the effect of plasma spray variables on the development...
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THE EFFECT OF PLASMA SPRAY VARIABLES ON THE
DEVELOPMENT OF CERAMIC COATINGS
ABDUL RAHIM MAHAMAD SAHAB
UNIVERSITI SAINS MALAYSIA
JANUARY 2008
THE EFFECT OF PLASMA SPRAY VARIABLES ON THE DEVELOPMENT OF CERAMIC COATINGS
By
ABDUL RAHIM MAHAMAD SAHAB
Thesis submitted in fulfilment of the requirements for the degree of Master of Science
JANUARY 2008
ACKNOWLEDGEMENTS
This thesis represents a total of almost twenty four months of effort; it could not
have been written and produced without the help of many colleagues and former
students. It gives me great pleasure to acknowledge the assistance of the following in
the completion and preparation of this MSc thesis:
I am very grateful and thankful to my supervisor, Associate Professor Dr. Luay
Bakir Hussain for his enthusiastic support, for his guideline throughout the whole of
academic semesters, and for his direct and indirect supervision in completing this
thesis. Many thanks as well to my co. supervisor Dr. Fazilah Fazan in guiding and
advising any matters occurred in the research study.
Special thank to Dr. Mohd Radzi Hj. Mohd Toff and Dr. Talib Ria Jaafar in
allowing me using the Praxair Plasma Spray System at SIRIM Berhad, Kulim Kedah.
Many thank to Pn Siti Mariam, Mohd Zakuan Abdullah and Mohd Hazri in operating
Praxair air Plasma Spray Equipment and CSEM Tribometer Equipment.
I gratefully acknowledge my Head at Plastics Ceramics Programme, SIRIM
Berhad, Shah Alam, Dr. Ahmad Fuad Md. Yusuf, my Coordinator Dr. Teng Wan Dung,
my colleagues Rafindde Ramli, Zarina Abdul Wahid, Zalena Saem, Zaini Rahmat and
Azura Ruhan in supporting the experiment done at the Ceramics Technology Group.
Students at the Faculty of Mechanical Engineering, Mara University of
Technology, Shah Alam, Hasanul Ariff Che Jawias and Muhamad Yusup Hj. Kamis for
their assistance in the early stage of my research with REMAKO, Klang.
Finally, to my beloved wife, Nor Saad for sharing her knowledge and giving the
constructive criticisms and suggestion in completing this thesis.
ii
TABLE OF CONTENTS
Page ACKNOWLEDGEMENTS ii
TABLE OF CONTENTS iii
LIST OF TABLES vii
LIST OF FIGURES ix
LIST OF SYMBOLS xiv
LIST OF ABBREVIATION xv
LIST OF APPENDICES xvi
LIST OF PUBLICATIONS & SEMINARS xvii
ABSTRACT xviii
CHAPTER ONE : INTRODUCTION
1.1 Surface Engineering 1
1.2 Thermal Spray Coating 1
1.3 Ceramic Coating and Application 2
1.4 Objective of the study 3
1.5 Significance of the Research 3
1.6 Scope of the Study 4
CHAPTER TWO : LITERATURE REVIEW
2.1 Introduction of Plasma 6
2.2 Thermal Spraying System 7
2.2.1 Flame Spraying 9
2.2.2 High Velocity Oxy-Fuel 10
2.2.3 Arc Spraying 11
2.2.4 Plasma Spray 12
2.3 Plasma Spray System 14
2.3.1 Plasma Torch System 15
2.3.2 DC Power Supply System 20
2.3.3 Control and Instrumentation System 21
2.3.4 Cooling Water System 22
iii
2.3.5 Gas Supply System 22
2.3.6 Material Feeder System 23
2.4 Coating Material 23
2.4.1 Coating Material Properties 23
2.4.2 Evaluation of The Coating Powder 25
2.5 Substrate Surface Preparation 25
2.6 Coating Deposition and Structure 26
2.7 Test Methodologies 28
2.7.1 Coating Characterisation 28
2.7.2 Coating Properties 29
2.7.3 Coating for Wear 30
2.7.4 Coating for Thermal Cycling Process 32
2.9 Coating Application 34
CHAPTER THREE: PRAXAIR PLASMA SPRAY SYSTEM AND COATING DEVELOPMENT
3.1 Introduction 36
3.2 Praxair Plasma Spray System 36
3.2.1 Plasma Torch 36
3.2.2 Power Supply 37
3.2.3 Automatic Controller (Main Controller) 38
3.2.4 Cooling System 41
3.2.5 Gasses System 42
3.2.6 Powder Feeder 42
3.3 Powder Selection 46
3.3.1 Ceramic 46
3.3.2 Bond Coating 47
3.3.3 Powder Selection of Coating 47
3.4 Important Parameters and Parameters Consideration 48
3.5 Other Variable Parameters Consideration 49
3.5.1 Robot Setting Programming 49
3.6 Ceramic Coating Deposition 51
3.7 Bonding Mechanism of Plasma Spray Coating 52
3.8 Stress 54
iv
CHAPTER FOUR: EXPERIMENTAL AND TESTING PROCEDURE
4.1 Introduction 56
4.2 Experiments 57
4.2.1 Coating Materials 57
4.2.2 Metal Substrate and Surface Preparation 60
4.2.3 Coating Deposition and Parameters Selection 63
4.3 Phase and Characterisation 68
4.3.1 XRD Analysis 68
4.3.2 SEM Analysis 68
4.4 Physical Testing 69
4.4.1 Adhesion Strength Test Measurement 69
4.4.2 Adhesion Test Procedure 70
4.4.3 Hardness Test Measurement 71
4.4.4 Bending Test Measurement 74
4.5 Wear Test 74
4.6 Thermal Resistance Test 75
4.7 Porosity 76
CHAPTER FIVE: TEST RESULTS AND DISCUSSION OF COATING SAMPLE
5.1 Overall Analysis 78
5.2 Metco 204NS Coating 78
5.2.1 XRD Analysis of Metco 204NS Coating 78
5.2.2 SEM Analysis of Metco 204NS Coating 80
5.2.3 Adhesion Strength and Hardness Analysis of Metco
204NS Coating
83
5.2.4 Bending Test Analysis of Metco 204NS Coating 84
5.3 ALO-105 Coating 85
5.3.1 XRD Analysis of ALO-105 Coating 85
5.3.2 SEM Analysis of ALO-105 Coating 90
5.3.3 Adhesion Strength Analysis of ALO-105 Coating 95
5.3.3.1 Adhesion Strength and Stand-Off-Distance 96
5.3.3.2 Adhesion Strength and Powder Flow Rate 97
5.3.3.3 Adhesion Strength and Current 98
v
5.3.4 Hardness Test Analysis of ALO-105 Coating 100
5.3.4.1 Hardness and Stand-Off-Distance 102
5.3.4.2 Hardness and Powder Flow Rate 103
5.3.4.3 Hardness and Current 104
5.3.5 Coating Thickness Analysis of ALO-105 Coating 105
5.3.5.1 Coating Thickness and Powder Flow Rate 106
5.3.5.2 Coating Thickness and Stand-Off-Distance 107
5.3.5.3 Coating Thickness and Current 108
5.4 Wear Test Analysis of ALO-105 Coating 109
5.5 Thermal Resistance/ Cycling Analysis of ALO-105 Coating 113
5.6 Coating Porosity Analysis of ALO-105 Coating 118
5.7 Summary of Coatings 118
CHAPTER SIX: CONCLUSION AND FUTURE RECOMMENDATION
6.1 Conclusions 121
6.2 Future Recommendation 123
REFERENCES 125
APPENDICES
1.1 Materials Properties Analysis 129
1.2 Profiling of Substrate and Coating Surface Using Mitutoyo Surftest SV600 Instrument
136
1.3 Typical Powder and Coating Properties from Powder Manufacture (Praxair Technologies Inc.)
138
1.4 Test Results of Coating 140
vi
LIST OF TABLES
Page
2.1 Typical industrial applications of plasma spray coating
35
3.1 Type of example (sample S1) for the gas flow rate setting of ALO-105 coating according to conversion from orifice flow chart gasses
41
3.2 Variables parameters of robotic arm setting for plasma sprayed coating
51
4.1 Coating powders specification for plasma spray coating
58
4.2 Variables parameters and roughness of substrate surface preparation for the ALO-105 and Metco 204NS
62
4.3 Chemical analysis of the metal substrate (JISG3103: SS400)
63
4.4 Plasma spraying parameters for the Amdry 962 and Metco 204NS coating using 3MB Gun type and Argon and Hydrogen gas mixture
65
4.5 Plasma spraying parameters for the NI-109 and ALO-105 coating using SG-100 Gun type and Argon and Helium gas mixture
66
4.6 Polishing parameters for specimen of microhardness test
73
4.7 Setting parameters of the hot mounting process with brown epoxy resin
73
4.8 Parameters of the wear test applied for the coating samples
75
5.1 Test results of mechanical performance (adhesion strength and hardness) for the plasma sprayed Metco 204NS coating
84
5.2 Test results of adhesion strength for ALO-105 coatings after plasma sprayed according to variable setting parameters in Chapter 4
96
5.3 Test results of hardness for ALO-105 coatings after plasma spayed according to variable parameters setting in Chapter 4
102
5.4 Test results of the thickness of NI-109 and ALO-105 coatings after plasma sprayed according to variables setting parameters in Chapter 4
106
5.5 Test results the friction coefficient of ALO-105 coating after plasma sprayed according to variable processing parameters setting in Chapter 4
110
vii
5.6 Results of thermal resistance/ cycling test at 800oC to room temperature
115
5.7 Porosity test result of ALO-105 coating as top coating and NI-109 coating as bond coating.
118
5.8 Summary of physical test results of plasma sprayed ALO-105 coating for samples S1 to S8
120
viii
LIST OF FIGURES
Page
2.1 Plasma flame or ‘fourth state of matter’ was formed when heat added
6
2.2 Temperature distribution and geometry of plasma jet
7
2.3 Group of thermal spraying process
9
2.4 Gun design and flame spraying coating was sprayed onto substrate
10
2.5 Gun design and HVOF coating was sprayed onto substrate
11
2.6 Gun design and wire arc spraying coating was sprayed onto substrate
12
2.7 Gun design and plasma spray coating was sprayed onto substrate
13
2.8 Basic system (gases, power, cooling, controller, feeder and plasma torch) of the plasma spray equipment
15
2.9 Schematic of plasma torch type F4 produced by Plasmatechnik Co.
17
2.10 Temperature distribution of plasma beam (a) for argon gas and (b) for nitrogen gas
18
2.11 Effect of plasma torch parameters on temperature characteristics of plasma beam.
19
2.12 Voltage-current characteristics of a torch and power supply
21
2.13 Control system 3710 of Praxair Technology
21
2.14 Plasma sprayed process for internal coating of turbine chamber
27
2.15 Schematic representation of coating structure
28
2.16 Typical interface geometries used for sliding friction and wear test
32
2.17 Turbine chamber was coated with ceramic powder
33
3.1 Plasma SG-100 Gun from Praxair and TAFA’s technology
37
3.2 Model 3710 Controller of Praxair Plasma Spray Equipment
39
3.3 Orifice flow chart conversions of gasses of Model 3710 Controller
40
ix
3.4 Cooling equipment of Praxair Plasma Spray Equipment
41
3.5 Gasses supply system of Praxair Plasma Spray Equipment Laboratory
42
3.6 Feeder system Model 1264 used to feed coating powders to spray gun in the gas stream
43
3.7 Plotted graph powder wheel speed vs. powder flow rate for ALO-105 and NI-109 powders
45
3.8 Estimated feed rate calibration for powder feeder system Model 1260 with 120-hole Wheel
46
3.9 Coating sample for (a) NI-109 coating and (b) both NI-109 and ALO-105 coating
52
3.10 Bonding formation and microstructure of plasma spraying process
53
3.11 Bonding interaction between particles and substrate surface to develop a coating layer
54
3.12 Effect of stress on the coating layer
55
4.1a Morphology of ceramic for (a) ALO-105 and (b) Metco 204NS powders
57
4.1b Morphology of bond coat for (a) NI-109 and (b) Amdry 962 powders
58
4.2a Phase composition of the Nickel base powders used as bond coating – Amdry 962
59
4.2b Phase composition of the Nickel base powders used as bond coating – Ni-109
59
4.3 Blasting machine (Zero Inex) which is used for the substrate surface preparation and cleaning
60
4.4a Feed of brown aluminium oxide powder (blasting media) which is used for the surface preparation and cleaning
61
4.4b Morphology of aluminium oxide powder (blasting media) which is used for the surface preparation and cleaning
61
4.5a Morphology of the metal substrate surface after grit blasted with selected brown aluminium oxide media - mesh 40
62
4.5b Morphology of the metal substrate surface after grit blasted with selected brown aluminium oxide media - mesh 24
62
4.6 Position of plasma gun and metal substrate fixed vertically on the fixture for coating deposition
67
x
4.7a Type of plasma spray guns (torches) used for coating process - SG-100 Gun
67
4.7b Type of plasma spray guns (torches) used for coating process - 3MB Gun
67
4.8 Self-Aligning device which is assembled together for the adhesion strength test
71
4.9 Failure locus of adhesion strength test
71
4.10a Schematic depth-sensing indention sensor of typical hardness tester
72
4.10b Example load displacement of hardness tester during indention
73
4.11 Arrangement of the wear test using pin-on-disc
75
5.1 Phase analysis of the Metco 204NS (YSZ) powder
79
5.2 Phase analysis of plasma sprayed Metco 204NS coating
79
5.3a Morphology of the coating surface without bond coat powder (Amdry 962)
81
5.3b Morphology of the coating surface with bond coat powder (Amdry 962)
81
5.4a Morphology of the sample cross section for coating without bond coat powder (Amdry 962)
82
5.4b Morphology of the sample cross section for coating with bond coat powder (Amdry 962)
82
5.5 The view of coating sample for both (a) Metco 204NS with bond coat and (b) Metco 204NS without bond coat after 3-point bending test
85
5.6 Phase analysis of the ALO-105 (Alumina 3%Titania) powder 86
5.7 Phase analysis of the plasma sprayed ALO-105 coating (S1, S2, S5 and S6) according to current setting 650 ampere
88
5.8 Phase analysis of the plasma sprayed ALO-105 coating (S3, S4, S7 and S8) according to current setting 550 ampere
89
5.9 Morphology the coating surface of ALO-105 for sample S1 to S8
91
5.10 Morphology the coating cross section of ALO-105 for sample S1 to S8
93
xi
5.11 Morphology the coating cross section of ALO-105 for sample S1 to S8 at 800 magnifications
94
5.12 Composite specimen for the adhesion strength test of coated sample
95
5.13a Adhesion strength versus stand-off-distance setting at 75mm and 90mm
97
5.13b Adhesion strength vs. powder flow rate setting at 22.5 g/min and 26 g/min
98
5.13c Adhesion strength vs. current setting at 550 ampere and 650 ampere
99
5.14a Examples of ALO-105 coating were pulled out after adhesion strength tested
100
5.14b Example of half of ALO-105 coating was pulled out from the metal substrate
100
5.14c Example of failure at adhesive epoxy
100
5.15 Example of diamond indentation profile of the hardness test measurement under SEM (a) for ALO-105 coating and (b) for NI-109 coating
101
5.16a Hardness vs. stand-off-distance setting at 75mm and 90mm
103
5.16b Hardness vs. powder flow rate setting at 22.5 g/min and 26 g/min
104
5.16c Hardness vs. current setting at 550 ampere and 650 ampere
105
5.17a Coating thickness vs. powder flow rate setting at 22.5 g/min and 26 g/min
107
5.17b Coating thickness vs. stand-off-distance setting at 75 mm and 90 mm
108
5.17c Coating thickness vs. current setting at 550 ampere and 650 ampere
109
5.18a The evaluation of the friction coefficient vs. the sliding distance for the S1, S2, S5 and S6 samples according to current setting at 650 Ampere
110
5.18b The evaluation of the friction coefficient vs. the sliding distance for the S3, S4, S7 and S8 samples according to current setting at 550 Ampere
111
5.19 Micrograph of coating sample after pin-on-disc test at surface view 20X (a), surface view 60X (b) and cross section view 50X for sample 7 (AF8-6)
113
xii
5.20 Morphology of coating surface after thermal cycles test for Sample S1 to S8
116
5.21 Morphology of coating surface for sample S3 after 50 thermal cycles test
117
xiii
LIST OF SYMBOLS
Tq : Temperature of plasma beam
Nq : Output of plasma beam
I : Arc current
L : Arc length
Cp : Latent heat of gas
Q : Gas flow rate
d : Nozzle diameter
Ax : Coefficient of thermal conductivity from plasma to nozzle
Ē : Gradient along the arc column axis
V : Plasma beam velocity
Qo : Volume of gas flow rate
T : Gas temperature
d : Nozzle diameter
A : Constant
M : Molecular weight of gas
σ : Adhesion Strength
F : Maximum load
A : Cross sectional area
HV : Vickers hardness
F1 : Testing load
S1 : Surface area of indentation
d1 : Average of the diagonal length of identation
θ : Facing plane angle of the diamond pyramid
µ : Friction coefficient
σ : Sigma
P : Porosity
W : Saturated weight
D : Dry weight
S : Suspended weight
xiv
LIST OF ABBREVIATION
DC : Direct Current
HVOF : High Velocity Oxy-Fuel
XRD : X-Ray Diffraction
SEM : Scanning Electron Microscopy
RF plasma : Radio Frequency plasma
DC plasma : Direct Current plasma
APS : Air Plasma Spray
RPM : Revolution Per Minute
TBC : Thermal Barrier Coating
MSDS : Material Specification Data Sheet
ASTM : International American Standard
Ar : Argon
He : Helium
N2 : Nitrogen
H2 : Hydrogen
SLM : Standard Litre per Minute
PSI : Pound per Square Inches
Amdry 962 : Nickel base powder
Metco 204NS : Yttria Stabilised Zirconia
YSZ : Yttria Stabilised Zirconia
ALO-105 : Aluminium Oxide 3%wt Titanium Oxide
NI-109 : Aluminium 5% Nickel
SiC : Silicone Carbide
POD : Pin-On-Disc
m : Monoclinic
t : Tetragonal
c : Cubic
ZrO2 : Zirconium Oxide
α-Al2O3 : Alpha alumina phase
γ-Al2O3 : Gamma alumina phase
TiO2 : Titanium Oxide
Al2O3 : Aluminium Oxide
xv
LIST OF APPENDICES
Page
1.1 Materials Properties Analysis 129
1.2 Profiling Of Substrate And Coating Surface Using Mitutoyo Surftest SV600 Instrument
136
1.3 Typical Powder And Coating Properties From Powder Manufacture (Praxair Technologies Inc.)
138
1.4 Test Results Of Coating 140
xvi
LIST OF PUBLICATIONS & SEMINARS
1.1 Development of Plasma Spray Ceramic Coating for Industrial
Application, Malaysian Science and Technology Congress 2004 (MSTC 2004), Selangor, Malaysia, pp. 5 – 7 October 2004.
1.2 The Performance of Plasma Sprayed Ceramic Coatings: With And Without Bond Coat, Proceedings: APSIM 2005, Advanced Processes and Systems in Manufacturing (Bangi, Malaysia), ISBN: 983-2975-40-9, pp. 121 – 129, 17 – 18 May 2005.
1.3 Mechanical Properties Investigation - Plasma Sprayed Ceramic Coatings, Proceedings of National Conference on Advances in Mechanical Engineering, Malaysia, ISBN: 967-958-177-2, pp 743 - 750, 18 – 20 May 2005.
1.4 The Effects of Spraying Distance in Formation of Plasma Sprayed Alumina 3%wt Titania Coating onto Metal Substrate, Proceedings Techpos’06, 16-17 May, 2006.
1.5 Plasma Spray Ceramic Coating and Measurement of Developed Coating Behaviour, ICOMAST 2006, 28-30 August, 2006, (In Press).
xvii
KESAN PEMBOLEHUBAH-PEMBOLEHUBAH SEMBURAN PLASMA DALAM MENGHASILKAN SALUTAN SERAMIK
ABSTRAK
Semburan plasma adalah salah satu sistem semburan panas yang boleh
meleburkan sebarang bahan untuk menghasilkan salutan. Suhu yang tinggi yang di
hasilkan oleh api plasma membenarkan bahan seramik seperti ‘zirconia’ dan ‘alumina’
dilebur dan disembur kepada permukaan bahan asas untuk menghasilkan salutan.
Pada peringkat permulaan, prestasi ciri-ciri mekanikal (kekuatan ikatan dan kekerasan)
salutan ‘Yttria Stabilised Zirconia’ (Metco 204NS) pada besi bahan asas yang disalut
dengan salutan ikatan dan tanpa salutan ikatan telah diuji dan disiasat kekuatannya.
Keputusan menunjukkan penggunaan salutan ikatan (serbuk utama nikel) membantu
menambah prestasi ciri-ciri mekanikal salutan. Kemudian, Al2O3 3%wt TiO2 (ALO-105)
disembur kepada besi bahan asas yang disalut dengan Ni 5%wt Al (NI-109) sebagai
salutan ikatan dengan menggunakan sistem semburan plasma ‘praxair’. Salutan-
salutan itu dihasilkan dengan melaraskan parameter-parameter pembolehubah salutan
seperti kadar aliran serbuk, karan elektrik dan jarak semburan untuk kegunaan seperti
tahan haus dan tahan panas. Struktur salutan telah di analisa dengan ‘X-ray
Diffraction’ (XRD) dan ‘scanning electron microscope’ (SEM). Salutan itu juga telah di
uji dengan ciri-ciri mekanikal (kekuatan ikatan dan kekerasan), rintangan haus (POD)
dan rintangan panas (ujikaji kitaran panas).
Keputusan menunjukkan semua salutan ALO-105 membentuk γ-Al2O3
walaupun serbuk asalnya adalah α-Al2O3. Salutan-salutan yang padat dan ketebalan
salutan yang berbeza telah dihasilkan dengan purata poros salutan ialah 7.7%.
Kekuatan ikatan, kekerasan dan sifat haus salutan akan bertambah dengan
pertambahan kadar aliran serbuk (22.5 g/min kepada 26 g/min) dan karan elektrik
(22.5 g/min kepada 26 g/min). Kekuatan ikatan yang tinggi didapati pada jarak
xviii
semburan 75mm dan kekerasan yang tinggi pula didapati pada jarak semburan 90mm.
Di dalam analisa rintangan panas, salutan telah membentuk retak-retak mikro dan
terkopek daripada permukaan bahan asas selepas melalui beberapa ujikaji kitaran
panas. Rintangan panas salutan yang paling tinggi diperolehi pada salutan yang
mempunyai ketebalan yang rendah.
xix
THE EFFECT OF PLASMA SPRAY VARIABLES ON THE DEVELOPMENT OF CERAMIC COATINGS
ABSTRACT
Plasma spray is one of the thermal spray systems which are able to melt any
material for coating. The high temperatures of the plasma flame permit ceramic
materials such as zirconia and alumina powders to be melted and sprayed toward
substrate in producing a coating. At the first stage, Yttria Stabilised Zirconia (Metco
204NS) coating over metal substrate with and without coating bond coat (nickel base
powder) was investigated for the mechanical properties performance (adhesion
strength and hardness). The results showed that applying bond coat coating (nickel
base powder) increased the performance of mechanical properties of ceramic coating.
Then, Al2O3 3%wt TiO2 (ALO-105) was deposited over metal substrate with Ni 5%wt Al
(NI-109) as a bond coat by the Praxair Plasma Spray System. The coatings were
produced and analysed for different variable coating parameters such as powder flow
rate, electric current and stand-off-distance. The ALO-105 coatings structure were
analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The
coatings also were tested for the mechanical properties (adhesion strength and
hardness), wear behaviour (POD) and thermal resistance (thermal cycling test).
The results revealed that all ALO-105 coating contained γ-Al2O3 although the
original powder was α-Al2O3. Dense and different thickness of coating was produced,
with low porosity level at the average of 7.7%. The adhesion strength, hardness and
wear behaviour of coating were improved when powder flow rate (22.5 g/min to 26
g/min) and electric current (550A to 650A) were increased. The high adhesion strength
(11.4 MPa) and high hardness of coating (772.7 Hv) were identified at stand-off-
distance 75mm and 90mm setting. In thermal resistance analysis, the ALO-105 coating
developed micro cracks and peeled off from substrate surface after a number of
xx
thermal cycles test. The highest thermal resistant of coating was identified at the lowest
thickness of the coating.
xxi
CHAPTER 1 INTRODUCTION
1.1 Surface Engineering
Surface engineering is an enabling technology applicable to a wide range of
industrial sector activities. It encompasses techniques and process capable of creating
and/or modifying surface to provide enhanced performance such as wear, corrosion
and fatigue resistance and biocompatibility [Burnell & Datta, 1996]. Surface
engineering now recognized as an enabling technology of major importance in the
successful, most effective and efficient exploitation of material in engineering practice
[Strafford et al., 1995]. Surface engineering enables the design and manufacture of
metallic, ceramic, polymeric and composite material. Most metals, alloys, ceramics and
some intermetallic compounds can be applied as coating either individually or as
mixtures [Grainger & Blunt, 1998]. Any surface engineering technique and process has
an advantage and limitation, which must be evaluated for a specific application. In
general surface technology is used to reduce the cost of component in service by
providing such as:
• Acceptable service life/ reduce downtime costs
• Wear and corrosion resistance on selected surface
• Repair a worn part surface
1.2 Thermal Spray Coating
Thermal Spraying is one of the advance hard facing technologies for surface
preparation and protection [Knotek, 2001]. The technology has been used seriously as
a remedy to combat wear, corrosion, heat, oxidation and other problems occurring
across the whole spectrum of the manufacturing and engineering industries [Harrison,
1996]. The diversity of thermal spraying processes used for hard coating is due to the
1
variety of applications and the required properties, as well as consideration of
economic aspects [Grainger & Blunt, 1998]. Basically, thermal spray coatings are
produced by melting and projecting a powder material and building up a surface
coating at the substrate [Berndt, 1980]. The different coating microstructures and
properties are dependent on the spray technique, powder properties and spray
parameters of the coating [Li et al., 2004]. Microstructure and properties of coating
should be examined in order to obtain good coating bonding. The coating condition
such as porosity, closed pores and un-melted particles are always the cause of defects
in coatings. There are advanced tests or performance tests techniques of plasma
sprayed ceramic coatings in order to determine the coating properties such as
mechanical tests, chemical tests and thermal tests [Herman et al., 1993].
1.3 Ceramic Coating and Application
Plasma spray technique is currently the primary method used commercially to
produce thick coating for ceramic materials. The ceramic coating basically is used to
extend product life, increase performance and reduce production maintenance costs.
The applications involve wear, heat and corrosion resistance, surface restoration and
others basically required in aircraft, automobile, power plant and oil and gas industries
[Reeve, 2001].
Alumina and zirconia ceramics are the most popular material used in plasma
spray coating. The selection of the material depends on the application of coating.
Alumina is mostly used on mating surface to resist abrasive wear, adhesive wear and
corrosion resistant. Zirconia is being increasingly used as thermal barrier coating
especially for gas turbine and diesel engine [Sulzer Metco Inc., 1998]. Effective
ceramic coating should exhibit low thermal diffusivity, strong adherent to the substrate,
phase stability and thermal shock resistance during thermal cycling and provide
oxidation wear and corrosion protection to the substrate [Herman et al., 1993].
2
1.4 Objective of The Study
In this study air plasma spray system is used for the ceramic coating. Ceramic
and bond coating (Nickel base) powders and three variables parameters (current,
powder flow rate and stand off distance) are investigated in order to develop a coating
surface onto low carbon steel substrate. To achieve high-quality coating, it is necessary
to understand the plasma system variables such as plasma spraying process and
coating parameters. The study is focused on the new machine, Praxair Plasma Spray
System with SG-100 gun. Mainly, there are two objectives:
1. To produce ceramic (Al2O3 3%wt TiO2) coatings onto metal substrate with selected
three variables parameters (current, powder flow rate and stand-off-distance) using
Praxair Plasma Spray System.
2. To analyse the relationship of current, powder flow rate and stand-off-distance
parameters of the coated samples for wear and thermal resistance performance.
1.5 Significance of The Research
Plasma spray coating is a unique process. It uses high direct current (DC) and
voltage according to power range to produce plasma flame for melting high
temperature of material [Dobler, 2003] such as ceramics. During process, the molten
powder accelerates towards the substrate, cools and anchors together to form a
strongly adhered coating with the principle technique of mechanical bonding adhesion.
Understanding the plasma system and coating process are the most important to
develop and improve the adherent bonding and quality of coating. Selected ceramic
coating material for specific application such as for wear and thermal resistance
application, type of metal substrate to reduce the constraint to manufacture
components from very high cost bulk material, surface preparation for the strong
adherence bonding between coating material and metal substrate and processing
plasma spraying parameters are related to each other in achieving the high
3
performance and high quality of coating. Selection and application of bond coating
layered between ceramic and metal substrate are also important in order to improve
high adherence coating bonding.
The analysis of phases exist and microstructures of coated samples are
important in order to determine the coating behaviour, coating structure and condition
of powder particles after plasma spray coating process. Measurement of mechanical
properties (hardness and adhesion strength) thickness and porosity of coating relates
to the coating performance such as in wear resistance and thermal resistance
application.
1.6 Scope of The Study
In developing high quality plasma spray ceramic coating, the first important
consideration is the selection of spraying process parameters and different type of
coating materials. In addition, substrate preparation and application of bond coating are
also considered in order to produce high strength and dense coating. In this study, the
coating samples were produced based on three variables which are current, stand-off-
distance and powder flow rate in order to obtain the best processing parameters in
term of coating performance in the wear resistance and thermal resistance application.
The coated samples were analysed for the phase, microstructure and the mechanical
properties (adhesion strength and hardness) and porosity. Finally the coated sample
were tested and analysed for the wear and thermal resistance application.
Overview and literature review of the plasma, thermal spraying, spraying
process and plasma spray system, coating materials and properties, substrate and
surface preparation, coating structure and properties and coating application are
highlighted in Chapter 2. This chapter emphasises more on plasma spray system and
ceramic coating.
4
Chapter 3 emphasises on air plasma spray system, Praxair Technology with
SG-100 Gun which is used in the study. The system such as plasma gun, system
controller, powder feeder, gasses system, cooling system and power system are
highlighted. The important parameters and some initial parameters setting related to
robotic arm (transverse speed, step distance and stand of distance) and powder flow
rate are discussed. Chapter 3 also discusses the selection of ceramic and bond coat
powders, bonding mechanism and the stressed of coating.
The analysis of the selected ceramic and bond coating powders such as
chemical compositions, particle sizes, phase present and microstructure are
highlighted in Chapter 4. The metal substrate preparation, experimental of coating
samples, coating methodology (parameters setting) and testing methodology (adhesion
strength, hardness, bending, thermal resistance, wear resistance and porosity) which
were carried out in this study are presented in Chapter 4.
Result and discussion are presented in Chapter 5. Phases present in the
ceramic coating powders and coated samples were analysed by X-Ray Diffraction
(XRD) equipment. Profile and cross section of the ceramic coated samples were
analysed and examined by Scanning Electron Microscopy (SEM). Results of the test
such as mechanical test (adhesion strength, hardness), wear resistance, thermal
resistance and porosity are investigated. Chapter 5 emphasises the relation of coating
performances (adhesion strength, hardness, wear and thermal resistance) and the
thickness of coating according to the selected variable parameters (current, powder
flow rate and stand-off-distance).
Conclusions and future recommendation of the overall study are discussed in
Chapter 6.
5
CHAPTER 2 LITERATURE REVIEW
2.1 Introduction
Plasma is an ionised gas. When gas is heated enough, the atoms collide with
each other and knocking their electrons off in the process and this will form a plasma
flame. In other definition, plasma is also known as ‘fourth state of matter [Seoul
National University, 2002]. Figure 2.1 shows the state of matters how plasma deforms
from solid, liquid and gas state when heat is added. Plasma is an electrically
conductive gas containing charged particles. When atoms of a gas are excited to high
energy levels, the atoms loose hold of some of their electrons and become ionised thus
producing a plasma containing electrically charged particles (ions and electrons)
[Matejka & Benko, 1989].
Figure 2.1: Plasma flame or ‘fourth state of matter’ was formed when heat added [Seoul National University, 2002].
Basically in plasma spray process, an electrical arc is struck between cathode
and anode of the plasma torch. When the plasma gas flows through the arc it gets
ignited. The plasma is initiated when electrons are accelerated from the cathode to the
anode. As the electrons speed towards the anode they collide with, excite and ionise
the atoms or molecules in the gas. The additional electrons freed by the ionisation are
also accelerated causing further ionisation. These collisions transfer the kinetic energy
of the electrons to the other species and raise the temperature of the gas. The ignited
6
gases come out of the nozzle in the form of a plasma jet of temperature above 15,000
K [Alex et al., 2000]. Figure 2.2 shows the temperature distribution and geometry of
plasma jet during ignition.
Figure 2.2: Temperature distribution and geometry of plasma jet [Knotek, 2001].
2.2 Thermal Spraying System
According to the literature, thermal spraying started in early 1900s by Swiss
engineer, M.U. At the time, lead and tin wires were melted using welding torch by the
energy of an acetylene/ oxygen flame. The wire-arc spraying process was patented in
1908 by Schoop for various metals feedstock [Knotek, 2001]. Development of spraying
techniques and equipment progressed slowly during 20s and 30s. Late 50s and early
60s the thermal spraying technology was expanded due to the increasing demand of
high temperature and wear resistant materials and coating systems [Burnell & Datta,
1996]. At present time, thermal spray processing is highly demanding in surface
engineering technology. Thermal spray coatings for ceramic, metallic and carbide help
to improve surface properties of parts such as in wear and corrosion resistance [Reeve,
2001]. The principle characteristics of thermal spraying coating process are as follows
[Grainger & Blunt, 1998]:
7
• The strength of the bond between coating and substrate is dependent on the
materials and process used.
• Able to apply any coating materials to the substrate that are unsuited compared to
welding process.
• Able to deposit coating material from thinner to thicker layer of coating.
• Almost all material compositions may be deposited such as metals, ceramics,
carbides, polymers or any combination.
• Most processes are cold compared to welding process.
• The process can be operated in air with great flexibility.
Thermal spraying is a generic term used to describe a group of processes.
Thermal spraying is an attractive coating technique as it offers a wide choice of
materials and processes that have a reduced impact on the environment when
compared to conventional plating processes. Figure 2.3 shows the group of thermal
spraying coating in various applications.
Basically there are 4 types of thermal spraying system, including flame spraying,
high velocity oxy-fuel, arc spraying and plasma spraying. All thermal spraying
processes rely on the same principle of heating a feedstock, (powder or wire) and
accelerating it to a high velocity and then allowing the particles to strike the substrate.
The particles will then form and freeze onto the substrate. The coating is formed when
millions of particles are deposited on top of each other. These particles are bonded by
the substrate by either mechanical or metallurgical bonding [Dobler, 2003].
8
Figure 2.3: Group of thermal spraying process [Knotek, 2001].
2.2.1 Flame Spraying
Flame spraying is a low cost process compared to other thermal spraying
processes. Flame spraying does not produce distortion or effect of heat treated on the
part during spraying process due to the temperature of flame produced at the nozzle tip
which is only 3,000oC. The flame spraying can be used for any metal, ceramic and
plastic substrate for coating process. The process produces thicker, contains high
levels of oxides and porous coating together with the option of achieving a rough
surface finish than other thermal spraying process [Knotek, 2001]. The process relies
on carefully control of chemical reaction between oxygen and a fuel (acetylene) to
produce heat with temperatures varying up to 3,000oC. In flame spraying process,
feedstock material is fed onto the flame in the form of wire or powder and compressed
air is used to atomise the molten metal and accelerate the particles onto the substrate
to produce a coating [Dobler, 2003]. Mostly spray coating is used for corrosion
9
resistance applications. Figure 2.4 shows a typically gun design and flame spraying
coating process.
Figure 2.4: Gun design and flame spraying coating was sprayed onto substrate [Plasma & Thermal Coatings Applied Surface Technology Ltd, 2005].
2.2.2 High Velocity Oxy-Fuel (HVOF)
The high velocity oxy-fuel (HVOF) has extended the range of thermal spray
applications. HVOF utilizes the combustion of gases, such as hydrogen or a liquid fuel
such as kerosene. A mixture of process gases is injected into the combustion chamber
of a torch at high pressure and ignited to produce the flame. The process creates a
very high velocity which is used to propel the particles at near supersonic speeds
before impacting onto the substrate and producing a coating. HVOF is designed to give
high levels of coating density and adhesion to a substrate [Grainger & Blunt, 1998].
Since the temperature of the flame is about 3,000oC, HVOF thermal spraying is
preferred for spraying tungsten carbide and/or corrosion-resistant carbides, alloy of
hastelloy, triballoy and inconel [Knotek, 2001]. Due to high kinetic energy and low
thermal energy of hydrogen, HVOF can produce high bond strengths, extremely high
coating density (less than 1% porosity) and low oxide content (less time speed of
particle spending within the heat source) [Grainger & Blunt, 1998; Knotek, 2001].
Figure 2.5 shows a typical gun design and HVOF coating onto substrate.
10
Figure 2.5: Gun design and HVOF coating was sprayed onto substrate [Plasma & Thermal Coatings Applied Surface Technology Ltd, 2005].
2.2.3 Arc Spraying
In arc spraying process, feedstock in the form of two wires is simultaneously
brought into contact with each other at a nozzle. The two wires with electric arc are
used to provide the heat source. As the wires are fed towards each other at a nozzle,
as it is touched, the electrical load placed on the wires causes the tips of the wires to
melt and to create a flame temperature around 4,000oC. An atomizing gas such as air
or nitrogen is used to strip the molten material from the wires and transport it to the
substrate [Dobler, 2003].
Arc spraying is among the lowest cost running process compared to the other
thermal spray systems. The materials applied by arc spraying are only for electrically
conductive wires such as stainless steel, hattelloy, nickel aluminides, zinc, aluminium,
and bronze. Arc spraying has the highest deposition rate of thermal spraying process.
It can be used to spray large areas or large numbers of components on repetitive
production line operation such as bridges and offshore fabrication with zinc and
aluminium coating to give corrosion protection on the structure. Other coating
applications are worn engineering components such as bearing and shaft with steel
and bronze alloys coating. [Knotek, 2001; Grainger & Blunt, 1998]. Figure 2.6 shows a
typical gun and coating process of wire arc system onto substrate.
11
Figure 2.6: Gun design and wire arc spraying coating was sprayed onto substrate [Plasma & Thermal Coatings Applied Surface Technology Ltd, 2005].
2.2.4 Plasma Spray
Plasma spray is regarded as the most versatile of all the thermal spray
processes e.g. flame spraying, arc spraying and HVOF. The plasma spraying process
involves the latent heat of ionised inert gas being used to create the heat source. The
most common gas used to create the plasma is argon as the primary gas and
hydrogen or helium as the secondary gas [Knotek, 2001]. However the gas usage
depends on the type of material to be sprayed and coating application.
Plasma spray is able to use and melt any variety of ceramic materials [Gansert,
2002]. The plasma spray system consists of an electronically controlled power supply,
a PLC-based operator control station, a gas mass flow system, a closed-loop water
chilling system, a powder feeder and a plasma gun. A primary inert gas, such as argon
is injected between two water-cooled electrodes (anode and cathode) in the gun,
where it is ionised to form a plasma jet when ignited. Any powders is injected into the
plasma flame will melt and subsequently deposited onto the component to form a
coating [Berndt et al. 1979].
Most of the ceramic materials used in plasma spray coatings are chromium
oxide, zirconium oxide and aluminium oxide [Knotek, 2001]. The particle velocities of
12
plasma spraying are higher than for those flame and arc spraying which produce
denser coatings and finer surface roughness [Dobler, 2003 & Fazan, 1997]. Figure 2.7
shows a typical gun and plasma spray coating process.
Figure 2.7: Gun design and plasma spray coating was sprayed onto substrate [Plasma & Thermal Coatings Applied Surface Technology Ltd, 2005].
The versatility of plasma spray has been widely recognised in many numbers of
industries. The unique features that characterise plasma spray processing are listed
below [Herman et al., 1993]:
• It can be used to deposit a wide range of ceramics and metals and any
combinations of the materials.
• It is able to develop homogeneous coating without composition change with coating
thickness.
• High deposition rates are possible without extreme investments in capital
equipment.
• Plasma spray can be applied at any environment conditions (air, vacuum and
underwater).
13
2.3 Plasma Spray System
Plasma system can be classified into RF plasma and DC Plasma. The
processes are described as below:-
• RF plasma – The gas is passed through a radio frequency field so that electrical
coupling occurs and energy is transferred to the gas.
• DC plasma – The gas is used as a medium in which a direct current arc is
established between two or more electrodes.
In RF plasma, no electrode is necessary to produce the flame, no contamination
of the working gas or materials injected into plasma occur during coating process
[Berndt, 1980].
Air plasma spray system used for the study is classified as DC plasma. The air
plasma spray consist of several principle components to form a complete system in
order to generate plasma flame, bring coating material to the plasma flame and
produce a coating onto a substrate. In general, the air plasma system consists of:
• Plasma torch
• DC power supply
• Control and instrumentation system
• Cooling water system
• Gas supply system
• Material feeder system
14
The principle components of the plasma spray system are arranged and linked
together in an optimum way in order to ensure maximum efficiency at minimum power,
pressure and space. Figure 2.8 shows a flow diagram of plasma spray operation.
Figure 2.8: Basic system (gases, power, cooling, controller, feeder and plasma torch) of the plasma spray equipment [Gordon England, 2004].
2.3.1 Plasma Torch System
Plasma torch is one of the most important systems of the plasma spray
technology [Matejka & Benko, 1989]. Plasma torch is designed purposely to produce
plasma flame jet at high velocity, high temperature, able to melt any coating material
and stable in operation [Herman & et al., 1993]. Basically plasma torch components of
the DC plasma torch are summarised as below:
• The cathode is usually made of tungsten enriched by 2% ThO2 (Thorium Oxide).
Basically tungsten serves to lower electron emission threshold from 4.52 eV to 2.63
15
eV, allowing an arc to be established at a lower voltage gradient [Herman et al.,
1993] and hindering cathode wear due to impurities in the plasma gases. The
oxygen and humidity present in the impurities gases will oxidise cathode rapidly at
high temperatures and reduce the electrode life.
• The nozzle which acts as anode is subjected to considerable heat effect and must
transfer several kW of output per each square centimetre. Basically anode is made
from high purity copper with high thermal conductivity and water cooled directly
[Matejka & Benko, 1989]. Normally the anode incorporates the nozzle or anode
throat [Herman et al., 1993].
• An insulating medium between cathode and anode – normally Teflon or ceramic
are used as insulating medium.
• A torch body is used to join and align the cathode, anode and insulating medium
into an integral assembly.
• Attachments for water lines and power cables.
Nowadays, there are two types of cathode design use in the plasma torch.
These are solid cathode tip and hollow cathode tip. Matjeka [1989] had pointed that the
use of solid cathode tip is much more better instead of the use of hollow cathode tip
due to cooling water could approach the tip surface. The tip shapes (e.g. acute, blunt or
hemispherical) depend on the loading and the plasma gas used. The nozzle profile
normally designed in cylindrical and conical shape [Berndt et al., 1979]]. A simplified
diagram of the plasma torch configuration for a DC plasma torch is shown in Figure 2.9.
The plasma arc is produced between two electrodes (i.e. cathode and anode)
when the gas flowed through the electric arc, ignited and transformed into ions of high
temperature. The anode and cathode have a limited life from 20 to 200 hours
depending on the operating power level of plasma torch. The anode and cathode have
to be replaced when it wears. Water leakages have to be avoided within cooling circuit
16
at plasma torch where it will lead to rapid erosion of the anode and cathode [Herman et
al., 1993].
Recently, the modern plasma torch design uses different combination of plasma
gases (Ar, N2, Ar + H2, N2 + H2, Ar + He) such as Torches 7 MB or 9 MB from Metco
cooperation. The principle of plasma torch designed is similar compared to the oldest
design. The different mainly are the shape of electrodes, the supply of plasma gas
between electrodes, the method of powder injection into plasma beam and geometry
design of the torch [Berndt, 1980].
Figure 2.9: Schematic of plasma torch type F4 produced by Plasmatechnik Co.
[Matejka & Benko, 1989]
The plasma beam temperature of plasma torch depends mainly on the
ionisation degree, type of plasma gas and working parameters. Figure 2.10 shows the
temperature comparison of argon (a) and nitrogen (b) gasses of plasma beams which
has been done by Jehn [1992]. The experiment used similar type of plasma torch
(6mm diameter of nozzle) and gas flow rate setting. The figure illustrates that the
temperatures measured in the nitrogen gas is substantially lower than argon gas. The
17
temperature of plasma beam e.g. 10,500 K of nitrogen and argon gasses are archived
at distance of 15mm and 20mm from the tip of plasma torch [Matjeka & Benko, 1989].
Dis
tanc
e fro
m n
ozzl
e (c
m)
Dis
tanc
e fro
m n
ozzl
e (c
m)
Figure 2.10: Temperature distribution of plasma beam (a) for argon gas and (b) for nitrogen gas [Matejka & Benko, 1989].
Figure 2.11 show how the change in gas flow rate, current, nozzle diameter and
arc length affect the temperature and output of plasma beam. If the current is
increased, the arc temperature and output of beam is increased. If the gas flow rate
increased, the arc temperature decreased and beam output increased [Matjeka &
Benko, 1989].
18
Tem
pera
ture
of
plas
ma
beam
L: arc length I: Arc current d: Nozzle diameter Q: Gas flow rate
Out
put o
f pla
sma
beam
Main parameters of plasma torch
Figure 2.11: Effect of plasma torch parameters on temperature characteristics of plasma beam (temperature and output of plasma beam) [Matjeka & Benko, 1998].
Plasma beam velocity also plays an important role in plasma spray coating. The
velocity of a plasma beam can be calculated in terms of the plasma beam output, gas
volume and its properties and the nozzle diameter based on the relationship below
[Matjeka & Benko, 1989]:
V = A. (Qo/ d2) . (T/M) (1)
Where;
V - Plasma beam velocity (m.s-1)
Qo - Volume of gas flow rate (m3.s-1)
T - Gas temperature (K)
d - Nozzle diameter (m)
A - Constant
M - Molecular weight of gas
Referring to equation (1), plasma beam velocity is directly proportional to the
gas flow rate and indirectly proportional to the square of nozzle diameter. This indicates
19
that increasing gas flow rate will decrease the plasma temperature [Matjeka & Benko,
1989].
2.3.2 DC Power Supply System
Power Supply is the main important component in plasma spraying system. It is
designed to suit the specific voltage and current characteristic of the plasma torch. In
order to produce superb plasma sprayed coatings using wide variety of materials,
plasma equipment must perform consistently, whether the energy level is as low as 15
kW or over 200kW [Praxair Surface Technology Inc., 2001]. Basically thermal plasma
spray technology used energy less than 100 kW [Herman et al., 1993]. An example in
Praxair Technology Inc. [2001], the energy used for the plasma torches SG-100 and
SG-200 is up to 80 kW and 40 kW. The power of DC circuit can be calculated from
Joule relation (voltage x current). The operating characteristic of a torch are given by
the voltage-current relationship and indicates the power level of the torch [Herman etc,
1993]. The arc characteristics and the power supply determine the stability of voltage
and operating current. The plasma arc is produced steadily when the arc voltage
equals to the voltage across the sources terminals at point A and B which is shown in
Figure 2.12.
At point B, any decrease in the current will extinguish the arc since the voltage
requirement of the torch cannot be supplied by the generator. If current increases then
the arc voltage decreases and equilibrium point is reached at A. At this point any
current change causes a voltage change which returns the current to point A. The torch
resistance is given by the slope (V/I) of the straight line as shown in Figure 2.12. The
maximum operating resistance of the torch is given by the tangent to the source
characteristics at point C.
20
Figure 2.12: Voltage-current characteristics of a torch and power supply [Berndt, 1980].
2.3.3 Control and Instrumentation System
The control and instrumentation system is used for controlling the overall
system efficiently during the operation of the system. All the switch systems such as
gasses, ignition and feedstock feeding are contained in one control box. However the
robotic arm control is separately from the control box system mentioned above. Figure
2.13 shows a typical example of a control unit which is used in a plasma spray system.
Figure 2.13: Control system 3710 of Praxair Technology [Praxair Surface Technologies, 2001]
21
2.3.4 Cooling Water System
Cooling system in plasma spray technology is used to cool the plasma torch.
Many problems associated with the running of torch can be attributed to a poor cooling
system and may result in overheating of anode, cathode and seals [Herman et al.,
1993]. Normally water is used as the agent of the cooling system. The cool water is
pumped to the torch in closed loop process from the cooling tower. The cool water flow
rate is very important in order to ensure the plasma torch cooled sufficiently during
spraying process. Otherwise, it may result wear of plasma torch component and need
to be replaced frequently especially for the tungsten electrode.
2.3.5 Gas Supply System
Gas supply system is linked with ignition system in order to ignite the plasma
arc. The gas will be injected to the plasma torch in order to produce plasma flame. The
gas also supplied to material feeder unit in order to seal coating powder from any
oxidation and assisted coating powder to be fed to the plasma torch. Basically plasma
spray gasses are divided into primary and secondary, i.e. gases such as Ar and N2 are
normally used as primary gas whereas gasses such as He and H2 are used as
secondary gas. The plasma forming gas is selected on the basic of the desired
temperature and velocity of plasma beam and the degree of inertness gas [Matejka &
Benko, 1989].
Application of gas mixture such as Ar + He and Ar + H2 with different properties
normally is used in order to increase the enthalpy and velocity of the plasma arc. The
temperature and enthalpy of gases can be controlled over a wide range by variation in
electrical input, flow rate and composition of plasma gases in the plasma arc.
22
2.3.6 Material Feeder System
The material feeder system is used for feeding an adjustable, uniform and
reproducible amount of powder to the plasma flame. Basically the feeder system is
designed and developed suitable for any coating powders and provided maximum
accuracy of the volume of powder fed to the plasma flame. The conventional feeder
system was upgraded and designed to modern feeder system which employs inert
carrying gas by which the powder supplied through a tube to the plasma gun. The
modern material feeder system was designed with automatic system to feed the
powder to plasma flame. The system normally includes powder container (in vacuum
condition), heater (attached to the container to heat the powders) and controller unit (to
feed powder to plasma gun). The velocity of the carrying gas is set in such a way that
powder remains suspended during the transfer and injected to the plasma gun at the
optimum kinetic energy [Matejka & Benko, 1989 & Praxair Technologies Inc, 1999].
2.4 Coating Material
Coating material is the main agent in order to develop a coating. The selection
of coating material is dependent on application of the coating such as wear, thermal
barrier, friction and the type of substrate material to be coated. In general most
powders used in advance thermal spray depositions have particle sizes in the range of
5 to 60 microns [Tucker, 2001]. Coating materials require quality control during storage
and handling to avoid outside contamination which will affect the coating quality
[Herman et al., 1993].
2.4.1 Coating Material Properties
Basically in thermal spray coating, available coating materials in market are a
spray-able form. The feedstock used in the thermal spray coating have to meet further
requirement such as chemical homogeneity, density, flow behaviour, size and shape
23
distribution of powders [Knotek, 2001]. For example, size and shape are particularly
significant for meltability considerations, and the shape of the particles will determine
the flowability of powder into the flame. Flake-shape powder will not flow smoothly,
resulting in a discontinuous, pulsing feed of powder into the flame, leading to a non-
uniform stream of molten particles and produce a poor coating. Spherical shape of the
coating powder enables smooth flow, uniform feeding and deposit with continuous flow
[Herman et al., 1993]. The techniques for producing powder materials are water and
gas atomization, crushing and milling and chemical techniques such as sol-gel,
agglomeration and spray drying [Knotek, 2001]. Now almost all material is available for
spraying application such as metals and alloys, hard compound or hard metals and
ceramics.
According to Herman [1993], the optimal particle size distribution of coating
powders is between 10 and 44 micrometers. Most of the powder used for advanced
thermal spray deposition falls between 5 and 60 microns in size [Tucker, 1994]. The
specific coating powders size range is related to torch or detonation gun design and the
heating characteristics of powder. To achieve uniform heating and acceleration of a
single component powder, it is advisable to have the size distribution as narrow as
possible [Tucker, 1994]. The coating sprayed using coarse powder produces very
porous and laced with cracks [Herman et al., 1993]. Fine powders are accelerated and
heated more rapidly, but they also tend to lose momentum more rapidly when spraying
at longer stand off distance. The coating results are dense and highly stressed
coatings [Tucker, 1994]. Coating powder specification such as chemical analysis,
shape characterisation, size distribution and flowability should be identified for better
coating produced. A wide variety of equipment is available for coating powder
analyses, and selection of a specific technique or type of test will vary with the type of
powder. The coating powder should be kept clean and dry. Contamination and moist
24