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A novel process for the liquid feedstock plasma spray of ceramic coatings with nanostructural features [Elektronische Ressource] / von Roberto Siegert

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191 pages
A Novel Process for the Liquid Feedstock Plasma Spray of Ceramic Coatings with Nanostructural Features Dissertation Zur Erlangung des Grades Doktor-Ingenieur Der Fakultät für Maschinenbau Der Ruhr-Universität Bochum Von Roberto Siegert Aus Deutschland Bochum, 2005 Tag der mündlichen Prüfung: 13. Oktober 2005 Erster Referent: Prof. Dr. D. Stöver Zweiter Referent: Prof. Dr. K. Landes Abstract I 1. Abstract This study is focused in the process optimization and implementation of the liquid feedstock plasma spray process conducted at atmospheric pressure, in which the feedstock is injected into a free expanding plasma jet generated by industrial-type DC torches, in the form of a suspension or solution-precursor. In here, a liquid carrier is used to transport feedstock particles with sizes falling into the submicrometric and nanometric range, into the useful region of the plasma jets. This condition brings about a series of important changes in the microstructure of the deposited coatings, highlighted by a reduction in coating-building splats of up to 3 orders magnitude as compare to those observed in standard plasma spray process, and by a fine porosity with pore sizes in the nanometer range. A universal apparatus for the precise dosage of the liquid feedstock was developed considering the geometrical constraints of the different torches under consideration.
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A Novel Process for the
Liquid Feedstock Plasma Spray
of Ceramic Coatings with
Nanostructural Features




Dissertation
Zur
Erlangung des Grades
Doktor-Ingenieur

Der
Fakultät für
Maschinenbau
Der Ruhr-Universität
Bochum

Von


Roberto Siegert

Aus Deutschland
Bochum, 2005





Tag der mündlichen Prüfung: 13. Oktober 2005


Erster Referent: Prof. Dr. D. Stöver

Zweiter Referent: Prof. Dr. K. Landes
Abstract I
1. Abstract
This study is focused in the process optimization and implementation of the liquid feedstock
plasma spray process conducted at atmospheric pressure, in which the feedstock is injected
into a free expanding plasma jet generated by industrial-type DC torches, in the form of a
suspension or solution-precursor. In here, a liquid carrier is used to transport feedstock
particles with sizes falling into the submicrometric and nanometric range, into the useful
region of the plasma jets. This condition brings about a series of important changes in the
microstructure of the deposited coatings, highlighted by a reduction in coating-building splats
of up to 3 orders magnitude as compare to those observed in standard plasma spray
process, and by a fine porosity with pore sizes in the nanometer range.
A universal apparatus for the precise dosage of the liquid feedstock was developed
considering the geometrical constraints of the different torches under consideration.
Moreover, various injection port designs, mainly grouped as atomizers or mechanical
injectors, were analyzed. A series of experiments, mostly based on the characterization of
the sprayed feedstock and resulting coatings, were design to isolate the most relevant
process parameters.
The results from the splats analysis were used to spatially resolve the trajectories of the
feedstock particles resulting from the interaction of the liquid carrier with the flow of the
plasma jet. SEM analysis of the splats and sprayed bulk material morphologies collected at
different standoff distances, was used to study the process of agglomeration and
consolidation processes that follow the fragmentation and evaporation of the feedstock
droplets. Image analysis tools were used to quantify and grouped into size classes the
detected splats. These results were to establish the correlations between feedstock
properties and the particle size distributions of the sprayed feedstock, which were positively
correlated to spray and process efficiencies, which lay bellow 25% and are relatively low
when comparing to the efficiency observed in conventional plasma spray. A model that
describes the fragmentation of the liquid feedstock and its evaporation was formulated. The
droplets size and time scales are proven to be in good agreement with the experimental
results.
Furthermore, the properties of the various coatings sprayed with the developed apparatus
were correlated to results from the splat analysis. The study of the cross-section and fracture
surfaces of the coatings served to establish the links between the coating microstructural
features and interaction of the liquid feedstock with the plasma jet, providing useful
information for the validation of the theoretical model. Abstract II
Macroscopic and microstructural properties of the sprayed coatings are in good agreement
with the results previously reported in literature, which are characterized by a columnar
growth between multi-lamellar structures reach in segmentation cracks and with density
-3values of about 4 to 4.3 g·cm . The resulting coatings are mainly porous, with porosity
values in excess of 30%. Moreover, various areas of implementation in which the new
microstructural features inherent to the LFPS process, have being explored. These areas
were mainly the spraying of thermal barrier coatings and the deposition of the various
functional layers of the solid-oxide fuel-cell systems, with an emphasis in the development of
gas-tight electrolyte layer based on yttrium-stabilized zirconia.
A process for the deposit of dense gas-tight layers with an average thickness of 50 µm
resulted from the parametrical study, which resulted in a patent application. This method had
the peculiarity of producing a bistratified layer system, in which the upper region of the
coating remelts due to the high thermal flux between the plasma jet and the substrate. In
here, the formation of the bistratified layer system during the uninterrupted spray process, is
inherent to the set of torch and feedstock parameters.












Index III
Index

i. Abstract I
ii. Index III
iii. List of Abbreviations VII
1. Introduction 1
2. Research Goals 3
3. Current State of the Art: Bibliographical Review 5
3.1 Process Description 5
3.2 Torch Architectures and Plasma Jets 8
3.2.1 Finger-Shaped Cathode Design 10
3.2.2 Triplex Torch 12
3.3 Plasma Forming Gases 15
3.4 Coating Properties 16
3.5 Plasma Spray Process Diagnostics 16
3.5.1 Jet Fluctuations 17
3.6 Different Approaches to the Atmospheric
Plasma Spraying of Liquid Feedstock 20
3.7 Plasma Spraying of Solid-Oxide Fuel Cell
Components 27
3.8 Plasma Spraying of Thermal Barrier Coatings 29
4. Experimental Setup 33
4.1 The Plasma Spraying Process –
DC Plasma Torches 33
Torch Fluctuations 34
4.2 Feeding Mechanism 35
4.2.1 External Injection 37
4.2.2 Internal Injection 38
4.3 Plasma Plume Characterization 39
4.3.1 “Splat” Analysis 39
Line-scan-test 40
Substrate Holder 4
Shutter Mechanism 1
4.3.2 Plasma Spray “Footprints” 42
4.3.3 Sprayed Particle Bulk Collection 43
4.3.4 Torch Extension Nozzle 45 Index IV
4.3.5 Screening Device 46
4.4 Materials and Liquid Feedstock 47
Suspensions 47
Feedstock Powder Characterization 48 Sol-Gels 9
4.5 Coating Deposition 50
4.5.1 Substrate Preparation 51
4.5.2 Control Unit 51
4.6 Coating Characterization 53
4.6.1 Cross-section and Fractographic Analysis 53
4.6.2 Metallographic Specimen Preparation 53
4.6.3 Light-Microscopy 54
4.6.4 SEM Analysis 54
Image Analysis and Particle Count 55 Shading Correction and Filters
4.6.5 Estimation of the Coating Density 57
4.7 Surface Glazing or Remelt 58
4.8 Thermal Cycling Specimens
5. Results 61
5.1 Influence of the Plasma Torch Architecture on
the Free-Expanding Plasma Jet – Region I 64
Plasma Jet Fluctuations 64
5.2 Injection Parameters – Region II 70
5.2.1 Feedstock Transport Mechanism 70
Atomizing Valve 70 Mechanical Injector 71
5.2.2 Location of the Injection Port 73
5.2.3 Liquid Feedstock and Plasma Interaction:
Operating Modes 75
5.3 In-flight Characterization of the Sprayed Feedstock –
Region III 78
5.3.1 Collection of Bulk Sprayed Particles 79
5.3.2 Splat Analysis 82
Large Clusters 84 Splats and Splat-Forming Agglomerates 84
Non-splashing Agglomerates 87
Overspray Index V
5.3.3 Influence of the Viscosity and Surface Tension 92
5.3.4 Spray Pattern 93
5.4 Influence of the Spray Parameters on the
Coating Microstructure – Region IV 94
5.4.1 Plasma Forming Gases and Air Entrainment 94
5.4.2 Injection Velocity and the Mass Flow 96
Coatings Sprayed with an Atomizer 96
Mechanical Injector
Adapted to the F4 Torch 98
Mechanical Injector
Adapted to the Triplex Torch 103
5.4.3 Liquid Feedstock Mass Loading 108
5.4.4 Influence of the Standoff Distance 110
5.4.5 the Substrate Roughness 112
5.5 Suspensions versus Sols 112
5.6 Process Efficiency and Deposition Rate 115
5.7 The Use of the Torch Extension Nozzle 117
5.8 The Use of a Plasma Plume Screen 121
5.9 Coating Glazing 122
5.10 Liquid Feedstock Plasma Spraying of Thermal
Barrier Coatings 123
5.11 Process Description – Synopsis 128
6. Discussion 131
6.1 Torch Parameters and Plasma Jet Properties – Region I 132
6.2 Injection Parameters – Region II 134
6.2.1 Feedstock Transport Mechanism 134
6.2.2 Feeding Rate 137
6.2.3 Injection Velocity 137
6.2.4 Influence of the Mass-Loading 139
6.2.5 Location the Injection Port 140
6.3 Liquid/plasma Interaction and Formulation of a
Deposition Model – Region III 141
6.3.1 Droplets Fragmentation Mechanism 142
Fragmentation of the liquid stream into droplets 143
Fragmentation of parent droplets into small satellites 144
Time Scale τ for Fragmentation 146
Index VI
6.3.2 Droplet Heating and Evaporation 147
Heating up the fragmented satellite to the evaporation
temperature of the solution 147
Evaporation Time 149
Summary of the Fragmentation Process 151
6.4 Spray Parameters and Coating Properties – Region IV 153
6.4.1 Feedstock Materials 154
6.4.2 Influence of the standoff distance 154
6.4.3 Substrate Roughness 156
6.4.4 Coating Density and Porosity 157
6.4.5 Microstructural Features 157
6.4.6 Splat Analysis 158
6.5 Coatings Sprayed with the F4 Torch 159
6.6 Coatings Sprayed with the Triplex I Torch 160
6.7 Areas of Implementation 161
6.7.1 Liquid Feedstock Plasma Spray of SOFC Components 162
6.7.2 Thermal Barrier Coatings 162
7. Conclusions 163
8. Reference 165
9. Acknowledgement 175
10. Appendix 17





Abbreviations VII
List of Abbreviations
APS Atmospheric Plasma Spray
BSE Backscattering Electron
CTE Coefficient of Thermal Expansion
CVD Chemical Vapor Deposition
DC Direct Current
EVD Electrochemical Vapor Deposition
FET Field Effect Transistor
LDA Laser Doppler Anemometry
LFPS Liquid Feedstock Plasma Spray
LTE Local Thermal Equilibrium
PETN Plasma Expanded Through a Nozzle
PIV Particle Image Velocimetry
PTV Particle Tracking Velocimetry
PVD Physical Vapor Deposition
Re Reynolds Number
RF Radio Frequency
SE Secondary Electron
SEM Scanning Electron Microscopy
SOFC Solid Oxide Fuel Cell
SPPS Small Particle Plasma Spray
TBC Thermal Barrier Coating
TGO Thermally Grown Oxide
TIG Tungsten Inert Gas
YSZ Yttrium Stabilized Zirconia

Units
A Amps
dB Decibel
g Grams
Hz Hertz
K Kelvin
kW Kilowatts
m Meters
Pa Pascal
SLPM Standard Liters per Minute
V Volts Abbreviations VIII