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

i

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.

iii

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

1

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

2

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.

3

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).

4

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

5

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.

6

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.

7

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

8

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%

9

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.

10

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.

11

Fig. 6. Temperatures distribution in plasma jet for 300A experiment.

Fig. 7. Velocities distribution in plasma jet for 300A experiment.

12

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.

13

Fig. 9. Temperatures distribution in plasma jet for 400A experiment.

Fig. 10. Velocities distribution in plasma jet for 400A experiment.

14

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

15

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).

16

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

17

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

20

[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

21

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.

22

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.

23

9.0 Appendix 2. Production drawings

This appendix consists of drawings submitted to the University of Toronto Machine Shop

for necessary modifications.

24

Fig. A2.1. Plasma torch modification, front view.

25

Fig. A2.2. Plasma torch modification, side view.