22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
New kind of plasma torch for supersonic coatings at atmospheric pressure
F.R. Caliari1, F. Miranda2, D.A.P. Reis1, G. Petraconi2, L.I. Charakhosvki3 and A.M. Essiptchouk3
1
2
Universidade Federal de São Paulo, São José dos Campos, SP, Brazil
Instituto Tecnológico Aeroespacial, São José dos Campos, SP, Brazil
3
Luikov Heat- and Mass Transfer Institute, Minsk, Belarus
Abstract: This work presents a novel kind of plasma torch for supersonic plasma spraying
at atmospheric pressure where the discharge chamber axis is perpendicular to powder
injection direction. The results of experimental investigation of the electrical, thermal and
kinetics characteristics of plasma torch are given.
Keywords: Atmospheric plasma spray; plasma torch; coating with supersonic flow
1. Introduction
Coating is the process where a thin layer of material
(metallic, ceramic and/or polymer) applied on a substrate
in order to get a protection against oxidation, corrosion,
abrasion, thermal loads, etc. By definition, [1], the
thermal spraying is “a group of coating processes in
which finely divided metallic or nonmetallic materials are
deposited in a molten or semi-molten condition to form a
coating”. Thus the coatings are produced when the
particles are heated and deformed at impact with the
substrate, which happens if they are not only softened but
have sufficient kinetics energy.
The thermal spraying encloses a family of processes
that use the thermal energy generated chemically (by
combustion) or electrically (mainly by electric arc
discharge) to soften and/or melt and accelerate fine
particles at high velocities from few tens to thousand
meters per second. The processes can be continuous or
pulsed.
In spraying based on chemical processes, the
temperature of flame depends on oxygen-to-fuel ratio.
The chemical compounds can react with sprayed material
and affect coating properties.
The plasma spray
techniques (more versatile but some expensive)
overcomes those drawbacks.
Among the processes, used for coating, the thermal
spraying has the largest range of materials, possible
coating thicknesses and features (Fig. 1). The high
temperature of the plasma jet is suitable, but is not
limited, for materials with high melting point: ceramics
and refractory materials. The process normally is realized
in the open-air environment, but for the improving the
coating quality a controlled atmosphere chamber (with
atmospheric or lowered pressure environment) are used.
Thermal energy converted from electrical one
determines the temperature of the jet and, therefore, the
efficiency of fusion of the particles. However, the kinetic
energy of the particles, their adhesion to the substrate and
porosity depend also of the plasma velocity. The
conventional plasma spray has higher thermal energies
and relatively low velocities of particles and concede to
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Fig. 1. Coating process comparison [2].
High Velocity Oxygen Fuel (HVOF), Detonation Gun (DGun) and Vacuum Plasma Spray process (VPS).
Most conventional torches for plasma spraying were
developed in decade 60. With the aim to improve the
plasma spraying technique, a considerable number of
plasma torches types, using different physical principles,
have been developed in the past, (see for example, [3]).
Most of them used linear-circuit plasma generators
where the electrodes (cathode and anode) arranged
axially. The cathode, normally, is inner electrode and
output nozzle is anode.
A discharge chamber of typical plasma torch consists of
one or more cathodes (normally tungsten rod) surrounded
by a concentric hollow anode, which acts as output
nozzle. An electric arc ignited between the electrodes and
the gas flow blowout a high enthalpy plasma jet. The
spraying powder is injected (radially or axially) into the
plasma jet where is melted and accelerated in the
substrate direction. The advantage of this type of plasma
torch is its simplicity, small number of parts and extensive
set of well-known operating parameters. The arc is
practically immobile on the cathode surface, while has
axial and radial motions on the anode surface. This
results in variations in the arc voltage and, hence, in the
enthalpy of the plasma jet.
1
In traditional plasma spray, material is injected radially
to plasma jet downstream to the nozzle. Longitudinally
blown arcs possess best energetic characteristics, but
radial injection of material stimulates its elevated
dispersion and spatial segregation by grain size
distribution. Besides, acceleration of particles at lowdensity plasma jet is not as effective as at high pressure
and high density. Radial injection of material at subsonic
part of nozzle is not effective due to precipitation of
particles onto the walls of the nozzle. In this work, a new
concept of plasma torch is presented.
2. Experimental setup
In this work, we present a novel design of plasma torch
for high velocity spraying. Development of this plasma
torches is motivated by the request to improve the arc
stability and to increase the range of powders to be used
and quality of sprayed coating surface. The plasma torch
is designed to work at sub- and supersonic regimes.
A principal difference from other construction is that the
discharge chamber axis is perpendicular to powder
injection line. The schematic diagram of plasma torch for
HVPS (High Velocity Plasma Spray) proposed in this
work is shown in Fig. 2. The mixing chamber is placed
between two tubular copper electrodes. The swirled gas
enters the electrodes (where formed reversed vortex flow)
and the mixing chamber (with direct vortex), stabilizing
the electric arc at the principal axis of the plasma torch.
The nozzle is arranged in mixing chamber
perpendicularly to the axis and according to the direction
of vortex rotation. Therefore, the arc plasma enters the
nozzle with no swirling. Powder material is injected
through nipple coaxially to the nozzle by the flow of
additional transporting gas. Elevated temperature of the
mixing chamber walls is necessary for preventing the
material sticking on the walls and possible clogging of the
nozzle. An additional magnetic field can be used for
anode axial position stabilizing. In our tests, in order to
fix the arc length and, thus, diminish the arc voltage and
enthalpy fluctuations, a button type anode was used.
Supersonic or hypersonic plasma jets permits to obtain
dense spray coatings, similar to kinetic spray [4, 5]. The
kinetic spray requires no heating particles up to the fusion
point, substituting the part of heat energy by kinetic
energy of particles. This simplifies the protection of
nozzle against clogging. One can hypothesize that
application of similar method of injection and
acceleration of particles combined with supersonic plasma
jet would make it possible fill up the gap between dense
but low melting metallic coatings obtained by lowtemperature kinetic spray and refractory coatings (e.g.,
from aluminum oxide) obtained by traditional plasma
spray with substantially higher porosity.
One preliminary cold flow modelling of HVPS is
presented on the Fig. 3. From the gas velocity pattern we
can see good vortex in the inner electrode channel that must
stabilize the arc on the axis. In the mixer chamber a vortex
is off-centered which may provoke oscillations of arc.
2
Fig. 2. Schematic diagram of plasma torch for high
velocity plasma spraying.
Fig. 3. Velocity flow pattern in transversal section of
mixer chamber (a) and axial section of electrode (b).
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Preliminary calculations show that at moderate, for
plasma gas, temperature 2000-3000 K and plenum
pressure 0.5 MPa, a velocity of nitrogen plasma achieve
1200-1600 m/s. Axial injection of particles at subsonic
part of jet at the direction of gas flow, the same as in
kinetic spray, must make more effective its acceleration.
Expected parameters of HVPS process in comparison
with traditional thermal spraying processes is shown in
Fig. 4.
Fig. 5. Supersonic plasma jet produced by HVPS.
Fig. 4. Gas temperature versus particle velocity of
thermal spray processes.
3. Results and Discussion
The HVPS plasma torch is ignited by a high frequency
high voltage simultaneous breakdown of two gaps formed
between mixing chamber and electrodes (cathode and
anode). Started main arc is blowing out and stabilizing on
torch axis by vortex.
The HVPS plasma torch is designed for up to 50 kW of
power inputs using nitrogen as plasma forming gas. The
power supply was arranged on the basis of six welding
transformers with magnetic shunts controlling impedance
and external characteristics. The transformers have been
connected in series-parallel groups and, together with the
standard three-phase rectifier, the dc power supply with
an off-load voltage of 675 V and descending V(I)
characteristics turned out.
In Fig. 5 a supersonic plasma jet produced by HVPS is
shown. A diamond-like structure is clearly observed.
Outstanding feature of HVPS is high voltage and low
current (see Fig. 6), which is exactly the opposite of
common plasma torches used for plasma spray. A low
current ensure low erosion rate of electrode, low
contamination of plasma and increase in working time of
equipment. For the supersonic regime, the arc current
was limited between 80A and 110A.
As the arc length was fixed, the arc voltage was nearly
constant for given gas flow rate and grew with gas flow
rate (Fig. 7) and, respectively, with pressure in the
discharge chamber, Fig. 8. An effective electric field
E = U/l, where U is total arc voltage and l = const is
interelectrode distance, show relatively high values that
are typical for developed turbulent flow. All that justify a
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Fig. 6. Volt-Current characteristics of HVPS.
practically linear increase of bulk enthalpy of plasma jet
with arc current (Fig. 9). HVPS plasma torch not only
produce a more stable jet but make it also easier to adjust
the enthalpy of the plasma gas by fine correction of the
arc current.
Fig. 7. Arc voltage versus gas flow rate.
Other important parameter is fluctuation of arc voltage
together with arc current. Without of arc length variations
it is expected that HVPS have low plasma jet temperature
fluctuations.
Thermal losses in the plasma torch were obtained from
the measurement of the cooling water flow rate and its
temperature.
For
temperature
measurement
chromel-alumel thermocouples (type K) were used. For
the torch power of 38-40 kW, the efficiency of
3
Fig. 8. Electric field strength versus pressure in discharge
chamber.
Fig. 10. Sprayed particle flow.
outlined from comparison with conventional linear
plasma spray torches are intermediate between PS and
HVOF.
5. Acknowledgement
The authors acknowledge FAPESP, CNPq and Capes
for financial support.
Fig. 9. Bulk enthalpy of plasma jet versus arc current.
transformation electrical energy in thermal one was 7580% that surpass the values of the linear plasma torches.
Preliminary studies of kinetic characteristics of HVPS
were carried out with application of an Inflight Particle
Sensor DPV-2000. For the particles Al 2 O 3 of 56 ± 28 μm
in the plasma flow conditions under study the particles
attained the velocity of 410 ± 40 m/s and temperature
2340 ± 330K that is typical for HVOF spraying. For
comparison, plasma bulk temperature of 3780 K and jet
velocity of 1224 m/s (M~1.5) were estimated.
One interesting phenomenon was observed during
supersonic spraying. Some particles directed out of
spraying axis were forced to merge back into the plasma
jet as it shown in Fig 10. That comportment can be
explained by the Magnus effect that determines force
acting on a rotating body in flow. The particles, crossing
a high velocity gradient in the supersonic jet boundary
layer, start to rotate and this create a pressure gradient on
particle surface directed to the jet axis. The acting force
is proportional to the upstream velocity and the vortex
strength established by particle rotation.
6. References
[1] F.J. Hermanek. Thermal spray terminology and
company origins. (Materials Park, OH: ASM
International) (2001)
[2] An Introduction to Thermal Spray. Sulzer Metco.
Thermal Spray Brochure. http://www.sulzer.com
[3] J.-L. Marqués, G. Forster and J. Schein . Open
Plasma Phys. J., 2, 89-98 (2009)
[4] A. Papyrin, V. Kosarev, S. Klinkov, A. Alkhimov
and V. Fomin. Cold Spray Technology, 1st edition.
(Elsevier Science) (2006)
[5] T.H.V. Steenkiste, J.R. Smith, R.E. Teets, et al.
Kinetic spray coatings. Surf. Coat. Technol., 111,
62 (1999)
4. Conclusion
A new design of plasma torch with discharge chamber
axis that is perpendicular to powder injection direction
has been discussed. The plasma torch has slightly
ascending current voltage characteristics and fixed arc
length. Electrical, thermal and kinetics characteristics
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