SLURRY SPRAYED THERMAL - University of Adelaide

_________________________________________________________________

SLURRY SPRAYED THERMAL

BARRIER COATINGS FOR

AEROSPACE APPLICATIONS

_________________________________________________________________

Phuc Nguyen

A thesis submitted in fulfilment of requirements

for degree of Doctor of Philosophy

School of Mechanical Engineering

The University of Adelaide

May 2010

Chapter 1 Introduction

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CHAPTER 1

1 INTRODUCTION


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1.1 Research Background

Thermal Barrier Coatings (TBCs) represent a relatively thin layer of a material with

high insulating properties, such as ceramics, that is bonded to a substrate, which is

usually a metal structure, to protect it during temperature excursions associated with

operating conditions or an accident. The application of TBCs can significantly

increase the operating temperatures for a number of practically important

applications, increase the efficiency and improve the durability of the structural

components and machine elements utilising thermal energy. There are many

applications, which have benefited from adopting TBCs, these include the

aeronautical aerospace, automotive and nuclear industries and heavy-duty utilities

such as diesel trucks [Koizumi, 1997; Padture, Gell and Jordan, 2002; Alhama and

Campo, 2003; Taymaz, 2007].

The development of TBCs has centred mostly on Partially Stabilised Zirconia (PSZ)

due to its unique physico-mechanical properties and has been led by its use in aircraft

engine combustion-path components. The significant advance in development of an

effective protective coating was associated with the development of Functionally

Graded (FG)–TBCs. FG–TBCs are multiphase composite materials that are

engineered to a have a spatial variation of material constituencies. Using FG–TBCs,

as an alternative to joining directly together two dissimilar materials such as

ceramics and metal, carries several advantages including: much lower thermal stress

distribution across the coating thickness; minimisation of stress concentrations at

interface corners; and an increase in bonding strength [Teixeira, 2001; Sahin and

Erdogan, 2004; Bialas, 2008].

There are many fabricating techniques for depositing ceramics or other coating

materials on metal substrates which have been developed over the past three decades,

[Niino, Hirai and Watanabe, 1987]. All fabricating techniques can be categorised in

three main groups: bulk processes, flame spray techniques and deposition techniques;

with each technique differing from each other greatly, in terms of physical principal

used, cost and simplicity [Kieback, Neubrand and Riedel, 2003]. However, the main


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obstacle in the widespread application of these techniques is usually a relatively high

cost of fabrication of TBCs, which include the use of sophisticated equipment and

need for highly trained personnel. Moreover, many of these techniques are not

applicable to cover large or curved areas. All these drawbacks formed the main

motivation for the current project.

The objective of this research is to develop and investigate a relatively simple and

cost effective technique for fabricating TBCs including FG-TBCs with the focus on

aeronautical and aerospace applications. This new technique is based on a traditional

Wet Powder Spray (WPS) technique and can be divided into four separate stages: (1)

slurry mixing, (2) spraying, (3) pressure stamping, (4) evaporation and sintering.

Previous studies have indicated that despite being very promising for a number of

industrial applications the quality of the fabricated coating utilizing the traditional

wet spray method is normally significantly lower than that obtained by other

methods in terms of fracture resistance and durability. Therefore, a significant effort

in the current research is directed to the characterisation and improvement of the

WPS technique to achieve a high quality TBC and make the modified method

suitable for the fabrication of thermal protection in various applications. The current

work includes the development of a new manufacturing procedure, extensive testing

of the mechanical and thermal properties of the manufactured coating, optimisation

of the fabricating parameters using experimental and theoretical approaches and a

numerical validation study.

The experimental approach includes a set of mechanical and thermal test procedures

as well as microscopic investigations to comprehensively characterise the quality of

the coating and understand the effect of various fabrication parameters and

composition on the fracture resistance, durability and apparent properties of the

coatings. At the final stage of the research, full-scale tests simulating the loading

conditions corresponding to aerospace applications were conducted to obtain the

overall assessment of the applicability of this technique in aerospace engineering.

The theoretical approach includes a multi-scale modelling of thermal field and

thermal stresses in FG–TBCs. The thermal stress is generally recognised to be the


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major factor responsible for mechanical failure of the coating. The aim of the

mathematical modelling was to understand the effect of fabrication parameters on the

intensity of the thermal stresses induced due to temperature excursion and to guide

the optimisation study to improve the overall quality of the coatings and produce the

technique as a cheap and robust alternative to the existing methods, which are

normally quite expensive and have many limitations.

1.2 Research Significance

Thermal Barrier Coatings (TBCs) are essential structural components in current

engineering applications associated with high temperatures or high thermal fluxes as

well as in future developments. These include thermal protection for rocket and

scramjet engines, re-entry space vehicles, gas turbines, diesel engines, nuclear power

plants and many other structures and machines.

Currently there are a number of well developed manufacturing techniques available

for fabricating TBCs including FG coatings. However, the main obstacle in the

widespread application of these techniques is a relatively high cost of fabrications

and equipment. For example the setup costs of Plasma Spray facilities start in the

millions of dollars. Moreover, many of these techniques are not applicable to cover

large, like in aerospace applications or produce FG–TBCs to increase the reliability

and improve the resistance to mechanical failures. All these drawbacks formed the

main objective of the current research.

From the above discussion it follows that the research and development of a new low

cost fabricating technique, which is the major objective of the current project,

represents a significant contribution in a number of current engineering applications

utilizing or experiencing high temperatures or temperature gradients as well as for

future developments focused on the achievement of high thermal efficiency and

performance.


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1.3 Research Objectives

The primary objective of this research is the development of a relatively simple, cost

effective technique with acceptable quality coating for fabricating TBCs with the

focus on aeronautical and aerospace applications. The specific research objectives

are as follows:

Development of a new, low-cost and effective fabricating technique for

manufacturing FG–TBCs.

Development of standardised experimental techniques for comprehensive

characterisation of the manufactured TBCs to determine thermo-mechanical

properties of the Slurry Based Technique.

Analysis of manufacturing parameters on the mechanical failure of the coatings

based on the combined experimental investigations and theoretical modelling.

Optimisation study with the main focus to improve significantly the fracture and low

cycle fatigue resistance of the TBCs.

Scale testing of Slurry based TBCs in high temperature and high temperature

gradient environments corresponding to a hypersonic flight.

Validation study utilising the micro-mechanical modelling and the Finite Element

Analysis (FEA).

1.4 Outline of Thesis

Chapter 1 gives an introduction to the research topic and presents the statement of

significance and objectives of the research conducted and concluded with outline of

the thesis.

Chapter 2 gives an in depth literature review of the research backgrounds of the


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current research topic. These include the current and future applications of Thermal

Barrier Coatings, the material aspects of TBCs and the primary properties, and the

different fabricating techniques, such as flame spray and deposition techniques.

Chapter 3 presents the Slurry Spray Technique for fabricating FG–TBCs, newly

developed at the University of Adelaide. The chapter focuses on the development

and improvement of this technique, with the focus on coating adherence and

survivability in comparison to the other traditional techniques for fabricating TBCs.

Chapter 4 presents the experimental investigation of the Slurry Based TBCs. These

experiments consist of standard thermo-mechanical tests including adhesion strength,

thermal cycling, thermal conductivity, Vickers micro hardness tests as well as

Scanning Electron Microscopy. These experiments aim to understand the effect of

various fabrication parameters and composition on the quality and effective

properties of the coatings, as well as characterise the TBC

Chapter 5 presents the development of a new test rig for scale tests simulating the

thermal loading conditions corresponding to the high temperature aerospace

applications. These experiments were conducted to obtain an overall assessment of

the applicability of this technique to produce TBCs for such sort of applications. The

test rig was based on a new concept and utilised a flat burner for generating the high

temperatures and temperature gradients.

This chapter also presents the outcomes of a virtual testing of the TBCs fabricated

with the new technique. This testing was conducted to evaluate the efficiency of

TBCs in conditions relevant to the hypersonic flight.

Chapter 6 presents the overall conclusions of the conducted research, along with

recommendations for future work.


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8

Chapter 2 Background and Literature Review

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CHAPTER 2

2 BACKGROUND AND LITERATURE REVIEW


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2.1 Introduction

This literature review is comprised of three sections. The first section is dedicated to

the current and potential applications of Thermal Barrier Coatings (TBC) focusing on

the aeronautics and aerospace industry. Particular attention is paid to Functionally

Graded Thermal Barrier Coatings (FG–TBCs). This type of coating shows

considerable promise for many current high-temperature applications as well as

future developments.

The second section of this chapter is devoted to the materials aspect of TBCs. Low

thermal conductivity, high melting point and good resistance against oxidation and

corrosion are all mandatory properties for materials used for TBCs. The material

which satisfies all these requirements is widely accepted as engineered ceramics.

Many types of ceramics have been investigated in the past, however, the major

development of TBCs is focused on Yttria Stabilised Zirconia (YSZ) [Clarke and

Phillpot, 2005], due to its unique mechanical and physical properties, which will be

critically discussed in this part of the literature review.

In the final section of the literature review, current techniques for manufacturing and

fabricating TBC are discussed in detail. The existing TBC techniques can be

separated into three main groups: bulk processes, flame spray processes and

deposition processes. Each group has its own distinct advantages and disadvantages,

however, all the reviewed manufacturing techniques result in high cost of fabrication

for TBC, complex process setups, and many of them are not applicable to cover large

areas. The current research is mainly driven by these drawbacks, with the main

objective to develop a new, simple and cost effective technique for manufacturing

TBC including functionally graded coatings applicable to cover curved and large

areas, which is specifically very important for future aerospace projects such as a

hypersonic scramjet project.


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2.2 Background – Applications

A Thermal Barrier Coating (TBC) is a relatively thin layer of a material with high

insulating properties, which is bonded to a substrate to protect the metal load

carrying structure during temperature excursions. The material used as the thin layer

is usually a ceramics, with the substrate usually being a metal structure. The

application of TBCs can significantly increase the operating temperatures up to

1400-1500ºC, increase efficiency of thermal processes and improve the durability of

the components. TBCs were originally developed for aerospace and power industry

applications. Currently, there are many other applications, which benefit from

adopting TBCs. These include applications in the automotive and nuclear industry

and heavy-duty utilities such as diesel trucks.

2.2.1 Aerospace and Aeronautical Applications

TBC were originally designed for use in turbine engines, and are currently finding

increased use in applications such as aeronautics, specifically in rocket and scramjet

engines [Toriz, Thakker and Gupta, 1989]. The duration of a mission cycle for an

aero gas turbine engine is typically several hours, although maximum gas

temperatures occur only for a matter of minutes during takeoff and landing [Abdul-

Aziz, Tong and Kaufman, 1989]. Repeated mission cycles result in thousands of

hours of operation between engine overhauls, with hundreds of hours being spent at

peak temperatures. The rocket combustors normally experience much higher thermal

loading than ground based turbines [Feuerstein, Knapp, Taylor, Ashary, Bolcavage

and Hitchman, 2008]. Traditional TBCs used in ground based turbines are usually

not able to cope within the hostile environments of much higher temperatures and

temperature fluxes experienced by air turbines. In many TBC applications, stresses

due to the difference in thermal expansion of the coating and the substrate can have a


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detrimental effect on the service life and safety of the component leading to

mechanical damage of the protective coating, such as spallation and cracking of the

coating [Mao, Dai, Yang and Zhou, 2008].

One effective way to reduce the adverse effect of thermal stresses is to use

Functionally Graded Thermal Barrier Coating (FG–TBCs), where thermal and

mechanical properties vary gradually through the thickness. In metal-ceramic FG–

TBCs, the ceramic-rich side is exposed to high heat fluxes from high temperature

applications. FG–TBCs are fabricated by directly joined together two dissimilar

materials, such as ceramic and metal powders, which are applied to a metal substrate

[Tamura, Takahashi, Ishii, Suzuki, Sato and Shimomura, 1999]. FG–TBCs have

many advantages over non graded TBC, for example these include minimisation or

elimination of stress concentrations, reduction of thermal stresses and singularities at

the interface corners. These advantages lead to the significant increase in the strength

and durability of the TBC [Koizumi, 1997]

2.2.2 Space Re-entry

Space vehicles travelling at hypersonic speeds, experience extremely high

temperatures from aerodynamic heating due to friction between the surface of the

vehicle and the atmosphere. Two types of space vehicles are categorised in this

section, the US space shuttles used for Apollo missions launched into space by a

vertical propulsion system and reusable spacecraft, which are based on horizontal

take off either from a ground based runway or horizontally flying carrier.

During re-entry, space vehicles travel at speeds of excess of 11 km/s [Miyamoto,

Kaysser, Rabin, Kawasaki and Ford, 1999]. At this stage the leading edges of the

vehicle rapidly heat up to where the heat protection reaches temperatures of up

2500°C. For example, if the space re entry vehicle is at an altitude of 120 km, re-

entry velocity may reach speeds of up to 8 km/s, where the temperatures may reach

up to 1500°C for a few minutes [Miyamoto, Kaysser, Rabin, Kawasaki and Ford,


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1999]. The structural components that experience the highest amount of heat are the

leading edges of the vehicles, for example, nose cones and rudders which are

constructed of carbon/carbon composites. Other areas of the space craft where the

temperatures are not as extreme (only up to 1200°C), ceramic tiles are used. For

temperatures ranging from 300 – 550°C, TBC based upon Ti sheets are used for

thermal protection [Koizumi, 1997].

2.2.3 Rocket Combustors

It has been demonstrated that the thermal performance of rocket engines may be

improved significantly by increasing the tolerance of the metallic walls of the nozzle

from the impact of the ultra-high temperatures produced by the stream of turbulent

combustion gases emerging from the combustor [Peters, Leyens, Schulz and

Kaysser, 2001]. One feasible approach for achieving ultra-high operating

temperatures in the combustor without damaging the structural integrity of the

metallic substrate of the nozzle wall is concerned with the application of a ceramic

coating to the exposed surface of the nozzle wall. The ceramic coating creates an

artificial thermal barrier which retards the heat flow from the stream of turbulent

combustion gases to the metallic substrate [Alhama and Campo, 2003].

Previous studies of Chemical Vapour Deposition (CVD) technique show that

manufactured Silicon Carbide/Carbon (SiC/C) Functionally Graded Thermal Barrier

Coatings (FG–TBCs) were used for rocket combustor tests with nitrogen tetroxide

and monomethyl hydrazine propellants, with firing cycles of 55 seconds

[Wakamatsu, Saito, Ono, Ishia, Matsuzaki, Hamamura, Sohoda and Kude, 1997].

The maximum outer wall temperature measured, ranged from 1376 – 1527°C, while

the inner wall temperatures reached 1677°C – 2027°C. With the protection provided

from the TBC, no damage to the combustors was observed.


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2.2.4 Gas Turbines

TBCs are used for military and commercial aeroengines as well as for gas turbine

engines for automobiles, helicopters and marine vehicles [Nicholls, 1991; Pichon,

Lacoste, Barreteau and Glass, 2006]. TBC are predominately used in areas where the

hot gas ways are located, to increase the operating temperature of the structure

[Gurrappa and Sambasiva, 2006]. TBC in gas turbines operate at a higher heat flux

and higher temperature range than diesel engines, as well as being subjected to hot

corrosion and erosion. For these applications, TBCs generally have relatively thin

coating thickness in the order of less than 400 �m, to reduce the possibility of

spalling [Duvall and Ruckle, 1982].

The temperature improvements of gas turbine alloys and coated alloys as a function

of the year of introduction are shown in Figure 2.1. In the majority of these cases, the

improvement of mechanical strength and creep properties at high temperatures is

connected with a decrease of oxidation resistance. With temperatures above 1100 °C

the super alloys have to be protected against oxidation and the mechanical strength

becomes critical, against high temperatures [Padture, Gell and Jordan, 2002].


15

Figure 2.1: Temperature Improvements of gas turbine alloys and years of

introduction into Rolls-Royce Engines [Stöver and Funke, 1999].

TBC systems in modern gas turbines consist typically of two layers, a bond coat

layer, and an isolative, ceramic top coat layer. The bond coat is often a metal and has

two major functions. It improves the bonding between the substrate and the topcoat

and it protects the substrate from corrosion and oxidation. For industrial applications,

Plasma spraying is widely used for the manufacture of both top and bond coatings.

This technique offers the possibility to deposit thick coating layers in the �m range.

These coatings can effectively reduce metal temperature in hot sections of the gas

turbine, such as combustion chamber liners, while keeping the level of cooling air on

a relatively low level [Osyka, Rybnikov, Leontiev, Nikitin and Malashenko, 1995].

1950 1960 1970 1940 1980 1990 2000

Tem

pera

ture

, °C

750

850

950

1050

1150

Directionally solidified Materials

Single Crystal Materials

Thermal Barrier Coatings Year

Wrought Materials

Conventionally Cast Materials


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2.2.5 Nuclear Industry

The evaluation of materials for nuclear waste disposal or transmutation lay in several

specific and highly desirable advantages. From studies by [Thomé and Garrido,

2001], the advantages include:

• high melting point

• good thermal conductivity

• absence of phase transformation at high temperatures

• stability against radiation

• good mechanical properties

• oxidation resistance

• low solubility in water

• retention of radiotoxic elements

• adequate neutronic properties

Yttria Stabilised Zirconia (YSZ) is considered an attractive matrix for nuclear

applications, such as inert matrix for the destruction of excess plutonium or good

host materials for nuclear waste storage [Degueldre, 2007]. This high temperature

refractory oxide is attractive because it presents a high radiation stability, a high

melting point, a small neutron capture cross section, and an ability to form solid

solutions with a wide range of solubility for actinide elements such as Plutonium,

Uranium and Thorium [Menvie Bekale, Legros, Haut, Sattonnay and Huntz, 2006].


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2.2.6 Diesel Engines

In diesel engine applications, ceramic coatings hold significant promise in the

reduction of wear and abrasion failure in reciprocating and rotary engines for

transportation and stationary power. TBCs are also employed in diesel engines for

trucks, buses, marine vehicles, tanks, military transport vehicles and farm vehicles

[Levy and Macadam, 1987; Hejwowski and Weronski, 2002; Taymaz, 2007]. They

also have application as thermal barriers to improve the efficiency of the engines, by

reducing energy loss and cooling requirements [Taymaz, 2007]. In addition to the

insulating attributes, TBCs improve combustion efficiencies through surface catalytic

and emissivity effects of the ZrO2 layer on combustion zone components. The

improvement in the efficiency ranges from 7 to 9 %, as reported from numerous

studies conducted, was normally achieved in the ceramic-coated diesel engines in

comparison with the similar uncoated diesel engines [Lackey, Stinton, Cerny,

Schaffhauser and Fehrenbacher, 1987]. This performance gain could potentially be

increased to an overall thermal efficiency of 54 % for advanced diesel concepts, with

FG–TBCs showing an increase in the lifetime of diesel engines [Uzun, Çevik and

Akçil, 1999; Taymaz, 2007]. These added benefits increase the potential for wider

commercial use in diesel engine applications.

2.2.7 Future Gas Turbine Systems

TBCs will play a crucial role in advanced gas turbine engine systems because of their

ability to significantly increase engine operating temperature and reduce cooling,

thus greatly helping to achieve low emission and high efficiency goals. Under the

NASA Ultra Efficient Engine Technology (UEET) program, advanced TBC systems

are being developed to provide vital thermal protection for components such as

combustor liner and vanes, for gas temperatures exceeding 1760°C in harsh oxidising

and water vapour containing combustion environments of the turbine engines. Higher

operating temperatures of turbine engines result in significant improvements in fuel


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consumption, efficiency, and emissions [Bansal and Zhu, 2008]. The temperature

gradient projected for a TBC system for future advanced turbine systems, is shown in

Figure 2.2.

Figure 2.2: Temperature Gradient over a TBC-coated substrate [Padture, Gell and

Jordan, 2002]

a1172507

Text Box

NOTE: This figure is included on page 18 of the print copy of the thesis held in the University of Adelaide Library.


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2.3 TBC Structure, Performance and Materials

2.3.1 Introduction

The development of TBCs has centred on Partially Stabilised Zirconia (PSZ) and has

been led by its use in aircraft-engine combustion-path components. PSZ is a unique

material used for many applications including engineering ceramics, TBCs, ceramic

implants, electro ceramics, high-temperature magnetohydrodyhamic electrodes, fuel-

cells, and oxygen sensors. This variety is grounded on use of a combination of

mechanical, electrical, thermal and other properties which will be considered in this

section [Beele, Marijnissen and van Lieshout, 1999].

The structure of ZrO2 (Zirconia) when heated above 1000°C changes from

monoclinic to tetragonal, the accompanying volume changes of 4 to 6 % can result in

severe spalling of the ceramic layer. Therefore, PSZ coatings, made from ZrO2

alloyed with stabilising oxides such as Y2O3, CeO2 and MgO, are used. Typical state-

of-the-art TBC utilise ZrO2 partially stabilised with 6-8 %wt Y2O3 [Duvall and

Ruckle, 1982]. The materials have been found to be best when deposited on a

metallic bond coating [Meier and Gupta, 1994]. Chromium and Aluminium elements

are added to the bond coat, and traces of Yttrium are added to form dense, well-

adhered, protective sub-TBC oxide scales. The smooth transition of the bond layer’s

Coefficient of Thermal Expansion (CTE) between that of the base metal and that of

the TBC (Y2O3 – ZrO2) is generally accepted to reduce the thermal stresses produced

during coating application and service thermal cycling [Teixeira, Andritschky,

Fischer, Buchkremer and Stöver, 1999].

Low thermal conductivity, high melting point and good resistance against oxidation

and corrosion are the required advantages of ceramic coatings applied in high

temperature applications. However, compared to metals, ceramics are not reliable

with respect to mechanical properties. This non-reliability hinders the use of bulk

ceramic parts in turbines and diesel engines, despite intensive research on structural

ceramics. Instead, the advantages of ceramics and metals are combined by utilising


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ceramic thermal barrier coated metallic substrates. Extremely low thermal

conductivity and phase stability makes Yttria-Stabilised Zirconia the most successful

ceramic top-layer, when combined with a metallic interlayer, as this interlayer acts

both as a bond coat and as an oxidation and corrosion protection barrier. The alloy

normally consists of a base of Molybdenum, Nickel, Cobalt and/or Iron, Chromium,

Aluminium, Yttrium and additional active elements such as Silicon, Titanium and

Rhenium. The bond and top coat can be applied by thermal spraying or by vapour

deposition techniques. The limited life-time of the TBC system forms the boundary

of this 40-year-old concept [Troczynski, Cockcroft and Wong, 1996]. Until the past

decade, the use of TBC on aircraft turbines blades was not design-integrated. The

TBCs are used frequently to lower the metal temperature, and therefore elongate the

life-time of a blade itself. If the coating spalls off, metal temperature will increase,

but not above a critical point. For design-integrated TBC with improvement of

efficiency, fuel consumption and exhaust pollution, 100% reliability is necessary.

The life-time of various TBC systems as a function of the operating temperatures

against cycles to failure is illustrated in Figure 2.3. For lower temperatures of 960°C

or less, the single layered TBC systems have comparable life spans with the double

layered coating systems. However, with the increase in temperature, the life span of

the single layered coating decreases significantly, with the double layered coating

exhibiting superior levels of life span at higher temperatures.


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Figure 2.3: Cycles to failure for different TBC systems. [Vaßen, Kaßner, Stuke,

Hauler, Hathiramani and Stöver, 2008]

Double layer

Double layer

Single layer La2Zr2O7

10

100

1000

10000

1250 1300 1350 1200 1400 1450

Cyc

les t

o Fa

ilure

T high (> 960°C)

T low (< 960°C)

Tsurface, °C

Double layer

Lifetime range of standard Lifetime range of double layer


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2.4 Material Properties

2.4.1 Modulus of Elasticity

The modulus of elasticity is an important material property which determines the

stress levels in the coating during fabrication and usage. The spallation resistance is

also greatly affected by the modulus of elasticity. At high temperatures, the modulus

of elasticity changes significantly due to the sintering densification technique [Zhu

and Miller, 2000]. Typical values of the modulus of elasticity used for TBC range

from 21 to 175 GPa, depending on the performance requirement of the coating. The

typical values of modulus of the elasticity of various materials can be found in Table

2.1 [Kokini, Takeuchi and Choules, 1996; Mesrati, Ajhrourh, Du and Treheux, 2000;

Vassen, 2000].

2.4.2 Thermal Conductivity

The thermal conductivity of a TBC is one of the most important material properties

governing the effectiveness of the coating to shield substrates from high temperature

experiences. Thermal conductivity governs heat conduction from the top layer of a

TBC to the structural material. As a result, thermal conductivity of TBC is an

important parameter to accurately compare the effectiveness of the coating produced

using different manufacturing methods and materials. Values of thermal conductivity

for ceramic coatings range from 2 to 9 W/mK [Padture and Klemens, 1997; Mesrati,

Ajhrourh, Du and Treheux, 2000; Vassen, Stuke and Stöver, 2009]. It was found that

in the fabricated TBC thermal conductivity gradually increases throughout the

thickness of the coating [Miller, Leissler and Jobe, 1993].


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2.4.3 Coefficient of Thermal Expansion

The thermal expansion is an important characteristic which affects mechanical

behaviour in severe thermal environments such as gas turbines and space structures.

The Coefficient of Thermal Expansion (CTE) of a material is defined as the linear

expansion of strain per unit of temperature change. Typical values of CTE for

various coating materials can be seen in Table 2.1. The thermal expansions of

metal/ceramic coatings have been studied extensively in order to optimise the graded

compositions through relaxation of thermal stresses [Lee, Miller and Jacobson, 1995;

Padture and Klemens, 1997; Cao, Li, Zhong, Zhang, Zhang, Vassen and Stoever,

2008].

2.4.4 Materials for Thermal Barrier Coatings

The most important component of a TBC material is the ceramic used to supply the

bulk material properties for the coating. Ceramics are ideally suited for use as TBCs

due to their high melting temperatures, toughness and typically low thermal

conductivity [Choi, Zhu and Miller, 2005]. Ceramics adopted for fabricating coatings

using the Slurry Spray Technique (discussed in section 3.3) also require additional

material properties to be used successfully. These additional material properties

include availability as a sinterable powder, resilience to thermal fatigue, ability to be

dispersed within a liquid and resistance to corrosion.


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Table 2.1: Properties of TBC Materials

Zirconia Garnet Ceramics Mullite La2ZrO7

Melting Temperature, °C 2700 1970 1850 2300

Thermal Conductivity, W/mK 2.0 3.0 3.3 1.6

Modulus of Elasticity, GPa 21

not applicable

30 175

CTE, (x 10-6) 1/°C 15.3 9.1 5.3 9.1

Poisson’s Ratio 0.25 not

applicable 0.25

not applicable

Yttria Stabilised Zirconia

The extensive use of Yttria Stabilised Zirconia (YSZ) in current TBC applications is

due to the ability to be deposed using many existing technologies such as Low

Velocity Oxygen Flame (LVOF) spray technique or Atmospheric Plasma Spray

(APS) [Kim and Kweon, 1999; Tamura, Takahashi, Ishii, Suzuki, Sato and

Shimomura, 1999; Lima and Guilemany, 2007]. The use of Yttria as the stabiliser for

Zirconia (ZrO2) in the ceramic compound is very beneficial for the quality of the

coating and was confirmed through numerous mechanical experimental

investigations and full-scale testing [Khor, Dong and Gu, 1999; Dobbins, Knight and

Mayo, 2003; Clarke and Phillpot, 2005]. YSZ offers the most reliable performance in

high temperature applications and the common use of this ceramic compound

provides a vast amount of information making it a standard coating material in most

TBC applications. However, the main disadvantage of YSZ is the inability to

withstand temperatures higher than 1200°C for prolonged use and phase

transformations of the material structure at temperatures greater than 1170°C [Cao,

Vassen and Stoever, 2004].


25

The stabilisation of Zirconia describes the addition, or doping, of Zirconia with a

metal oxide addition to increase the resilience of Zirconia to stresses introduced

during cooling after exposure to high temperatures. These stresses are the result of

phase changes within the Zirconia microstructure as thermal energy is dissipated.

During cooling from temperatures less than 1200°C, the unstabilised Zirconia

undergoes a phase change from cubic to monoclinic phase. Transition between these

two phases results in a 3 % volume reduction, which in turn produces tensional

stresses within the Zirconia microstructure. These tensional stresses then proceed to

enhance crack propagation along grain boundaries within the Zirconia microstructure

and ultimately result in universal fracture and failure [Mao, Dai, Yang and Zhou,

2008].

Stabilisation of the Zirconia through the addition of small quantities of metal oxide

prohibits the phase change of Zirconia between monoclinic and cubic phases.

Therefore stresses associated with this phase change are reduced. A common metal

oxide addition used to stabilise Zirconia is yttria, which is typically used to dope the

Zirconia in concentrations between 1 – 8 % wt [Majumdar and Jana, 2000].

Mullite

Mullite represents an important material for use in TBC applications. Mullite is low

density, has high thermal stability, maintains stability in severe chemical

environments, has low thermal conductivity and favourable creep strength

[Torrecillas, Calderón, Moya, Reece, Davies, Olagnon and Fantozzi, 1999; Brunauer,

Frey, Boysen and Schneider, 2001]. Mullite is a composed of SiO2 and Al2O3, with

the composition of 3SiO2 + 2Al2O3. In comparison to YSZ, Mullite has a much lower

CTE, and higher thermal conductivity, as seen in Table 2.1, and has the advantage of

being more resistant to oxidation. Mullite is an attractive alternative to YSZ for

applications in diesel engines, where the temperatures are much lower than gas

turbine engines; however, the temperature variations are much larger. Tests have

shown that Mullite coatings in diesel engines have a longer lifespan than YSZ

[Kokini, Takeuchi and Choules, 1996; Gilbert, Kokini and Sankarasubramanian,


26

2008]. However, the thermal cycling life of Mullite has been found to be much

shorter than the YSZ when temperatures exceed 1000°C. At this temperature, Mullite

crystallises, which is followed by volume contraction, causing cracking and

spallation of the TBC [Rendtorff, Garrido and Aglietti, 2008].

Yttrium Aluminium Garnet (YAG) - Y3Al5O12

Garnet ceramics was developed as a TBC material, as shown in [Padture and

Klemens, 1997]; YAG (Y3Al5O12) exhibiting superior mechanical performance in

high-temperature applications, outstanding thermal stability up to the high melting

point of 1970°C and low thermal conductivity (in the order of < 3 W/mK) [Clarke

and Phillpot, 2005]. It also has a significantly lower oxygen diffusivity coefficient

compared to pure Zirconia, implying higher resistance towards oxidation. However,

the relatively large value of the CTE of 9.1 x 10-6 1/°C has posed severe limitations

in the use of this material in many practical applications [Cao, Vassen and Stoever,

2004].

LZ – La2ZrO7

LZ is a promising material for use as a TBC, due to its excellent physico-mechanical

properties and microstructure [Cao, Li, Zhong, Zhang, Zhang, Vassen and Stoever,

2008]. The LZ has a cubic pyrochlore structure, making LZ phase stable up to its

melting temperature. With this feature, LZ is an attractive material for use as a TBC

and also has a thermal conductivity which is lower than that of YSZ [Clarke and

Phillpot, 2005]. Another advantage is that LZ has relatively low sintering

temperature in comparison to other ceramics. However, the main disadvantage of the

coating material is the relatively short thermal cycling life, due to the relatively low

CTE and low fracture toughness [Cao, Vassen, Tietz, Jungen and Stoever, 2001].


27

Partially Stabilised Zirconia

Yttria Partially Stabilised Zirconia (PSZ) powder TZ-3Y-E produced by Tosoh,

Tokyo, Japan has been used for coating development. TZ-3Y-E is a mixture of

partially-stabilised Zirconia powder with 3 mol% yttria which exhibits superior

sintering properties and higher aging resistance at lower sintering temperature of

1350ºC [Tosoh, 2008]. PSZ has a density of 6050 Kg/m3, thermal conductivity of 2.2

W/mK, and melting point of 2680ºC [Yoshida, 2005]. PSZ has an average particle

size of 0.6 microns [Antou, Hlawka, Cornet, Montavon, Coddet and Machi, 2004].


28

2.5 Fabricating Methods

2.5.1 Introduction

There exist many methods of joining and fabricating Functionally Graded Thermal

Barrier Coatings (FG–TBCs). These fabricating methods can be separated into three

categories: Bulk Processes, Flame Spray and Deposition techniques. Each technique

differs greatly from one another, in terms of fabricating method, cost and simplicity.

The following section provides an overview of the processing techniques available

for fabricating TBC.

2.5.2 Bulk Processes

Powder Stacking

Bulk processing of ceramic coatings by powder stacking involves the following

sequential steps with the selection of ceramics and metals, as seen in Figure 2.4.

Initially the depositions of powders on each layer are of a different composition,

which is then compacted and sintered.


29

Figure 2.4: Processing steps in fabrication of ceramic coatings

The deposition can be done under normal gravity, centrifugal forces and applied

pressure [Leushake, Winter, Rabin and Corff, 1999]. The multi-layered powder

configurations contain discrete compositions in each layer, and stepwise change in

composition from each layer to the next.

The compacting and sintering behaviour varies from layer to layer. If this variety is

not taken into consideration different sintering behaviour will cause various localised

sintering faults, which include warping, necking, splitting and crack formation. The

sintering behaviour is controlled by the following three parameters of the shrinkage

curve: the onset temperature of shrinkage, the slope of shrinkage curve as a function

of temperature and the integral net shrinkage [Watanabe, 1995].

(1) Select Powder

(3) Mix Intermediate Compositions

(2) Add sintering Aids for ceramic

(4) Lay powders in die

(5) Compaction

Sintering Hot Press


30

Laminate Sheet Stacking

With more advanced techniques, thin sheet lamination can be formed from powder

slurries, to form 100 to 1500 �m thick coating [Jin, Takeuchi, Honda, Nishikawa and

Awaji, 2005]. The thin sheets are produced by laminating or stacking these layers

with different compositions, as shown in Figure 2.5. Mixtures of these compositions,

usually with Zirconia and Nickel powder, are processed into slurries containing

binder, deflocculate and plasticiser additives. Air and excess water are then removed

by evaporation before film casting. Next the individual sheets are stacked by pressing

them together, followed by drying of the stack with slow heating using a low

temperature oven, which is followed by sintering. The green sheets are able to be

moulded and formed into various geometries. The number of sheets would be limited

mainly by costs of the fabrication process [Zhang, Han, Zhang, He, Li and Du,

2001].

Figure 2.5: Production of compositionally layered TBC by sheet lamination

Laminating Debinding and Sintering


31

2.5.3 Flame Spray Techniques

Flame Spray is a group of TBC fabricating techniques where a high energy source is

used to melt (or heat) the ceramic and metal powders, and sprayed onto a metallic

substrate or structure. There exist many methods exist within this group, which will

be discussed in the current section. All of these processes require a fuel source,

compressed air, and a method of decomposing the material, which is usually from a

combustion source.

Low Velocity Flame Spray

The Low Velocity Oxygen Flame (LVOF) spray technique involves spraying molten

material onto a surface to produce a ceramic coating. Material (ceramics mentioned

in the previous section) in powder form is melted in an oxy-acetylene flame to form a

fine spray, as depicted in Figure 2.6.

Figure 2.6: Schematic of LVOF spray gun

When the spray material contacts the prepared surface of a substrate, the fine molten

droplets rapidly solidify to form a coating [Sampath, Herman, Shimoda and Saito,

1995]. The LVOF spray technique is considered a cold process, due to its low

substrate temperature during fabrication, in comparison to other techniques. The key

components of the LVOF spray technique consists of the compressed air, fuel gas

supply, powder feeder, control equipment and the powder flame spray gun, as seen in

Powder Coating

Substrate

Fuel Gas


32

Figure 2.7.

Figure 2.7: Key components of LVOF process

The advantage of the LVOF spray technique over other manufacturing methods is

that a much wider range of materials can be easily processed into powder form,

giving a larger choice of coatings [Kieback, Neubrand and Riedel, 2003]. The flame

spray technique is only limited by materials with higher melting temperatures than

the flame can provide or materials that decompose during heating, therefore, use of

LVOF is limited in industry.

High Velocity Oxygen Fuel Thermal Spray

High Velocity Oxygen Fuel (HVOF) thermal spray process is fundamentally the

same concept as the LVOF process, with the main difference being the production of

an extremely high spray velocity upon application of the coating. The technique

involves a high pressure water cooled HVOF combustion chamber with an extended

nozzle. Fuel (such as kerosene, acetylene, propylene, hydrogen) and oxygen are fed

into the chamber and with the resulting combustion producing a hot high-pressured

flame which is forced down a nozzle, increasing its velocity [Hasan, Stokes, Looney

Control Equipment

Fuel Gas

Compressed Air

Powder Flame Spray Gun

Powder Feeder


33

and Hashmi, 2008]. The ceramic and metal powder is fed axially into the HVOF

combustion chamber under high pressure or fed through the side of the nozzle where

the pressure is lower [Lima and Guilemany, 2007], as shown in Figure 2.8.

Figure 2.8: Schematic of HVOF spray gun

With the increase of the oxygen and fuel required to sustain the high velocities, the

process is more complicated than the LVOF technique, involving high energy

consumption. Due to the high kinetic energy of the system, adequate cooling must be

supplied, or risks failure of the HVOF spray gun and equipment. A detailed

schematic of the HVOF technique can be seen in Figure 2.9.

Figure 2.9: Key components of HVOF process

Powder Coating

Substrate

Fuel Gas

Expansion Nozzle

Control Equipment

Fuel Gas

Compressed Air

Powder Feeder

Water

HVOF Spray Gun


34

The TBC produced by the HVOF technique are very dense, strong and have low

residual tensile stress or in some cases compressive stress. This enables much thicker

coatings to be applied than previously possible with other processes [Dobbins,

Knight and Mayo, 2003; Bolelli, Lusvarghi, Varis, Turunen, Leoni, Scardi, Azanza-

Ricardo and Barletta, 2008]. The high kinetic energy of particles striking the

substrate surface does not require the particles to be fully molten to produce high

quality coatings.

Atmospheric Plasma Spray

The Atmospheric Plasma Spray (APS) process utilises a high frequency arc, which is

ignited between an anode and a tungsten cathode [Khor, Dong and Gu, 1999]. The

gases flowing between the electrodes are ionised, where the plasma plume developed

is several centimetres in length. The temperature within the plume can range from

6000°C to 15000°C [Guo, Kuroda and Murakami, 2006]. The spray material is

injected as a powder outside the gun nozzle into the plasma plume, where it is

melted, and propelled by the gas onto the substrate surface, as seen in Figure 2.10.

Figure 2.10: Schematic of APS gun

Plasma Gas

Coating

Substrate Cathode Anode

Powder


35

The component of the APS consists of the same components as the LVOF spray

process. However, the APS is inherently more complex than flame spray, and

requires additional components, as seen in Figure 2.11.

Figure 2.11: Key components of Atmospheric Plasma Spray Process

Unlike other thermal spray processes, the APS process has the advantage that it is

able to spray very high melting point materials such as tungsten and ceramics.

Plasma sprayed coatings are generally much denser, stronger and cleaner coatings

produced by other thermal spray processes, with the exception of HVOF and

detonation processes [Kieback, Neubrand and Riedel, 2003]. However, with the

sophisticated setup of the APS, comes considerably greater setup and running costs,

which generally is in excess of a millions dollars.

Vacuum Plasma Spray

The Vacuum Plasma Spray (VPS) technique is essentially a modified plasma spray

technique; with the main difference being that the VPS is conducted under low

pressure or in a vacuum; which is the key component for producing higher quality

coatings, as seen in Figure 2.12. At lower pressures ranging from 10 KPa to 50 KPa,

Control Equipment

Fuel Gas

Compressed Air

Powder Feeder

Water

Plasma Spray Gun

Heat Exchanger


36

the plume has a larger length and is used in conjunction with a nozzle modified for

high pressure expansion ratios; the nozzle itself contains a higher gas speed. These

differences between APS and VPS techniques allow extremely clean thermal

coatings with virtually no oxides and porosity less than 1% to be produced [Guo,

Kuroda and Murakami, 2006].

Figure 2.12: Key components of Vacuum Plasma Spray Process

VPS has the advantage of the being able to spray broader and longer spray jets in

comparison to APS, producing virtually oxide free coatings and low residual stress

[Guo, Kuroda and Murakami, 2006]. However, the VPS technique is inherently more

expensive to operate in comparison to other known thermal spray technique. This

particular technique is only used when the benefits of the produced coating

outweighs the price disadvantage. For example this fabricating technique is quite

commonly used for advanced aerospace components, where advanced materials such

as refractory metals and reactive materials are needed. Like the APS, the VPS has

considerably larger setup and running costs, which is attributed to the complex and

sophisticated setup of the atmospheric chamber required for the technique.

Control Equipment

Fuel Gas

Compressed Air

Powder Feeder

Water Heat Exchanger

Plasma Gun and Atmospheric Chamber


37

Cold Gas Spray

The Cold Gas Spray (CGS) technique utilises a high-pressure compressed gas to

propel fine powder particles at very high velocities, from 500 to 1500 m/s. High

pressure, compressed gas travels though a heating unit into the gun, where the gas

exits through a nozzle, as seen in Figure 2.13. A high-pressure powder feeder is used

to introduce powder material into the high velocity gas jet, depicted in Figure 2.14.

The powder particles are heated in the gas heater, then accelerated through the spray

gun, where upon impact with the substrate, deform and bond to create a coating

[Papyrin, 2001].

Figure 2.13: Schematic of Cold Spray Gun

The CGS is technically not a flame technique, as it uses high kinetic energy from the

process, instead of deformation of material by high temperatures to create a well

adhered coating. However, the CGS process has the advantage of being a low

temperature technique, and does not require bulk particle melting, retaining the

composition and phase of the initial particle with minimal amounts of oxidation.

Unlike other processes, such as the LVOF spray techniques, the CGS is essentially a

cold process, and cooling equipment is not required, as shown in Figure 2.14. CGS

has the advantage of producing TBCs with surface coatings of a high hardness, cold

worked microstructure [Kreye and Stoltenhoff, 2000]. The disadvantages of the CGS

process are that hard brittle materials such as ceramics cannot be sprayed without

using ductile organic binders to create the initial bond to the substrate. As it is still an

emerging technology, and still in the research and development stage, little coating

Coating

SubstratePowder

Heated Gas


38

performance data is available on this technique.

Figure 2.14: Components of Cold Spray Process System

Detonation Technique

The Detonation Gun Technique (DGT) employs a long water cooled barrel with inlet

valves for gases and powder, as seen in Figure 2.15. Oxygen and fuel (acetylene

most common) is fed into the barrel along with a charge of powder. A spark is used

to ignite the gas mixture and the resultant detonation heats and accelerates the

powder to supersonic velocity down the barrel. A pulse of nitrogen is used to purge

the barrel after each detonation; this process is repeated many times a second

[Cannon, Alkam and Butler, 2008]. The high kinetic energy of the hot powder

particles on impact with the substrate result in a build up of a very dense and strong

coating [Ke, Wu, Wang, Gong, Sun and Wen, 2005]. Due to the high running costs

of this technique, minimal amounts of research have been conducted using the DGT.

Control Equipment

Gas Supply N2

Gas Heater

Powder Feeder

Cold Spray Gun


39

Figure 2.15: Schematic of Detonation Gun

2.5.4 Deposition Techniques

Physical Vapour Deposition

Vapour deposition of the coating material on exposed substrate surfaces is conducted

by vaporising the coating material within an evacuated chamber and projected the

coating vapour particles onto the substrate. The most common vapour deposition

method is Electron Beam Physical Vapour Deposition (EB–PVD) where a focussed

beam of electrons is used to melt and vaporise a small quantity of the coating

material within an evacuated chamber. After being vaporised the coating material

forms a dissociated cloud which then precipitates to form a uniform layer upon the

exposed substrate surface, which generally occurs in a straight line matter [Movchan

and Yakovchuk, 2004]. In a majority of cases, coatings will consist of metal oxides,

nitrides, carbides and other similar materials [Bouzakis, Lontos, Michailidis, Knotek,

Lugscheider, Bobzin and Etzkorn, 2003]. The atoms of metal will then react with the

appropriate gas during the transport stage, as depicted in Figure 2.16. The reactive

gases used to transport the material may be oxygen, nitrogen or methane [Kieback,

Neubrand and Riedel, 2003].

Spark Plug Coating

Substrate

Powder

Nitrogen Purge

Fuel Gas

Oxygen

Cooling Water


40

Figure 2.16: Schematic of EB–PVD

The EB–PVD has the advantages of smoother surface finishes and superior erosion

resistance, in comparison to APS with the main advantage being that the EB–PVD

produces TBCs with outstanding shock resistance, and a considerably longer life

span [Toriz, Thakker and Gupta, 1989]. The long service life and high shock

resistance is related to the columnar microstructure of the coatings. However, TBC

fabricated by EB–PVD is only able to produce relatively thin coatings (> 100 �m), in

comparison to APS, and substrate dimensions are limited by the evacuated chamber.

Chemical Vapour Deposition

Chemical Vapour Deposition (CVD) is a process that involves depositing a solid

material from a gaseous phase; this process is similar to EB–PVD. Although it is

considered a thin-film layering process, the CVD technique has been known to

produce coating thickness of up to 25 �m. EB-PVD differs from CVD in that the

precursors are solid, with the material to be deposited being vaporized from a solid

target and deposited onto the substrate [Choy, 2003]. The CVD process is an omni

directional process, meaning that all exposed surfaces such as holes and porous

surfaces are all coated. Precursor gases (often diluted in carrier gases) are delivered

into the reaction chamber at ambient temperatures. As the precursor gases pass over

Electron Source

Coating

Vapour

Substrate


41

or come into contact with a heated substrate, they react or decompose forming a solid

phase which is deposited onto the substrate [Eroglu and Gallois, 1991]. CVD is a

high temperature process; therefore the substrate material is limited by the substrate

melting temperature [Vargas Garcia and Goto, 2003].


42

2.6 Conclusion and Research Motivation

Relevant literature has been examined in regards to current and future applications of

TBC’s, fabricating techniques and material properties of the constituencies. The

development of TBC has had a huge impact in many industries providing new design

opportunities, higher efficiency and better longevity of the structural components. It

was demonstrated that a notable advances in this area will be significant and will

have a profound impact on many industries and future applications.

From the review of the literature, it was found that the current techniques of

manufacturing TBC are normally expensive and not practical in covering large areas.

They often need expensive equipment and highly trained personnel. These

circumstances were the main motivation behind the current research, which aims to

develop a simple and cost effective fabricating technique based on conventional

methods of depositing and sintering of ceramics powder.

There exist many suitable materials, mostly ceramics, for fabricating TBCs, but it

was noted that for a successful development of a new low cost technique based on

conventional heat sources, such as oven or oxy torch, the sinterability is the key issue

in the manufacturing procedure. From the review, Zirconia has the best sintering

properties and, subsequently, the development of the new fabricating technique, as

described in the next chapters, was focused on this type of ceramics.


43


44

Chapter 3 Development of Slurry Spray Technique

45

CHAPTER 3

3 DEVELOPMENT OF SLURRY SPRAY TECHNIQUE


46

3.1 Introduction Slurry Spray Technique (SST) is a manufacturing process developed by the

candidate at the University of Adelaide for fabricating thin ceramic coatings for their

primary use in aerospace applications, in particular for thermal protection of

hypersonic vehicles. The concept of the SST originated from feasibility studies

conducted by [Ruder, Buchkremer, Jansen, Malléner and Stöver, 1992], on Wet

Powder Spray (WPS) technique. The suggested technique has many advantages

being simple and cheap; however, the quality of the coating at the initial stage of the

current work as well as in the previous studies was found to be quite poor and

inapplicable for most industrial applications. The idea to improve the quality of the

WPS coating and make it comparable with the quality achieved by other

manufacturing techniques was the main driving force behind the research undertaken

in this thesis.

This chapter will provide an overview of the WPS technique, which provides the

groundwork for the current study. In this Chapter the physics and mechanics behind

each stage of the fabrication will be examined and the possible ways for the

improvement of the quality of the fabricated TBCs will be discussed. Majority of

them were implemented into the development of a new technique based on the WPS

technique. This new technique will be presented in this current Chapter. The new

manufacturing technique can also be used for the fabrication of multi-layered

Functionally Graded Thermal Barrier Coatings (FG–TBCs) which have many

advantages in comparison with the deposited and sintered ceramics; in particular,

have much higher fracture resistance.


47

3.2 Wet Powder Spray Technique

The Wet Powder Spray (WPS) technique is divided into four main stages: mixture

preparation; application; drying (carrier removal); and debinding and sintering. This

process is schematically shown in Figure 3.1.

Figure 3.1: Flow Diagram of the WPS technique

3.2.1 Stages of WPS Technique

The principal stages of the WPS technique and their purpose are briefly described

below.

Mixture Preparation: Powder, binder and carrier (alcohol or distil water) are mixed

until complete dissolution of the components and the forming of a uniform mixture.

The volume ratio of the binder and binder-dissolving carrier is normally in the range

from two to five percent of the total volume. The optimal volume ratio will be

investigated later in this chapter.

Spray Application: An air brush or spray gun is used to deposit the mixture on the

(2) Spraying and Brushing

(3) Drying

(1) Mixing

(4) Furnace Sintering


48

substrate to be coated. Properties such as particle size, density and ratio of solid to

liquid play an important role in this deposition stage. Their influence can be

controlled by adequate selection of parameters of the deposition technique (e.g.

pressure, distance, flow rate for the spray gun deposition).

Drying (Carrier Removal): The drying stage starts immediately after the deposition

of the mixture solution. The mixture is normally deposed on the substrate in a semi-

dry condition so that any uncontrolled dripping and wetting can be avoided. The

deposed coatings using WPS technique have a thickness range from 20 to 200 µm.

The drying process for this range of thicknesses is normally from half an hour to two

hours in open air in ambient humidity (~40%), depending on the thickness of the

coating deposited.

Debinding and Sintering: A binder is used to hold the deposited coating together

homogeneously, and prevent cracking during the drying process. However, the

presence of binder in the coating during the sintering stage is undesirable as it

normally leads to cracking and other coating defects. Therefore, prior to sintering of

the deposited ceramics, the binder must be removed from the coating. Debinding and

sintering can both be completed in the same furnace. The debinding involves heating

the coated specimen up to 400°C and normally holding for 2 hours or more, during

which the binder slowly vaporises from the coating. The temperature is then

increased until the desired temperature is reached, at which time the TBC is sintered.

Sintering temperatures are dependent on the substrate and coating properties, which

will be further investigated and discussed in this chapter. Sintering times are

normally limited to within fifteen hours as longer times can increase the risk of

cracking of the coating [Ruder, Buchkremer, Jansen, Malléner and Stöver, 1992].


49

3.2.2 Literature Review – Previous Results

Typical results of the application of the WPS to fabricate ceramic coatings will now

be critically discussed based on the work by Ruder et al. [1992]. In this work, the

coatings were fabricated with a base mixture consisting of Zirconia (ZrO2) and

Nickel (Ni) powders. Incoloy 800 substrate was coated with a double layer of the

mixture deposited on both sides of the specimen. Between the application of the first

and second layer, the initial layer was left at ambient temperature to make sure that

the water component of the deposited coating will be fully evaporated. Once the

layers were free of moisture, the deposed ceramic layers were then sintered at

1300°C in a furnace, where the specimens were placed in a vertical position fully

imbedded in alumina sand. After sintering, specimens were taken and prepared for

cross sectional observation and micro-examination. The most notable feature is

rather high volume porosity, which reaches approximately 45 %. The high porosity

was explained by poor control of the spraying conditions. Since the sprayed mixture

was deposited in semi-dried conditions, the powder particle was interconnected by

the organic binder, which had a large volume fraction. The ceramic coating lacked a

fluid medium; where the particles were not able to migrate and rearrange to form a

more compact configuration. However, as noted in [Dahl, Kaus, Zhao, Johnsson,

Nygren, Wiik, Grande and Einarsrud, 2007], the problems with the densification of

the ceramic coating fabricated by WPS can be partially avoided by a better control of

the spraying and longer sintering times.

Micro-examinations of SEM reveal a smooth and continuous morphology along the

sintered particles. Another interesting observation is that the brush technique of the

deposition resulted in a much more uniform coating thickness rather than air gun

deposition. Further, some qualitative mechanical tests were conducted for the

fabricated coatings, such as scratching and peeling, demonstrating a reasonably good

adhesive strength and scratch resistance of the coating fabricated with WPS.


50

3.2.3 Summary

The WPS technique proposed by [Ruder, Buchkremer, Jansen, Malléner and Stöver,

1992], for fabrication of thin ceramic coatings, has shown to be a very promising

technique for manufacturing a low cost TBCs without the need of sophisticated

equipment. However the technique that was suggested almost two decades ago has

many drawbacks. The fabricated coatings were generally of very poor quality with

unacceptably high levels of the porosity. To achieve a minimum required quality

with the WPS technique, Ruder suggested to use longer sintering times and a better

control of the ceramic powder deposition. For research conducted by Ruder it was

concluded that the WPS technique required further research, development and

experimentation, before the WPS could become a viable option for fabricating TBC.


51

3.3 Development of WPS Technique

The initial step in development of the fabrication technique was to reproduce the Wet

Powder Spray (WPS) technique [Ruder, Buchkremer, Jansen, Malléner and Stöver,

1992] and identify the range of fabrication parameters that could be changed and

examined with the main objective being the improvement of the quality of the

coating. At the initial stage, an extensive literature search was conducted on coating

materials, deposition regimes and mechanisms of sintering of ceramics. In this

section, each aspect of the manufacturing process will be examined and various

options potentially leading to improvement of the quality of the coating will be

discussed.

3.3.1 Fabrication Parameters - Materials

The following section examines the materials and constituents that could be used for

the solution mixture to improve the quality of the deposition stage. The solution

mixture consists of ceramic powder, dispersants and organic binders. In addition, the

substrate material that the coating is to be applied to plays a key role in the selection

of the coating constituents, thus, it is also discussed here.

Powders – Ceramic

Ceramics are ideally suited as a base for TBC due to their high melting temperatures,

and exceptionally low thermal conductivity [Choi, Zhu and Miller, 2005]. The

physico-mechanical properties of ceramics used for fabricating TBC, typically by

Flame Spray techniques, are well investigated and widely available in the literature.

However, for use with the WPS technique the ceramic powder must also posses

some additional characteristics, which are critical for the quality of the final coating.

These include the ability to be dispersed within slurry solution and sintered at


52

relatively low temperature. In addition, the deposited and sintered ceramics have to

generate low levels of residual stresses, which are critical for WPS.

In the previous chapter, large ranges of ceramics were found to be available for

fabricating TBC; these include conventional ceramic compounds, chlorites, and

pyrochlore to rare earth oxides. Rare earth oxides are particularly promising

materials for use as a TBC, due to the low thermal conductivity and high thermal

expansion inertness [Cao, Vassen and Stoever, 2004]. Based on the requirements

discussed above, Partially Stabilised Zirconia (PSZ) ceramics were selected by the

candidate for the use in WPS as they possess all the critical properties for this

technique, which was discussed in Chapter 2, section 2.4.4.

Powders – Metals

In the current section, the variation in the slurry composition of ceramic with a metal

powder will be introduced. The purpose is to reduce the mismatch between the

Coefficient of Thermal Expansion (CTE). The Nickel powder selected by the

candidate, closely resembled the thermo-mechanical properties of the substrate

material (as seen further in this section). Selection of the Nickel powder effectively

reduces the residual thermal stresses experienced by the coating and the substrate, by

reducing the mismatch in the CTE [Zhang, Xu, Wang, Jiang and Wu, 2006].

Dispersants

To prevent agglomeration of the ceramics within the aqueous slurry solution, a

dispersant is required to be added to introduce repulsive ionic forces between the

powder particles via steric stabilisation [Greenwood and Kendall, 1999]. The total

amount of dispersant added to the slurry is dependent upon the mass ratio of ceramic

powder to mixing agent. At low mass ratios the electrostatic stabilisation is induced

by the mixing agent and is able to hold the ceramic in solution mixture.

Hydrolysed organic polymers are typically added as dispersants to form slurries.

Non-uniform polarised regions of the polymer molecules allow these organic


53

compounds to graft themselves onto ceramic powder particles through ionic

exchange [Khan, Briscoe and Luckham, 2000].The trailing section of the polymer

molecules retains their ionic charge and repulses other powder particles encapsulated

by the polarised dispersant molecules [Khan, Briscoe and Luckham, 2000].

The dispersant chosen for the slurry based solution mixture was tetra sodium

pyrophosphate. This dispersant was chosen based upon studies by [Briscoe, Khan

and Luckham, 1998], successful use in the creation of yttria stabilised zirconium

beads during the sintering of a slurry mixture. The use of different dispersants led to

no significant changes in the solution mixture properties, such as viscosity and did

not significantly affect the quality of the fabricated TBC, which will be examined

further in the following section. With the use of the dispersant for the solution

mixture, a binder is also needed to hold all components together once the coating has

been applied to the substrate.

Binder

The binder has an integral function in the formation of sintered TBCs by maintaining

the structural integrity of the coating prior to sintering. In essence, the binder and

ceramic powder particles form a soluble composite as the ceramic powder is acting

as a reinforcement phase and the binder as a matrix phase. The binder needs to be

vaporised before sintering at temperatures lower than for sintering the ceramic

coating. This process of vaporising of binder is called debinding [Tanaka, Pin and

Uematsu, 2006]. If the debinding stage prior to sintering is omitted, contaminants

could form within the sintered ceramic microstructure. This would result in

significant degradation of the quality of the TBC and, potentially, cause premature

failure of the coating.

The binders for the WPS slurry were selected based upon past success with similar

studies using Yttria Stabilised Zirconia (YSZ). One study in particular showed

promising results in the manufacture of sintered YSZ beads [Roy, Bertrand and

Coddet, 2005] using a nylon based on a copolymer of styrene, acrylic ester or hydro


54

soluble polyvinyl alcohol. These binders were therefore considered suitable by the

candidate for use with the WPS slurry.

Mixing Agent

Distilled water was used as the mixing agent throughout the experimental

development of the WPS technique. Ionic disassociation within the distilled water

provides a limited form of electrostatic stabilisation of the ceramic powder particles.

The pH of the slurry solution is therefore adjusted using sodium hydroxide or

hydrochloric acid to create a slightly alkaline solution. The distilled water is also

capable of being fully evaporated from the slurry without leaving any residue as all

dissolved salts are eliminated during distillation [Narita, Hébraud and Lequeux,

2005].

Substrate Material

Materials commonly coated in hypersonic and industrial applications materials were

considered for the substrate material in the initial stages of experimental

development. This enables comparison between TBC produced using the WPS to

existing techniques. The candidates include stainless steel, Inconel and aluminium.

High carbon steel was not considered due to corrosion issues during application of

the aqueous solution mixture prior to the evaporation stage.

It is desirable for the substrate material have similar CTE to that of the coating

material, to prevent development of thermal stresses during fabrication and

temperature excursions during operation. As ceramics typically have very low CTE,

the coatings produced on Stainless Steels (opposed to Aluminium Alloy) will be the

most resilient to cracking at fabrication stage and to thermal fatigue due to repetitive

thermal excursions. Typical material properties for various substrate materials are

displayed below.


55

Table 3.1: Substrate material properties

Substrate Material Melting Point, °C

CTE, (x 10-6) 1/°C

Thermal Conductivity, W/mK

Aluminium Alloy 660 23.1 51.9

Stainless Steel 316 1400 10.8 16.2

Inconel 601 1610 12.8 9.8

From Table 3.1, the Inconel 601 was shown to have a low CTE and thermal

conductivity, which is beneficial for the reduction of the induced thermal stresses.

Inconel 601 also has high melting temperature and resilience to corrosion. For these

reasons Inconel is predominantly used for load-bearing structures in aerospace

applications [Song, Lee, Lee, Kim, Kim and Lee, 2002; Zhang, Li, Li, Zhang, Wang,

Yang and Li, 2008]. Therefore, for this investigation Inconel 601 was selected by the

candidate as the substrate material.

3.3.2 Mixture Preparation

The first stage of the WPS involves dispersing the slurry constituents within a mixing

agent to form an aqueous solution capable of being sprayed onto exposed surfaces.

The most important factors in this stage are the level of dispersion of the constituents

within the slurry and the final viscosity of the mixture.

Firstly, a dispersant is added to the slurry mixture to allow the ceramic powder to be

dispersed within the working fluid. The dispersion of the slurry components, i.e. the

ceramic powder and binder, is important in determining their ability to be used in the

fabrication of thermal barrier coatings via the WPS. All coating components must be

capable of being fully dispersed homogeneously within the working fluid medium, in

order to produce coatings with uniform mechanical properties.


56

The pH of the slurry is a further aspect that can be controlled to assist the dispersion

of the ceramic powder within the mixing agent. In addition, if the pH level of the

mixture is too high, this may produce solutions with increased relative sediment

height (RSH) [Mahdjoub, Roy, Filiatre, Bertrand and Coddet, 2003]. Appropriate pH

levels must therefore be maintained to produce solution mixtures that are able to be

applied to the substrate by a spray gun.

The final viscosity of the slurry can be adjusted by varying the composition of the

mixture to allow the slurry to be sprayed. This is discussed below, along with further

details on the dispersion process.

Dispersion

The ability of a non aqueous solution to be dispersed uniformly within a fluid is

dependent upon the attractive Van der Waals forces between powder particles within

the slurry mixture [Briscoe, Khan and Luckham, 1998]. The presence of large inter

particle forces can cause agglomeration of the ceramic where powder particles group

together to form an insoluble sedimentary layer. The development of a sedimentary

layer within the slurry must be avoided due to the increased viscosity of the slurry

mixture and material inhomogeneity within the sprayed coating.

To effectively negate the attractive inter-particular forces, repulsive forces have to be

introduced, which will keep the ceramic particles in suspension. There are two major

methods of introducing these forces: electrostatic stabilisation and steric stabilisation.

Both methods utilise ionic forces between powder particles by assigning ‘like ionic’

charges to the particles. These repulsive ionic forces increase with the increased

proximity between powder particles, and are able to overcome the attractive Van der

Waals forces [Greenwood and Kendall, 1999].

Electrostatic stabilisation of the ceramic powder particles is achieved by adjusting

the pH of the mixing agent to create a polarised environment around the particles.

The polar environment strips oppositely charged ions from the powder particles,

therefore leaving the particles themselves with a like charge. Repulsive ionic forces


57

are then induced between the ionised powder particles to prevent agglomeration [Lan

and Xiao, 2007].

Similar to electrostatic stabilisation, in steric stabilisation an ionic dispersant is added

to the slurry mixture to deter agglomeration of the powder particles. The dispersant

itself is a polymeric entity composed of long polymer units capable of forming

intermolecular bonds with the powder particles. The dispersant is grafted to the

powder particle surface where the ionic charge of the exposed polymer layers

provides a repulsive ionic force between the engulfed particles [Greenwood and

Kendall, 1999].

Optimum Viscosity

To produce a slurry mixture with an optimum viscosity, the composition of the

mixture is varied until the best quality deposited layer is achieved. Each component

in the slurry mixture has a varying degree of influence on the viscosity of the slurry

solution. The binder composition has the most significant effect on the viscosity of

the mixture and quality of the applied coating layer.

The viscosity of the slurry mixture is also an important parameter governing the

selection of the spray process used to apply the slurry mixture to the substrate

surface. That is, the ability of the spray apparatus to uniformly apply a slurry mixture

is dependent on the mixture’s viscosity. The effects of ceramic, binder and dispersant

composition on solution viscosity will be examined in the following section.

The following sections of this chapter outline the effects of the composition of the

ceramic powder (i) and binder (ii) on the overall solution viscosity. These viscosity

measurements adhere to ASTM standard D1200 – 05: Standard Test Method for

Viscosity by Ford Viscosity Cup. The apparatus for the experiments is a ford cup,

which is a cylindrical vessel with conical bottom leading to a drain orifice. The

viscosity is rated via the ford index as determined by the time required for the full

cylinder to completely drain.


58

i. Viscosity Measurements – Ceramic and Metal Powder

The composition of the ceramic powder plays a major role in the quality of the

applied solution mixture using a spraying system. Viscosity experiments were

conducted by the candidate to determine the optimum percentage of ceramic powder

in the slurry mixture. The composition of the solution, other than the ceramic and

Nickel composition remain unchanged. The testing regime of the ceramic powder

compositions can be seen in Table 3.2.

Table 3.2: Testing of Ceramic Powder Compositions

Composition Test Binder (%) Dispersant (%)

ZrO2 (%) Ni (%)

1 4 0.4 33 66

2 4 0.4 66 33

3 4 0.4 100 0

Through experimentation with the variation of percentages of the powder

composition in the slurry, it was noted that viscosity decreases proportionally with

ceramic content in the slurry mixture. The results from the experiments can be seen

in Figure 3.2. They show that in the case of TBC with Functionally Graded (FG)

layers (discussed in section 3.4.1), it will be necessary to adjust the percentage of the

binder content to maintain optimum viscosity of around 25 mm2/s to minimise

porosity induction.


59

Figure 3.2: Graph of ceramic powder composition vs. viscosity of the slurry solution

If the composition of the slurry solution exceeds 50% ceramics, a flocculated mixture

is normally produced. This will, in turn, introduce surface irregularities during the

application of the slurry mixture to the substrate via clumping of material mass, as

opposed to a well dispersed medium. Such a situation creates problems including

uneven pressure application during the pressure stamping stage (as discussed further

in Chapter 3, section 3.4.2), which include cracking. The optimum percentage of the

ceramics in the slurry solution was determined experimentally. From a qualitative

examination of the deposited layers, the optimum ceramic and metal composition

was found to be approximately 45%.

ii. Viscosity Measurements - Binder

The binder is one of the most important components that will affect the viscosity of

the mixture. Therefore the variation of the binder percentage, as part of the slurry

mixture, is examined. Experiments were conducted to determine the optimum

percentage of binder in the slurry solution mixture. For benchmarking and

experimental consistency the composition of the solution, other than the binder,

remained unchanged as described further in WPS. The slurry compositions for

Vis

cosit

y, m

m2 /s

Percentage of Ceramic

10

20

30

40

20 40 60 80 100

50


60

viscosity experiments can be seen in Table 3.3.

Table 3.3: Viscosity measurements of the solution mixture for various binder

compositions

Test Ceramic Powder (%) Binder (%) Dispersant (%)

1 45 2 0.4

2 45 3 0.4

3 45 4 0.4

4 45 5 0.4

From the experimentation of the variation of percentages of the binder, it was

observed that the viscosity of the slurry mixture increases roughly linearly with the

percentage of binder. The results of the tests conducted can be seen in Figure 3.3.

Figure 3.3: Graph of binder percentage versus viscosity for the slurry solution. Error

bars represent maximum and minimum values obtained

Vis

cosi

ty, m

m2 /s

20

40

60

80

2 3 4 5Percentage of Binder

100


61

If the binder percentage of the slurry solution was 2 % or less, the slurry mixture

became excessively fluidic, preventing its application the substrate with the current

spray equipment. If the percentage of binder in the slurry solution exceeded 4 %,

agglomeration in the slurry mixture increased such that the mixture was too

flocculated to be applied with the spray gun. Therefore, the used percentage of binder

used in the solution was determined to be 3 %.

3.3.3 Spray Application

The application of the aqueous slurry mixtures to substrate surfaces is conducted

directly by atomising the slurry within a pressurised air stream before being

projected upon the substrate surface. The spray process can be conducted utilising

pre-existing general spraying techniques and equipment allowing simple application

of the slurry to the substrate. Importantly, spray application of the coating offers the

opportunity to coat surfaces with irregular or complex geometries, and large surface

areas, which is important specifically for aerospace applications.

Spray Gun Selection

In order to achieve sufficient atomisation, the slurry has to be either drawn up from

below the conventional feed spray gun or fed from the top down into the nozzle from

the gravity feed. In order to achieve sufficient atomisation with siphon feed, greater

air pressure must be used in order to attain a strong enough vacuum to pull the slurry

up the feeding tube from below. The main problem with the siphon feed spray gun is

the requirement of high pressure, which is the main limitation for their use in High

Velocity, Low Pressure (HVLP) systems. It is much more difficult to keep low air

pressure and achieve adequate vacuum, with varying paint hopper sizes. Unlike the

siphon fed system, the gravity feed spray gun requires less air pressure to atomize the

solution mixture.


62

Spraying with less air pressure has the advantages of less overspray, less waste and

greater control during application of the slurry. For the siphon fed spray gun, it is

possible to achieve atomisation at lower air pressures by pressurising the cup. For

application in the current research, a gravity fed spray gun was selected by the

candidate to apply the slurry-based coatings.

Spraying Techniques

Spraying techniques for the WPS closely follow those used in traditional spray

coating applications. The aqueous slurry is fed into a high velocity air stream via

small tube orifices and directed at a target surface to be deposited. As the slurry is

fed into the air stream via small orifices it is atomised, where the fine slurry mixture

is initially disassociated into small discrete molecules by shear forces generated

during introduction to the turbulent airflow. Reduction of the disassociated molecule

size allows a finer distribution of the sprayed coating along the target substrate.

However this reduction comes at the cost of increased slurry losses to spray

dispersion in the surroundings. The size of the disassociated slurry molecules is

controlled by the pressure and thus velocity of the air stream. Equipment utilised

during the spray process included a standard variable pressure air compressor and

gravity fed paint spray gun.

3.3.4 Drying (Carrier Removal)

The evaporation of binder out of the sprayed coating slurry represents an important

stage of the WPS where the wet coating layers remain in a fragile state until the

binder and water has fully evaporated. Once the stage is completed, the sintering

stage is able to commence. During this period the coating is susceptible to

disturbance from both, stresses as a result of external forces and stresses produced by

the evaporation of the water and binder.


63

Evaporation Mechanism

The evaporation mechanism of water from the sprayed coating slurry is a lateral

drying process where, in the case of coatings sprayed on flat substrates, dry areas

first develop along the coating edges and, propagate inwards towards the centre of

the coated specimen. The evaporation process involves three distinct slurry stages as

shown in Figure 3.4. The first stage reflects the water-supersaturated conditions of

the coating, where the water composition is greater than that of the saturated region

and allows the powder particles to migrate freely within the slurry media. The second

stage is considered ‘fully saturated’ where the water content within the slurry fills all

pore space between the coating particles. The third stage refers to the dried powder

state, when the water has fully evaporated from the slurry to form a packed

arrangement of powder particles.

Figure 3.4: Slurry regions during evaporation

1. Super-Saturated Region 2. Saturated Region

3. Dry Saturated Region


64

Prior to evaporation the entire sprayed coating represents a single water-

supersaturated region. During evaporation water is initially evaporated over the

entire coating surface. However the rate of evaporation is greatest at the coating

edges due to the greater exposed area of the coating. The edges develop into

saturated regions and continue to be evaporated, however, as the saturated regions

continue to evaporate, water is drawn to the saturated regions from the water-

supersaturated regions by capillary forces [Tanaka, Pin and Uematsu, 2006]. The

saturated regions then shrink inward toward the centre of the coating as dry regions

begin to form on the coating edges. Once all water-supersaturated regions have

developed into saturated regions, dry regions advance towards the coating centre

until the coating is fully evaporated.

After evaporation the coating is left as a continuous arrangement of loosely packed

powder particles. This arrangement is commonly described as a ‘green form’ or

having a ‘green body’ referring to the evaporated, but not yet sintered state. To

increase the strength of the green form a binder is added to the slurry mixture to bond

the evaporated powder particles together [Tanaka, Pin and Uematsu, 2006]. The

powder particles combine to form a composite with the binder acting as a matrix

phase to stabilise the powder particles substitute as a particulate phase as seen in

Figure 3.5. Prior to coating application, substrate surface preparation is needed to

prevent any contaminants during the spraying stages.


65

Figure 3.5: Binder acting as matrix phase within powder/binder composite

Surface Preparation

Proper surface preparation prior to spraying is crucial to the integrity of the TBC.

Specific steps are required to be undertaken in order for the coating to perform to

optimum standards. The amount of time used for surface preparation may account for

just over 50% to complete the fabrication of the TBC [Berndt and Lenling, 2004].

Therefore, substrate surface preparation represents substantial investment,

considering the time and labour required during fabrication of TBC.

From the literature, it was concluded that sand blasting the substrate surface before

application is the preferred method, as opposed to sanding the surface [Kawamura,

Okado, Nishio and Suzuki, 2004]. Sand blasting removes all the grit, as well as

creating a very porous surface, enabling the solution mixture to seep into the pores of

the substrate surface during the spraying stage. The solution seeping into the pores

assists the coating to bond to the substrate during the sintering stage. The type of

bonding that occurs between the substrate and the coating is known as ‘diffusion

bonding’ [Bernard-Granger, Monchalin and Guizard, 2007].

Surface preparation helps promote bonding between coating and the substrate;

however, the bonding will be adversely affected by substrate thickness variations,

which would result in uneven application of the slurry solution. Uneven substrate


66

surface can result in failure of the ceramic coating, if not properly addressed during

substrate preparation prior to coating. These issues are of concern during the pressure

stamping stage (discussed in the current Chapter, section 3.4.2), as the uneven

surface will cause specific areas to have non-uniform pressure applied to the surface,

causing uneven contact and growth of grain boundaries between the ceramic and

Nickel powder particles, and the metallic substrate. This is crucial during the

fabrication of the TBCs, as this phenomenon affects the stress distribution during the

sintering stage. These stress concentrations act as the catalyst for the coating failure,

leading to cracking and spallation of the coating.

Formation of Stress during Evaporation

During the initial development of the WPS, TBCs with thicknesses of between 1-2

mm were sprayed onto the metallic substrates, and allowed to evaporate under

ambient atmospheric conditions. During evaporation of the sprayed slurry coatings,

cracks were observed to develop within the semi-dried coating as seen in Figure 3.6.

Cracks initially developed along boundary edges of the partially dried coating before

propagating inwards to the centre of the coating surface.

Figure 3.6: Cracking within evaporated coating of excessive thickness


67

The development of cracks within the evaporating slurry coating can be attributed to

the relief of stress resulting from the reduction of the coating surface thickness

during evaporation. As water is evaporated from the TBC, the coating volume

decreases, however, the coating itself is constrained by the substrate, which restricts

lateral reduction of the coating volume. Therefore while the coating is allowed to

reduce in thickness, lateral stresses are introduced by the substrate to the coating as

the water evaporates.

With adequate water composition these lateral stresses are relieved by viscous flow

of the slurry. After sufficient evaporation, the coating begins to form a continuous

particle structure held together by attractive van der Waals forces capable of

supporting the lateral stress [Lan and Xiao, 2007]. As evaporation continues, the van

der Waals forces increase as the powder particles are drawn into closer proximity by

the shrinking coating volume. Cracking of the evaporated coating occurs where the

lateral stress produced by the reduced coating volume becomes greater than the local

cohesive strength of the coating. Capillary forces during evaporation complement the

lateral stresses induced via volume reduction of the coating. Investigation by [Lan

and Xiao, 2007], determined the maximum lateral stresses that occur when the

coating transitions from a fully saturated state to a dry state, where the capillary

forces are maximised. The volume reduction and lateral stress profile for Yttria

partially stabilised zirconium slurry coatings are shown below in Figure 3.7 for a

decreasing residual water content of the coating [Lan and Xiao, 2007].

Two major factors affecting the ability of the evaporated coating to resist the lateral

stresses induced during the evaporation stage include the surface tension of the slurry

solution and the coating particle size. To minimise capillary forces during

evaporation, slurries with small surface tensions are preferred to decrease the

resultant lateral stresses produced during evaporation of the coating. By increasing

powder particle sizes, an increase in van der Waals forces between particles can be

achieved, leading to a higher cohesive strength of the evaporated coating.


68

Figure 3.7: Volume reduction and lateral stress within YSZ for decreasing water

content

An alternative approach to prevent cracking of the evaporated slurry coating is to

utilise smaller coating thicknesses to reduce the lateral stresses developed during

evaporation. Smaller thicknesses lead to a reduced initial layer volume per unit area

and thus reduce lateral stress generation caused by the constrained volume reduction

of the coating. Coating thicknesses greater than 150 �m, were observed to crack

during the evaporation stage, as seen in Figure 3.6. The optimum coating thicknesses

were found to range from 50 to 150 �m. For these coating thicknesses, cracking of

the coating surface during the evaporation stage was minimised.

Coating

Volume

Slurry water composition, %vol

Late

ral S

tress

, MPa

Coa

ting

Vol

ume,

% o

f ini

tial

40 30 20 10 00

20

40

60

80

100

0

0.4

0.8

1.2

1.6

2.0


69

3.3.5 Debinding and Sintering

Before sintering of a green body can begin, an initial debinding process is undertaken

to remove the binder component from the coating. The binder must be removed from

the coating microstructure to prevent contamination within the final sintered coating.

Contamination of the coating microstructure includes either the introduction of

inclusions within the coating material, such as dust particles or surface lubricants, or

the formation of an additional ceramic phase.

Inclusions within the coating microstructure develop from the concentration of the

binder within porous regions of the coating during sintering. As grain boundaries

grow between powder particles during sintering, pores form between the coalescing

powder particles. The binder is driven into the porous regions ahead of the advancing

grain boundaries where, under the increased pressure and high temperature

environment, the binder is able to transform into a distinct particulate within the

coating microstructure [Konyashin, 2001]. These inclusions form areas of stress

intensification and are possible initiation points for coating failure during thermal

cyclic loading.

The debinding process involves combustion of the binder from the green body at

temperatures much lower than the sintering temperature of the ceramic powder.

Typically organic polymers, such as Polyvinyl Alcohol, are commonly utilised as

binding agents due to their low melting points of approximately 600°C, and ability to

form stable green bodies through the formation of secondary bonds between the

binder monomers and the ceramic powder particles.


70

Sintering

The sintering stage of the WPS is the point at which the evaporated slurry coating is

transformed from a weak green body to a solid coating able to resist high

temperatures and corrosive environments. The sintering process of the powder

particles transitions between three distinctive periods as depicted in Figure 3.8.

Figure 3.8: Powder coalescence during sintering

Initially, the deposited and dried powder represents a loosely separated pack of

powder particles with few contact regions. With the application of thermal energy,

the grain boundaries expand along these contact areas to form connected regions

between the particles. As more thermal energy is supplied, the grain boundaries

continue to create a spherical pore within the final microstructure [Choi, Zhu and

Miller, 2005]. The principal effect that drives the sintering process is the reduction in

exposed surface area, and consequently the surface energy, of the powder particles,

in comparison to their original unsintered states [Bernard-Granger, Monchalin and

Guizard, 2007].

Sintering of a specimen can be conducted using a variety of methods. Traditionally

sintering is performed using high temperature ovens or kilns, which have an

advantage over other sintering methods as the specimen can be sintered in a

Unsintered Semi - Sintered Fully - Sintered


71

controlled thermal environment at a wide range of temperatures for any time duration

required. Another method involves using high intensity lasers to sinter thin material

volumes. In this case sintering takes place for very short durations under very high

temperatures in a localised region on the surface of the specimen. Lastly sintering

can also be conducted on a specimen using a direct combustion stream, such as oxy-

acetylene or propane. Direct combustion stream temperatures can reach up to 3000°C

however, while being the simplest sintering method, creating a uniform heat flux on

the specimen is difficult and the control of the sintering temperature range is limited.

Sintering Temperatures

Sintering temperatures required to sufficiently promote grain growth amongst

powder particles typically require temperatures above 70% of the melting

temperature of the material (in Kelvin). This temperature is below melting

temperature, but is high enough to allow solid state sintering [Degarmo, Black and

Kohser, 2003]. However, for cases where two dissimilar materials are to be sintered,

the sintering temperature must be increased to where liquid state sintering occurs.

Liquid state sintering is essentially in a semi molten state to allow the material to fill

the voids between the two different materials. Theoretically, minimum sintering

temperatures occur below this limit, however as sintering times are inversely related

to the sintering temperatures [Choi, Zhu and Miller, 2005], the associated long

sintering durations limit the practicality of using such low sintering temperatures.

High Temperature Furnace

A high temperature furnace was used to sinter specimens, which were produced in

tandem with the oxy-acetylene torch. The furnace provides a uniform heat

distribution over the slurry coated substrates, minimising the induced thermal

stresses from differences in temperature. In contrast, the oxy-acetylene torch creates

uneven temperature distribution over the coating throughout the sintering stage. The

specimens sintered using the high temperature furnace allowed comparative


72

assessment between the two sintering methods on the quality between the Slurry

Sprayed TBC.

3.3.6 Summary

The WPS technique was found able to successfully fabricate ceramic TBCs. Through

analysis of the WPS technique, the fabrication stages were refined allowing

production of the TBC. In the WPS technique, the solution mixture was applied to

the substrate in a semi-dry state. The coating relies on the binder to act as the

interlayer bonds between the particles, which were responsible for the high levels of

porosity in the coating. After examination of the spraying stage, a dispersant was

included in the slurry solution, to aid in the powder dispersion, allowing the slurry to

be produced in a more aqueous medium. Therefore, the solution was applied as a wet

medium, allowing the particles to migrate freely in the slurry medium, reducing the

porosity of the TBC and creating a denser coating. This in turn reduced the sintering

times needed for densification of the coating. However, the produced TBC spalled

during and after sintering. Qualitative adhesion tests (scratching and peeling) have

shown that the coating was still easily damaged, and the overall quality of the coating

was poor.

It was concluded the technique required further development, research and

experimentation, before the technique could fabricate TBC without coating failure

during the fabrication stages. The following section aims to address the majority of

these issues with the WPS technique. Further research and experimentation was

conducted into each aspect of the WPS technique, leading to the development of the

SST.


73

3.4 Slurry Spray Technique

The Wet Powder Spray (WPS) technique has been shown to be capable of producing

TBC, however, many problems still exists during the fabrication stages of the Slurry

based technique. These problems include failure of the produced Slurry based TBC,

with regards to spallation of the coating, during, after sintering, long sintering times,

and general problems which arose during the fabrication process. These problems

and issues are addressed with the Slurry Spray Technique (SST).

An overview of the SST can be seen in Figure 3.9. The SST incorporates additional

steps throughout all the stages of the fabrication process, which include a multi-layer

spraying stage, pressure stamping stage and the use of an automated sintering

platform.

Figure 3.9: Stages of the SST

After initial tests conducted by the candidate, it was observed that thermal cycling

tests on TBC specimens fabricated using the SST were unable to withstand exposure

(2) Multilayered Spraying (1) Mixing

(4) Stamping

Pressure

(3) Drying(5) Torch and

Furnace Sintering


74

to thermal cycles. Upon inspection of the failed specimens, large spall regions along

the substrate and coating interface were observed over the coating surface area. The

TBC failure, through spallation of the coating, was the result of stresses induced by

the thermal expansion mismatch between the substrate and coating material during

the stages of fabrication. To reduce the stresses associated with the mismatch in the

Coefficient of Thermal Expansion (CTE) of the coating material and substrate,

Functionally Graded Thermal Barrier Coatings (FG–TBCs) were introduced

[Polanco, Miranzo and Osendi, 2006].

Functionally Graded Thermal Barrier Coatings

To produce FG–TBCs, the compositions of the coating were varied using a metal

additive with a CTE similar to that of the substrate. Metal rich regions were

deposited close to the substrate surface, altering the effective difference in the

coefficient of thermal expansion within that locality. At regions further away from

the substrate the metal composition was reduced, allowing the material properties of

the ceramic to dominate. The net result was a coating with a varied thermal

expansion coefficient over the thickness of the Functional Grading Coating. The

presence of a large coefficient differential at the coating and substrate interface is

avoided by varying the thermal coefficient gradually through the coating. Thus the

thermal stress is distributed more evenly through the coating thickness, effectively

reducing the maximum induced thermal stress.

During fabrication of the FG–TBCs using the SST, Nickel powder was selected as an

appropriate grading material. The Nickel powder has a CTE of 12 x 10-6 1/°C, which

is sufficiently close to that of the Inconel substrate material. This effectively

minimises the difference between the CTE of the coating layers and the substrate

material. Additionally the inclusion of Nickel within the ceramic coating does not

lead to the creation of additional phase variations within the coating microstructure

of the TBC material [Polanco, Miranzo and Osendi, 2006].

Experiments were conducted by the candidate to determine the optimum percentage


75

of ceramic and Nickel powder for the slurry solution mixture. The composition of the

solution, other than the Zirconia and Nickel remain unchanged as described in the

original technique, and is illustrated in Table 3.4. The variation in the composition of

ceramic and metal powders was chosen for each test, to minimise thermal stresses.

Table 3.4: Testing of Graded Compositions

Composition Test No. Layers

Section ZrO2 (%) Ni (%)

Top 50 50 1 Double

Base 100 0

Top 33 66

Mid 66 33 2 Triple

Base 100 0

Top 25 75

Mid 1 50 50

Mid 2 75 25 3 Quadruple

Base 100 0

Fabrication of two and three layered coatings proved successful, with both the

coatings fabricated without spallation and delamination, during and after the

sintering stage. However, the four layered coating spalled during and after sintering

stage, which was limited by the thickness of the coating. From qualitative analysis,

the maximum numbers of coating layers was found to be three layers.


76

3.4.1 Spraying

During application of the slurry to the substrate, it was observed that there was a

substantial amount of excess slurry solution. These wastages can be attributed to the

design of the gravity fed spray gun. The gravity fed spray gun has limitations in the

directions and angles the slurry can be applied to the substrate as it was designed to

apply sprayed material to vertical standing structures. The application of the slurry

solution requires the substrate to be positioned horizontally. Therefore excess slurry

mixture remained in the paint pot, unable to be sprayed to the structure. It was

concluded that the current spray gun was highly inefficient and added unnecessary

cost to the spraying stage of the SST. These issues led to the selection of a spray gun

which addressed the aforementioned problems.

Spray Gun

The Anest Iwata gravity fed spray gun (RG.3L.3C) has a fluid nozzle size of 1.0 mm

and a fluid output of 80 ml per minute, as per manufacturer’s specifications. The

sprays gun has a fan pattern which covers an area of 35 mm and is able to achieve a

fine atomisation spray. The utilisation of the Anest Iwata gravity fed spray gun has

the benefit of the ability to spray 90 degrees perpendicular to the surface of the

substrate. The spray gun is equipped with a side mounted gravity pot that can be

rotated thus allowing for flexibility in all angles of spraying: horizontal, vertical and

hard to reach angles. This helped to overcome the issues of wastage of the slurry

mixture and, at the same time, allow the slurry mixture to be applied to surface

structures without the limitation of vertical application. The gun is also equipped

with a 130 ml pot ideal for spraying on smaller surface areas or lesser substrates.

Wet Interlayer

The base layer must be fully evaporated prior to coating of any subsequent ceramic

layer, otherwise a wet interlayer will be present. This will affect the pressure

stamping stage (discussed in section 3.4.2), creating a dark indention mark with


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loosely packed particles visible in the coating and, in turn, interferes with the

sintering stage. The loosely packed particles of the coating is caused by the

compaction of the porosity left by evaporated water between the layers, which can be

seen in Figure 3.10.

Figure 3.10: Stamped substrate with mark of wet inter-layer.

3.4.2 Pressure Stamping

Before pressure stamping of the coating was introduced into the fabrication

procedure, long sintering times were experienced in order to adequately sinter the

coatings. The long sintering times were attributed to the remoteness of the powder

particles in respect to each other within the unsintered body, which increased the

thermal energy that was required to grow the grain regions prior to intersection of the

grain boundaries and coalescence [Hirvonen, Nowak, Yamamoto, Sekino and

Niihara, 2006]. As a result, extended sintering durations were required to initially

supply thermal energy to grow the grain regions before coalescence could begin.

To reduce the sintering times required to grow the grain boundaries prior to

coalescence, the overall proximity of the powder particles to each other, needed to be

Wet inter-layer


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increased. Hence, an increase in the contact area between the powder particles was

required. To increase the contact areas between powder particles a pressure force is

applied utilising a universal pressure stamping machine. The application of pressure

to the coating results in mechanical densification, as shown in Figure 3.11, in

contrast to the surface diffusion densification achieved through sintering.

Figure 3.11: Densification during pressure application

Tests were conducted to determine the optimum compression pressure to be applied

to the coating surface. For benchmarking and experimental consistency, the

composition of the solution remained constant as described in the WPS technique,

section 3.3.2.

The aim of the following experiments was to determine the most optimal pressure to

decrease the sintering times without causing any damage to the coating. The

experiments conducted with compression pressure ranging from 20 to 60 MPa, as

seen in Table 3.5.

Table 3.5: Testing of surface pressures

Test 1 2 3 4 5

Pressure [MPa] 20 30 40 50 60


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From a qualitative examination of the experiments conducted, it was observed that

the compression pressures that ranged from 20 to 30 MPa showed improved

coalescence of grain boundaries, as shown in Figure 3.12. With the application of

surface pressures of 40 to 60 MPa improved grain boundary growth was observed,

however during the sintering stages, spallation and delamination of the TBC was

seen along specimen surface.

The pressure stamping of the unsintered coating with the arrangement of powder

particles progressed from a loose configuration to densely packed volume with

reduced porosity. The reduction in porosity is visible in comparisons of Scanning

Electron Microscope (SEM) images of a pressure stamped and unstamped coating in

Figure 3.12. Essentially mechanical rather than thermal energy is utilised for the

coating densification. Once mechanical densification has occurred, continued

densification of the green body can begin through the sintering of the tightly packed

particle arrangement from the supplied thermal energy.


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Figure 3.12: SEM porosity comparison between 30 MPa pressed (top) and un-

pressed (bottom) coatings

3.4.3 Sintering

During the initial phase of the sintering stage, the binder is vaporised from the

coating surface using an oxy-acetylene torch, fixed above the centre of the specimen.

From examination of the coating surface, it was observed that the particles in the

centre of the coating showed a good coalescence between the ceramic powders.

However as the distance from the centre of the coating increased, the level of

coalescence between particles was observed to drop dramatically. From examination

of the sintering method of oxy acetylene, it was deduced that the coating was


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experiencing uneven heat distribution throughout the coating surface. With uneven

heat distribution lays a greater difference of temperature along the surface of the

coating, leading to the variation in the coalescence ceramic particles.

After examination of the various sintering durations, the coatings were observed to

be adequately sintered after 30 minutes. However the percentage of the

Zirconia/Nickel composition of the base layer is restricted by certain constraints,

which include the mismatch of thermo-mechanical properties, a source of thermal

stress, which normally leads to the spallation of the coating during the fabrication

stages. Moreover, if the Zirconia/Nickel content of slurry mixture exceeds fifty

percent, a flocculated slurry is produced which introduces surface irregularities

during application of the coating. These surface irregularities on the coating resulted

in uneven pressure application during pressure stamping stage of the SST. This

introduced stress concentrations within the surface of the coating, which eventually

led to cracking and spallation of the TBC during the sintering stage.

The current sintering method (oxy-acetylene torch) posed a major issue in the

development of the SST. With the variation in temperature along the coating surface,

thermal stresses are experienced along the surface resulting in greater chances of

spallation. Other sintering methods were considered to increase the coating

coalescence between the ceramic particles and apply a uniform heat distribution over

the coating surface, minimising the induced thermal stresses [Basu, Funke and

Steinbrech, 1999]. This led to the use of a high temperature oven and the

development of the automated sintering platform.

Automated Sintering Rig

To overcome the problems of uneven sintering of the TBC, an automated sintering

rig was designed by the candidate and fabricated at the University of Adelaide,

School of Mechanical Engineering. The sintering rig was designed to apply heat

uniformly to the surface of the coating, as opposed to from a stationary position

which focuses on the centre of the specimen. The design of the automated sintering


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rig consists of a platform, microcontroller package and a laptop for real-time

debugging of the automated software, as shown in Figure 3.13.

Figure 3.13: Automated sintering platform setup

Positional Oxy-Acetylene Torch

The first prerequisite of the automated sintering platform was that it allowed the

position of the oxy-acetylene torch to move freely, to enable the manipulation of the

flame position and height. This would take any changes in ambient temperature,

humidity and external forces into consideration. The automated sintering platform

allows the positioning of the oxy-acetylene torch along all three axes, allowing

substrates with various dimensions to be sintered

Sliding Platform

A sliding platform was designed such that the oxy-acetylene flame remained

stationary, while the substrate moved in a predetermined pattern on the sliding

platform, with the specimen being uniformly heated, as shown in Figure 3.14. The

platform can move in the X and Y direction, through the use of two stepper motors,

allowing substrates of various dimensions to be sintered. The position, distance and

speed of the stepper motors are controlled through a microcontroller package.

Laptop Microcontroller

Package

Automated Sintering Platform

Oxy Acetylene

Torch Holder


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Figure 3.14: CAD drawings of the moveable platform

Microcontroller Package

The microcontroller package contains a microcontroller and a motor controller,

connected to two stepper motors, which is attached to the rig. Information and

control is transferred into the microcontroller package from the laptop and

communicated to the rig. The sintering time can be set and adjusted using the

debugging software as well as the heating pattern, which can be set and modified to

the desired sintering characteristics.

The moving platform provides a uniform heat distribution to the coating surface

during the sintering stage. Figure 3.15 shows a typical heating pattern programmed

into the microcontroller, during use of the automated sintering platform for the

sintering stage of the SST. The heating pattern (as shown in Figure 3.15) used for the

sintering of TBC, produced coatings with greater levels of particle coalescence

throughout the coating surface. Along with the higher levels of particle coalescence,

fewer specimens were observed to spall during and after the sintering stage. This was

attributed to the even application of heat to the coating, effectively reducing the

residual thermal stresses of the coating. Since a noticeable improvement over the

original sintering method was observed, this sintering method and pattern was

adopted by the candidate for the fabrication of Slurry Based TBC over the use of the

oxy acetylene torch.


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Figure 3.15: Automated Sintering Rig Oxy Acetylene Torch Path

3.4.4 Development of Fabricating Parameters

After extensive research and experimentation in the fabricating stages of the Wet

Powder Spray (WPS) technique, the SST was developed. The developed technique is

comprised of five stages: slurry mixing, multi layered spraying, evaporation, pressure

stamping and sintering. The additional stage of pressure stamping was necessary to

reduce the long sintering times experienced with the original technique. In the

following section, each of stage of the SST will be summarised with the optimal

fabricating parameters of each stage shown.

Slurry Solution Mixture

Through experimental research and examination detailed in current and previous

sections (3.3.2), the optimum composition was determined for each of the key

components for the slurry spray mixture. The optimum percentage of ceramic and

Nickel powder, binder and dispersant determined is shown in Table 3.6.

Oxy acetylene Torch

Substrate

FinishStart


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Table 3.6: Optimum slurry mixture by volume

Ceramic and Nickel Powder Binder Dispersant Mixing Agent

45 % 3 % 0.4 % 51.6 %

Spraying process

The optimum parameters during the spraying process were identified after

experimental investigations of spraying parameters. These parameters include the

spraying height, compressor pressure and also the orifice size of the spray gun

nozzle, which are shown in Table 3.7 below. These parameters will directly affect

the area, thickness and the performance of the coating.

Table 3.7: Optimum slurry spraying parameters

Spraying Height Number of Grader Layers Coating Thickness

200 – 300 mm 2 – 3 50 – 150 µm

The most notable development of this stage was the addition of multilayered

spraying. This development effectively reduced the drying stresses during the

evaporation stage, allowing the fabrication of thicker coatings. The introduction of

FG–TBCs was observed to improve the coating’s resistance to failure during

fabrication by reducing the thermal stresses induced during the fabrication stage.


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Pressure Stamping

Compression pressures ranging from 10 to 60 MPa were systematically applied to

the coating specimens, and based upon coating failure during and after the sintering

stage, the optimum parameters were determined. Through examination of the

specimens after the pressure stamping stage, the optimum value of applied pressure

to the TBC specimen was determined to be 30 MPa.

Sintering

The introduction of the automated sintering rig effectively reduced the number of

specimen failures during the stages of fabrication, by minimising the thermal stresses

experienced by the TBC. With this sintering approach, significantly lower amounts

of coating failure occurring during the fabrication of the TBCs. This sintering

method was adopted for the fabrication of TBC with the SST over the oxy acetylene

torch.

3.4.5 Summary

From extensive investigation and research, the WPS progressed into the SST. With

the additional stage (pressure stamping) implemented in the SST, the increase in

quality of the produced TBC was evident with significantly lower coating failures

during the fabrication stage. The application of compression pressure to the

evaporated coatings and development of functionally graded coating layers was

found to greatly reduce sintering times and increase the coating resilience to peak

thermal stresses during and after the sintering stages. The peak thermal stresses were

reduced with the introduction of the automated sintering rig. This effectively reduced

the number of specimen failures during the stages of fabrication by minimising the

thermal stress experienced by the TBC. The primary differences between WPS and

SST are highlighted in Figure 3.16.


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(a) WPS

(b) SST

Figure 3.16: Technique Comparison of WPS and SST

(2) Multilayered Spraying (1) Mixing

(4) Stamping

Pressure

(5) Torch and Furnace Sintering

(3) Drying

(3) Drying (4) Torch Sintering

(2) Spraying (1) Mixing


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The addition of multi-layered spraying to the fabrication process, allowed the

production of FG–TBCs. This novel addition to the fabrication process opened up the

SST to a variety of applications. Therefore, FG–TBCs can be engineered to suit the

needs of the situation, and not limited by the material properties of the ceramic

coating.


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3.5 Conclusion

A literature review into current TBC fabrication techniques demonstrated that a niche

exists to benefit many industrial applications, as seen in Chapter 2, section 2.2. There

is a need for a new fabrication technique aimed at the depositing thermal protection

coatings over large complex surfaces in an economical manner.

The examination of the WPS technique was conducted through the analysis of the

previous experimental results conducted by [Ruder, Buchkremer, Jansen, Malléner

and Stöver, 1992], and new investigations. This led to the refinement of each of the

fabricating stages. These included the examination of the slurry solution mixtures,

spraying parameters, evaporation stages and sintering methods. While the WPS

technique were able to produce low cost TBCs, in terms of general structural

integrity, the coatings were of very low quality which include high levels of porosity

and low adhesive strength, which resulted in failure of the coating even during the

fabrication stages.

These deficiencies led to the modification of WPS technique and development of the

SST. The SST includes the new fabricating stages such as multi-layered spraying,

which allows the fabrication of FG–TBCs. With the addition of the pressure

stamping stage, the sintering times was reduced considerably and provide a higher

quality coating.

For the development of the SST, the effects of a number of fabricating parameters on

the quality of the coating were investigated and the optimal characteristics for every

stage of manufacturing determined through experimentation.

SLURRY SPRAYED THERMAL - University of Adelaide

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