RELATION OF THE ADHESION t-t Jcjfl OF PLASMA SPRAYED

237 Pages·2014·8.53 MB·English

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RELATION OF THE ADHESION
OF PLASMA SPRAYED COATINGS
TO THE PROCESS PARAMETERS
SIZE, VELOCITY AND HEAT CONTENT
OF THE SPRAY PARTICLES
J.M. HOUBEN
The iron hand of the brave german
knight Gotz von Berlichingen"' 1535.
Mutantur tempora, quae homines sentiunt non mutantur:
A manu ferrea Godefridi, equitis Berlichingensis. ad art em
plasmatis spargendi.
RELATION OF THE ADHESION OF PLASMA SPRA YEO COATINGS
TO THE PROCESS PARAMETERS
SIZE, VELOCITY AND HEAT CONTENT
OF THE SPRAY PARTICLES
RELATION OF THE ADHESION OF PLASMA SPRAYED COATINGS
TO THE PROCESS PARAMETERS
SIZE, VELOCITY AND HEAT CONTENT
OF THE SPRAY PARTICLES
PROEFSCHRIFT
ter verkrijging van de gra.ad van doctor a.an de
Technische Universiteit Eindhoven, op gezag van
de rector magnificus, Prof. ir. M. Tels, voor een
commissie a.angewezen door het College van
Decanen in het openba.ar te verdedigen op
vrijdag 9 december 1988 te 16.00 uur.
door
JOHAN MARTIN HOUBEN
geboren te Oirsbeek
drutc:: wibro (lf&&ef't&t:iedrukkerij, helrnond
Dit proefschrift is goedgekeurd
door de promotoren:
1e promotor: Prof. dr. ir. J.A. Klostermann
2e promotor: Prof. dr. R. Metselaar
Aan Johanna, Tim, Rita, Max en Inez
CONTENTS page
0 GENERAL INTRODUCTION 1
1 INTRODUCTION TO PLASMA SPRAYING:
4.
1.1 Macro characterization of the plasma
spray process.
1.2 Micro characterization of the thermal 14
interaction between a single plasma spray
particle and the substrate.
1.3 Material transport across the interface 28
between spray material and substrate.
1.4 Background of this thesis. 37
2 COLLISION THEORY:
2.1 Introduction. 39
2.2 Thermodynamical aspects of the collision 39
of a spherical spray particle onto a rigid,
smooth surface.
2.2.1 Compression. 39
2.2.2 Relaxation. 56
2.2.3 Final energy balance 67
2.3 Mechanics of a colliding spherical particle. 95
3 EXPERIMENTS:
3.1 Introduction. 114
3.2 First order of spread particle morphology. 114
3.2.1 How sprayed coatings are built up 125
3.3 Second round categorization op spread 129
particle morphology.
3.3.1 Experimental approach. 129
3.3.2 Determination of the particle heat content 133
subsequently the temperature at the moment
of impact.
3.3.3 Estimation of the particle velocity. 145
3.3.4 Experimental results regarding the morphology. 146
3.3.5 Concluding remarks regarding the morphology. 171
3.4 Mechanical testing of sprayed coatings. 172
3.4.1 Test method. 174
3.4.2 Test results. 175
4 DISCUSSION:
4.1 On the relation between the determined 178
morphologies and the theoretical model.
4.2 On the relation between the potential energy, 181
or for weak shocks, the kinetic energy of a
particle and the final surface energy of a
spread particle.
5 CONCLUSIONS 183
6 APPENDIX 185
7 LIST OF SYMBOLS 212
8 REFERENCES 215
9 SUMMARY 220
10 ACKNOWLEDGEMENT 222
11 SAMENV ATTING 223
12 CURRICULUM VITAE 225
13 DANKWOORD 226
0 GENERAL INTRODUCTION
This thesis is dedicated to the technology of plasma spraying which is a method
to apply coatings to a substrate. All thermal spray techniques, and plasma
spraying is one of them, use the thermal and kinetic energy of a combustion flame
or an electric arc discharge to accelerate and to heat solid powdered material
which is to be projected onto a substrate. In this way machine parts (or
constructions) can be improved regarding their surface properties, since very often
a machine part fails due to surface attacks such as fatigue, wear, erosion,
corrosion, oxidation or a combination of some of these phenomena. A coating also
may be applied to insulate or to conduct heat and electricity. At last, thermal
spray techniques are employed to produce free standing bodies, mostly made of
refractory or ceramic materials. Plasma spraying is a technique where an inert, or
partly inert gas is heated by a controlled arc discharge to such a temperature
level(~ 10.000 K) that partly ionisation of the gas occurs. This gas has a very
high enthalpy and is called a plasma, the fourth state of matter. The plasma
generating device is called a plasma torch.
Apart from the high energy content an arc plasma for thermal spraying is
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characterized by it's relatively high torch-outflow velocity(~ 200 ms - ). The
plasma leaves the torch as a jet stream with an effective diameter of
approximately 5 mm and a length around 45 mm. Powdered material, size
5 - 100 pm, is injected side on into the plasma jet which accelerates and heats
the powder. The energy transfer from the plasma to the spray particles consists of
a thermal part and a kinetic part. Both energies are strongly coupled. If the
control setting of the torch is changed, the heat content and the velocity of the
particles change simultaneously, which makes the tracing of optimum spray
conditions for the achievement of pre-set coating proporties rather elaborate, if
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possible. The built up of a plasma sprayed coating can be reduced to the many
fold repeated process of interaction of one single particle with the substrate as far
as the adhesion to the substrate is concerned.
The subject of this thesis is the investigation of the relation between the input
parameters velocity, size and heat content of a spray particle and the morphology
of the spread particle. Subsequently, the relation between the morphology and the
adhesion will be determined in order to find the optimum combination of the
spray parameters with regards to the adhesion strength. The individual influence
of heat content and particle velocity can be determined by the application of a
rotating substrate support. Four polished substrates are fixed at the perimeter of
a disk shaped substrate holder. The disk is mounted on the shaft of a grinding
device, which permits the application of rotational speeds up to 24.000
revolutions per minute. With a disk diameter of 100 mm, the rotation velocity of
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the substrate amounts up to 125 ms- . This velocity can be added to or
subtracted from the standard flight velocity of a particle with a pre-set heat
content. So, the impact velocity can be varied over rather a wide range,
independent of the standard heat content. The third main input parameter is the
particle size. It can be fixed within a narrow range by sieving. In this way it is
possible to determine experimentally the particle morphologies and the related
adhesion strengths as a function of the impact velocity or as a function of the
heat content.
To back up these experiments, the thermal energy transfer of the particle to the
substrate will be described. Subsequently, a collision theory elucidates the
mechanical and thermodynamical effects which are coherent with a shock
compression and the relaxation after the shock wave. The splash models,
emanating from this theory, will be experimentally tested on the basis of
photographic pictures of spread particles and their cross sections.
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Finally adhesion tests will be carried out to determine the specific strength level
of particular morphologies. The final aim of the whole work is to find the
optimum combination of the three input parameters in order to design a new
generation of plasma spray equipment with sufficient control over these
parameters.
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