high-power plasma torch optimization · plasma spraying is a process intended for a...
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HIGH-POWER PLASMA TORCH OPTIMIZATION
by
Nikolay Grisha
A project report submitted in conformity with the requirements for the degree of Master of Engineering
Graduate Department of Mechanical and Industrial Engineering University of Toronto
© Copyright by Nikolay Grisha 2011
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Abstract
Plasma spraying is a process intended for a component's surface improvement by
means of particles deposition. It includes surface strengthening, and adding rust-
preventing, dielectric, fire safety and other properties to a component's surface.
A high power plasma torch designed in Centre for Advanced Coating
Technologies (CACT), University of Toronto, for the purpose of thermal waste treatment
is proposed to be used in a high-volume plasma spraying process. Modification of the
existing plasma torch is required in order to accommodate feedstock material delivery
into the plasma jet.
A series of experiments were conducted and results were evaluated in order to
verify if the torch is suitable for plasma spraying. Results and modification of the
existing torch are presented.
ii
Acknowledgments
I would like to thank Javad Mostaghimi, my supervisor, for the opportunity to work on
this project.
Also I would like to thank Larry Pershin for his guidance throughout the project.
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Table of Contents
Abstract i
Acknowledgments ii
Table of Contents iii
List of Tables iv
List of figures v
1.0 Introduction 1
2.0 Plasma torch modification 3
3.0 Experimental setup 4
3.1 Particle sensor DPV-2000 7
3.2 Feedstock material 7
3.3 Experiment parameters 8
4.0 Experiment results 10
4.1 Experiment 1. Arc current 300A 10
4.2 Experiment 2. Arc current 400A 12
4.3 Processed powder evaluation 14
5.0 Conclusion 17
6.0 Future work 18
7.0 References 19
8.0 Appendix 1. 3-D models 21
9.0 Appendix 2. Production drawings 23
iv
List of Tables
Table 1. Feedstock powder properties 7
Table 2. Test parameters 8
Table 3. Experiment 1 results 10
Table 4. Experiment 2 results 12
v
List of figures
Fig. 1. Proposed existing torch modification schematics (front view) 4
Fig. 2. Experimental setup diagram 5
Fig. 3. Experimental setup 6
Fig. 4. Experiment 9
Fig. 5. DPV-2000 results screen-shot for 300A experiment 10
Fig. 6. Temperatures distribution in plasma jet for 300A experiment 11
Fig. 7. Velocities distribution in plasma jet for 300A experiment 11
Fig. 8. DPV-2000 results screen-shot for 400A experiment 12
Fig. 9. Temperatures distribution in plasma jet for 400A experiment 13
Fig. 10. Velocities distribution in plasma jet for 400A experiment 13
Fig. 11. Unprocessed Al2O3 powder magnification 14
Fig. 12. Captured processed Al2O3 powder magnification 15
Fig. 13. Al2O3 single splats collected on a stainless steel substrate
during a single “swipe test” 16
Fig. 14. Multiple “swipe test” results, coating structure 16
Fig. A1.1. Existing plasma torch, 3-D model 21
Fig. A1.2. Plasma torch modification, 3-D model 22
Fig. A1.3. Plasma torch modification, front view 22
Fig. A2.1. Plasma torch modification, front view 24
Fig. A2.2. Plasma torch modification, side view 25
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1.0 Introduction
“Thermal spraying process is understood as a process of particulate deposition in
which the molten, semi-molten or solid particles are deposited onto substrate and the
microstructure of the coating results from the solidification and sintering of the particles”
[1]. This process uses electrically generated plasma to treat feedstock material.
Feedstock melts rapidly within the plasma torch and then propelled as small molten
droplets via a gas towards the target material. When the molten droplets contact the
target material they flatten, rapidly solidify to form coating that remains on the surface of
the target material. Deposits having a thickness from just a few micrometers up to
several millimeters can be produced using a variety of feedstock materials, including
metals and ceramics. The feedstock material is normally presented to the plasma torch in
the form of a powder or wire [2].
A high-power DC plasma torch with numerous advantages was developed and
tested in Centre for Advanced Coating Technologies (CACT), University of Toronto.
Originally it was designed for materials thermal treatment, such as waste destruction.
This torch has the following features. It has a graphite water-cooled cathode. It operates
on carbon oxide – hydrocarbons gas mixture not only to increase torch power and
improve heat transfer into the treated materials but also to prolong cathode's life due to
the deposition of carbon onto the cathode during operation. Plasma forming gas mixture
is delivered into the torch in tangential direction to stabilize arc column and initiate anode
attachment rotation what increases anode's longevity. Further, to increase anode lifespan
in comparison with other torches a solenoid was connected to the anode in series
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configuration. Solenoid's magnetic field forces arc root to rotate, and therefore, erosion
of the anode is reduced. Moreover, the solenoid significantly decreases arc voltage
fluctuations what resulted in greater arc stability [3]. This type of torches has thermal
efficiency of 60% – 70 % [3, 4], whereas conventional argon-operated torches are
reported to be only ~35% efficient [4, 10]. Additionally, according to [4] coating
deposition rate per one torch pass is 3 – 4 times higher for the torch which operates with
CO2 – CH4 mixture than argon.
Spraying technologies with high output are desired when a necessity of spraying
large area surfaces arises, such as big rolls in pulp and paper industry. Although, a high
velocity oxygen fuel (HVOF) technologies capable of producing high-quality coatings
are frequently used for these purposes, they are limited to metal coatings mostly due to
relatively low operational temperatures up to 2900°C. It is very challenging to spray
ceramics and refractory metals using HVOF due to short exposure of particles to the jet
[5]. This makes plasma spraying technologies more suitable for coating of large surfaces
due to larger operational temperatures range, order of 17000°C [5].
The objective of this research is to build a 3-D model of the existing CACT
plasma torch, find a way to modify it in a cost-effective way to be suitable for high-
output plasma spraying process. Furthermore, it is desired to conduct several spraying
tests and measure crucial parameters of plasma plume (e.g. temperature and velocity
distributions). Finally, it is necessary to evaluate processed feedstock material
characteristics to estimate the potential of this torch and need for future studies.
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2.0 Plasma torch modification
First, a 3-D model of the existing plasma torch was built by using Solid Works
CAD software, see Fig. A1.1 of Appendix 1. Further, this model was reviewed to find
ways to convert this torch to plasma spraying technological device. The following ideas
were guidelines for the modification:
• It was proposed to introduce feedstock material into plasma on the exit of the
torch in order do not suppress electric arc.
• The feedstock material is a ceramic powder which described in details in section
3.2 of this report
• Since a high-performance for the spraying is a requirement it is desired to have
three ports to deliver feedstock powder into the plume.
• The ports should be spread equally around the plume for more uniform load of
plasma with particles
The simplest way to attach powder-feeding ports found is to drill orifices through
the torch body right after the anode’s exit without destructing existing cooling system and
insert the delivery ports as shown schematically on Fig. 1. Each port should be
comprised of a stainless steel tube and an adapter for connection to the existing powder
feeding system. It is recommended to fix each port with a tightening bolt with the
distance from tip to the anode's orifice of 3 – 4 mm to avoid tubes meltdown. 3-D model
of the modified torch is shown in Appendix 1, (Fig. A1.2 and A1.3).
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Fig. 1. Proposed existing torch modification schematics (front view).
Drawings to support production and modification can be found in the Appendix 2,
(Fig. A2.1 and A2.2).
3.0 Experimental setup
In order to measure temperatures and velocities in the jet, and also to evaluate
processed particles, the experimental set up diagram is proposed (Fig. 2). The main idea
of the experiments is to deliver Al2O3 powder (5) into the plasma torch (1) and to collect
processed particles for further study. The powder is delivered by means of carrier gas. In
this research only two out of three ports available were utilized. Feedstock powder was
supplied through ports (a) and (b) which are fixed 180° relative to each other, as shown in
Appendix 1, (Fig. A1.2 & A1.3). In-flight particle sensor was employed to measure
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temperatures and velocities fields. The measuring head was placed 150 mm away from
the anode’s exit. The system was set up to provide measurements from the jet area 40 by
40 mm consisting of 91 points and with detection rate of 5 particles per second. Particles
which passed through the plasma jet are proposed to be captured into a container with tap
water. They need to be dried and reviewed later under a scanning electron microscope.
Fig. 2. Experimental setup diagram. 1 – plasma torch, 2 – plasma arc stabilization
solenoid, 3 – torch holder, 4 – particle sensor, 5 – powder feeding system, 6 – plasma
plume loaded with particles, 7 – container with tap water to capture processed particles.
The actual laboratory equipment placement can be found on Fig. 3.
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Fig. 3. Experimental setup. 1 – water-cooled high-power plasma torch with
electromagnetic arc stabilization, 2 – base, 3 – in-flight particle sensor DPV-2000
(150mm of the nozzle exit), 4 – one of the powder-feeding tubes, 5 – water supply tube
for cooling, 6 – solenoid casing.
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3.1 Particle sensor DPV-2000
In flight particle sensor for thermal spraying systems DPV-2000 manufactured by
Tecnar Automation Ltée (St-Bruno, Canada) allows to determine the characteristics of
sprayed particles in a set up area of the plume. This system is capable of simultaneous
monitoring of particles' velocities, temperatures and diameters.
According to [6] velocity measurements are the simplest and most precise with
order of 0.5%. DPV-2000 is a high-speed, high precision two colors pyrometer with the
possibility to measure temperatures of particles ranging from 1000°C to 4000°C. If
measured temperature is within the range then the lowest precision is 3%.
For detailed principles of measurement and derivations for particles' velocities Vp,
temperatures Tp and diameters Dp it is recommended to refer to [6].
3.2 Feedstock material
Ceramic coatings are the most interesting for this project. Al2O3 rein powder
manufactured by GTV company was selected for the experiments. Melting point of solid
material is 2072°C. Powder physical properties can be found below in Table 1.
Table 1. Feedstock powder properties [7].
Material Chemical
composition Particle size
Al2O3 powder,
fused and crushed Al2O3 , 99%
+15 µm
-45 µm
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A coating of this material is recommended to be applied in atmospheric pressure
spraying (APS) processes and has the following properties/application fields [7]:
• High wear resistance except for fatigue load conditions
• Coating hardness 600-1200 HV0.3
• Applicable up to 1500°C
• Excellent dielectric strength, especially at elevated temperature
• Electrical resistance 1015 Wcm
• High chemical resistance except for bases
Initial powder structure was observed under a scanning electron microscope.. The
magnified unprocessed powder images are presented on Fig. 11 (x150 magnification on
the left and x500 magnification on the right). One can notice that particles have arbitrary
shape typical for fused and crashed powders [1, 5].
3.3 Experiment parameters
There were conducted two APS experiments with the following
parameters (Table 2).
Table 2. Test parameters
Exper. Arc
current, A
Arc voltage,
V
Arc power,
kW
Gas composition, lpm
Thermal efficiency
1 300 254 76.2 CO2 – 77, CH4 – 21 67.70%
2 400 279 111.6 CO2 – 77, CH4 – 21 65.60%
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Powder feed rate total from two ports was 8.2 kg/h for both experiments. At the same
time, it is known that argon operated conventional torches are capable of spraying of
approximately up to 4 kg/h. Experiments conduction (Fig. 4) is depicted below.
Fig. 4. Experiment. 1 – water-cooled high-power plasma torch with electromagnetic arc
stabilization, 2 – plume loaded with Al2O3 particles, 3 – in-flight particle sensor
DPV-2000. 4 – reservoir with tap water to collect melted particles for future analysis,
5 – powder feeding port.
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4.0 Experiment results
There were conducted two experiments with different volt-ampere characteristics
of plasma arc to determine if CACT plasma torch has a potential for high-volume plasma
spraying applications.
4.1 Experiment 1. Arc current – 300A
Maximum plasma jet parameters for the first experiment can be found below
(Table 3). A partial screen shot of test results from DPV-2000 system is reflected
on Fig. 5.
Table 3. Experiment 1 results
Parameter Value Max. temperature, °C 2453 ± 130.01 Max. particles' velocity, m/sec 124 ± 19.86 Max particles' diameter, µm 22 ± 7.35
Fig. 5. DPV-2000 results screen-shot for 300A experiment.
Temperatures and velocities fields are presented on Fig. 6 and Fig. 7 correspondingly.
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Fig. 6. Temperatures distribution in plasma jet for 300A experiment.
Fig. 7. Velocities distribution in plasma jet for 300A experiment.
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One can notice from the temperatures distribution that in every point of the
measured area of the plasma plume temperatures are higher than melting point of the
selected powder material (Tm= 2072°C). At the same time values of measured velocities
are close to the range (150 to 300 m/s) of characteristic velocities for DC atmospheric
plasma spraying processes [8].
4.2 Experiment 2. Arc current – 400A
Second experiment maximum parameters of plasma plume are shown in Table 4. A
partial screen shot of test results from DPV-2000 system is presented on Fig. 8.
Table 4. Experiment 2 results
Parameter Value Max. temperature, °C 2597 ± 134.59 Max. particles' velocity, m/sec 171 ± 42.09 Max particles' diameter, µm 39 ± 12.23
Fig. 8. DPV-2000 results screen-shot for 400A experiment.
Temperatures and velocities fields are presented on Fig. 9 and Fig. 10 correspondingly.
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Fig. 9. Temperatures distribution in plasma jet for 400A experiment.
Fig. 10. Velocities distribution in plasma jet for 400A experiment.
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As it was expected, higher temperature values were detected in the measured field
due to higher input power. All temperatures are higher than melting point of Al2O3
feedstock powder (Tm= 2072°C). Velocities are typical for DC APS technologies and
within the range of 150 – 300 m/s [8].
4.3 Processed powder evaluation
Further, analysis of both unprocessed and processed powders was conducted
under scanning electron microscope (SEM) Hitachi S2500.
The unprocessed powder has typical “sharp edges” characteristic due to the nature
of its production, fusion and crash [1, 5] (Fig.11).
Fig. 11. Unprocessed Al2O3 powder magnification
The magnified by SEM processed powder image with x150 magnification is on the left
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and x1200 magnification is on the right of Fig. 12.
Fig. 12. Captured processed Al2O3 powder magnification.
One can notice that the most of the particles melted-down during spraying process
and have spherical shapes. Particles became spheres during flight due to surface tension
of the molten material. Also it can be observed that in the processed powder there are no
untreated particles. Even the largest in size particles do not have sharp edges as before
spraying. Their edges were fused.
Furthermore, a series of “swipe tests” was conducted to verify the particles'
condition inside the plasma jet. During a single “swipe test” a metal substrate was moved
rapidly through the plasma jet in order to capture molten particles. For all tests a
rectangular polished stainless steel substrates were used. The results are presented below
(Fig. 13 & 14).
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Fig. 13. Al2O3 single splats collected on a stainless steel substrate during a single “swipe
test”, with x1100 (left) and x1200 (right) magnification
Fig. 14. Multiple “swipe test” results, coating structure with x400 magnification (left),
x500 (middle), x700 (right).
A single “swipe test” resulted in deposition of singular droplets (Fig. 13). The
form of solidified splats in this test suggests that particles were fully melted before hitting
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the substrate. The shape of the splats is typical for spraying on a cold substrate [9].
A multiple “swipe test” implies that a substrate was moved through the plasma jet
several times in order to obtain a coating. The resulting coating structure under the SEM
is presented on Fig. 14. One can notice that despite the low velocities in the plasma jet,
the coating is relatively dense and has low porosity. The right picture (Fig. 14) shows
that this coating has inclusions of partially melted particles similar in shape to those on
Fig. 12 on the left.
5.0 Conclusion
During this research a 3-D model of the existing plasma torch was built. Torch's
geometry allowed modifying it for high-volume plasma spraying processes in the fastest
and cheapest way found. Two experiments with different arc parameters were conducted
and results were analyzed to determine if the torch is suitable for spraying technology.
According to the observations above, the torch with chosen volt-ampere
characteristics was capable of melting almost all particles with 8.2 kg/h feed rate. The
observations of single splats after a “single swipe” test and the coating after multiple
“swipe test” suggest that this torch has a potential for high-output plasma spraying
application, and therefore, a number of further experiments and optimization steps is
suggested.
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6.0 Future work
Several torch optimization steps are recommended to improve spraying process quality:
• It is necessary to increase particles velocity. This could be done either by
reducing the outer diameter of the nozzle or by increasing gas flow rate.
• In order to make the plasma torch more compact it is proposed to review the
possibility of removing one layer of coils from the solenoid.
Some additional tests are required to fully evaluate the plasma torch:
• Since the plasma torch of this configuration is new it is recommended to
conduct more tests with different arc volt-ampere characteristics and different
feedstock powders.
• Peel adhesion test (PAT) is recommended to determine coatings adhesion
characteristics. This test has numerous advantages compare to traditional
ASTM tensile pull test [11].
• It is required to utilize all three powder-feeding ports to estimate maximum
possible feed rate.
• A number of experiments is required to determine electrodes longevity.
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7.0 References
[1] L. Pawlowski, The Science and Engineering of Thermal Spray Coatings. John Wiley
& Sons Ltd., 1995. p. xv, 6.
[2] Zircotec official web page 2011, visited 17 March, 2011;
http://www.zircotec.com/page/plasma-spray_processing/39
[3] L. Pershin, J. Mostaghimi, N. Grisha, Carbonaceous Gases for DC Plasma
Generation Centre for Advanced Coating Technologies, University of Toronto, 2009:
http://www.ispc-conference.org/ispcproc/papers/537.pdf
[4] L. Pershin, L. Chen and J. Mostaghimi, Plasma spraying of metal coatings using CO2
based gas mixtures, ITSC-2008 conference proceedings, 2008.
[5] R. P. Krepski, Thermal Spray Coating Applications in the Chemical Process
Industries. MTI publication No.42 by NACE International 1993. p. 40, 43, 61
[6] Tecnar official web page 2011, visited 16 March, 2011;
http://www.tecnar.com/DATA/DOCUMENT/DPV_Calculation_Principles.pdf
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[7] GTV official web page, visited 18 March, 2011;
http://www.gtv-mbh.de/cms/upload/downloads/GTV_Spray_Powder_Catalogue_08-
2006.pdf
[8] B. Dzur, Atmospheric IC-plasma spraying of coatings – a too little attended
alternative?, ITSC-2008 conference proceedings, 2008.
[9] S. Goutier, M. Vardelle, J.C. Labbe, and P. Fauchais, Alumina Splat Investigation:
Visualization of Impact and Splat/Substrate Interface for Millimeter-Sized Drops, Journal
of Thermal Spray Technology, Volume 19(1-2) January 2010, p. 49-55
[10] Yttria deposition by a novel plasma torch, L. Pershin and J. Mostaghimi, ITSC-
2010 conference proceedings, 2010.
[11] A. C. Siegel, MEng project report: Peel Adhesion Test of Thermal Spray Coatings,
University of Toronto, 2000
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8.0 Appendix 1. 3-D models
This 3-D model was developed in Solid Works CAD software.
Fig. A1.1. Existing plasma torch, 3-D model. 1 – body, 2 – solenoid casing, 3 – base,
4 – water-cooling tubes with adapters, 5 – water-cooled solenoid, 6 – port for plasma
forming gas, 7 – anode.
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Fig. A1.2. Plasma torch modification, 3-D model. a, b, c – powder feeding ports.
Fig. A1.3. Plasma torch modification, front view, d – tightening bolts.
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9.0 Appendix 2. Production drawings
This appendix consists of drawings submitted to the University of Toronto Machine Shop
for necessary modifications.
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Fig. A2.1. Plasma torch modification, front view.
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Fig. A2.2. Plasma torch modification, side view.