st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Influence of plasma generation conditions on flow field of a plasma jet and particle in-flight behavior and oxidation in water-argon plasma spraying
T. Kavka1, A. Mašláni1, P. Ctibor1, J. Matějíček1
1
Institute of Plasma Physics AS CR, v.v.i., Prague, Czech Republic
Abstract: The paper describes features of the plasma spraying by a hybrid water-argon DC
arc plasma torch. In the torch there are two parameters to be changed to modify properties of
generated plasma namely arc current and secondary gas flow rate - argon. Primary gas is
steam and its flow rate is determined mainly by arc current. Modification of both arc current
and argon flow rate result in modification of particle temperature, velocity and oxidation rate.
Keywords: Plasma spraying, particle oxidation, water-argon plasma.
1. Introduction
In the plasma spraying technology choice of the plasma
torch parameters is crucial to obtain coatings with desired
properties and composition. Indeed, properties of coatings
depend on their structure, which in turn is governed by
particle state at the moment of approaching a substrate.
Particle state gradually changes while flying through the
plasma jet from the point of its injection. In the ideal case
particles are heated and undergo melting before they meet
the substrate. Under insufficient conditions particles may
be either overheated that intensive evaporation and thus
big material losses take place, or they may not reach desired degree of melting to produce good splats and provide sufficient cohesion with the substrate. The state of
particles as well as their in-flight parameters can be
changed to wide extend by varying plasma torch or
spraying parameters, namely torch power, plasma gas
type and flow rate, powder injection location and angle
etc. Knowledge of effect of each single parameter on
plasma properties and particle in-flight behavior is a key
to development the spraying process with the best output
under the reasonable costs.
It should be noted that each plasma torch is limited by
its geometry and electrodes to certain levels of power/plasma gas flow rate, which in turn restrict energy
available for plasma processing. The present paper is devoted to study of the plasma torch based on Gerdien arc
principle, which is characterized by high power and energy output. The torch can operate up to input power of
180 kW already at 600 A with efficiency of 60 %. Such
high power is mainly due to two factors: stabilization of
arc by water wall allows extension of the arc length, while
presence of steam in plasma gas requires higher voltage.
Some amount of steam is evaporated alone the water stabilizing channel, where it is mixed with argon, which is
supplied in the cathode part of the torch and acts as a protective and stabilizing medium. Thus, the plasma gas is a
mixture of evaporated steam and supplied argon. Evaporation is governed by the arc itself, while argon flow rate
can be changed by an operator. Modification of either
argon flow rate or arc current result in complex interdependent changes inside the arc chamber, which in turn
result in modification of properties of generated plasma.
The present paper summarizes effect of these two parameters on properties of generated plasma, particle in-flight
behavior and their oxidation.
2. Plasma torch features
The main distinction feature of the hybrid water-argon
torch is a long arc channel created by a water vortex. Water prevents metallic parts of the torch from overheating,
which allows generation of long stabilized arcs. The anode of the torch is positioned outside the torch chamber to
prevent strong erosion in the aggressive steam environment. The detailed description of the torch is given in [1].
Under present configuration the arc length varies from 60
to 80 mm. The length of the water stabilized part is 50
mm and water is evaporated alone the whole channel.
In the present experiments the arc current was varied
between 300 and 500 A, while the argon flow rate between 12 and 40 slm. As it was shown in [2] modification
of argon affects only little power losses and torch energetic characteristics because of low enthalpy of argon.
However, change of argon flow rate affects strongly
plasma gas composition and flow rate. Indeed, increase of
argon flow rate results in increase of both total plasma gas
flow rate and argon molar ratio in it. For the present conditions, the plasma gas changed from almost pure argon to
almost pure steam. Amount of evaporated water is proportional to energy released by arc and depends on the arc
current. For studied conditions water evaporation rate was
between 0.1 and 0.3 g/s. It should be mentioned that only
a part of the whole supplied argon and evaporated water is
present at the torch exit in the plasma jet, because some
amount of it is exhausted into the water stabilizing circuit.
Amount of exhausted water depends on pressure difference between the water tank and the arc chamber, thus it
is not constant, but rather depends on plasma torch conditions as well.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Modification of the plasma torch parameters result in
modification of properties of generated plasma. Fig. 1 and
2 show how plasma temperature and velocity change depending on arc current and secondary gas flow rate. Centerline values were chosen for illustration to emphasize
processes taking place. Temperature was measured by
means of optical emission spectroscopy [3]. Velocity was
calculated from measured temperature profiles and mass
and energy balance at the torch exit using constant Mach
number approach [4]. Arc current has much bigger effect
on both plasma temperature and velocity, which is mainly
due to higher energy released in the arc. The biggest effect of argon consists in modification of plasma composition. For small argon flow rates (e.g. 12 slm) the argon
content is below 30 vol. %, while for high argon flow
rates it can be as high as 80%. This factor affects one of
the most crucial parameter responsible for momentum
transfer – viscosity.
Fig. 1 Centerline plasma temperature at the torch
exit as a function of arc current and secondary gas
flow rate.
influence on momentum transfer. Indeed, increase of arc
current result in higher temperatures and higher content of
steam in plasma. If plasma temperature exceeds 10,000 K
viscosity goes down with the temperature. Temperatures
at the torch exit are well above this critical value. However, in contrast to gas torches, in the hybrid torch particle
injection does not take place immediately at the torch exit,
but usually at some distance from the nozzle. This distance depends on material to be sprayed, as it will be
shown later. As soon as plasma jet leaves the arc chamber
it starts interacting with the surrounding atmosphere. The
result of this interaction is entrainment of cold air into the
plasma jet, which leads to modification of all plasma
properties. Temperature and velocity of the plasma go
down with a distance, while amount of air in the jet increases [5]. The particles are injected into the plasma at
distances from several millimeters from the nozzle up to
several centimeters, when the plasma jet is fully turbulent.
Thus, at the place of particle injection temperature of the
plasma gas is already much lower than at the exit nozzle
and the viscosity dependence on temperature may turn out
to be in the region of a rising slope.
It should be pointed out that the process of interaction
of plasma jet with the surrounding is very crucial for the
plasma spraying applications. This process is very complex and is still not fully understood and described. The
intensity of interaction and modification of plasma properties depend on all parameters the present article deals
with – plasma temperature, velocity and composition as
well as the nozzle geometry and/or pressure outside the
torch. The presence of air in the plasma jet is becoming
especially undesirable if metallic particles are to be
sprayed. Indeed, air contains around 20 vol.% of oxygen,
which can react very intensively with the heated and
melted surface of the injected particles. This process results in oxidation of material during particle flight, which
in turn may have a negative effect on coatings. The next
chapter analyses how plasma torch parameters influence
properties of the particles introduced into the plasma jet.
Fig. 2 Centerline plasma velocity at the torch exit
as a function of arc current and secondary gas
flow rate.
Temperature dependence of viscosity on plasma gas
composition is shown in Fig. 3. Addition of argon into
water plasma results in increase of plasma gas viscosity
and thus improves momentum transfer between particles
and plasma. Increase of arc current may have a negative
Fig. 1 Temperature dependence of the viscosity of
Ar-H2O mixture at atmospheric pressure
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
3. Particle in-flight parameters and oxidation
Particle in-flight behavior was studied within a wide
range of parameters. Two types of powder were used –
copper and tungsten with granulometry between 100 and
125 µm and 63 and 80 µm respectively. Particles were
injected transversally into the plasma jet under the angle
of 70°. Tungsten is a refractory material requiring higher
amount of heat for melting. That is why it was injected
into the hot region of the jet at 25 mm from the nozzle.
Copper has much lower melting point and good thermal
conductivity. Its injection at very close positions resulted
in particle overheating and strong evaporation. Thus, for
copper the injector was located at 90 mm from the nozzle.
Argon was used as a carrier gas. The powder feed rate
was 8.5 kg/h of Cu and 13.5 kg/h of W. It should be noted
that the feed rate was reduced for the present measurement to facilitate particle in-flight diagnostic, which was a
primary goal of the study. In-flight parameters were
measured by DPV-2000 particle diagnostic system. We
scanned particle jet at the distance 320 mm from the torch
to examine particle parameters at the moment they exceed
a substrate during plasma spraying. Obtained data were
statistically processed in Matlab considering only conditions with enough of monitored particles (for ‘too cold
conditions’ particles were not bright enough to be observed). Influence of plasma torch parameters on mean
particle temperature and velocity is shown in Fig. 4.
Mean values have been calculated through the whole set
of measured individual particles.
Big temperature difference between Cu and W parti-
Fig. 2 Particle mean temperature and velocity
at 320 mm from the nozzle as a function of
torch parameters.
cles is because of their difference in melting point. Cu
melts already at 1085 °C, while W only at 3422 °C. Increase of both arc current and argon flow rate resulted in
higher particle velocities. Effect of arc current is more
pronounced, which can be linked to increased plasma
velocity as it was shown before. Effect of argon flow rate
is less pronounced and is more likely to be linked to modification of plasma jet composition with increased amount
of argon and thus increased viscosity. Influence of both
parameters on particle mean temperature was rather small.
While increase of arc current resulted in slight increase of
temperature of particles of both materials, argon flow rate
had almost no effect.
Fig. 5 Oxygen content in Cu particles as a function of
their in-flight mean temperature and velocity.
Temperature and velocity of particles determine rate of
particle oxidation during flight through the jet. Indeed,
particle velocity determine the time of particle flight
through plasma. The longer the time the longer particle is
exposed to oxygen, which is entrained into plasma, the
higher the probability of particle oxidation. Particle temperature is important because materials under higher
temperatures are more prone to oxidation. In the present
work effect of particle temperature and velocity on
in-flight oxidation was studied through freezing of particle chemical state. This procedure is possible if particle
after their flight through plasma are collected into the
liquid nitrogen. To do so the torch was positioned vertically above a vessel, which was filled with liquid nitrogen.
The surface of nitrogen in the vessel approximately corresponded to the position of the substrate under spraying
conditions. Particles were collected for 10-20 s. After that
they were analyzed on oxygen content using the inert gas
fusion technique. The results of evaluation for Cu are
shown in Fig. 5 correlated to particle in-flight mean temperature and velocity. Oxygen content in a feedstock
powder was 0.41 %wt. As it was expected higher particle
velocities and lower temperatures are preferential with
respect to metallic particle oxidation. There was much
less oxide found in tungsten particles, which amount was
negligible. This phenomenon is a special feature of the
material. Tungsten also actively interacts with oxygen.
However, both types of oxides, which may be formed
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
(WO2, WO3), has a boiling temperature much lower than
the melting temperature of tungsten. Thus, as soon as an
oxide is created it is evaporated from the particle surface.
Thus, the collected particles exhibited almost no oxide for
the whole set of studied conditions.
One more characteristics, which can be crucial is a
feeding distance. It determines into which region of the
plasma jet the particles are injected and how far they
should fly to reach the substrate. Some results on effect of
feeding distance were reported in [6]. The main dependences are similar to that discussed above: the longer the
particle flight time the more oxide was observed in the
collected particles.
4. Discussion on coating properties and conclusions
The main goal of the present study was to understand
interconnections between the plasma torch parameters,
properties of generated plasma, behavior of different particles injected into the plasma jet and determine their effect on properties of sprayed coatings. The main
cross-correlations are depicted in Fig. 6 for copper together with examples of coatings and oxygen content in
them. The pictures of the macroscopic appearance of
coatings as well as their microstructure are shown for
several plasma spray conditions. X-ray diffraction indicated Cu2O as a dominant oxide phase and also showed
presence of some CuO as well. Higher amount of oxide in
coatings in comparison to in-flight particle analysis is due
to post-impact oxidation of coating surface. The best obtained coatings were chosen as basic: for copper powder it
was the smallest studied arc current and the highest argon.
As argon flow decreased amount of oxygen increased,
which was connected to longer particles flight in plasma.
Increase of arc current, which is connected with higher
plasma and particle temperatures resulted in higher oxide
content despite increased particle velocity. Besides domination of particle surface temperature on material oxida-
tion this phenomenon may be explained by higher entrainment rate for higher arc current. However, the entrainment process into the argon-water jet is not fully
studied and understood. Preliminary results showed more
intensive mixing with the surrounding for higher arc currents especially because of huge density gradients existing
at the torch nozzle. Decrease of feeding distance in principle meant prolongation of particle dwell time, which
allowed their better heating, but also oxidation.
The hybrid water-argon plasma torch exhibits a variety
of conditions, which might be beneficial for spraying of
some materials. For each material a detailed study is necessary to find optimal conditions because variation of
either arc current or argon flow rate may change the
spraying pattern and coating properties in a wide range.
5. Acknowledgement
The authors gratefully acknowledge financial support of
Technology Agency of the Czech Republic under project
no. TA01010300.
6. References
[1] M. Hrabovský, V. Kopecký, V. Sember, T. Kavka, O.
Chumak, M. Konrád; IEEE Transactions on Plasma
Science. 34 (4), 1566, (2006)
[2] T. Kavka, J. Matejicek, P. Ctibor, A. Maslani, and M.
Hrabovsky; J. Thermal Spray Technol. 20 (4), 760
(2011).
[3] V. Sember and A. Mašláni; Temp. Material Proc., Vol.
13 (No. 2), 217 (2009)
[4] T. Kavka, O. Chumak, V. Sember, M. Hrabovský; Proc.
ICPIG-XXVIII, 15-20.07.2007, Cancun, Mexico
[5] E. Pfender, J. Fincke and R. Spores, Plasma Chem.
Plasma Process. 11 (4), 529 (1991)
[6] T. Kavka, J. Matějíček, P. Ctibor, M. Hrabovský: J.
Thermal Spray Technol., 21 [3-4], 695 (2012)
Fig. 6 Correlation between plasma spray parameters and obtained copper
coatings.