repair of damaged mcraly coatings targeting petroleum...

Repair of Damaged MCrAlY Coatings Targeting Petroleum Industry Applications

by

Rabab Farhat

Master of Science

Mining and Materials Engineering

McGill University

Montreal, Quebec

March 2012

A Thesis submitted to McGill University in Partial Fulfillment of the Requirements of the Degree of Master of

Science in Materials Engineering

©Rabab Farhat, 2012

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Acknowledgement

First and foremost, I would like to express my deepest gratitude to Prof. Mathieu

Brochu (McGill University) for his support and continuous guidance throughout

my studies. Also, I would like to thank Jesus Portillo for his assistance in

performing the chemical analysis shown in Chapter 3.

I would like to thank the Libyan ministry of education and scientific research

(MOESR) for their financial support provided under the form of a complete

scholarship.

I would like to thank all the people from the Materials Engineering Department

and a special thanks to the NIAN-lab group, for their friendship, help, and

outings, which were very memorables. I cannot forget my officemates, and all

the good times we had together.

Finally, I would like to thank my parents, brothers, and sisters for their endless

love and absolute support at all times.

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ABSTRACT

The increase in efficiency of furnace and refinery components in petroleum

industries has been the target of many studies. However, the repair technology

for damaged pieces is still to be developed. During prolonged service, a

degradation of developed coatings occurs as a result of the harsh environment.

Therefore, a repair technology, which can extend the life of the coatings, is now

under consideration. In this work, electrospark deposition (ESD) has been

investigated to understand the solidification behavior and its possibility to

repair damaged MCrAlY coatings.

Ni-based alloys with different compositions were deposited on Ni substrate

using ESD to understand crystal structure of the solidified deposit and the effect

of the dissimilar weld composition on dilution. The electrode samples were

prepared by spark plasma sintering (SPS). Firstly, different coatings with single

and bi-phase microstructure were deposited on pure Ni substrate. Secondly,

NiCoCrAlY and CoNiCrAlY were deposited on the damaged spot of the oxidized

NiCoCrAlY and CoNiCrAlY respectively. A fine microstructure of metastable

phases obtained from each deposit. Also, it was found that an epitaxial growth of

NiCoCrAlY and CoNiCrAlY were obtained on the damaged spots. In addition, α-

Al2O3 was obtained on the surface of the deposit after 24hr oxidation at 10000C.

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RÉSUMÉ

L'augmentation de l'efficacité des fours et des composantes de raffinage dans

l’industrie du pétrole a été le sujet de nombreuses études. Cependant, la

technologie de réparation des pièces endommagées reste toujours à développer.

Durant un service prolongé, une dégradation des revêtements développés se

produit en raison de l'environnement hostile. Par conséquent, une technologie

de réparation qui peut prolonger la vie des revêtements est actuellement à

l'étude. Dans ce travail, le dépôt électro-étincelle (ESD) a été étudié pour

comprendre le comportement de la solidification ainsi que son potentiel pour la

réparation des revêtements MCrAlY.

Différentes alliages à base de nickel ont été déposés sur des substrats de

nickel via ESD afin de comprendre la structure cristalline du dépôt solidifié et

l'effet du changement de composition de la soudure sur la dilution. Les

échantillons d'électrodes ont été préparés par frittage flash (SPS). Tout d'abord,

différents revêtements avec une microstructure monophasée et biphasée ont été

déposés sur un substrat de nickel pur. Deuxièmement, du NiCoCrAlY et du

CoNiCrAlY ont été déposés à l'endroit endommagé sur les oxides de NiCoCrAlY

et de CoNiCrAlY respectivement. Une fine microstructure de phases métastables

a été obtenue à partir de chaque dépôt. Également, il a été constaté qu’une

croissance épitaxiale de NiCoCrAlY et de CoNiCrAlY a été obtenue sur les

surfaces endommagées et une couche de α-Al2O3 a été obtenue sur la surface du

dépôt après une oxydation de 24 heures à 10000C.

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Table of Contents

Acknowledgement .......................................................................................................................... ii

ABSTRACT ........................................................................................................................................ iii

RÉSUMÉ ............................................................................................................................................. iv

Table of Contents ............................................................................................................................ v

List of Figures .................................................................................................................................vii

List of Tables .................................................................................................................................... xi

Chapter 1: Introduction ............................................................................................................. 1

Chapter 2: Literature Review ................................................................................................. 3

2.1 High Temperature Coatings ....................................................................................... 3

2.1.1 Diffusion Coatings ................................................................................... 4

2.1.2 Overlay Coatings ..................................................................................... 4

2.1.3 Thermal barrier coatings (TBC) ............................................................... 5

2.2 Al-Containing Ni-based Superalloys........................................................................ 6

2.3 Oxidation of MCrAlY ...................................................................................................... 7

2.4 Spark Plasma Sintering (SPS) ................................................................................... 9

2.5 Coatings Lifetime ......................................................................................................... 10

2.6 The Failure Mechanism ............................................................................................. 11

2.7 Electrospark Deposition ESD .................................................................................. 15

2.7.1 ESD process and Coating Characteristics .............................................. 16

2.7.2 ESD Applications ................................................................................... 18

2.8 Electrospark Deposition As A Rapid Solidification Process. ........................... 20

2.9 Electrospark Deposition of MCrAlY Coatings ................................................... 21

2.10 Thesis Outline ............................................................................................................ 22

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Chapter 3. Solidification and Thermal Stability of Metastable Phases obtained

through Rapid Solidification in Al-Containing Ni-based systems ............................. 23

Preface ......................................................................................................................................... 23

3.1. INTRODUCTION .......................................................................................................... 24

3.2. EXPERIMENTAL PROCEDURES ........................................................................... 26

3.3. RESULTS AND DISCUSSION ................................................................................... 29

3.3.1 Solidification structure and microstructure analysis ............................... 29

3.3.2 Thermal decomposition of the metastable coatings ............................. 46

3.4. CONCLUSION ............................................................................................................... 47

Chapter 4. Utilisation of Electrospark Deposition to Restore the Local

Oxidation Resistance Properties in Damaged NiCoCrAlY and CoNiCrAlY Coatings

............................................................................................................................................................. 49

Preface ......................................................................................................................................... 49

4.1. INTRODCUTION ........................................................................................................ 50

4.2. EXPERIMENTAL PROCEDURES .......................................................................... 52

4.3. RESULTS AND DISCUSSION ................................................................................... 54

4.3.1 Characterisation of starting materials ....................................................... 54

4.3.2 Characterisation of the microstructure of the oxidised substrates ....... 55

4.3.3 Characterisation of the Electro Spark Deposits microstructure. ............ 59

4.3.4 Characterisation of the oxidation behaviour of the ESD repaired

location. ............................................................................................................... 62

4.4. CONCLUSION .................................................................................................................... 68

Chapter 5. Summary ................................................................................................................ 69

Chapter6. REFERENCES ....................................................................................................... 71

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List of Figures Figure 1. Family of coatings suggested for oxidation and/or corrosion

resistance…………………………………………………………………………………………………...….3

Figure 2. Comparison of the life of different coatings…………………………………….....6

Figure 3. A schematic of the SPS process……………………………………………………….10

Figure 4. Schematic of two possible pre-failure mechanism…………………………...12

Figure 5. Schematic of a typical TBC failure sequence………………….........................14

Figure 6. A schematic of the ESD process……………………………………….………………15

Figure 7. Schematic of single-phase solidification morphologies………………..…...20

Figure 8. XRD pattern of the starting Ni substrate………………………...………………..28

Figure 9. Equilibrium phase diagram obtained using FACTSage and metastable

phase diagram also modeled by FACTSage, and represented by the dash

line………………………………………………………………………………………………………….…..30

Figure 10. XRD patterns of the NiCr electrode (lower) and NiCr deposited on the

Ni substrate (upper)……………………………………...……………………………………………..31

Figure 11. SEM micrograph of (a) the NiCr deposit on the Ni substrate and (b) a

higher magnification micrograph depicting the columnar solidification

structure. …………………………………………………………………………………………………….32

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Figure 12. XRD patterns of the CoNiCrAlY electrode (lower) and CoNiCrAlY

deposited on the Ni substrate (upper)………………….……………………………………….33

Figure 13. SEM micrograph of (a) the CoNiCrAlY deposit on the Ni substrate and

(b) a higher magnification micrograph depicting the columnar solidification

structure………………………………………………………………………….…………………………..34

Figure 14. XRD patterns of the NiCoCrAlY electrode (lower) and NiCoCrAlY

deposited on the Ni substrate (upper)…………………………………………………..………36

Figure 15. SEM micrograph of (a) the NiCoCrAlY deposit on the Ni

substrate…………………………………………………………...……………………………….………38

Figure 16. Relation between dilution of the electrode material as a function of

the melting temperature difference between the electrode and the

substrate……………………………………………………………………………………………….……..40

Figure 17. XRD patterns of the NiCrAl electrode (lower) and NiCrAl deposited on

the Ni substrate (upper)…………………………….…………………………………………………41

Figure 18. Cross-section of a NiAlCr deposit on Ni substrate………………..…………42

Figure 19. XRD patterns of the NiAl electrode (lower) and NiAl deposited on the

Ni substrate (upper)………………………………….…………………………………………………44

Figure 20. SEM micrograph of a NiAl deposit on Ni substrate…………………………45

Figure 21. XRD spectra of the NiCoCrAlY material at different steps: starting

powder (lower), SPS consolidated sample (middle), and ESD (upper)…………....54

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Figure 22. XRD spectra of the CoNiCrAlY material at different steps: starting

powder (lower), SPS consolidated sample (middle), and ESD (upper)………..…..55

Figure 23. Micrographs of the substrates after the oxidation treatment of 24

hours at 10000C; (a) NiCoCrAlY and (b) CoNiCrAlY……………………..…………………56

Figure 24. XRD spectra of the substrates after the oxidation treatment: (a)

NiCoCrAlY and (b) CoNiCrAlY……………………………………………...………………………..57

Figure 25 surface morphology of the oxide scale on the substrates after

oxidation at 10000C for 24hrs (a) NiCoCrAlY and (b) CoNiCrAlY……………………58

Figure 26. Microstructure of a CoNiCrAlY substrate after simulated damaged and

repair with ESD process………………………………….……………………………………………60

Figure 27. Micrograph of a NiCoCrAlY weldment on a NiCoCrAlY substrate

depicting the change of microstructure from a two-phase substrate to a single-

phase weldment……………………………………………………………………...……………………62

Figure 28. Micrograph depicting the precipitation of the β phase from the

metastable γ solidification structure during the second oxidation

treatment……………………………………………………………………………………………………63

Figure 29. Micrograph presents the microstructure evolution of the repaired

NiCoCrAlY after the oxidation treatment of 24 hours at 10000C………….………….64

Figure 30. XRD spectra of the ESD deposit after 24hours at 10000C: (a)

NiCoCrAlY and (b) CoNiCrAlY……………………...……………………………………………..64

x

Figure 31 surface morphology of the oxide scale on the ESD deposit after

oxidation at 10000C for 24hrs (a) NiCoCrAlY and (b) CoNiCrAlY…………………..66

Figure 32. The calculated thickness of the depleted layer and oxide layer of both

the NiCoCrAlY and CoNiCrAlY substrates, ESD deposits and ESD deposits

exposed to grain growth prior to oxidation. …………………………………………….……68

xi

List of Tables

Table 1. ESD process Parameters……………………………………..…………………………17

Table 2. Substrate Alloys Coated by ESD to Date………………………………………….18

Table 3. ESD Coatings Applied to Date…………...……………………………………………19

Table 4. Nominal chemical composition of the starting powders…………………...27

Table 5. Quantitative chemical analysis of the spots highlighted in Figure

4a…………………………………………………………………………………………………….……….32

Table 6. Chemical analysis of a CoNiCrAlY deposit on Ni substrate………..……..35

Table 7. Quantitative Chemical analysis of a CoNiCrAlY deposit on Ni

substrate…………………………………………………………………………………...…………..…38

Table 8. Chemical analysis of a NiAlCr deposit on Ni substrate………………...…..43

Table 9. Chemical analysis of a NiAl deposit on Ni substrate…………….………….46

Table 10. Thermal decomposition of the metastable coatings……………………..47

Table 11. Nominal Chemical composition of starting powders……………………..52

Chapter 1 Introduction

1

Chapter 1: Introduction

In the petroleum industry, maintenance costs associated with the corrosion and

degradation of materials and equipment constitute a significant portion of the

operational budget of production facilities. Therefore, coating technologies have

been developed over the last 60 years to increase the lifetime of the power

components. High temperature coatings are used to protect bulk materials

against high temperature oxidation and hot corrosion. The coatings limit the

surface degradation of the material to increase its service time. Unfortunately,

like substrate materials, the coatings will also degrade during prolonged service.

Since the lifetime of coatings is mostly controlled by the extent of surface

degradation, coatings are typically prone to fail at hot spots, due to temperature

exposure above the design condition. The current way of dealing with damaged

coatings is by removing the entire coating and applying a new one[1]. The re-

coating method is expensive and causes frequent shutdowns of the refining

operation [2]. Hence, any development of a coating technology that can reduce

shutdown time, such as on-site localized repair of damaged coatings, is of great

interest.

Al-containing Ni-based superalloys, which are mostly used as the bond coat

in turbine blade applications for both in-land and aerospace engines, are

proposed for use in the petroleum industry, due to their very high resistance to

hot oxidation and corrosion [3, 4]. MCrAlY systems, where M= Ni, CoNi, NiCo, or

Co, could be used as standalone coatings due to its ability to form an aluminum

Chapter 1 Introduction

2

oxide layer, a very high oxidation resistance scale [5, 6]. MCrAlYs are commonly

deposited by conventional thermal spray methods such as vacuum plasma

spraying (VPS)[7-9], air plasma spray (APS)[10], and high velocity oxygen fuel

(HVOF)[9-15]. These coatings are known to form the α-Al2O3 scale to protect the

substrate. However, with time, the α-Al2O3 scale will fail either through cracking

due to thermal cycling or through the formation of undesirable mixed oxides or

spinel above the alumina layer. Therefore, a technique that can locally repair, i.e.

replenish the Al concentration of the damaged area is needed.

Electrospark deposition (ESD)[16] is a known technique capable of

repairing defective spots of surfaces. The process has an inherently low heat

input and high cooling rate, which helps to solidify deposits with homogenous

composition. The process is also known to exhibit lower cracking, and is hence

attractive for superalloys.

The main objective of this project is to investigate the possibility of

repairing MCrAlY coatings using ESD. To do so, two sub-projects were designed:

(1) understanding the solidification behavior of Al-containing Ni-based alloys to

evaluate the level of supersaturation and phase separation occurring in these

metastable deposits, and (2) to study the oxidation behavior of MCrAlY repaired

locations to investigate if the properties are better, similar or worse than bulk

MCrAlY samples. This thesis is structured as follows: a literature review chapter,

two papers presented as separate chapters, and a summary.

Chapter 2 Literature Review

3

Chapter 2: Literature Review

2.1 High Temperature Coatings

There are various types of protective coatings that are used to protect metallic

substrates against high temperature oxidation and hot corrosion [17, 18]. These

coatings fall into three basic types: diffusion coatings, overlay coatings, and

thermal barrier coatings (TBCs). Figure 1 shows a plot highlighting various

proposed compositions for applications needing oxidation or corrosion

resistance or both. The next few sections will describe these main types of

coatings.

Figure 1. Family of coatings suggested for oxidation and/or corrosion

resistance [19]


4

2.1.1 Diffusion Coatings

These coatings are widely used in gas turbine engines and turbine blades [20].

They are formed through a surface enrichment of a substrate by an alloying

element forming a protective oxide scale, such as Al (aluminides), Cr

(chromized), or Si (sililconized). Pt-modified diffusion aluminide is one of the

diffusion coatings that classically contains only a single phase (β), with Pt in

solid solution [21-23]. Furthermore, a combination of these elements is

occasionally used in some systems [5]. They can be applied by different

techniques such as pack cementation [24] and chemical vapor deposition [25].

Although diffusion coatings offer good protection against high temperature

oxidation [26], they do have a limitation due to their high ductile-to-brittle

transition temperature [27].

2.1.2 Overlay Coatings

These coatings are typically used in superior high-temperature applications [5,

28]. The most common overlay coatings are the MCrAlYs families, where M is

the base metal (Ni, Co, or both). They can be applied by different techniques

such as electron beam physical vapor deposition (EB-PVD) [29-31], and

conventional plasma spray techniques such as vacuum plasma spray (VPS) [7-

9], air plasma spray (APS)[10], and high velocity oxy-fuel (HVOF), being the

most commonly used technique [9-15]. The main advantage of overlay MCrAlY

coatings is that their chemical composition is not necessarily dictated by the


5

substrate composition, which is not the case for diffusion coatings [32].

Generally, overlay coatings perform better at higher temperature than diffusion

coatings [33], but its performance is obviously dependent on the deposition

process.

2.1.3 Thermal barrier coatings (TBC)

These coatings are widely used in gas turbines for propulsion and power

generation [34-37] to reduce the temperature imposed on the component,

prolonging its lifetime. They are a layer-based coating system, which is

composed of an outer ceramic coating, typically an yttria-stabilized zirconia YSZ,

which is deposited on the bond coat.

The morphology and the chemical composition of the bond coat are crucial

for the performance and lifetime of the TBC coatings as shown in Figure 2. These

bond coats, which are either diffusion coatings or overlay coatings, have a big

influence on the structure and morphology of the TGO [38].

In general, it can be noticed that Ni-based superalloys are used to protect

the hot components in high temperature applications due to their high melting

points and capability to resist hot corrosion. These coatings can be used as a

bond coat in TBC systems or as a standalone coatings depending on the targeted

application.


6

Figure 2. Comparison of the life of different coatings [28]

2.2 Al-Containing Ni-based Superalloys

Al-containing Ni-based superalloys have been used in this research project.

These coatings consist of single and dual phases as follows:

NiCr (100%γ)

NiAl (100%β)

NiAlCr (25%γ-75%β)

NiCoCrAlY (45%γ-55%β)

CoNiCrAlY (73%γ-27%β)

The variations in the amounts of nickel, cobalt, chromium, and aluminum

have been provided to understand the effect of the composition of each coating

on its performance and to tailor the coating to specific applications [28]. Al,


7

whose percentage is usually less than 14wt% [33], is the main element that

delays the oxidation and degradation of the coatings. The protection mechanism

is achieved by the formation of the -Al2O3 oxide scale. However, the existence

of the other alloying elements affects the performance of Ni-based superalloys,

where chromium is added to accelerate the formation of the α-Al2O3 scale [39].

Yttrium is also present in very low concentrations, to enhance the adhesion of

the TGO with the applied coating [40-42]. Moreover, the coating microstructure

also plays an important role in the development and growth of the TGO. The

TGO forms at the surface of the coatings upon exposure to high temperatures in

oxidizing environments. The formation of this scale is usually affected by the

characteristics of the MCrAlY and its surface preparation [43, 44]. This alumina

scale is fed by the β phase of the applied coating [43]; thus, the percentage of the

β phase in the coating is crucial. The impact of the β phase on the characteristics

of the formed alumina scale is clearly shown in chapter 4 of this thesis.

The formation of the protective oxide scale varies from one study to another

depending on the starting compositions and the testing environment [10, 11, 13,

44-46]. The variation in oxidation behavior of these alloys is not well

understood and needs more investigations. In this work, different Ni-based-

superalloys have been used to further investigate the effect of the starting

composition and experimental conditions on the formation of the alumina scale.

2.3 Oxidation of MCrAlY

The main goal in applying high-temperature alloys and metallic coatings is to

form a surface alumina scale, which protects the coatings from excessive


8

degradation and spallation [47]. In a perfect case, this oxide scale should be

dense, continuous, highly stable, free from cracks or pores, adherent, and

consistent. α-Al2O3 is the only desirable stable phase that has the ability to resist

hot corrosion in high temperature applications [47].

When MCrAlYs are exposed to high temperatures, an alumina scale initially

forms on the top of the bond coat and for extended periods of service duration,

the Al2O3 growth is followed by the appearance of undesired oxides such as

NiAl2O4, NiO, and other spinels. The oxidation of alumina-forming alloys occurs

through the formation of metastable phases such as γ-phase and θ-phase in the

early stage of the oxidation or when it oxidizes at low temperatures [39, 47-49].

Many studies have reported that the α-Al2O3 forms at longer oxidation periods

and at higher temperatures. However, the formation of this oxide scale and its

transformation mechanism to the stable phase depend on the composition and

the microstructure of the applied coating, including the Al content and the grain

size, the deposition technique, and the followed process conditions [30, 50-56] .

Different researchers have reported that nanostructured coatings can form

a more consistent and continuous alumina layer than their conventional

counterparts [44]. The refinement of the grain size results in an increase of the

grain boundary areas that can assist and accelerate the aluminum diffusion. The

Al content in the coatings also plays an important role in the alumina formation

mechanism.


9

Mercier et al. [43, 57] have studied the oxidation behavior of NiCoCrAlY and

CoNiCrAlY coatings. It was shown that the nanostructured NiCoCrAlY and

CoNiCrAlY coatings developed a single phase α-Al2O3 scale whereas the coatings

with micro-scale microstructure developed a duplex oxide scale, which is

composed of an inner Al2O3 layer and an outer layer of mixed oxides. They have

also shown that the oxidation of polished coatings developed a monolayer oxide

scale. These coatings were deposited by HVOF technique where the droplets

exposed to high temperature during spraying promote the formation of mixed

oxides. It was found that the oxides formed during spraying have a pronounced

impact on the formation of the alumina scale [43, 52]. The substrates used in

this research were fabricated by spark plasma sintering (SPS), where fully dense

and pore-free samples were obtained. The sintering was done under vacuum, so

no artifacts modifying the oxidation behavior should be present.

2.4 Spark Plasma Sintering (SPS)

Spark plasma sintering (SPS) is a relatively new rapid sintering technique. This

process is used to consolidate a powder bed under vacuum in short time to yield

dense bulk samples. The sintering of the powder bed is based on high

temperature (spark plasma) which generated within the gaps between the

particles by the electric discharge [58]. The pulsed electric current passes

through the conductive graphite die and punches containing the pressed

powder. This current results in heating of the contact points between the


10

particles due to the Joule effect. This self-heating style results in a rapid heating

rate, which can reach 1000 0C/min and terminates with high density

components [59]. A general schematic of the SPS apparatus is shown in Figure 3.

Figure 3. A schematic of the SPS process [60]

The SPS process occurs under vacuum, thus eliminating the effect of

internal oxidation that could be present in other conventional sintering

processes.

2.5 Coatings Lifetime

In coatings developing a surface Al2O3 scale as a protection mechanism there are

several failure mechanisms that can explain the degradation of the coating

which depend on the different testing and operating scenarios. In general, the


11

failure is correlated with imperfections located at the interface between the

applied coating and the oxide scale, especially the large residual compression

stress in the TGO from its thermal expansion misfit or from the stresses that

arise during the TGO growth [34].

Consequently, to try to eliminate the problem, many studies were intended

to extend the life of MCrAlYs by using nanostructured coatings, due to the better

matching of their physical and mechanical properties, and also their ability to

produce more consistent alumina scale [11, 44, 54, 61] . However, during the

service time of the coating system, the coatings will be in the micron size region

due to grain growth arising from the exposure to the hot environment. In this

circumstance and after long times, the coatings may behave as conventional

coatings do and then may start to spall in the highly stressed areas, often

associated with hot spots.

2.6 The Failure Mechanism

The failure mechanism of the standalone Ni-based superalloys is not discussed

in the open documented literatures. However, the failure in the TBC system,

where the Ni-based superalloys are used as bond coats, is well deliberated.

There are two models that demonstrate the TBC failure, based on the

microstructure of the applied coatings. These models, as shown in Figure 4,

display the mechanism of the pre-failure that happens during service [62].


12

Model I presents the case when the applied coating is strong, but its interface is

weak. The pre-failure develops at the interface through delamination cracks.

The detachment of the coatings occurs as a result of the formation of these

cracks. This model assumes that the cracks form in the TGO layer or occur in the

surface scale and go through the coating until a complete detachment of the

coating is observed. On the other hand, Model II presents the case when the

applied coating is weak, but its scale/coating interface is strong. The pre-failure

develops through cracks which are perpendicular to the interface; these cracks

extend to the interface where they cause detachment of the coating.

Figure 4. Schematic of two possible pre-failure mechanism [62].


13

The failure mechanisms may differ for different TBC systems, depending on

the structure of the TBC and its area of application. Correspondingly, the

character of the TGO is crucial for the durability of the coating system.

Consequently, the failure mechanism of the standalone MCrAlY coatings is

described in model I, where the defects happen on the TGO scale. The defects

occur as cracks in the TGO scale due to the applied stress or as a result of the

complete consumption of the β phase. When the Al concentration reaches a

critical minimum value, the depleted coatings will form other oxides like Cr2O3

and/or other spinels besides the alumina scale. The time when a complete

consumption of the β phase occurs is believed to be the lifetime end of the

coatings [63].

Also, the failure of the coatings is correlated with the formation mechanism

of the thermally grown oxide scale. Upon exposure to high temperature, the TGO

forms on the surface of the bond coat [BC] in the TBC system. The thickness of

TGO increases with the exposure time until reaching the parabolic growth law

[34, 45, 64, 65]. It has been reported that TBCs fail when the TGO reaches a

critical thickness which is in the range of 5.5 µm [66]. As the TGO thickens, small

cracks nucleate at developed stress spots near or in the TGO. These cracks

extend and coalesce with time causing buckles or delamination that ends with

spallation at these spots. However, the TBC remains in the remnant ligaments as

shown in Figure 5 [38].


14

Figure 5. Schematic of a typical TBC failure sequence [38]

In general, the spallation and the failure of the TBC system and MCrAlY

standalone coatings occur in the thermally grown oxide TGO scale. This

spallation occurs in the highly stressed hot spots only. Therefore, it’s highly

appreciated to repair these defects by use of a cost-effective deposit technique

to prolong the life of the TBC.


15

2.7 Electrospark Deposition ESD

Electrospark Deposition (ESD) is a low heat input, high energy density, micro

welding process that is traditionally used to deposit, repair, and extend the life

of wear and corrosion resistance coatings [16, 67]. A schematic of the process is

shown in Figure 6.

Figure 6. A schematic of the ESD process [68]

ESD process consists of a rotating or vibrating electrode of the desired

composition that melts due to the short duration of the electric pulses ranging

from milliseconds to microseconds and deposit onto the substrate [16]. The

short pulse duration and high-pulse frequency, which ranges between 0.1-4

kilohertz [69], results in the transfer of a small amount of material during each

pulse. The approximate thickness per pass varies between 1-15µm, which is

function of the deposition parameters and the system [16]. The low heat input,


16

which reduces the heat affected zone (HAZ), high cooling rate which reaches

105-106 K/s [69], and the ability to produce metallurgical bonding, render this

technique attractive for applications in corrosion and wear environments. ESD

can also be applied for restoration and refurbishment of damaged coatings,

tools, or equipment.

In the ESD process, the electrode material solidifies quickly due to the short

duration of the electrical arc between the working electrode and the substrate.

Therefore, the rapid solidification of the deposit restricts the chemical diffusion

and results in a non-equilibrium fine homogenous microstructure. The change

of the chemical composition leads to an extension of the solid solubility of the

resulting phase in some systems. In general, the character of the solidified

deposit depends on the solidification parameters and the alloy system used.

2.7.1 ESD process and Coating Characteristics

ESD process is well known as a rapid solidification technique, which when

appropriately controlled, can result in metastable structures, extension in solid

solubility, quasi-crystalline structures, amorphous deposits, and nanostructured

deposits, depending on the alloy system that is used [1, 70-78]. As experienced,

the deposition parameters have a huge impact on the characteristics of the

deposit and the efficiency of the process as listed in Table 1 [16]. Although ESD

is one of the easiest and fastest processes to be used, it is necessary to sustain a

continual light electrode motion during deposition to prevent grinding the

surface of the substrate by the electrode or sticking it to the substrate surface. In


17

addition, the deposition environment has a high impact on the features of the

deposit. Therefore, a change in any of the process parameters will affect the

quality of the deposit.

Moreover, the high cooling rate and low heat input inherent to ESD allow

the repair of materials that have high tendency to crack during welding. It is

worth mentioning that ESD is only applicable when the defect region is in the

millimeter range.

Table 1. ESD process Parameters [16]

Electrode Substrate Environment Electrical Others

Material Material Gas

Composition Power Input

System

Efficiency

Geometry Surface finish Flow rate Voltage Number of

Passes

Motion Cleanliness Temperature Capacitance Overlap of

Passes

Speed Temperature Flow

Geometry Spark Rate

Spark

Duration

Contact

Pressure Geometry

Orientation


18

2.7.2 ESD Applications

The ESD process has been used to deposit a wide range of materials, of both

similar and dissimilar material combinations. Nearly any electrically conductive

material which can be melted through an electric current is suitable for ESD [67,

69, 79, 80]. For instance, metals, carbides, borides, intermetallics or cermets,

and hard alloys such as MCrAlY and Inconel that are considered to be

unweldable by conventional welding techniques due to their high cracking

tendency, can be deposited by ESD[1, 70, 71, 73]. Table 2 and 3 show a list of

various substrate and coating material combinations reported possible by ESD

process.

Table 2. Substrate Alloys Coated by ESD to Date [69]

High and Low Alloy Steels

Stainless Steels

Tool Steels

Zirconium Alloys

Nickel and Cobalt Alloys

Titanium Alloys

Aluminum Alloys

Copper Alloys

Refractory Metals (W,

Re, Ta, Mo, Nb)

Chromium

Uranium

Erbium


19

Table 3. ESD Coatings Applied to Date [69]

Wear Resistance Coatings Corrosion Resistance

Coatings

Build-up or Social

Surface Modification

Hard carbides(a) of: W,Cr,

Ti, Ta, Hf, Mo, Zr, V, Nb

Stainless steels,

Hastelloys(b), Inconel(b),

Monels(b)

Ni-base and Co-base

superalloys

Hardfacing alloys:

Stellites(b), Tribaloys(b)

Colmonoys(b), etc

Aluminides of: Fe, Ni, and

Ti

Refractory Alloys (W,

Ta, Mo, Nb, Re, Hf)

Borides of: Cr, Ti, Zr, and Ta FeCrAlY, NiCrAlY,

CoCrAlY

Noble metals (Au, Pt,

Ag, Pd, Ir)

Intermetallics and Cermets

Al and Al Bronze Alloys

Other Alloys (Fe, Ni,

Cr, Co, Al, Cu, Ti, V, Sn,

Er, Zr, Zn)

(a) With metal binders. Usually 5-15% Ni or Co

(b) Trademarks: Hastelloy- Haynes International. Kokomo. IN

Inconel & Monel- International Nickel Co., Huntington. WV

Stellite & Tribaloy- Deloro-Stellite Co., Goshen. IN

Colmonoy- Wall Colmonoy Corp., Detroit. MI


20

2.8 Electrospark Deposition As A Rapid Solidification Process.

In general, rapid solidification processes have become important industrial

techniques due to their ability to refine the microstructure, extend the solid

solubility, suppress the microsegregation, and form the metastable phases in the

solidified solid [81]. The rapid solidification limits the time for melts to

equilibrate thus it inherently favors the formation of non-equilibrium phases.

The solidification microstructure is directly influenced by the solidification

conditions, specifically the temperature gradient G and solidification rate V [82].

Figure 7 shows different solidification morphologies and their dependence on

the characteristic solidification conditions G and V, where G/V value influence

the growth morphology and G*V determine the scale of microstructure with its

arm spacing [70].

Figure 7. Schematic of single-phase solidification morphologies [82].


21

During the electrospark deposition, the cooling rates may reach 105-106 C/s,

thus a homogenous deposit can be obtained, depending on the equilibrium time

of the liquid droplet prior to solidification. However, the solidification

microstructure can be varied depending on the system used and the

solidification conditions. Also, the high cooling rate or the short time exposure

to high temperature during rapid solidification widens the applications of these

processes mainly for the superalloys which are considered unweldable [70].

2.9 Electrospark Deposition of MCrAlY Coatings

Electrospark deposition (ESD) has been successfully used to deposit MCrAlY

coatings [1, 70-73, 78]. Xie et al [1] have shown the possibility of depositing

NiCoCrAlTaY on IN-792. It was found that the deposition rate increased with

increasing spark pulse energy, but the surface roughness also increased

correspondingly. Columnar γ phase structure was observed, which indicates

that the ESD can be used to form a homogenous coating of single γ phase from

an electrode with a dual phase γ/β structure. After oxidation at 1000 °C for

100h, a dense and adherent α-Al2O3 scale was observed on the top of the deposit

with an upper scale of θ-Al2O3 and spinel. The composition of the scale shows

the capability of the ESD to form homogenous coatings with the ability to

incorporate the Al in the microstructure.

Goodall et al [78] have shown the formation of nanostructured deposits. In

this work, nanostructured γ/β NiCoCrAlY was used as an electrode and


22

substrate material. A homogenous nanostructured γ phase structure was

obtained. The formation of the nanostructure of the deposit is referred to the

ultrafast solidification of the molten metal. This high cooling rate was sufficient

to form a superfine grain structure with a preferential growth that matches the

cellular structure. After oxidation at 1000 °C for 6h, a consistent α-alumina was

observed with some traces of θ-alumina on the top of it.

Due to the wide variety of the MCrAlY compositions, results can be different

from one study to another. Also, the difference in the deposition parameters

plays an important role in the performance of ESD.

2.10 Thesis Outline

This thesis is a manuscript-based thesis that contains two manuscripts. Each

manuscript is presented in a separate chapter in its as-submitted format. A

preface is added for the two manuscripts to demonstrate its connection to the

previous chapter and to show the contribution to original knowledge of the

manuscript. Due to the format of the thesis, the introduction of each manuscript

may contain the same information as the one present in the literature review.

The thesis ends with a summary for the entire thesis and its contribution to

original knowledge.

Chapter 3 Solidification and Thermal Stability of Metastable Phases obtained through Rapid Solidification in Al-Containing Ni-based systems

23

Chapter 3. Solidification and Thermal Stability of Metastable Phases obtained through Rapid Solidification in Al-Containing Ni-based systems R. Farhat, J. Portillo and M. Brochu1

1Mining and Materials Engineering Department, McGill University, 3610 University

street, Montreal, QC, Canada, H3A 2B2

Preface This chapter demonstrates the feasibility of using the electro-spark deposition to

deposit Al containing Ni-based superalloys on Ni substrate. The protective Ni-based

superalloys which were damaged by in-service hot corrosion will require a repair by

a process that has the ability to restore the fine microstructure to provide a

consistent and continuous alumina scale and prove the extending of the lifetime of

the coatings. After the spallation of the thermally grown oxide (TGO), the depleted

zone of the damaged coatings will be exposed to the hot environment. This region is

the interested situ for reparation, which presents the γ phase. Depending on this

fact, Ni substrate, which represents the γ phase, was chosen to be the substrate for

NiAl, NiCr, NiAlCr, NiCoCrAlY, CoNiCrAlY electrodes. The deposition process was

done under the same parameters to study the effect of electrode composition on the

characterization of each deposit.


24

3.1. INTRODUCTION

Aluminum-containing Ni-based superalloys are designed to resist hot corrosion and

high temperature oxidation. MCrAlY alloys, where M is Ni, CoNi, NiCo or Co are one

of the most important coating materials used as a bond coat in gas-turbine-engine

disc components for land-based power generation and air craft propulsion [83].

These alloys mainly consist of a γ phase (Ni- or Co-based solid solution), with either

a β phase (NiAl) and/or a γ’ (Ni3Al) strengthening precipitates. The phases present

is dictated by the chemical composition of the alloy. It has been shown that rapid

solidification processing is capable to form high quality coatings with fine and/or

metastable microstructures, which could improve control of phase evolution during

decomposition of the metastable microstructure. Previous works on rapid

solidification of Ni-based superalloys include techniques such as laser cladding [84],

melt spinning [85], and electro-spark deposition [16]. Rapid solidification

microstructures often deviate from the prediction made from equilibrium concepts,

thus the analysis of the microstructure should be carried out using metastable phase

diagram concepts for which two primary hypotheses are considered: homogeneous

composition in the liquid and no atomic diffusion in the liquid nor the solid is

occurring during solidification [74].

Electro-spark deposition (ESD) is a rapid solidification process used to deposit

unweldable materials or materials that tend to crack during welding [70]. ESD is a

pulsed arc-micro welding process that uses short-duration high-current electrical

pulses to deposit an electrode material on a metallic substrate [16]. The process


25

uses high pulse frequency in the 0.1 to 4-Kilohertz range applied for a few

microseconds which allow dissipating the heat up during approximately ~99% of

the duty cycle. The outcome is a high cooling rate that may is quoted to reach 105 to

106 C/sec [86]. The combination of low heat input and small volume of deposited

material, hence the high cooling rate results in ‘’metastable’’ structures such as non-

equilibrium phases, extension of solid solubility of the system, amorphous deposit,

and nanostructured deposit [1, 70-78]. However, the comparison of ESD

solidification structures with metastable phase diagram is not as straight forward

like in the melt spinning process. In ESD, the electrical arc between the working

electrode and the substrate is present for only micro- to milliseconds; hence the

time for ordering of the liquid will be significantly reduced. Consequently the actual

chemical composition may vary for each droplet and will differ from the equilibrium

melt, which may result in variation in the rapid solidification microstructures for

multi-weld deposit.

Compared to the conventional welding processes, ESD suffers from a significant

lack of predictability in terms of phase composition after solidification in dissimilar

materials welds. Several reasons are reported to explain this phenomenon, such as

evaporation of one or more constituent from either the electrode or the substrate,

and/or the splashing effect arising from the movement of the electrode and/or

interaction with the environment [87]. Consequently, in dissimilar materials

combinations, the electrode materials and the deposit generally have different

compositions, and this observation was reported for various combinations: Ag-Ni on

Cu [87], WC92-Co8 on steel and Ti [88], Vitreloy 1 on AlCeCo alloy [75].


26

Unfortunately, in all these works, no relationship underlying the deposition

parameters, the electrode and substrate composition to the composition of the

deposited materials could be extracted, hence the need to generate a larger

database of experimental results to try to elucidate trends and behaviors. This is

strengthened by the lack of understanding of the transfer mechanism in ESD, which

definitely complicate the study.

This work investigates the correlation between the starting electrode crystal

structure, the solidification microstructure and their relationship with metastable

phase diagrams for various Ni-based compositions deposited using the ESD process.

Five different alloys possessing various ratios of γ and β (NiCoCrAlY, CoNiCrAlY,

NiAl, NiCr, and NiAlCr) have been deposited on Ni substrate (γ). Particularly, the

solidification structure, chemical composition and the solidified phases, were used

to experimentally verify the non-equilibrium predictions and their thermal stability.

3.2. EXPERIMENTAL PROCEDURES

Different powder mixtures were used to produce the single and two-phase

microstructures investigated. Table 4 presents the chemical composition,

equilibrium phase composition and their ratio for the investigated systems. The

NiCoCrAlY and CoNiCrAlY powders were supplied by Paraxair surface technologies

Cs36-65 (NI-171) and Sultzer-Metco (9954), respectively. The NiAl and NiCr

powders were supplied by Alfa Aesar and were used as control composition for the


27

γ and β trials, respectively. The NiAlCr composition was obtained by mixing NiAl

and NiCr powders. Fully dense pucks, precursor to the electrodes, were obtained by

spark plasma sintering using a Thermal Technology SPS 10-3 press. The electrodes

were machined from the consolidated pucks. The substrate used during the ESD

trials was commercial pure Ni (γ). The Ni substrate (17 x 12mm) was grinded to 800

grit followed by cleaning in acetone. The XRD spectrum of the Ni starting plate is

presented in Figure 8. As expected, the pattern is in accordance to the JCPDS card

001-1258. Based on the similarity in the reflections, the pattern suggests a fully

random texture.

Table 4. Nominal chemical composition of the starting powders.

MCrAlY Compositional %

Ni Co Cr Al Y Phase %

NiCr 80 - 20 - - 100 γ

CoNiCrAlY 32 38.5 21 8 0.5 73γ -27 β

NiCoCrAlY 46.5 23 17 13 0.5 45γ- 55β

NiAlCr 75 - 10 15 - 25 γ-75β

NiAl 70 - - 30 - 100β


28

Figure 8. XRD pattern of the starting Ni substrate.

The deposition experiments were accomplished using a SparkDepo 500 ESD

machine. The microwelding trials deposits were performed in an argon atmosphere

to prevent oxidation. The processing parameters were 100V, 390Hz, and 210µF, and

kept constant for all deposits. Assuming full discharge and recharge of the

capacitors at cycle, a single spark energy of 1.05J was determined using E=1/2 CV2

[71]where C is the capacitance in farads (F) and V is the voltage in volts (V).

The deposited microstructure was observed by x-ray diffraction (XRD) and Field

Emission Scanning Electron Microscopy (FE-SEM) analysis. The XRD was performed

using a Bruker D8 discovery X-ray diffractometer, to identify the phase composition

of the substrate, electrodes and the deposits before and after heat treatment. The

FE-SEM was performed on cross- section of each deposit using a Hitachi 4700

microscope equipped using backscattered (YAGBSE) imaging mode and Philips XL-


29

30 FE-SEM for the chemical analysis. Putronic grade (>99.995%) metallic samples of

Ni, Al and Cr were used as standard for quantitative chemical analysis.

Since metastable solidification structures were obtained from the NiCoCrAlY,

CoNiCrAlY, NiAl, and NiAlCr electrodes, the coatings were heat treated to evaluate

the thermal stability of the solidified non-equilibrium structure. The ESD deposit

were heated in a vacuum furnace (mechanical pump), in the temperature range of

400-8000C for 5min holding time. After heat treatment, the samples were analysed

by XRD and the patterns were compared to the as deposited spectra.

3.3. RESULTS AND DISCUSSION

3.3.1 Solidification structure and microstructure analysis

3.3.1.1 Solidification of a 100% FCC electrode (Ni-20Cr)

This series of trials was used as control experiments to evaluate the solidification

behaviour of a single-phase FCC electrode (Ni-20Cr material) on a pure Ni sample.

Considering the rapid solidification nature of the ESD process, the metastable phase

diagram for a Ni-rich Cr system was modeled and superimposed (dashed line) on

the corresponding equilibrium diagram, and the results are presented Figure 9. The

metastable phase diagram indicates that for this composition, even if rapid

solidification is imposed, the FCC structure should be obtained.


30

Figure 9. Equilibrium phase diagram obtained using FACTSage and metastable

phase diagram also modeled by FACTSage, and represented by the dash line.

Figure 10 shows the XRD patterns of the starting electrode and the ESD deposit,

respectively. Firstly, it can be observed that the electrode mostly possesses a γ (111)

orientation owing to the fact that the electrode was processed using powders, which

possess this crystallographic orientation arising from the atomisation process. With

respect to the deposit, the relative intensity of the peaks shifts from a γ (111)

orientation to the γ (200) orientation. This most intense diffracted peak is indicative

of a preferential orientation along the 1 0 0 crystallographic direction. This

preferential orientation coincides in fact with the columnar growth direction of the

FCC materials. In such system (γ deposited on γ), the rapid solidification imposed by

the ESD process would only affect the size of the solidification structure, but not its

crystallographic structure nor its solidification direction.


31

Figure 10. XRD patterns of the NiCr electrode (lower) and NiCr deposited on

the Ni substrate (upper)

Figure 11a presents a SEM micrograph of a multi-pass deposit on the Ni

substrate. Epitaxial growth was observed as the grain structure is continued after

the interface, which can be better imaged in Figure 11b. The later micrograph also

depicts the high quality of the coating, free of cracks and solely containing few sub-

micron pores. Table (5) shows a quantitative chemical analysis corresponding to the

points highlighted in the micrograph of Figure 11a. Spots analysis A and B show

high purity Ni, as expected since the analysis were performed in the parent material.

Spot C was taken near the interface and is showing enrichment in Cr, up to 6.5%.

This analysis would suggest a certain level of dilution during ESD. Going further into

the coating, the concentration in Cr increases progressively to reach a plateau near

the spot analysis F. As depicted, the equilibrium concentration is slightly enriched in

Ni, when compared with the starting electrode composition.


32

(a) (b)

Figure 11. SEM micrograph of (a) the NiCr deposit on the Ni substrate and (b) a

higher magnification micrograph depicting the columnar solidification

structure.

Table 5. Quantitative chemical analysis of the spots highlighted in Figure 4a.

NiCr Ni Cr

Point A 99.9 0.1

Point B 99.8 0.2

Point C 93.5 6.5

Point D 85.6 14.4

Point E 84.1 15.9

Point F 81.6 18.4

Point G 81.6 18.4

Point H 81.3 18.7

Electrode 80.0 20.0


33

3.3.1.2 Solidification of a 73% FCC/ 27% BCC (CoNiCrAlY)

The x-ray diffraction of the starting electrode and coating are shown in Figure 12. It

can be seen that the electrode is mainly consisting of γ/β phases, where the γ (111)

structure is the main component. The solidified structure is mainly composed of the

γ (200) orientation, corresponding to a preferential solidification orientation along

the <100> crystallographic direction. This preferential orientation matches with the

dendritic growth direction of the FCC material structure.

Figure 12. XRD patterns of the CoNiCrAlY electrode (lower) and CoNiCrAlY

deposited on the Ni substrate (upper).

Figure 13a presents a low magnification cross-section micrograph of the ESD

deposit while Figure 13b shows a higher magnification micrograph of the ESD

deposit, depicting columnar grains. Also, as shown, this multipass weldment is

characterised with a low level of porosity and defects. The quantitative chemical

spot analysis values corresponding to the location shown in Figure 13a are listed in


34

Table 6. The spot analyses A and B are from a location near the interface, while the

Spot C, approximately 10 microns within the coating, are highlighting the dilution

region arising from the mixing of the substrate and the electrode material. Based on

the chemical analysis of this dilution region, the Ni-Co-Cr solid solution would

inherently have a γ primary solidification structure, which should not be affected by

imposing rapid solidification. Also, the low concentration in Al detected should be

incorporated in the solid solution. Consequently, the similarity in the crystal

structure allow for the development of an epitaxial layer followed by a columnar

growth of γ dendrite throughout the mixing region, as observed in Figure 13a.

Starting from spot analysis D, the chemical composition reached a steady chemical

composition. As depicted by comparing the starting electrode chemical composition

with the steady state chemical composition of the coating, an enrichment in Ni is

also observed, like for the NiCr system, but the dilution effect is more pronounced.

(a) (b)

Figure 13. SEM micrograph of (a) the CoNiCrAlY deposit on the Ni substrate

and (b) a higher magnification micrograph depicting the columnar

solidification structure.


35

Table 6. Chemical analysis of a CoNiCrAlY deposit on Ni substrate.

CoNiCrAlY Ni Co Cr Al

Point A 79.5 10.8 6.2 3.6

Point B 52.6 26.1 14.6 6.8

Point C 39.1 33.8 18.6 8.5

Point D 41.6 32.3 18.0 8.1

Point E 39.9 33.0 18.6 8.5

Point F 41.7 32.4 18.0 8.0

Point G 41.3 32.6 18.0 8.2

Electrode 32.7 37.7 20.9 8.8

In the equilibrium solidification of the CoNiCrAlY alloy, as reported by Bezençon

et al [84], the primary solidification is γ, which is similar to the solidification

structure of the dilution region. Since no significant change in composition is

occurring and the same crystal structure for the substrate and the electrode is

present, this will allow for the sequence of epitaxial layer followed by a columnar

growth of γ dendrite, as shown in Figure 12. Similarly to the NiCr system, the

utilisation of a rapid solidification process will have no influence on the primary

solidification phase and only the size of the solidification structure will be affected.

In addition, the solidification condition inherent to ESD, i.e. temperature gradient

and solidification velocity, caused that solute rejection of the element forming the β


36

phase in CoNiCrAlY is negligible; hence the absence of β peaks in the ESD spectra

shown in Figure 12.

3.3.1.3 Solidification of a 45%FCC/55%BCC electrode (NiCoCrAlY)

Figure 14 depicts the XRD pattern of the electrode and the coating. It can be

observed that the electrode has both β and γ phases with a major peak of β (110)

orientation, consistent with the crystal structure ratio of the electrode material. The

primary structure shifted to the γ (200), which corresponds to the preferential

orientation along the <100> crystallographic direction known for FCC systems.

Figure 14. XRD patterns of the NiCoCrAlY electrode (lower) and NiCoCrAlY

deposited on the Ni substrate (upper).

Figure 15 presents a cross-section micrograph of the ESD deposit, depicting

columnar grains. Also, as shown, this multipass weldment is characterised with a

low level of porosity and defects. The quantitative chemical spot analysis values

corresponding to the location shown in Figure 15 are listed in Table 7. The spot


37

analyses A and B correspond to the dilution region arising from the mixing of the

substrate and the electrode material. The chemical analysis reveals an enrichment

in Co and Cr while moving further into the deposit. Since phase equilibrium

possessing this range of Ni-Co-Cr composition is a solid solution, it is expected that a

similar phase will be present when rapid solidification is used. Like for the

CoNiCrAlY sample, the low concentration in Al present in this region should be part

of the solid solution. Consequently, the similarity in the crystal structure allow for

the development of an epitaxial layer followed by a columnar growth of γ dendrite

throughout the mixing region, as observed in Figure 15.

Starting from spot analysis C, the chemical composition reached a steady

chemical composition throughout the remaining thickness of the deposit. In the

equilibrium solidification of the NiCoCrAlY alloy, as reported by Bezençon et al [84],

the primary solidification in this alloy is β. They also reported that when welded

onto a γ substrate, the dilution and mixing produces a transition of solidification

structure from γ dendrites to β dendrites. However, in the present case, no

indication of the presence of the β phase was observed in the XRD shown in Figure

14. One observation to be highlighted is the significant dilution observed between

the plateau and the electrode material. When translated to the corresponding phase

diagram [84], this composition gets really near the eutectic point of this system, and

by extrapolating the concept of metastable phase diagram; the beta primary

solidification is believed to be suppressed by the high cooling rate as the alloy is

near-hypereutectic. Finally, similarly to the CoNiCrAlY system, the rapid


38

solidification of the ESD process is believed to inhibit solute rejection of the Al,

hence the absence of β peaks after complete cooling.

Figure 15. SEM micrograph of (a) the NiCoCrAlY deposit on the Ni substrate

Table 7. Quantitative Chemical analysis of a CoNiCrAlY deposit on Ni substrate.

NiCoCrAlY Ni Co Cr Al

Point A 91.3 3.3 2.6 2.8

Point B 72.1 10.9 9.2 7.8

Point C 66.5 13.2 11.2 9.1

Point D 67 12.8 11.2 9

Point E 62.4 15.1 12.7 9.8

Point F 64.1 14.2 12.3 9.4

Point G 64.1 14.3 11.8 9.8

Electrode 47.9 20.5 17.9 13.7


39

As observed for the three system presented, a certain level of dilution is

occurring during ESD. Previous researchers have reported that the difference in

melting point between the substrate and the electrode is responsible for this

dilution [88]. This dilution was associated with an increased lost in electrode

material through evaporation and splashing [87]. Figure 16 shows the relationship

between the plateau composition measured for the NiCr, CoNiCrAlY and NiCoCrAlY

alloys. Unfortunately, since very few reports on ESD contained reliable chemical

composition analysis, only one additional point (ESD of Ag on Cu [87]) could be

added to highlight trends. The first observation arises for the single-phase electrode,

namely NiCr and Ag. The two-point trend seem to show that for a single phase

material, a relatively constant dilution of composition of up to 2wt% is observed in

the coating, and that the difference of temperature between the substrate and the

electrode seems to have no influence. The trend is very different for two-phase

materials, like the γ/β alloys used in this study. With increasing the temperature

difference between the solidus of the alloy and the substrate melting temperature,

the level of dilution seems to increase linearly. The explanation for this increased is

believed to come from the increased loss of the solid phase (β phase in the present

case), which is expelled once the lower melting point phase (γ phase in the present

case) starts melting. With the reduction of the volume fraction of matrix (γ phase),

and increased lost of the solid phase is occurring, hence the higher dilution for the

NiCoCrAlY system than for the CoNiCrAlY system. The trend observed in Figure 16

for two-phase materials is also strengthened by the work of Paustovskii et al [89,

90], in which they measured for various Ni-Cr-Al-Y alloys deposited on steel that the


40

ratio of electrode loss to cathode gain during ESD increases as the melting

temperature difference between the substrate and the solidus of the electrode

increases. However, more work must be performed in ESD of non-reactive two-

phase system to confirm this hypothesis.

Figure 16. Relation between dilution of the electrode material as a function of

the melting temperature difference between the electrode and the substrate.

3.3.1.4 Solidification of a 25%FCC/ 75%BCC electrode (NiCr/NiAl)

A combination of the above phases NiCr (γ) & NiAl (β) with a composition of Ni-

15Al-10Cr has been deposited on Ni substrate (γ). Figure 17 presents the XRD

patterns of the starting electrode and the ESD deposit. It can be seen that the XRD of

the electrode consists of γ/β phases whereas the deposit mainly possesses the γ

solid solution with small volume fraction of γ’ phase. As it can be observed, the β is

fully suppressed during the solidification, but that one small peak corresponding to

the Ni3Al phase is present. Due to the thin deposit obtained in this case, it is

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150 200

NiCr Ag [A]

CoNiCrAlY

NiCoCrAlY

% i

ncr

ease

of

the

sub

stra

te

con

cen

trat

ion i

n t

he

dep

osi

t

Difference in melting temperature between the

substrate and the electrode (oC)


41

hypothesized that the majority of the intensity of the (111) reflection is arising from

the Ni substrate. In such case, the bulk of the intensity from the (200) reflection,

indicates a preferential solidification orientation along the <100> crystallographic

direction. This preferential orientation is in concurrence with dendritic growth

direction of the FCC material.

Figure 17. XRD patterns of the NiCrAl electrode (lower) and NiCrAl deposited

on the Ni substrate (upper).

Figure 18 presents a micrograph of the cross section of the NiAlCr deposit. A

deposit presents a columnar homogenous aspect with a small volume fraction of

porosity. Table 8 shows the quantitative chemical analysis values corresponding to

the location shown in Figure 18. Spot A shows high Ni purity, as expected because

the analysis was performed in the substrate material, whereas the spots B, C, and D

correspond to the dilution region between the substrate and the electrode. The

chemical analysis shows an enrichment in Cr and Al. Based on the ternary phase

diagram, this composition remains in the Ni solid solution range, hence the primary


42

solidification phase remains γ. However, starting from spot E to J, the concentration

in Al and Cr increases further and reaches the position where intermatallic may

from based on the phase diagram. However, as mentioned earlier, deviation occurs

due to the rapid cooling rate, thus the homogeneous composition yields a small

volume fraction of γ’ phase has depicted by one small peak in the XRD scan. The

similarity in the crystal structure throughout the deposit shows that the Al detected

is incorporated in the solid solution and the β phase is suppressed due to the rapid

solidification of the ESD deposit.

Figure 18. Cross-section of a NiAlCr deposit on Ni substrate


43

Table 8. Chemical analysis of a NiAlCr deposit on Ni substrate

NiAlCr Ni Al Cr

Point A 98.9 0.5 0.6

Point B 83.4 10 6.6

Point C 77.1 13.3 9.6

Point D 75.3 14.9 9.8

Point E 70.6 18.1 11.3

Point F 70.9 17.6 11.5

Point G 71.1 17.5 11.4

Point H 71.1 16.7 12.2

Point I 71.6 17.2 11.2

Point J 71.8 17.6 10.6

Electrode 72.6 17.7 9.7

3.3.1.5 Solidification of a 100% BCC electrode (NiAl)

Figure 19 presents the phase structure of the electrode and of the ESD deposit,

respectively. The electrode pattern is in good agreement with the JCPDS 044-1188

and consists of β phase possessing the (110) orientation. In the deposit, the

solidification structure is composed of two phases, namely the two peaks a γ/γ’

(111) and (200), in conjunction with the β (110) reflection. Again, due to the

thickness of the deposit, it is believed that the peak corresponding to the (111)


44

phase arises from the substrate. The peak (200) for the gamma structure indicates

the dendritic growth of the γ phase during solidification according to the

preferential orientation. Moreover, in the present case, two additional phases are

present, γ’ and β. These phases must arise from a larger change in chemical

composition due the dilution between the substrate and the electrode than for the

lower β-content alloys.

Figure 19. XRD patterns of the NiAl electrode (lower) and NiAl deposited on

the Ni substrate (upper).

Figure 20 presents a cross-section micrograph of the ESD deposit, depicting

columnar grains at the interface region, and as a result of composition variations, a

transition to superfine γ/β structure was observed. Also, as shown, this ESD deposit

is characterised with a low level of porosity. Table 9 shows a quantitative chemical

analysis corresponding to the points highlighted in the micrograph of Figure 20.

Spots analysis A, B and C correspond to the dilution region between the substrate


45

and the electrode material. Based on the chemical analysis of the dilution region, the

enrichment of Ni and the columnar structure is referred to contain primary γ phase.

Starting from spot analysis D, the chemical composition is within the range NiAl-

Ni3Al. The XRD seem to indicate that during rapid solidification, the γ’ phase is

partially suppressed while the β phase is suggested as the primary solidification

phase. This could be expected as the composition is on the hypo-side of the eutectic.

Moreover, by comparing the starting electrode chemical composition with the

plateaued chemical composition of the coating, an enrichment in Ni is still observed.

Figure 20. SEM micrograph of a NiAl deposit on Ni substrate


46

Table 9. Chemical analysis of a NiAl deposit on Ni substrate.

NiAl Ni Al

Point A 90.5 9.5

Point B 87.1 12.9

Point C 83.6 16.4

Point D 81 19

Point E 80.6 19.4

Point F 80.7 19.3

Point G 80.5 19.5

Point H 77.7 22.3

Point I 80.3 19.7

Point J 77.2 22.8

Point K 76.6 23.4

Electrode 68.3 31.7

3.3.2 Thermal decomposition of the metastable coatings

In order to demonstrate the metastable nature of the CoNiCrAlY, NiCoCrAlY,

NiAlCr and NiAl deposits produced by ESD, heat treatments at intervals of 50oC

were performed in a vacuum furnace. The samples were then analysed to

identify if the equilibrium phases would develop and at which temperature the

equilibrium microstructure would be present. The summary of the results is


47

presented in Table10. In all cases, the measured equilibrium compositions

corresponded to the expected phases for the respective constant chemical

composition found in deposit and the decomposition occurred between 550 and

800oC. For the case of the NiAl system, the decomposition of the γ reflection into

the γ/γ’ reflections was used as transition state, taking into account that the γ

reflections come from the substrate.

Table 10. Thermal decomposition of the metastable coatings.

System

As-

solidified

phase

Equilibrium

phases

after heat

treatment

Transition

temperature

(oC)

Reported

equilibrium

phases

Reference

CoNiCrAlY γ γ, β, γ’ 800 γ, β [84]

NiCoCrAlY γ γ, β, γ’ 800 γ, β [84]

NiAlCr γ, γ’ γ, β, γ’ 700 γ, β, γ’ [91]

NiAl γ, β, γ’ γ, β, γ’ 550 β, γ’ [92]

3.4. CONCLUSION

Electrospark deposition has been successfully used to form metastable phases

through the rapid solidification of Al-containing Ni-based superalloys. The

solidification microstructure and its relationship with metastable phase

diagrams for various ratios of γ and β (NiCr, CoNiCrAlY, NiCoCrAlY, NiAlCr, and


48

NiAl) were identified. Due to the dilution with the substrate, the primary

solidification phase of tested γ/β superalloys is γ. However, in the case of the

NiAlCr (25%γ/75%β) and NiAl (100%β) a formation of the second phase were

detected. The high cooling rate in the ESD process facilitates the formation of

the metastabe phases, which represents an extension in the solid solubility with

the respect of preserving the Al in the coating. The thermal decomposition of the

metastable coatings was performed under vacuum. The equilibrium phases

were restored for each deposit between 550 and 8000C. The β phase re-

established of each deposit.

Chapter 4. Utilisation of Electrospark Deposition to Restore the Local Oxidation Resistance Properties in Damaged NiCoCrAlY and CoNiCrAlY Coatings

49

Chapter 4. Utilisation of Electrospark Deposition to Restore the Local Oxidation Resistance Properties in Damaged NiCoCrAlY and CoNiCrAlY Coatings R. Farhat and M. Brochu1

1Mining and Materials Engineering Department, McGill University, 3610 University

street, Montreal, QC, Canada, H3A 2B2

Preface

This chapter reports the feasibility of using the ESD technique to repair MCrAlY

coatings. According to the previous chapter, ESD has been proven to repair Al

containing Ni-based superalloys. However, to emphasis this feasibility to repair

MCrAlY coatings, a study has been done to repair a damaged MCrAlY substrate

with the same parameters. NiCoCrAlY and CoNiCrAlY were deposited on the

damaged region of the NiCoCrAlY and CoNiCrAlY respectively. It was noticed

that the ESD deposits have fine grain size comparing to the substrates due to the

rapid solidification that occurs during the ESD process. Also, an improvement in

the oxide scale was observed on the repaired region.


50

4.1. INTRODCUTION

MCrAlY coatings (M: Ni, Co, NiCo, or CoNi) are widely used as a bond coat

between a substrate and ceramic in thermal barrier coatings (TBCs) to protect

gas turbines and aircraft engine components from hot corrosion and high

temperature oxidation [43, 93, 94]. It has also been proposed that these coatings

be used as standalone coatings due to their ability to resist highly oxidative

environments [43]. In both cases, the protection against oxidation and hot

corrosion is accomplished by the formation of a thermally grown oxide (TGO).

Upon exposure to high temperatures, an Al2O3 scale initially forms at the surface

of the coating, from the diffusion of Al from the β phase. With further oxidation

the surface and near surface Al is fully consumed, and other oxide phases, like

spinels and Cr2O3, start developing above the original Al2O3 TGO. Consequently,

for stand-alone applications, the oxidation resistance becomes a function of the

integrity of the scale, and the eventual development of this two-layer TGO

indicates the beginning of the end for the coating [93].

During prolonged service time, the coating is prone to failure due to a

combination of an environment causing metal oxidation and thermal-expansion

mismatch stresses, caused by a large variation in microstructure and

composition [94]. The degradation of the oxidation resistant scale usually

begins at the Al2O3 layer due to either the local residual stresses being at their

highest, or due to poor interfacial strength between the bond coat and TGO [44,


51

95, 96]. If spallation of the original TGO occurs, the metallic Al-depleted layer

becomes exposed to the oxidative environment. Consequently, the depleted

zone directly develops the deleterious mixed oxides, like spinels and/or Cr2O3,

or other oxides; the developed phase being dictated by the bond coat’s starting

composition. The formation of the mixed oxides-based TGO accelerates the

damage through the MCrAlY coating. The principal method of salvaging the

underlying component is to strip-down the MCrAlY coating and deposit a new

one, which can be labour intensive, in addition to the cost associated with the

downtime of the equipment [1]. In such a case, the development of a localized

repair technique, which would permit the replenishment of Al-concentration of

these depleted and exposed metallic zones, becomes an attractive, cost-saving

alternative.

Electrospark deposition (ESD) is a micro-arc welding process that is used to

repair, and thus extend the lifetime of damaged components [97]. The very

small heat affected zone (HAZ) inherent to the ESD process renders this

technique highly applicable to those materials possessing a high tendency to

crack during welding [16], such as superalloys [70] and MCrAlY [78]. The

welding process consists of the melting, metal transfer, and solidification onto

the substrate, from a rotating electrode. The high energy, high frequency

electrical pulses induce rapid solidification, and depending on the chemistry of

the starting material, can result in nano-structured deposits [76, 77], metastable

structures [74], or epitaxial deposits [71, 73, 78].


52

This work is aimed at demonstrating that the repair of a locally damaged

NiCoCrAlY or CoNiCrAlY coating can be done by ESD to further extend the

lifetime of coatings. This proof of concept was done by studying the autogeneous

solidification behaviour on an oxidised sample, investigating the interfacial

microstructure, and characterising the evolution of the oxide scale formed at the

surface of the repair zone upon re-oxidation.

4.2. EXPERIMENTAL PROCEDURES

Table 11 presents the chemical composition of the NiCoCrAlY and CoNiCrAlY

powders, which were supplied by Praxair surface technologies Cs36-65 (NI-

171) and Sultzer-Metco (9954), respectively. Both the substrate and electrode

were fabricated using spark plasma sintering (SPS) with a Thermal Technology

SPS 10-3 press. The consolidated pucks were used as substrates, and the

electrodes were machined from the same samples.

Table 11. Nominal Chemical composition of starting powders

Ni Co Cr Al Y

NiCoCrAlY 46.5 23 17 13 0.5

CoNiCrAlY 32 38.5 21 8 0.5


53

Prior to the simulated repair trials, the SPS consolidated substrates were

polished to a mirror finish using conventional metallographic techniques. The

initial step of the simulated service condition was to impose an oxidation

treatment at 10000C for 24 hrs in ambient air in a Blue M box furnace, in order

to generate the protective surface oxide scale and an Al-depleted layer

underneath. The second portion of the simulated service was to simulate TGO

spallation, which was performed by scratching the oxidised surface (removal of

the Al2O3 surface layer) using a diamond knife. Thirdly, the repair of the

damaged coating (simulated using a scratch test) was carried out using a

Technocoat SparkDepo 300 ESD unit. The micro-welding repair consisted of

filling the scratched area, which was done in an argon atmosphere to avoid

oxidation. The deposited layers were obtained using the following parameters:

100 V, 390Hz, and 210µF. Assuming the welding frequency allows for complete

charge and discharge of the capacitors, a spark energy of 1.05J can be assumed

using E=1/2 CV2 [71], where C is the capacitance and V is the voltage,

respectively. Finally, following the repair trial, a similar oxidation treatment to

the one previously described was imposed on the replenished samples to

investigate the oxidation behaviour of the repaired zone, and was compared

with the oxidation behaviour of the substrate.

Cross-sections of the samples examined at various stages of the work were

polished using standard metallographic procedures, down to 0.05m polishing

suspension. The microstructure analysis was performed using scanning electron


54

microscopy (SEM) with a Hitachi S-4700 microscope equipped with an Oxford

EDS system. The phase composition identification was performed by x-ray

diffraction (XRD) using a Bruker D8 Discovery x-ray diffractometer.

4.3. RESULTS AND DISCUSSION

4.3.1 Characterisation of starting materials

The XRD patterns of the starting NiCoCrAlY powder and the SPS bulk sample are

shown in Figure 21 (lower and middle spectra); these indicate that the starting

microstructure is composed of β and γ phases. Also, it can be noted that the

starting powder orientation was not affected by the SPS consolidation.

Figure 21. XRD spectra of the NiCoCrAlY material at different steps:

starting powder (lower), SPS consolidated sample (middle), and ESD

(upper).

XRD spectra of the starting CoNiCrAlY powder, and the SPS bulk sample, are

shown in Figure 22 (lower and middle spectra), and indicate that the starting


55

microstructure is composed of γ and β phases. Also, similarly to the NiCoCrAlY

system, no change in microstructure orientation was observed after the SPS

consolidation for the CoNiCrAlY system.

Figure 22. XRD spectra of the CoNiCrAlY material at different steps:

starting powder (lower), SPS consolidated sample (middle), and ESD

(upper).

4.3.2 Characterisation of the microstructure of the oxidised substrates

Figure 23a and b present comparative micrographs of the oxide layer from the

NiCoCrAlY and CoNiCrAlY substrates after exposure to air at 10000C for 24 hrs,

respectively. Firstly, in both cases, the micrographs are showing that fully dense

samples were produced using the SPS process, thus no interference from

internal porosity during the oxidation analysis should be present. Also, in both

micrographs, the light and dark phases observed in the substrate are the γ and β

phases, respectively. In terms of oxidation product, for both systems, a single


56

oxide layer is observed. For the NiCoCrAlY system (Fig 23a), upon oxidation, the

surface oxide scale averaged 1.3±0.2m, while the corresponding Al-depleted

layer, caused by the consumption of Al for the formation of the Al2O3 scale,

averaged 2.0±0.3m. For the CoNiCrAlY system (Fig 23b), the Al2O3 scale

averaged 1.8±0.1m, while the Al-depleted layer averaged 6.6±0.6m.

(a) (b)

Figure 23. Micrographs of the substrates after the oxidation treatment of

24 hours at 10000C; (a) NiCoCrAlY and (b) CoNiCrAlY.

Figure 24 depicts the XRD analysis carried out to investigate the crystal

structure of the oxide layer developed during the oxidation treatment

performed at 10000C for 24 hours for the NiCoCrAlY substrate (Fig 24a) and the

CoNiCrAlY substrate (Fig 24b). In the case of the NiCoCrAlY system, with respect

to the penetration depth of the XRD, the oxidation treatment caused an almost


57

complete reduction of the beta phase, favoring the formation of the α-Al2O3

scale. For the CoNiCrAlY system, also with respect to the penetration depth of

the XRD, the beta phase completely disappeared, and similarly, an α-Al2O3 scale

developed.

(a) (b)

Figure 24. XRD spectra of the substrates after the oxidation treatment: (a)

NiCoCrAlY and (b) CoNiCrAlY.

Figure 25a and b show the comparative surface morphologies of the

developed oxide scale after exposure to air at 10000C after 24hrs on the top of

the NiCoCrAlY and CoNiCrAlY substrates, respectively. In both cases, the

surfaces are covered by a uniform and continuous Al2O3 layer, the smooth ridges

indicates to the formation of the α-Al2O3 [98]. A clear difference in the surface

density of the examined substrates is observed, this refers to the difference in


58

the Al percentage in the substrates. The transition to the dense α-Al2O3 oxide

layer is believed to be reached with the tested time.

(a) (b)

Figure 25 surface morphology of the oxide scale on the substrates after

oxidation at 10000C for 24hrs (a) NiCoCrAlY and (b) CoNiCrAlY.

An in-depth explanation of the oxidation behaviour of MCrAlY can be found

elsewhere [43, 99], but a brief overview will be presented. The development of

the alumina scale in MCrAlY occurs through a series of different metastable

structures such as θ-Al2O3, γ-Al2O3, and δ-Al2O3, which will eventually transform

into the stable α-Al2O3 phase [100]. The metastable alumina phases are

responsible for an initial transient stage in oxidation, while the final corundum

structure dictates overall kinetics [101]. Therefore, looking at a system that

would prone an early α-Al2O3 formation is desired as oxygen diffusion is limited

in this dense crystal lattice, which would yield in a slow growing, dense, and

protective oxide layer. The results obtained in this work show that after only 24


59

hours of exposure, no trace of the intermediate metastable phases remains and

that both substrates are in the steady-state oxidation.

4.3.3 Characterisation of the Electro Spark Deposits microstructure.

Figure 26 shows a low magnification micrograph of the CoNiCrAlY ESD deposit

on the CoNiCrAlY substrate. Firstly, this multipass weldment is characterised

with a low level of porosity and defects. Furthermore, the interface between the

substrate and the ESD deposit can be seen clearly due to the fact that a two-

phase material is observed within the substrate while the deposit seems to be

composed of a single phase material. Another important feature to observe is

that the ESD process, which inherently causes partial melting of the substrate,

was performed with sufficient energy to fully melt any remnant of the Al-

depleted zone caused by the initial oxidation. When comparing this multi-

deposit repair thickness, which ranges between 100-200m, with the Al-

depleted zone shown in Fig. 23b (6.6±0.6m), the potential dilution of Al can be

considered negligible; thus it can be expected that the ESD deposit will possess

an Al concentration similar to the original base material. A similar behaviour

was observed for the NiCoCrAlY system.


60

Figure 26. Microstructure of a CoNiCrAlY substrate after simulated

damaged and repair with ESD process.

Figure 27 shows a higher magnification micrograph of the interface

between the substrate and the ESD deposit for the NiCoCrAlY system. From the

micrograph, it can clearly be observed that the deposit is composed of only a

single-phase material. To further validate this observation, XRD analysis of the

micro-weldments was performed and the results are shown in Figure 21a) and

b), respectively, on the upper spectra. It is worth mentioning that in the present

case, the welding combinations are autogeneous in nature; hence no

complication from substrate dilution is occurring, facilitating the analysis in

terms of primary solidification phase. For both systems, it can be observed that

the primary solidification structure shifted from β and γ phases to the γ (200),

which corresponds to the preferential orientation along the <100>


61

crystallographic direction known for FCC systems. For the CoNiCrAlY system,

based on the equilibrium diagram presented in [84], the primary solidification

phase should be γ. For such a system (γ/β), the appearance of the β as a

solidification structure will be a function of solidification parameters such as

cooling rate, temperature gradient, and solidification front velocity [78]. For

conventional welding processes, cooling rate, temperature gradient, and

solidification front velocity have low values, hence the possibility of detecting β

phase upon solidification. On the other hand, due to the solidification conditions

inherent to ESD, solute rejection of the element forming the β phase in

CoNiCrAlY, becomes negligible. Hence, the ESD allowed for the development of a

full γ columnar structure throughout the deposit. The effect of cooling rate on

the deviation of the primary solidification structure is even more pronounced

for the NiCoCrAlY system. The equilibrium phase diagram shows, that for this

system, the primary solidification phase is β [84]. However, the results

presented here, which are not influenced by the welding of dissimilar materials,

indicate that the solidification conditions remain sufficiently in the non-

equilibrium state so the development of a full γ columnar structure throughout

the deposit can be observed. In an attempt to explain this solidification, the

concept of a metastable phase diagram will be used. In such treatment, two

hypotheses are required: (1) the liquid phase must be homogeneous and (2), no

atomic diffusion in the liquid or in the solid occurs during solidification [74]. In

such case, solely Gibbs free energy curves representing the various phases are

used. Consequently, the change in solidification structure occurs at the


62

crossover of the Gibbs free energy curves for the two components and not with

the tie-line [74]. It is worth mentioning that conceptually, the crossover point

possesses a less negative free energy value than the regular tie-line would have

in equilibrium solidification. Hence, the metastable γ solidification structure

observed indicates that for the composition range of the NiCoCrAlY alloy, the

crossover in solidification structure, to see a β phase as a primary solidification

phase, would occur at a higher Al concentration.

Figure 27. Micrograph of a NiCoCrAlY weldment on a NiCoCrAlY substrate

depicting the change of microstructure from a two-phase substrate to a

single-phase weldment.

4.3.4 Characterisation of the oxidation behaviour of the ESD repaired location.

As demonstrated in the previous section, the ESD process induced a single γ

phase solidification structure, which is metastable in nature, as both systems

should contain γ and β phase at equilibrium. Figure 28 shows that the


63

precipitation of the β phase has occurred during the second oxidation treatment

at 10000C for 24 hours. The respective phase size of the γ and β phases is

smaller in the ESD deposit than in the substrate material.

Figure 28. Micrograph depicting the precipitation of the β phase from the

metastable γ solidification structure during the second oxidation

treatment.

Figure 29 shows the microstructure of the NiCoCrAlY repaired location after

the oxidation trial at 10000C for 24 hours. As expected, a consistent layer of α-

Al2O3 formed on the top of the deposit and a depleted zone was developed

beneath the oxide scale. A similar oxidation response, in terms of

microstructural development, was obtained for the CoNiCrAlY sample.


64

Figure 29. Micrograph presents the microstructure evolution of the

repaired NiCoCrAlY after the oxidation treatment of 24 hours at 10000C

Figure 30 presents the XRD analysis of the oxide scale formed after 24 hours

at 10000C for the NiCoCrAlY deposit (Fig30a) and the CoNiCrAlY deposit (Fig

30b). Both of the oxidized deposits show an almost complete depletion of the β

phase and a formation of the corundum stable α-Al2O3 scale. The performance of

the crystal structure of the oxide layer of the ESD deposits is similar to the

oxidized substrates shown above.

Figure 30. XRD spectra of the ESD deposit after 24hours at 10000C: (a)

NiCoCrAlY and (b) CoNiCrAlY


65

Figure 31 presents the oxide surface formed from the repaired regions of

NiCoCrAlY (Fig31a) and CoNiCrAlY (Fig 31b). A uniform, continuous and dense

Al2O3 can be observed on repaired regions. As it is shown in the micrograph, the

oxide scale has same smooth ridges that observed on the substrates but it is

denser on the repaired location than on the substrates. The fine grain size of

the deposit region developed more consistent oxide scale, this results are in

agreement with Ajdelsztajn work [44] where the nanostructured coatings

produced a continuous alumina layer than the conventional one. The

nanostructured or the very fine grain size results in increasing the number of

diffusion paths for aluminum in the coatings and therefore accelerates the

formation and densification of the thermodynamically stable α-Al2O3 phase.

Xie and Wang [1]also have reported that due to the fine grain size deposit that

obtained from electrospark deposition, a fast diffusion of Al occurs during the

oxidation. This fast diffusion of Al has promoted the formation of the fast

growing θ-Al2O3 in the early stage of the oxidation and with time θ-Al2O3

transforms to α-Al2O3. Despite the absence of θ-Al2O3 peaks from the XRD

analysis shown in Figure 30, the surface micrographs are indeed showing some

remnants of the θ-Al2O3, indicating that the transformation towards α-Al2O3 is

not completed after a hold of 24 hours. This result is in line with the work of Xie

and Wang [1], which showed that after 100 hours, still remnants of θ-Al2O3 were

present. However, based on qualitative comparison of micrographs, the

approximated quantity of θ-Al2O3 remaining after 24 hours in the present work

seems to be lower than in the case of Xie and Wang [1].


66

(a) (b)

Figure 31 surface morphology of the oxide scale on the ESD deposit after

oxidation at 10000C for 24hrs (a) NiCoCrAlY and (b) CoNiCrAlY.

The measurement of the layers developed during oxidation, namely the

depleted layer and the surface oxide scale, was performed in order to verify

whether the ESD repair would possess similar oxidation behaviour and the

results are presented in Figure 32. An expected result arises from the fact that

within the standard deviation, the respective ratio of the thickness of the scale

to that of the depleted layer is constant for the substrate and the ESD deposit,

and this for both systems studied. The interesting fact from this comparison

arises for the statistical difference in the thickness of the oxide scale, where the

scale for the repair location is thicker than for the substrate after 24 hours.

During the oxidation of the γ/β, the nucleation of the Al2O3 scale initiates from

the beta phase (Al reservoir), and then grows to fully cover the surface

corresponding to the γ phase. In the present case, the very fine grain size


67

combined with a metastable structure causing a homogeneous distribution of Al

throughout the deposit, resulting from ESD, should serve to enhance oxidation

kinetics by increasing the number of diffusion paths for Al within the coating

and reducing the overall diffusion distance for the Al to reach the surface. To

strengthen the effect of the supersaturated solid solution on the scale growth

and thickness of the depleted layer, ESD coating where heat treated in vacuum

at 1000oC for 2.5 hours prior to oxidation to induce grain growth and phase

separation, and the results are presented in Figure 31 under the caption ESD +

HT. As demonstrated, the oxidation behavior of these coatings is similar to their

original substrate counterparts, illustrating that indeed, the enhanced

thickening is caused by the Al-supersaturated FCC phase. Krukovsky [102] has

shown that the thickness of the oxide scale and its phase composition depends

on the aluminum content in the coatings during the first 20,000 hours of the

oxidation at 10000C, therefore in this case, the constant ratio difference of the

oxide scale thickness between the ESD deposit and the substrate for both

systems refers to the Al content in the starting composition for both systems.


68

Figure 32. The calculated thickness of the depleted layer and oxide layer of

both the NiCoCrAlY and CoNiCrAlY substrates, ESD deposits and ESD

deposits exposed to grain growth prior to oxidation.

4.4. CONCLUSION

Electrospark deposition (ESD) has been used to repair the damaged regions of

NiCoCrAlY and CoNiCrAlY coatings. An epitaxial growth is obtained with a

primary solidification phase of γ. A consistent monolayer α Al2O3 has been

obtained on the top of the deposit, which indicates that ESD can replenish the

damaged surface. Moreover, the scale evolution of the repaired location is

similar to the one known for the substrate, but with an increased thickening due

to the homogeneous Al distribution arising from the Al-supersaturated solid

solution and shorter diffusion path inherent of the small solidification structure.

0

2

4

6

8

10

12

14

16

NiC

o

Su

bstr

ate

NiC

o E

SD

NiC

o E

SD

+ H

T

Co

Ni

Su

bstr

ate

Co

Ni E

SD

Co

Ni E

SD

+ H

T

Depleted layer thickness

Surface scale thickness

mic

rons

Chapter 5 Summary

69

Chapter 5. Summary

The previous Chapters have shown that the electrospark deposition technique

has the capability to be used as a deposition process for Al containing Ni-based

superalloys. In Chapter 3, it was shown that the ESD process can be used to

deposit similar and dissimilar materials. It has been used to deposit NiCr (100

γ), NiAl (100β), NiAlCr (25γ-75β), NiCoCrAlY (45γ-55β), CoNiCrAlY (73γ-27β)

on the Ni substrate. In all the deposits, formations of thin homogenous deposit

with metastable phases were obtained. The chemical analyses, which were done

throughout the vacuum annealing at different temperatures, have shown the

exact decomposition temperature of each deposit. It’s believed that the

difference in the thermal stability of each deposit is related to the composition

of the deposit and the morphology of the deposit.

In Chapter 4, the same NiCoCrAlY and CoNiCrAlY were used in the deposition

process, but in this time, they were used as electrode and substrate. However,

the depositions were accomplished on the damaged region of the coatings. It

was obvious throughout the scanning electron microscopy that the

microstructures of the coatings are fine microstructure. Therefore, the oxide

Chapter 5 Summary

70

scale evolution of the repaired locations was more consistent in both coatings.

However, the difference in the oxide evolution between the NiCoCrAlY and

CoNiCrAlY coatings is referred to the difference in the Al percentage.

The ESD technique was the state of the art of this work, the experimental work

showed a good results, the NiAl (β) electrode was successfully deposited on the

Ni (γ) substrate in chapter 3. Consequently, the results in chapter 4 were

expected to be similar to what has been seen in the previous chapter.

Future work on using ESD as a technique to repair Al containing Ni-based

superalloys and other wear and corrosion resistance coatings should be applied.

However, more studies are required to investigate the microstructure

morphology of the deposit. Moreover, a consideration of the rapid solidification

of the ESD process and the influence of the solidification conditions must be

understood to optimize the properties of the deposits.

Chapter 6 References

71

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