. When the engine is in

operation, CMAS will be ingested along with air by engines [31-33]. Table2.1 is

the compositions of CMAS deposited on different engines, generally, the

composition of CMAS depends on the place where engine is being in operation.

Thus, the composition of CMAS will be slightly different between one another.

Besides CaO, MgO, Al2O3, and SiO2 as shown on the name already, CMAS also

contains large amount of Fe2O3 and other metal oxides [32-34]. Because of the

variation in composition, the melting point of CMAS is in the range of 1240 -

1260 °C [35].


Page 31

Table 2.1 CMAS composition from service retired turbine blades

composition CMAS deposit from

Borom [32]

CMAS deposit from

Vidal-Setif [33]

CMAS deposit from

Braue [34]

CaO 28.7 20-27 33.6

MgO 6.4 4.5-9 9.9

SiO2 11.1 12.26 10.1

Al2O3 43.7 28-36 22.4

TiO2 - 2-5 3

Fe2O3 8.3 9-14 15.4

NiO 1.9 <1 0.8

ZrO2 - 1-4 0.9

Y2O3 - <1 -

Others - - 3.9

2.2.2 Degradation Mechanisms of CMAS attack

According to the results reported in the literature, CMAS stays in liquid phase

and penetrates through the holes, pores or even columnar boundaries into the

YSZ top coat when the engine is in operation as shown in Figure 2.8. Eventually,

YSZ grain boundaries have been filled up by molten CMAS glass and become

highly dense, regardless of whether the YSZ top coat was made by APS or

EB-PVD [29,32,36]. Due to this phenomenon, strain tolerance of APS YSZ top

coat layer has been reduced because the microstructure has been affected by the

molten CMAS glass [37-39]. On the other hand, although it is really hard to

remove EB-PVD TBCs from substrate because of their columnar structure, after

CMAS penetration, YSZ top coat can be separated into two different zones, which


Page 32

are the reaction zone and un-reacted zone [40]. In the reaction zone, columnar

gaps and pores are filled by molten CMAS glass [41-43]. In contrast, there is little

damage happening in the un-reacted zone. During thermal cycling, the top coat

becomes denser and results in higher thermal mismatch with the substrate. This

increases the internal stress of YSZ top coat and leads to the eventual spallation

[35,44].

Figure 2.8 Cross-sectional image of TBC top coat penetrated by CMAS [42]

Besides the penetration through the columnar gaps and pores, CMAS may

also penetrate into the vertical cracks. Vertical cracks are generated when YSZ is

sintered, which help the YSZ top coat to release strain energy [45,46]. However,

vertical cracks also provide a pathway for liquid CMAS and facilitate the spallation

of the top coat.

Moreover, the liquid CMAS can dissolve the YSZ and extract the yttrium from

stabilized zirconia, which means that when engine is in operation, liquid CMAS

starts to extract yttrium, until its concentration of yttrium is low enough that the

metastable tetragonal YSZ turns into tetragonal zirconia [40]. During the cooling

process, the tetragonal phase zirconia turns to monoclinic phase, which led to a 4


Page 33

- 6% of volume expansion [36,38].

Figure 2.9 Schematic image of cross-sectional engine turbine blade [33]

Table 2.2 The thickness of CMAS deposit on the blade and its correlated point

shown in Figure 2.9 [33]

Point number 1 2 3 4 5 6 7 8 9 10

CMAS thickness (μm) 45 0 0 0 0 0 12 10 20 10

2.2.3 CMAS Penetration on Service Retired Engine Turbine

Observation of a service retired turbine blade showed strong adherence of

CMAS on the pressure side and the leading edge of turbine blades [33-35] as

shown in Figure 2.9 and Table 2.2. In the leading edge (point 1), CMAS formed as

a porous and inhomogeneous phase deposited on the YSZ top coat. Near point 8

and 9 in Figure 2.9, where this area is considered as the hottest section of the

pressure side, CMAS formed a homogeneous surface structure, which indicates

the melting of CMAS and solidified because the surface temperature is higher

than the melting temperature of CMAS when the engine is in operation [33]. Also,

CMAS penetrated down to the YSZ top coat, reached to the TGO layer and led to


Page 34

some vertical cracks on TBC columnar gaps and the delamination crack on the

upper layer of the top coat. Near point 7 and 10 in Figure 2.9, where it is the less

hot area of the pressure side, the surface morphology of CMAS at these points is

not as homogeneous as in point 8 and 9. And in the other places, there is no

CMAS deposition.

In the hot region of the pressure side, the original porous structure of TBCs

top coat was filled by molten CMAS and vertical cracks were formed in columnar

gaps. Besides, new phases were found in interface no matter in columnar gaps or

in the pores inside of TBC columns. The top coat morphology also changed to

dense rounded grain as a result of reactions between YSZ and CMAS. According

to the literature, the concentration of yttrium inside YSZ decreased to about 1.5 -

2 mol.% from the original 6 - 8 mol.% [36,38]. In CMAS/YSZ interface, besides

the tetragonal prime phase of YSZ, the monoclinic phase of ZrO2 was formed,

corresponding to the yttrium depletion. For the composition of CMAS, it is really

depending on the place where the engine is served for. In service retired hot

pressure turbine, anorthite (CaAl2Si2O8) and monoclinic Fe-bearing diopside-type

phase (Al0.6Ca0.96FeO0.51MgO0.44Si1.4O6.12) were found in side of the CMAS

deposit. In the area close to the YSZ/CMAS interface, the phase and composition

of molten CMAS glass depended on the place where the engine was served at.

For instance, tetragonal Ca2Zr5Ti2O16 and fluorite-related superstructure

Ca2Zr2Ti4O14 have been found by Vidal-Setif [33], and Ca-Zr-Fe silicate, CaSO4,

CaZrO3 and kimzeyite-type garnet phase (Ca,Mg)3(Zr,Ti,Fe)2(Al,Fe,Si) were


Page 35

found by Braue [34,47]. All of them are the products of YSZ/CMAS interaction and

were observed in different turbine blades.

By observing the engine turbine, we can know that the degradation

mechanism is highly dependent to its surface temperature [33]. In the hottest area

of the pressure side, CMAS degradation is much more severe than other places

because its surface temperature is high enough for the CMAS/YSZ reaction to

occur. CMAS becomes less active away from the hotter area.

2.3 Prevention of CMAS degradation

With the demand of higher engine operating temperature, CMAS

degradation is becoming a severe problem for TBCs especially for the engines

which are served in sand and ash rich environment [32,48]. Recently, scientists

have paid more attention into this inevitable problem and come out with several

solutions to mitigate the CMAS attack on YSZ top coat, which can be separated

into three categories. First, coating an overlay coating on the YSZ top coat, which

can block the pathway for CMAS to penetrate or react with YSZ top coat or it can

also be treated as a sacrificial layer [49,50]. The sacrificial layers can catch the

CMAS by chemical reaction and some sacrificial layers can also raise the melting

temperature of CMAS. The second way is trying to modify the chemical

composition of the YSZ top coat. Generally, this is done by doping Al2O3 and TiO2

[38,51]. The third way to reduce the CMAS damage is to make stabilized zirconia


Page 36

top coat using other stabilizers. According to the literature report, Gd2Zr2O7 and

La2Zr2O7 provide a good CMAS degradation resistance function for TBC system

[52,53].

2.3.1 Protective Overlay

Protective overlay is one of the most common ways to protect the YSZ top

coat from CMAS attack. Among many different kinds of materials, alumina is a

common protection material used to mitigate CMAS effectively which image is

shown in Figure 2.10 [49]. Reaction between CMAS and alumina prevents the

penetration of CMAS into the YSZ top coat. Near the CMAS/alumina interface,

anorthite (Ca2Al2Si2O8) and spinel (MgAl2O4) are typically formed [49]. For

Ca2Al2Si2O8 and MgAl2O4, the melting temperature is 1553 °C and 2135 °C

respectively which are higher than the surface temperature of engine turbine,

which is able to block the path of CMAS penetration. The phase of

CaO-Al2O3-SiO2 inside of CMAS is pseudo-wollastonite [43], which is really hard

to be crystalized. However, after the reaction with Al2O3, phase transformation

happens from pseudo-wollastonite to anorthite. This means the phase will

transform from a hard-to-crystalize phase to a crystalizable phase after the

chemical reaction. On the other hand, MgO will also react with Al2O3 and form

MgAl2O4. Compared with the traditional YSZ TBC top coat, alumina protective

overlays can effectively stop the CMAS degradation.

Although protective coating is the easiest way to protect the YSZ top coat


Page 37

from CMAS degradation, some factors should also be considered such as

adhesion, surface hardness or even thermal-mechanical mechanisms.

Figure 2.10 Cross-sectional image demonstrates the complete suppression of

CMAS melt ingression into the coating assembly [49]

2.3.2 Doping Elements

Doping Alumina and titanium oxide is another way to reduce the effect of

CMAS penetration, although alumina and titanium oxide have little solubility in

YSZ [41]. The atomic level mixing of the constituent elements (i.e. ZrO2, Y2O3,

Al2O3, and TiO2) in the chemically prepared feedstock and rapid fabrication

process kinetically suppresses the precipitation and extends the solubility [54,55].

According to Drexler, doped alumina and titanium oxide top coat provides high

resistance to the CMAS penetration. CMAS only penetrates the APS YSZ doping

Al2O3 and TiO2 TBCs sample 40 - 50 μm depth in 24 hours isothermal heat

treatment at 1200 °C. After interaction, YSZ remains in the tetragonal phase and

the reaction product anorthite has been found in the arrest area [38].


Page 38

Figure 2.11 (a) Cross-sectional image of APS top coat made by YSZ doping

alumina and titanium oxide interact with CMAS, and (b) its EDS mapping of Si

The mechanism of dopants during the CMAS/YSZ interaction is really similar

to the protective overlay that was mentioned in the last session. When the surface

temperature reaches the CMAS melting point, CMAS starts to penetrate into the

porous structure and generate cracks inside TBC. The molten CMAS also reacts

with top coat. Because molten CMAS dissolved YSZ and released dopants from

YSZ, the concentration of aluminium and titanium are increased during the heat

treatment, since the concentration of alumina has been increased, phase

transformation of CMAS from pseudo-wollastonite to anorthite will occur.

Pseudo-wollastonite is known as a hard to crystallize phase, it changes

abnormally and rapidly hundreds of degrees below its melting point. By adding

alumina, CMAS would transformed from pseudo-wollastonite to crystalizable

anorthite [43]. Also, the solid anorthite will stay at its interface and block the

pathway of CMAS penetration. During the interaction, titanium is treated as a

nucleation agent [38,41,43], and the precipitated alumina still reacts with the

CMAS, and facilitates the crystallization.


Page 39

2.3.3 New Top Coat Materials

Recently, rare earth zirconate has been considered as the future materials

for TBC top coat. Compared to traditional YSZ top coat, rare earth zirconate

provides higher sintering resistance and lower thermal conductivity [52,56-58].

Moreover, rare earth zirconates stay in the stable single crystal phases during

thermal cycling [59,60] which is beneficial for the future engine design aiming at

higher operating temperatures [1]. Today, Gd2Zr2O7 (GZO) has been widely

tested and it provides good property for CMAS resistance [41,60,61]. To observe

its interaction with CMAS, the degradation mechanism is really like alumina

protective coatings. For CMAS, it only penetrates into the 30 μm of the depth and

attracted by top coat. The chemical reaction could be written as below.

CMAS + Gd2Zr2O7 C0.15Gd0.42Zr0.05Si0.38O1.64 (apatite)

+ Zr0.77Gd0.2Ca0.03O1.87 (fluorite)

+ Mg0.33Al0.63Zr0.02Gd0.02 (spinel)

+ residual elements

In this reaction, apatite plays an important role as a key product [62,63]

because its melting temperature is 1930 °C [64] and the place it deposited highly

affects the degradation of CMAS penetration. The interaction mechanism

between CMAS and Gd2Zr2O7 is really similar to the case between Al2O3 and

CMAS. As illustrated in Figure 2.12, in the early stage, CMAS and Gd2Zr2O7 top

coat interact in the columnar gaps and nucleate the products within the columnar

gaps. Once the gaps are blocked by apatite and other elements, CMAS starts to


Page 40

attack columnar tips, and formed a product overlay on the top. Because the

melting temperature of the reaction product is much higher than the engine

operating temperature, it stays in the solid phase when the engine is in operation

and forms a layer in the interface of CMAS/Gd2Zr2O7. The thick layer of reaction

products can effectively protect the top coat surface by blocking the pathway of

CMAS penetration and provide the longer lifetime of top coat. Actually, not only

gadolinium zirconate provides good resistance against CMAS, other rare earth

zirconates also provide similar protection mechanism [41,61].

Figure 2.12 Mechanism of CMAS/Gd2Zr2O7 interaction. (a) In early stage,

CMAS/Gd2Zr2O7 react in TBCs columnar gaps, (b) columnar gaps are filled up

with reaction products and the columnar tips just receive small attack from

CMAS, (c) columnar tips is severely attacked by CMAS, (d) additional

crystalline forms during cooling [60]

CHAPTER 3 RESEARCH PROPOSAL

Page 41

Chapter 3

Research Proposal

3.1 Experimental Scope

CMAS-induced degradation is a serious issue for TBCs. Besides, sands and

debris in the air, volcanic ash is also included in the CMAS category. Although the

degradation of TBC by volcanic ash is really similar to that done by traditional

CMAS, the degradation by volcanic ash goes much severer and their structure

and composition are much more complicated than the traditional CMAS. Since

the disruption of air travel and loss of 2 billion U.S. dollars associated with the

eruption of Eyjafjallajökull volcano in Iceland in 2010 [54,65], materials scientists

have been focusing on the effect of volcanic ash degradation.

In this research, instead of formulating artificial CMAS compositions, real

volcanic ash are utilized to simulate naturally occurring CMAS particles and

understand the degradation effect of volcanic ash on the YSZ top coat. Corrosion

and phase transformation-induced degradation of EB-PVD TBCs are studied in

detail.

The second half of the research focuses on engineering preventative

techniques against volcanic ash attack by the sol-gel fabrication of metal-oxide


Page 42

protective coatings on the surface of the YSZ top coat to prevent volcanic ash in

contact with the YSZ top coat using two different ideas. The first idea is using a

dense YSZ overlay to block the columnar gaps and the porous structure of the top

coat to protect the top coat structure from CMAS attack. Also, this layer is treated

as a sacrificial layer, which means CMAS will react with the dense YSZ overlay

before reacting with the YSZ top coat. The second Idea is much related to the

property of volcanic ash. Because of its complicated composition, it is nearly

impossible to find a ceramic material, which does not react with volcanic. Thus,

the mechanism of the second type of protective coating will be based on

protective overlay materials which react with volcanic ash, but result in reaction

products with higher melting temperatures capable of staying in the solid phase

when the engine is in operation. For the material of the second type, coating an

Al2O3 film can restrain CMAS infiltration into the YSZ top coat effectively,

according to the literature [49].

3.2 Experimental Procedure

The research procedure can be separated into six different parts: 1)

characterization of the volcanic ash, 2) study of the CMAS-induced TBC

degradation by the use of volcanic ash, 3) penetration test of volcanic ash in

different materials, 4) differential thermal analysis (DTA) analysis for the mixture

of volcanic ash and candidate materials, 5) X-ray diffraction (XRD) analysis of


Page 43

volcanic ash-YSZ mixture and volcanic ash-Al2O3, and 6) sol-gel based synthesis

of metal-oxide based protective coatings.

1. Characterization of the volcanic ash

It is necessary to carry out initial measurement of the melting temperature

and thermo-physical properties of volcanic ash by using DTA. Characterization of

the surface morphological change and surface roughness of ash after thermal

exposure will also be done using high magnification digital microscope (Keyence

VHX-5000). Although the surface of volcanic ash shows a significant roughness

and great variation of height, by compiling lots of images at different focal planes,

highly focused images can still be obtained. Also, focal position data on image

can be utilised to build up a 3D model.

On the other hand, the composition will be measured by X-ray fluorescence

Spectrometer (XRF) and the phase will be determined using XRD. XRF is a

technique used for elemental quantitative analysis which mechanism is similar

with energy dispersive spectroscopy (EDX). The principle of XRF is using an

X-ray to irradiate the sample and make electron to be ejected from its original

atomic orbit. In order to fill up the space, electron coming from higher energy level

orbit will drops to lower energy level orbit and release fluorescent X-ray which is

characteristic and can be defined to a particular element. By analysing these

characteristic fluorescent X-rays, the composition of the sample and the

concentration of elements can be measured. For EDX, an electron gun built up in


Page 44

SEM emits a focused electron beam to irradiate the sample and release the

characteristic X-rays. The composition and concentration can be confirmed by

analysing the characteristic X-rays. The main difference between XRF and EDX

is the radiation resource using to hit the sample. XRF uses X-ray and EDX uses

electron beam. Due to the larger size of radiation resource, XRF is able to do the

larger area (28 mm2) of measurement which is more suitable to find out the

general composition of volcanic ash. Once better understanding of the volcanic

ash has been established, it will allow us to determine experimental parameters

for the subsequent TBC testing.

2. Study the CMAS-induced TBC degradation by the use of volcanic ash,

In this part, the TBC samples used are 8 wt.% YSZ deposited by EB-PVD on

a nickel-based superalloy. To begin with, TBCs surface is covered with a thin

layer of volcanic ash, before heating samples in a furnace at 1250 °C for different

exposure times. SEM and energy dispersive X-ray spectroscopy (EDX) will be

used to analyse the degradation of YSZ top coat by volcanic ash. In addition,

molar ratio of zirconium, yttrium, and oxygen would be measured using EDX point

analysis, and the ratio change of Y2O3 in TBC samples after different hours of

thermal exposure would also be calculated using following formula:

Y2O3(wt%) =XYMY + 1.5(XyMO)

[XZrMZr + 2(XZrMO)] + [XYMY + 1.5(XyMO)]

(X=molar ratio, M= atomic mass, MY=88.90585, MZr=91.224, MO=15.9994)

3. Penetration test of volcanic ash in different materials


Page 45

To better understand the ability of protective coatings against volcanic ash,

dense alumina and YSZ plates were used to simulate the ideal fully-dense YSZ

and alumina protective coatings. In this study, great amount of volcanic ash was

put on the top of alumina and YSZ plates, and heat-treated to observe the

penetration depth of volcanic ash in different materials. SEM and EDX mapping

were used to determine the penetration depth of volcanic ash in alumina and YSZ

plates.

4. DTA analysis of volcanic ash and YSZ/Al2O3

DTA was used to determine the reactivity between volcanic ash-YSZ, and

volcanic ash-Al2O3 in different temperatures. For volcanic ash-YSZ DTA analysis,

the purpose is trying to find out the chemical reaction and phase transformation at

different temperatures and understanding the mechanisms of CMAS penetration.

On the other hand, according to the result of volcanic ash-Al2O3 DTA analysis,

chemical reaction temperatures will be obtained and the melting point of

reactants and reaction products might also be determined. For this series of

experiments, samples were prepared by 1:1 weight ratio of volcanic ash and

YSZ/Al2O3, and the heating profile was set from room temperature to 1480 °C

(instrument limit) with the heating rate 10 °C per minute was set from.

5. XRD analysis of volcanic ash-YSZ mixture and volcanic ash-Al2O3

After DTA analysis, the reaction products of volcanic ash-YSZ and volcanic

ash-Al2O3 were measured using XRD analysis. For volcanic ash-YSZ XRD


Page 46

analysis, in order to get more understand for volcanic ash-YSZ reaction, 2

samples were made for this experiment. The first sample was made from powder

mixture of volcanic ash and YSZ mixed in 1:1 weight ratio were prepared, and the

other sample was pure YSZ powder which made as a comparison. After sample

preparation, heat treatments proceeded from room temperature to different target

temperatures with a heating rate 10 °C per minute. Upon reaching the target

temperature, samples were held at target temperature for 3 minutes for better

reactions. The target temperatures were chosen from the peaks and valleys

shown in DTA curves. In volcanic ash-YSZ system, peaks and valleys were

located at 490 °C, 950 °C, 1160 °C, and 1350 °C chosen from the peaks and

valleys shown in DTA curve.

For the sample preparation of volcanic ash-Al2O3, it is the same with volcanic

ash-YSZ. The sample was made from powder mixture of volcanic ash and Al2O3

mixed in 1:1 weight ratio and heat treated from room temperature to different

target temperatures which are 410 °C, 525 °C, 1060 °C and 1310 °C respectively

shown in volcanic ash-Al2O3 DTA curve. After Sample preparation, XRD analysis

was conducted and the reaction products were obtained. XRD semi quantitative

analysis would be used to volcanic ash-YSZ and pure YSZ patterns to compare

the ratio change of tetragonal phase to monoclinic phase YSZ after different

temperature of heat treatments. XRD semi quantitative analysis is a technique

uses diffraction peaks to identify the fraction of selected phases inside the sample.

For the mixture of two crystalline phases, the fraction of each phase can be


Page 47

calculated by determining the strongest peaks from each phase and comparing

each peak intensities with their correlated max intensities (when they are in single

phase situation) using Klug’s equation:

f1 =

(I1mix

I1max)A2

A1 − (I1mix

I1max) (A1 − A2)

(f1 = fraction of phase 1, Imix = intensity in mix situation, Imax = intensity in max

situation, A= mass absorption coefficient)

In this experiment, the peak selected for monoclinic phase ZrO2 is located at

28°(-1 1 1). And the peak selected for tetragonal phase ZrO2 is located at 30°(1 1

1) in its XRD pattern.

6. Sol-gel based synthesis of metal-oxide based protective coatings

The final part of this research is making a protective coating on the top of

TBCs using sol-gel based methods. YSZ and Al2O3 were chosen as the materials

for protective overlays. And the ideal morphology for protective coatings is a

dense overlay with thickness 20 - 50nm.

Sols should be prepared first for both dip coating and spray coating. The sols

for fabrication of YSZ protective coatings is made from the mixture of zirconium(III)

chloride (ZrCl3), yttrium(III) chloride hexahydrate (YCl3∙6H2O) propylene oxide,

and deionized water [66]. The sol of Al2O3 coatings is made from mixing

aluminium chloride hexahydrate (AlCl3∙6H2O), and propylene oxide (CH3CHCH2O)

into deionized water [67].


Page 48

Dip coating was made by dip coater, after substrate immersed into the sol for

1 minute, the machine pulled the sample up with a speed 0.03 cm/sec. For spray

coating, it is made by air gun spraying with an emitting pressure 0.4 MPa and

spraying time 5 seconds. After each time of dipping or spraying, 75 °C of heat

treatment for 15 minutes was proceeded. The amount of the materials produced

for coatings are controlled by the dwell time and pull up speed for dip coatings,

and spraying time and emitting pressure for the spray coating.

This experiment can be separated as two parts, the first half focused on

parameters defining (i.e. gelation time of sols, the thickness measurement for

each time coating and surface morphologies of protective coatings made by dip

coatings and spray coatings) and stainless steel was chosen as the substrate.

The better sols for sol-gel coatings and the better coating technique can be

confirmed after all parameters were defined.

In second half, protective coating would be coated on the top of TBCs using

better sols and coating technique. In order to get dense coatings, one-hour

thermal heat treatment was conducted after the last 15 minutes heating.

CHAPTER 4 VOLCANIC ASH AND ITS PENETRATION MECHNISMS IN TBCS

Page 49

Chapter 4

Volcanic Ash and Its Penetration Mechanisms in TBCs

4.1 Study of Volcanic Ash

Unlike traditional artificial CMAS, real volcanic ash is used in this experiment

to simulate the CMAS attack. And In this session, morphology of volcanic ash and

their degradation mechanisms to TBCs are discussed.

4.1.1 Structure and Surface Morphology

The volcanic ash was coming from Sakurajuma, Japan, which erupted on the

10th of March, 2009. The surface morphology and composition were studied by

SEM and XRF as shown in Figure 4.1 and Table 4.1 respectively. Compared with

the volcanic ash from other places [68,69], it contains less silica. Also, in

comparison with traditional artificial CMAS, the volcanic ash contains seven more

elements (i.e. Na, P, K, Fe, Ti, Mn, and Ta), which may result in different

degradation mechanisms to TBCs. Regarding its phase structure, the result of a

XRD analysis (shown in Figure 4.2) shows that the volcanic ash has a complex

crystal structure containing cristoblaite (SiO2), anorthite sodian (CaNaAl2Si2O8),

and Pigeonite (FeMgCaSiO3) and other silicon contained oxides. However, it is

very difficult to fully reveal the complete phase structures due to the complexity of


Page 50

the XRD peaks and the phase structure.

Figure 4.1 SEM image of volcanic ash

Table 4.1 Composition of the volcanic ash from Sakurajima volcano, Japan

Element Na Mg Al Si P K Ca Ti Mn Fe Ta O

Volcanic Ash

(at.%)

0.57 4.02 10.27 35.44 0.19 1.99 6.39 0.94 0.22 9.46 0.03 bal

Figure 4.2 XRD pattern of volcanic ash


Page 51

4.1.2 Melting Point of Volcanic Ash

The melting point of volcanic ash plays an important role in determining the

type of degradation induced on TBCs. If the melting temperature of the volcanic

ash is higher than the surface temperature of the engine turbine, there should be

no or very little penetration happening between volcanic ash and YSZ top coat of

TBCs. Figure 4.3 shows the result of DTA analysis on volcanic ash. It is noticed

that a sharp valley has been formed at the temperature around 1235 °C, which

can be explained by the transformation from solid phase to liquid phase (i.e.

melting). Besides, another experiment has been carried out for better

understanding of the melting temperature of volcanic ash. Isothermal heat

treatments at four different temperatures 1150 °C, 1200 °C, 1250 °C, and 1300 °C

have been done on volcanic ash for 30 minutes and their surface morphology and

surface roughness were shown in Figure 4.4 - Figure 4.7. By comparing the

heat treated samples, obvious change of their surface morphology and surface

roughness were obtained. If volcanic ash starts melting, its structure would

transform from rough particles to a much flatter plane. Figure 4.4 shows the

volcanic ash image took after 1150 °C of heating. It shows that the surface was

inhomogeneous and the volcanic ash was sintered already. By observing the

surface roughness, a 291.6 μm height difference from the highest point to the

lowest point was noticed. By referring to Figure 4.5, it can be seen that volcanic

ash had partially started melting at 1200 °C because its surface structure became

smoother and its surface morphology looked much more homogenized after


Page 52

1200 °C heat treatment. Also, its height difference reduced from 291.6 μm to

113.8 μm. After heat treatment at 1250 °C, the surface structure of volcanic ash

became much smoother and by observing its surface roughness, there is only

36.4 μm height difference. In Figure 4.7, the sample has been heated at 1300 °C

and the surface structure and roughness looked almost the same with the sample

which heated at 1250 °C. All of these observations demonstrate that the volcanic

ash melts between 1200 to 1250 °C which agrees well with the result of the DTA

test (melting point 1235 °C).

Figure 4.3 DTA curve for volcanic ash

Figure 4.4 (A) Surface image of volcanic ash and (B) its surface roughness

A B


Page 53

after heat treatment at 1150 °C for 30 mins


heat treatment at 1200 °C for 30 mins





B

B

A

A

A B


Page 54

4.2 Mechanisms of volcanic ash attack

After the study of the melting temperature of volcanic ash, the next step was

to understand the degradation mechanism of volcanic ash on TBCs. According to

the results, there are two different kinds of degradation mechanism, delamination

and penetration.

Figure 4.8 SEM image of the as-received TBC sample

Figure 4.9 SEM image of TBC after 30 minutes heat treatment at 1250 °C


Page 55

4.2.1 Delamination Type of Degradation Mechanism on TBCs

Unlike traditional CMAS penetration, delamination happens when the

thickness of the deposited CMAS/volcanic ash reaches a certain thickness,

delamination occurs after only one thermal cycle [44]. This phenomenon has also

been noticed in previous experiments for volcanic ash degradation tests. Figure

4.8 shows the cross-sectional morphology of the as-received TBC sample. By

covering the thermal barrier coatings by a thick layer of volcanic ash (30mg/cm2),

the YSZ top coat spalled off after one thermal cycle at 1250 °C as it shown in

Figure 4.9.

Xi Chen [44] has explained the relationship of the delamination crack and the

thickness of CMAS deposit using finite element calculation. According to the finite

element simulation, bending stress is formed due to the thermal expansion

mismatch between CMAS, YSZ top coat, and bond coat layers and concentrate

more on the upper-left and lower-right side of YSZ columns shown in Figure 4.10.

Due to the high bending stresses, columns adjacent to the sintering edge cracks

from the right side to the left side near the interior edge, as it shown in Figure

4.11A. By calculating individual YSZ columns, energy release rate G is increased

along with the crack length (length of crack/diameter of YSZ column) with a fast

speed, once the length of crack has been propagated for almost an entire column,

G goes to infinity. By simulating the long delamination cracks for the whole TBC

model shown in Figure 4.11B, once the crack starts propagating, energy release


Page 56

rate is increased and reaches its steady state (GSS) when the crack propagates to

a long distance.

𝐺𝑠𝑠𝑡ℎ𝑒𝑜𝑟𝑦

=(∆𝑇∑ 𝐸𝑖ℎ𝑖∆𝛼𝑖)

3𝑖=1

2∑ 𝐸𝑖3𝑖=1 ℎ𝑖

2

In the formula, ΔT is the temperature change, i refers to the various layers

(e.g. CMAS, TBC, BC), Δαi is the mismatch of CTE between each layer and the

substrate, Ei is the Young's modulus of corresponding layers, and hi refers to the

heights of each layer. In Figure 4.12, it can be seen that when the crack length

reaches the length of 30 - 50 times of the YSZ columns diameter, their G would

be close to its GSS and once GSS is over the fracture toughness of YSZ columns

(60 - 80 J/m2), the crack will propagate throughout the TBC YSZ top coat and lead

to the spallation of entire YSZ top coat. By observing the diagram in Figure 4.12,

once the height of CMAS reaches 20 - 30 percent of YSZ top coat height, the

steady state energy release rate would reach the fracture toughness of YSZ and

result in spallation. But in real situations, delamination cracks are induced by

thinner layer of CMAS deposit [48].

Owing to the similarity of volcanic ash and CMAS in their composition and

the degradation mechanisms on TBCs, the relationship between CMAS thickness

and delamination crack propagation reported by Xi Chen [44] can still be used in

this study. According to literature [70], the density of crystalized volcanic ash is in

the range between 2700 - 3300 kg/m3, which means the amount of volcanic ash

(30mg/cm2) using in this experiment provides a thickness about 91 - 110 μm, and


Page 57

is much thicker than the 20 - 30 percent of top coat height which makes top coat

totally spalled off after only one thermal cycle [44].

Figure 4.10 Schematic image of the distribution of stress on YSZ columns [44]

Figure 4.11 Schematic image of A) short column cracks near the sintering

separation B) long delamination crack [44]

Figure 4.12 The diagram of energy release rate and CMAS thickness [44]

A B


Page 58

4.2.2 Penetration Type of Degradation Mechanism on TBCs

By reducing the amount of volcanic ash (3 mg/cm2) introduced to the YSZ top

surface, the mechanisms of volcanic ash penetration could be revealed clearly.

Figure 4.13 shows that the TBC columns were penetrated by volcanic ash when

heated to 1250 °C for 15 minutes. In addition, volcanic ash also melted and got

evenly covered on top of the YSZ columnar tips. Through the EDX mapping

shown in Figure 4.14, it can be observed that the penetration of molten volcanic

ash went as far as 40 μm already, although there was no severe crack or

observed spallation.

Figure 4.13 TBC sample under the heat treatment at 1250 °C for 15 minutes

with volcanic ash

Figure 4.14 EDX mapping of (A) Zirconium and (B) Silicon for the TBC sample

under the heat treatment at 1250 °C for 15 minutes with volcanic ash

A B


Page 59

SEM image of volcanic ash penetration into TBC top coat after exposed at

1250 °C for 3 hours is shown in Figure 4.15. It can be seen that volcanic ash

penetrated into YSZ columnar gaps and pores, and great amount of volcanic ash

was found at the bottom of YSZ top coat. In addition, spallation of TBC top coat

was found in some part of the interface of top coat and TGO. It is believed that the

aggregation of volcanic ash at the bottom of top coat might have caused the

degradation of coating adherence and accelerated the spallation of TBC.

Figure 4.15 SEM image of volcanic ash attack on TBC after 3 hours heat

treatment at 1250 °C

Figure 4.16 shows the cross-sectional microstructure of the volcanic ash

penetrated TBC heat treated at 1250 °C for 10 hours. It was also noticed that

considerable amount of volcanic ash was concentrated near the bottom of YSZ

top coat which would eventually lead to the delamination of the top coat. However,

unlike the CMAS attack on TBCs [40,42], there is no molten volcanic ash glass

stayed inside of YSZ columnar pores. Referring to the EDX point analysis shown


Page 60

in Figure 4.17 and Table 4.2, point 1 shows the composition of volcanic ash glass.

It can be seen that besides the original elements existed in the volcanic ash,

17.67 at.% of zirconium was detected although zirconium signals might come

from the surrounding YSZ columns. Compositions of point 2 to 6 are also

included, which indicates that the concentration of yttrium got reduced

considerably. Compared with the original 8YSZ (8 wt.% of Y2O3 induced,

approximately equal to 2.9 at.% of Y in its composition), only 1.1 - 1.6 at.% of

yttrium remained which means that the concentration of yttrium had been

decreased by almost 1.6 at.%. Although further phase detecting experiments

haven’t been done on TBC samples, according to the literature review [36,38],

molten CMAS is able to deplete the yttria from YSZ which cause stabilized

zirconia turned to Y-lean zirconia. Also, its phase will be transformed from

tetragonal prime phase to monoclinic phase along with the composition change.

And this phenomenon not only exist in CMAS attack but also volcanic ash

degradation [41,69].

Figure 4.16 SEM image of YSZ top coat after 10 hours heat treatment with

volcanic ash at 1250 °C


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Figure 4.17 EDX point analysis of YSZ top coat after 10 hours heat treatment

with volcanic ash at 1250 °C

Table 4.2 Data of EDX point analysis

Element ( at.%) Zr Y O Al Si

Pt.1 17.67 - 66.72 0.39 15.22

Pt.2 33.75 1.45 64.80 - -

Pt.3 33.78 1.37 64.85 - -

Pt.4 34.29 1.64 64.07 - -

Pt.5 33.74 1.39 64.87 - -

Pt.6 32.74 1.12 66.14 - -

The concentration change of Y2O3 in YSZ top coat after different hours of

heat treatment at 1250 °C measured by EDX point analysis is presented in Figure

4.18. The results show that the concentration of Y2O3 decreases with increasing

heat treatment time. For the original TBC top coat, 8 wt.% of Y2O3 was added into

ZrO2 as the phase stabilizer. However, the concentration decreased to 7.77 wt.%

after 1 hour thermal exposure. After 3 hours of heat treatment, the concentration

of Y2O3 reduced to 6.47 wt.% and only 3.71 wt.% of Y2O3 left in YSZ top coat

columns after 10 hours thermal exposure. All these results suggest that two


Page 62

mechanisms are involved in degradation of TBC by volcanic ash. The first

mechanism is physical penetration: molten volcanic ash penetrates into TBC

columnar gaps and pores, which combines and stiffens the top coat by sintering.

Second, molten volcanic ash also attack YSZ top coat through chemical reactions:

it depletes the yttria, which may lead to the harmful phase transformation of YSZ

[40]. Further experiments would be discussed in next section.

Figure 4.18 Concentrations of Y2O3 after thermal exposure to volcanic ash at

1250 °C for 1, 3, 10 hours

4.3 Investigation of the Reaction between Volcanic Ash and YSZ

by DTA and XRD Analysis

In order to get more information of volcanic ash degradation, further

experiments for reaction temperatures and reaction products measurement will

be discussed in this section. The reaction temperatures between volcanic ash


Page 63

and YSZ were measured by DTA, and their reaction products were observed

using XRD after reaction temperatures were determined. The DTA curves of

volcanic ash, YSZ, and volcanic ash-YSZ mixture sample were shown in Figure

4.19. In the DTA curve of volcanic ash-YSZ mixture sample, there are four peaks

or valleys located at 490 °C, 950 °C, 1160 °C, and 1350 °C. By comparing with

these three patterns, the melting point of volcanic ash and the phase

transformation point of YSZ might be shifted. For the melting point of volcanic ash,

it was reported to be at 1235 °C in chapter 4.1. However, the endothermic valley

which is able to represent the melting of volcanic ash was shifted to about

1160 °C in volcanic ash-YSZ mixture DTA curve.

Figure 4.19 DTA curves of volcanic ash, Al2O3, and volcanic ash-Al2O3 mixture

For the phase transformation of YSZ, it has been reported in Figure 2.3 [12]

that phase transformation from monoclinic to tetragonal would happen when the

temperature reaches around 1050 °C. Also, the DTA curve of YSZ showed an

exothermic peak at 1050 °C which corresponds to the phase transformation from

monoclinic YSZ to tetragonal phase YSZ. This result is in accordance with the


Page 64

YSZ phase diagram shown in Figure 2.3. However, the phase transformation

point seemed to have decreased from 1050 °C to 950 °C by mixing volcanic ash.

Further details would be explained using XRD analysis.

Figure 4.20 XRD spectra of (a) volcanic ash-YSZ mixture samples and (b) pure

YSZ samples before and after heat treatment at 490 °C, 950 °C, 1160 °C, and

1350 °C

The XRD patterns of volcanic ash-YSZ mixture are shown in Figure 4.20(a).

Signals coming from YSZ were too strong, which made most peaks of volcanic

ash invisible. Thus, XRD pattern of pure YSZ is shown in Figure 4.20(b) as a

comparison. Comparing with the XRD patterns of volcanic ash-YSZ mixture

samples and pure YSZ samples, two changes are identified: decreasing of SiO2

signal and the phase transformation of YSZ. In Figure 4.20(a), SiO2 peak was

observed in as prepared volcanic ash-YSZ mixture sample, but it decreased


Page 65

severely after heat treatment at 490 °C. This phenomenon demonstrated that the

reaction between volcanic ash and YSZ started at 490 °C which is far below the

melting point of volcanic ash. On the other hand, the volcanic ash would also

influence the phase transformation of YSZ. In Figure 4.20(a), monoclinic YSZ

peaks became stronger after heat treatment at 490 °C. They decreased after

950 °C thermal exposure, and slightly increased after 1160 and 1350 °C heat

treatment. However, as it is shown in Figure 4.20(b), strength of monoclinic YSZ

peaks keeps decreasing in pure YSZ samples with the increasing of thermal

treatment temperature.

Table 4.3 Ratio of tetragonal phase YSZ to monoclinic phase YSZ for volcanic

ash-YSZ mixed sample and YSZ measured by using XRD Semi quantitative

measurement before and after thermal exposure at different temperatures

Heating temperature (°C ) As prepared 490 950 1060 1310

Volcanic ash + YSZ 66:34 53:47 71:29 68:32 66:34

YSZ 67:33 75:25 81:19 100:0 100:0

The ratio of tetragonal phase YSZ to monoclinic phase YSZ measured by

XRD semi quantitative analysis were shown in Table 4.3. For the volcanic

ash-YSZ mixed samples, the percentage of monoclinic YSZ was increased from

original 34% to 47% after thermal exposure at 490 °C and decreased to 29% after

heat treatment at 950 °C. After heat treatment at 1160 °C and 1350 °C, the

concentration of monoclinic YSZ increased slightly to 32 and 34%. On the other

hand, the concentration of monoclinic phase YSZ kept decreasing in pure YSZ

samples, and it reached 0% after heat treatment at 1160 °C. According to the


Page 66

phase diagram shown in Figure 2.3 [1,12], phase transformation of YSZ occurred

at around 1050 °C. Once the monoclinic phase YSZ transformed to tetragonal

phase, it would be really difficult for YSZ to transform back to monoclinic phase

because yttria stabilizes the phase of zirconia, allowing tetragonal phase able to

stay in lower temperature for a long time. Thus, no monoclinic phase YSZ could

be observed when heat treatment temperature was higher than 1050 °C. For the

case of volcanic ash-YSZ mixed sample, the percentage of monoclinic phase

kept increasing except for the sample heat treated at 950 °C. Compared with the

as prepared sample, the percentage of tetragonal phase was 13% lower after

heat treated at 490 °C. The increase in monoclinic phase can be explained by the

depletion of yttria. When the heat treatment was carried out above 490 °C, the

reaction between volcanic ash and YSZ caused the depletion of the yttria content,

and subsequently led to transformation of monoclinic phase in YSZ at lower

temperature. For volcanic ash-YSZ mixed sample heat treated at 950 °C, the

monoclinic phase YSZ was 18% lower than volcanic ash-YSZ mixed sample

thermal exposure at 490 °C. This is because monoclinic phase YSZ was already

existed in as-prepared sample. For 490 °C heat treated sample, monoclinic

phase YSZ could be separated as YSZ with yttria and YSZ with no yttria, which

schematic diagram was shown in Figure 4.21. According to the previous DTA

curve, it was demonstrated that the phase transformation point of YSZ was

shifted from 1050 °C to 950 °C, all monoclinic phase YSZ powder transformed to

tetragonal. However, when the temperature was cooling down, only


Page 67

yttria-depleted YSZ transformed back to monoclinic phase. Yttria-rich YSZ stayed

in the tetragonal phase because yttria stabilizes the tetragonal phase, which

made the decrease of 18% of monoclinic phase yttria.

Figure 4.21 Schematic diagram of monoclinic phase YSZ after 490°C thermal

exposure

Table 4.4 The comparison of degradation mechanisms by volcanic ash from

Sakurajima, Eyjafjallajökull and artificial CMAS

Melting

temperature

Depleting

of Yttrium

Phase

transformation

Penetration

speed

Volcanic ash-

Sakurajima

1235 °C √

t’-ZrO2 to

m-ZrO2

40μm/

30mins

Volcanic ash-

Eyjafjallajökull

[41,69]

1200 °C √

t’-ZrO2 to

m-ZrO2

150μm/ 1hr

Artificial CMAS

[40]

1240~1260 °C √ t’-ZrO2 to

m-ZrO2

20μm/ 4hrs

Table 4.4 is a comparison between artificial CMAS [40], Volcanic ash from

Eyjafjallajökull volcano, Iceland [41,69], and Volcanic ash from Sakurajima

volcano, Japan. Basically, volcanic ash and artificial CMAS share almost the

same degradation mechanisms such as depleting of YSZ and lead the phase

transformation. However, there is an important difference between the other two,

besides the melting temperature. Compared with artificial CMAS shown in

literature [40], volcanic ash has a faster penetration speed than artificial CMAS.


Page 68

The reason why volcanic ash shows a faster penetration might be related to its

composition and phase structure. The viscosity of volcanic ash might be smaller

than artificial CMAS due to the 7 more composition inside of volcanic ash (i.e. Na,

P, K, Fe, Ti, Mn, and Ta) and its complicated phase structures. Because further

research hasn’t been done, the exact reason is still needed to be confirmed. This

is the reason why so many airline companies decided to cancel flights when the

Eyjafjallajökull volcano erupted.

The degradation mechanisms of volcanic ash attack on YSZ top coat was

clearly shown in this chapter. Indeed, results showed that the degradation

mechanisms of volcanic ash attack were quite similar to the CMAS attack on

TBCs presented in previous publications [32,40,41,71]. Except for the detrimental

physical penetration effects such as sintering and spallation made by molten ash,

content of Y2O3 was depleted by molten volcanic ash. For the characterization of

YSZ, zirconia is known as an unstable material, which undergoes the phase

transformation between monoclinic phase and tetragonal phase under thermal

cycles if there is no yttria to stabilize the tetragonal phase. Accompanied with the

phase transformation, 4 - 6% of volume change in the unit cell occurs which is

extremely harmful to the TBC lifetime [72]. Moreover, literature also claims that

molten volcanic ash will react with ZrO2 to form ZrSiO4 as their reaction product

[69], but this reaction product has not been found in our experiments.

CHAPTER 5 CHARACTERIZATION OF PROTECTIVE COATING MATERIALS

Page 69

Chapter 5

Characterization of Protective Coating Materials

In order to prevent the direct attack of volcanic ash on TBC top coat, YSZ

and Al2O3 protective coatings were fabricated on TBCs. Before the fabrication

started, the capabilities of YSZ and Al2O3 protective coatings against volcanic ash

were carried out and will be discussed on in this section.

5.1 Reaction and Permeability of Volcanic Ash in YSZ and

Alumina Plates

In this study, YSZ and Al2O3 plates were operated to simulate the ideal

protective coatings, which were expected to be dense and impermeable. Ideal

YSZ protective coatings were simulated by YSZ plates cut from an 8YSZ ingot.

For the ideal Al2O3 protective coating, an α-phase alumina plates of purity 99.9%

was simulated. Volcanic ash was placed on the surface of these two different

plates with a concentration of 20 mg/cm2, and heat treated at 1250 °C for 5 and

20 hours. SEM and EDX were utilized to characterize their microstructure and

chemical morphology, and understand their ability against volcanic ash

degradation.

To systematically study the protective effect of protective coatings, reactions

and permeability of volcanic ash in YSZ and alumina plates were carried out. The


Page 70

cross-sectional SEM images and EDX mappings of YSZ plates heat treated with

volcanic ash on the top of the surface at 1250 °C for 5 hours and 20 hours are

shown in Figure 5.1. The results showed that YSZ plates provide a poor ability

against volcanic ash penetration. After the 5 hours and 20 hours heating, volcanic

ash penetrated to about 118.35 μm and 296.44 μm. Following the same

experimental procedure with YSZ plates, heat treatments were carried out in

alumina plates.

Figure 5.1 Volcanic ash attack on YSZ plates after thermal exposure at 1250 °C

for (a) 5 hours and (b) 20 hours

Compared with YSZ plates, alumina plates showed very different ability

against volcanic ash. After 20 hours of heat treatment, volcanic ash was still not

able to penetrate the alumina plates which SEM images and EDX mappings are

shown in Figure 5.2. During the heat treatment, two layers were formed between

the volcanic ash and Al2O3 plate. The first layer contained the mixed oxides of Mg,

Al, and Fe, with a thickness of 1.31 and 1.75 μm were observed to lie on the

alumina plate after 5 hours and 20 hours of thermal exposure respectively. On


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other hand, the second layer is located between volcanic ash glass and the first

layer, which contains the mixed oxides of Ca, Si, and Al. The thicknesses of the

second layer after 5 and 20 hours of heat treatment were 1.22 μm and 1.70 μm

respectively. These two reaction layers are going to be explained in subsequent

sections.

Figure 5.2 Volcanic ash attack on Al2O3 plates after thermal exposure at

1250 °C for (a) 5 hours and (b) 20 hours

5.2 Investigation of the reaction between volcanic ash and

alumina by DTA and XRD

The reaction temperatures between volcanic ash and Al2O3 were measured

by using DTA analysis, and the corresponding reaction products were analysed

using XRD. The DTA curve of volcanic ash, alumina and volcanic ash-Al2O3

mixed samples are shown in Figure 5.3. In the DTA curve of volcanic ash-Al2O3

mixed sample, four peaks and valleys were found, which were respectively

located at 410 °C, 525 °C, 1060 °C, and 1310 °C. There is no sharp endothermic

valley which is able to represent the melting point of volcanic ash-Al2O3 mixed

sample on their DTA curve even until 1480 °C. It can be confirmed that compared


Page 72

with the melting point of pure volcanic ash (1235 °C), the melting point of volcanic

ash was increased by mixing it with alumina powder. Even though there is an

endothermic region starting at 1310 °C, it might represent the beginning of the

melting process and it is still higher than the original melting point of volcanic ash.

Figure 5.3 DTA curves of volcanic ash, Al2O3, and volcanic ash-Al2O3 mixture

Figure 5.4 XRD spectra of volcanic ash-Al2O3 mixture before and after heat

treatment at 410 °C, 525 °C, 1060 °C, and 1310 °C


Page 73

The XRD spectra of volcanic ash-Al2O3 were shown in Figure 5.4. Actually,

weaker peaks coming from volcanic ash were covered by Al2O3 signals in their

mixed DTA patterns, but the formation of anorthite and other reaction products

can still be discovered. After 1310 °C of heat treatment, signals coming from

volcanic ash were getting weaker in the XRD pattern after 1310 °C heat treatment,

which it is believed to be caused by two reasons: The first reason is because of

the formation of a new reaction product. Under high temperature, strong reactions

between volcanic ash and Al2O3 led the formation of new reaction products and

decreased the signals of volcanic ash. The second reason is that the volcanic ash

started to melt partially, which can be explained by the endothermic region

starting from 1310 °C in the DTA curve shown in Figure 5.3. By analysing the

XRD spectra, anorthite (Al2CaSi2O8), magnetite (Fe3O4), and spinel

(Al1.75Mg0.889Mn0.351O4) were found as reaction products after 1310 °C of thermal

exposure. These results correspond to the EDX results shown in Figure 5.2. The

layer upon alumina plate shown in Figure 5.2 is made from magnetite and spinel

and the layer below volcanic ash is made from anorthite. After thermal exposure

at 410 °C, the composition of anorthite sodian transferred to labradorite

(Al0.81Ca0.325Na0.16Si1.19O4) by reducing the ratio of Na, Si and adding the ratio of

Al, and Ca in its composition. Indeed, major peaks of anorthtite sodian and

labradorite are almost located in the same position, but it is still able to be

differentiate by comparing their peak strength. For instance, there are three peaks

located between 22 and 25 degree, if the strength of the right peak is almost the


Page 74

same with the middle peak, it would be associated with anorhtite sodian. However,

it could be considered as labradorite if the middle peak is much stronger than the

other two. Except of the transformation of labradorite, FeSi peak was also

identified in 490 °C sample. Signals of magnetite were found after 1060 °C

thermal exposure. After 1310 °C heat treatment, signals of anorthite were

identified by replacing labradorite, and spinel was found in the same XRD pattern.

According to literature [38,41,43,49,73], alumina has been widely proved to

have a good ability against CMAS and volcanic ash penetration, and it has been

fabricated on the top of TBC as a protective coating or doped inside YSZ top coat.

In an alumina-rich environment, volcanic ash reacts with alumina forming

anorthite, spinel, and magnetite. Besides, the melting points of their reaction

products (i.e. anorthite (1553 °C), spinel (2135 °C), magnetite (1538 °C)) are

higher than the typical surface temperature of operation jet engine turbine, which

is able to prevent the volcanic ash penetration when engine is in operation.

By studying the volcanic ash penetration in both YSZ and Al2O3 plates, it can

be observed that even though the density of YSZ plate is higher than TBC

columns, severe penetration was still happening. It clearly demonstrates that

besides the density, the materials chemistry also plays an important role to

prevent volcanic ash degradation. During the heat treatment with Al2O3, volcanic

ash reacted with Al2O3, and their reaction products (anorthite, spinel and

magnetite) were formed in their interface and blocked the pathway for molten


Page 75

volcanic ash penetration. According to Aygun et al. [43], CaO-SiO2-Al2O3 the

ternary phase diagram shown in Figure 5.5 demonstrates that typical CMAS is

located into pseudo-wollastonite field, whose glass composition is difficult to

crystallize. It has been reported that, the crystals of pseudo-wollastonite start to

change abnormally and rapidly hundreds of degrees below its melting point

(1544 °C [74]) [75]. However, the difficult-to-crystallize pseudo-wollastonite would

transform to a crystallisable anorthite by providing alumina, and the physical

properties of anorthite are much stable at high temperature. Thus, it can be note

that Al2O3 provides a good ability to mitigate volcanic ash and CMAS penetration.

Figure 5.5 CaO-SiO2-Al2O3 ternary phase diagram [43]

CHAPTER 6 FRABRICATION OF PROTECTIVE COATINGS BY SOL-GEL BASED METHOD

Page 76

Chapter 6

Fabrication of Protective Coatings by Sol-Gel Based

Method

The sol-gel based method is a relatively simple way to fabricate coatings,

which has been widely used for producing metal oxide coatings. Compared with

EB-PVD or APS method, the cost of sol-gel method is much cheaper and it does

not need an evacuated working environment which is needed for EB-PVD

method. Figure 6.1 is the schematic image of the sol-gel based method. It shows

that the precursor stays in liquid phase in the beginning, and the elements inside

of precursor polymerize through internal chemical reactions. With further

polymerization, the viscosity is increased and the morphology turns from a liquid

solution to a jelly-like gel.

Parameters for controlling the gelation time are really complicated which is

related to volume, concentration, temperature and other factors. In general,

coating deposition is carried out when the precursor is close to gelation because

higher viscosity of the precursor allows the deposition of thicker coating. Sol-gel

based dip coating and spray coating of YSZ and Al2O3 have been carried out in

this research.


Page 77

Figure 6.1 Schematic image of the sol-gel based coating method

6.1 YSZ protective coatings

A dense YSZ protective coating is the first idea for mitigating CMAS/volcanic

ash attack. During the interaction of CMAS/YSZ protective overlay, YSZ

protective overlay is treated as a sacrificial layer, which not only blocks the

pathway of CMAS penetration, but also prevents the interaction of CMAS/YSZ

top coat directly. The research procedure is shown as follows.

6.1.1 Precursor for YSZ coatings

For the fabrication of a protective coating using a sol-gel based approach, it

is necessary to determine the gelation time in the beginning. In this experiment,

the reactions of solutions are followed from the formulas showing below:


Page 78

[M(H2O)x]n+ + A- + [M(OH)(H2O)x-1](n-1)+ +

[M(OH)(H2O)x-1](n-1)+ further reaction→

M(OH)n

M(OH)n

heating→ MO(n/2) + H2O(g)

By ring-opening (propylene oxide), hydrolysis, and heating, metal oxides would

form as reaction products [76,77].

For YSZ precursor, it was made by the mixture of zirconium(IV) chloride

(ZrCl4), yttrium(III) chloride hexahydrate (YCl3∙6H2O), propylene oxide (C3H6O)

and deionized water [66] which formulas are shown as followed:

ZrCl4 + YCl3∙6H2O + ZrOH3+ + YOH2+

ZrOH3+ + YOH2+ further reaction→ Zr(OH)4·Y2(OH)3

Zr(OH)4·Y2(OH)3

heating→ ZrO2·Y2O3 + H2O(g)

Table 6.1 shows the parameters of all experimental trials for the preparation

of YSZ precursors. The volume of propylene oxide was varied in order to produce

an optimized precursor for the preparation of YSZ coatings. From the

experiments, two phenomena were observed. The first one was found by

comparing trial 9 to trial 11. It could be noted that the gelation time was highly

affected by the composition of propylene oxide. The gelation time could be

significantly shortened even by adding 0.5 ml of additional propylene oxide into


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the precursor. The second phenomenon was observed by comparing trial 2 and

trial 6. The results showed that the gelation time was not strongly related to the

ratio of precursor composition. Although trial 2 and trial 6 had the same molar

ratio, gelation time was different by 12 times. The reason behind this is still not

fully confirmed. After many trials, the precursor trial 11 was determined to be the

best solution for dip coating and spray coating.

Table 6.1 Parameters of YSZ precursors

ZrCl4 (g) YCl3·6H2O

(g) H2O (mL)

Propylene

Oxide (mL)

Gelation

time

trial 1 0.583 0.152 5 0.64 >24hr

trial 2 0.583 0.152 5 1.07 35mins

trial 3 1.165 0.303 10 3.61 <5mins

trial 4 1.165 0.304 10 3.85 <5mins

trial 5 1.165 0.304 10 4.09 <5mins

trial 6 4.661 1.214 40 8.56 3mins

trial 7 4.660 1.214 40 3.50 >12hrs

trial 8 4.661 1.215 40 5.00 >12hrs

trial 9 4.661 1.215 40 6.50 >12hrs

trial 10 4.661 1.214 40 7.50 7mins

trial 11 4.661 1.215 40 7.00 95mins

6.1.2 Characterization of YSZ coatings

After the composition of the best precursor had been determined, the next

step was focused on the fabrication of YSZ protective layer by sol-gel dip coating

and spray coating. Before coating on the TBC sample directly, YSZ was coated


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on stainless steel plates with the surface polished to #800 grit finish. For dip

coating, the precursor was prepared 30 minutes prior to the coating procedure,

and was used to dip coat samples for 1 and 5 times with withdraw speed 0.03

cm/min. Figure 6.2 and Figure 6.3 show the morphology of YSZ protective

coating made from 1 and 5 times sol-gel dip coating. Figure 6.2 is the

cross-sectional and surface image of YSZ coating made from 1 time sol-gel dip

coating, a 2 μm thick overlay with cracks was observed on the stainless steel

substrate. On the other hand, the cross-sectional and surface images of YSZ

coating made by dip coat of 5 times are shown in Figure 6.3. After 5 times dip

coating, YSZ had larger thickness of about 5 μm but more cracks were found on

its surface.

Regarding sol-gel spray coating, there was only one sample made by air gun

spray coating for 6 times. The reason why 1 time spray coating sample was not

prepared was because the coverage of precursor on the stainless steel substrate

was really random by spray coating for only one time. However, with more

spraying time, it was believed that the overlay would be much flatter and the

distribution of precursor would be more even. Figure 6.4 shows the coating

morphology made by spray coating for 6 times. A 6 μm thick coating was

observed with comparatively less crack. From the preliminary observation, it is

confirmed that the coatings made by spray coating would offer better surface

finish, which is much more suitable for fabricating protective coatings.


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by Sol-gel based dip coating for 1 time on stainless steel substrate


by Sol-gel based dip coating for 5 times on stainless steel substrate


by Sol-gel based spray coating for 6 times on stainless steel substrate

A B

A B

A B


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6.2 Alumina protective coatings

Alumina protective coating can effectively protect YSZ top coat from CMAS

degradation. According to the literature review [67], it is not possible for alumina

coatings to prohibit the CMAS penetration. However, after chemical reaction

between the two, their reaction product, anorthite, magnetite, and spinel stay in

the CMAS/Al2O3 interface. With higher melting temperature than CMAS, these

reaction products block the pathway of CMAS penetration.

6.2.1 Precursor for alumina coatings

The precursor of alumina coating is made from mixing aluminium chloride

hexahydrate (AlCl3∙6H2O) and propylene oxide added into deionized water which

chemical formula can be presented as:

AlCl3∙6H2O + AlOH2+ +

AlOH2+ further reaction→ Al2(OH)3

Al2(OH)3

heating→ Al2O3 + H2O(g)

For the precursor of Al2O3 coatings, it was really hard to find an optimized

solution because the propylene oxide effect was not as clear as in sol-gel YSZ

protective coatings fabrication for alumina precursor fabrication. As shown in

Table 6.2, the gelation time was really long and unpredictable because unlike

sol-gel YSZ coatings fabrication, great amount of propylene oxide was added into


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the solution and yet, there was no obvious change in the gelation time, especially

for producing larger volume of precursor needed for coatings deposition. After

evaluating different parameters, trial 9 was selected as the precursor for the

fabrication of alumina protective coatings.

Table 6.2 Parameters of alumina precursors

AlCl3·6H2O (g) H2O (mL) Propylene Oxide (mL) Gelation time

trial1 0.724 5 1.29 >12hrs

trial2 0.724 5 1.07 >12hrs

trial3 0.724 5 0.86 >12hrs

trial4 0.740 5 3.01 160mins~ 5hrs

trial5 0.740 5 4.52 160mins~ 5hrs

trial6 0.740 5 6.02 160mins~ 5hrs

trial7 0.740 5 9.04 160mins

trial8 2.960 20 7.86 2hr~24hrs

trial9 2.960 20 11.00 2hr~24hr

trial10 5.794 40 21.00 >12hrs

trial11 5.794 40 35.00 >12hrs

6.2.2 Characterization of Alumina Coatings

For the alumina protective coatings fabrication, alumina has only been

fabricated by spray coating. As shown in Figure 6.5, 5 μm thick alumina coating

with little crack was observed on the substrate after 5 times spraying. Table 6.3

shows the result of XRF analysis. It can be seen that the concentration ratio of

alumina composition was really close to the ideal stoichiometric ratio.


Page 84

Observation of all coatings indicated that the coating adhesion was poor, but

the adhesion could be improved by heat treatment. Also, if the substrate was

TBCs, probably the adhesion of protective coatings might be better than metal

substrate.

Figure 6.5 Sol-gel based spray coating for 6 times on the stainless steel substrate

Table 6.3 XRF results for alumina coating

Element (at.%) Al O

Pt.1 35.20 ± 3.73 64.80 ± 0.45

Pt.2 35.57 ± 3.25 64.43 ± 0.38

Pt.3 35.16 ± 3.88 64.84 ± 0.47

Pt.4 31.61 ± 2.73 68.39 ± 0.42

Pt.5 32.82 ± 3.35 67.18 ± 0.47

Avg 34.07 ± 3.39 65.93 ± 0.44

A B


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6.3 Preliminary Fabrication and Characterization of YSZ and

Alumina Protective Coatings on TBC Top Coats

After measurement of parameters and protective coatings fabrication on

stainless steel, experiments of protective coatings were fabricated on the top of

TBCs. The ideal thickness of protective coatings is in the range between 20 to 50

μm.

6.3.1 Fabrication and Characterization of YSZ Protective Coatings on

TBC Top Coats

For YSZ protective coating, the precursor is made from the mixture of trial 11

shown in Table 6.1 using sol-gel based spray coating. In order to reach the ideal

thickness, 20 cycles of spraying and drying were proceeded followed by 1 hour

ageing at 600 °C. Figure 6.6 shows the surface and cross-sectional images of

YSZ overlay coated on the top of TBC top coat after 600 °C, and its EDX mapping

results are shown in Figure 6.7. The composition of YSZ protective coating

measured by EDX point analysis was shown in Table 6.4, the weight ratio of Y2O3

in protective coating is 8.72 wt.% which is not far from the weight ratio of YSZ top

coat (8 wt.%). As shown in Figure 6.6, the thickness of YSZ protective coating is

in the range between 20 to 30 μm. However, the surface and cross-sectional

images demonstrates that the morphology of YSZ protective coating was too

damaged to prevent volcanic ash or CMAS attack although the distribution of

every element is balanced and the composition is close to ideal according to


Page 86

EDX.

Figure 6.6 (a) Surface morphology and (b) cross-sectional image of YSZ

protective coatings

Figure 6.7 EDX mapping of YSZ protective coatings

Table 6.4 Composition of YSZ protective coating measured by EDX point

analysis

Elements (at.%) Zr Y O

Point 27.88 ± 0.33 2.91 ± 0.35 69.21 ± 0.12

6.3.2 Fabrication and Characterization of Al2O3 protective coating on

TBC top coat

Following the same precursor with alumina coating fabricated on the

stainless steel, the precursor of alumina coatings was made from the mixture of

trial 9 in Table 6.2. 15 cycles of spraying and drying were proceeding on The TBC


Page 87

top coat. 600 °C ageing for 1 hour was carried out after the last drying. Surface

morphology and cross-sectional images of the alumina protective coating are

shown in Figure 6.8. The average thickness of alumina protective coating is in the

range between 10 to 20 μm. Following the same problem, alumina protective

coating is not able to protect YSZ top coat from volcanic ash attack according to

its damaged surface. Moreover, severe spallation was found in alumina protective

coating, which will be explained in coming paragraph. Although EDX mapping

and point analysis shown in Figure 6.9 and Table 6.5 demonstrate that the

distribution and composition of alumina is even and close to ideal, the spalled

surface structure is still not able to prevent volcanic ash attack.

Figure 6.8 the (a) surface morphology and (b) cross-sectional image of Al2O3

protective coatings

Figure 6.9 EDX mapping of Al2O3 protective coatings


Page 88

Table 6.5 Composition of Al2O3 protective coating measured by EDX point

analysis

Elements (at.%) Al O

point 33.05 ± 2.18 66.95 ± 0.24

The spallation and damage of protective coatings could result in the thermal

expansion mismatch of different materials, which schematic diagram was shown

in Figure 6.10. During the ageing process at 600 °C, columnar gaps stretched

along with the thermal expansion of the superalloy substrate. Although protective

coatings would stretch during heat treatment, different level of thermal expansion

still damaged the protective coatings and led to the spallation.

Figure 6.10 Schematic image of the damage mechanism of protective coatings

during thermal exposure

CHAPTER 7 CONCLUSIONS AND SUGGESTED FUTURE WORK

Page 89

Chapter 7

Conclusions and Suggested Future Work

7.1 Conclusions

Volcanic ash attack is one of the most serious problems for aero engine

turbines. Basically, the degradation mechanism is quite similar to that by artificial

CMAS, but volcanic ash results in more severe damage than CMAS because the

penetration rate of volcanic ash into TBC columns is much faster. When the

engine is in operation, volcanic ash stays in liquid phase and penetrates into the

columnar structure of YSZ top coat. After 3 hours heat treatment, crack was

formed. In addition, it was found by SEM observation that volcanic ash penetrated

through the columnar gaps of TBC and destroyed the microstructure of YSZ top

coat after a 10 hours isothermal heat treatment at 1250 °C. Moreover, molten

volcanic ash would concentrate in the bottom of YSZ and accelerate the

spallation of YSZ top coat.

By observing its chemical composition, EDS results shows that great amount

of zirconium stays in the volcanic ash glass which means that the volcanic ash

dissolved the YSZ top coat during the heat treatment. Also, by observing the

composition of the 10 hours heat treated TBC sample, the concentration of yttria


Page 90

(3.71 wt.%) was much lower than as received TBC samples (8 wt.% Y2O3), which

agreed with the understanding that the concentration of yttria can be depleted by

molten volcanic ash.

More details of volcanic ash/YSZ interaction have been found using DTA and

XRD analysis. Reactions between volcanic ash and YSZ started at 410 °C which

is lower than the surface temperature of engine turbine. In addition, phase

transformation was found in XRD analysis. Accompanied with the depletion of

yttria, zirconia was no longer stable and phase transformations occurred during

thermal cycles. For zirconia, it stayed in tetragonal phase when temperature is

higher than 1050 °C and returned to monoclinic phase in low temperature. Along

with phase transformation, 4 - 6% volume change would occur which is extremely

harmful for the TBC lifetime.

For the protective mechanisms, alumina has been proved to have a good

ability against CMAS and volcanic ash penetration. In penetration experiments,

YSZ showed a very poor ability against volcanic ash penetration, even though the

density of YSZ plate is much higher than YSZ top coat. Unlike YSZ, alumina

showed a good ability to prevent volcanic ash penetration, which formed two

layers after reacting with volcanic ash. The layer upon alumina plate is mainly

made from magnetite and spinel, and the layer below volcanic ash is mainly

made from anorthite. These three reaction products are able to protect TBCs

against volcanic ash because their melting temperature is much higher than the


Page 91

operating surface temperature of jet engines turbines. Hence, when the engine is

in operation, these reaction products will block the pathway of CMAS penetration.

In order to understand more of the reaction of volcanic ash and alumina, DTA

and XRD analysis had been carried out. It has been found that by mixing alumina,

the melting point of volcanic ash was increased from 1235 °C to 1310 °C. In

addition, anorthite (CaAl2Si2O8), magnetite (Fe3O4), and spinel

(Al1.75Mg0.889Mn0.351O4) were found after heat treatment as reaction products,

which correspond to the penetration experiment.

For the protective coatings, YSZ and alumina coatings have been made

using sol-gel based spray coating method. For both YSZ and Al2O3 coatings,

adhesion is a big problem, which needs to be improved. During the thermal

exposure, columnar gaps in YSZ top coat would extend accompanied with the

thermal expansion of the superalloy substrate. Thus, different level of thermal

expansion broke the structure of protective coatings on TBCs.

7.2 Suggested Future work

I. New TBCs design against volcanic ash

According to previous experiments, thermal expansion mismatch is a big

problem for protective coatings and leads the spallation of protective coatings

after thermal cycles. Thus, new techniques against CMAS need to be developed.


Page 92

In literature, doping alumina into TBC columns has been done by Drexler et al.

and shows a good effect against volcanic ash and CMAS [38,41]. For my idea,

CMAS and volcanic ash degradation could be mitigated by modifying the top coat

structure. Figure 7.1 is the schematic image of the new design of TBCs. In order

to avoid the thermal expansion mismatch between YSZ columnar gap and the

protective coating, the protective coating could be coated directly on the top of

each YSZ columns using EB-PVD method, which can not only avoid the thermal

mismatch from columnar gap but also strongly improves the adhesion. However,

thermal expansion mismatch between YSZ top coat and protective material,

coating technology, and the cost are still needed to be considered.

Figure 7.1 schematic image of the new design of TBCs

II. New materials against volcanic ash

According to the literature review [53], gadolinium zirconate (Gd2Zr2O7)

provides a good volcanic ash resistance. Fig 7.2 is the illustration of the

YSZ/volcanic ash and Gd2Zr2O7/volcanic ash interaction. Unlike YSZ, Gd2Zr2O7

provides a really good volcanic ash resistance, their reaction products will stay in


Page 93

the interface and block the pathway. The protection mechanism is very similar to

the protective mechanism of alumina, but less Gd2Zr2O7 is needed for

consumption when reacting with volcanic ash. Thus, Gd2Zr2O7 shows a high

value to be further investigated. For this part, TG/DTA analysis will be done in the

beginning in order to get more information of Gd2Zr2O7/volcanic ash reaction.

Figure 7.2 Illustration of the YSZ/volcanic ash and Gd2Zr2O7/volcanic ash

interaction [41]

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