Key Factors for Dense Copper Coating by HVOF Spraying

Key Factors for Dense Copper Coating by HVOF Spraying
K. Isoyama
Tokyo University of Science, Noda, Japan
J. Kawakita, S. Kuroda
National Institute for Materials Science, Tsukuba, Japan
H. Yumoto
Tokyo University of Science, Noda, Japan
Abstract
For thermal sprayed coatings, compactness of their constituent
particles is required in many applications, e.g. to obtain
impermeable anticorrosion coating in marine use. We
investigated key factors to improve compactibility of deposited
particles in HVOF sprayed coatings by condition
measurements of spray particles. The results revealed that
plastic deformability of the sprayed particles as well as their
molten fraction was important to obtain the dense VHOF
coatings.
Introduction
One of the objectives of the “Ultra Steel” research (STX-21)
project started in 1995 at Japan’s National Institute for
Materials Science was to improve the corrosion resistance of
structural steels in marine environments by depositing
anticorrosion materials through thermal spraying. Both
impermeability and cleanliness are necessary for such an
anticorrosion coating formed on the structural steel. Therefore,
we have used High Velocity Oxy-Fuel (HVOF) thermal
spraying technique. Its characteristic is that it owes both heat
source and acceleration force to a jet flame made from
high-pressured mixture of oxygen and fuel. This technique
enables us to obtain sprayed particles with a higher speed over
500 m·s -1 and with a lower temperature up to 2000°C,
compared to other conventional method such as plasma
spraying. Such particles are impinged to a target substrate in
the semi-molten state and piled up, leading to form coatings.
This method gave us a dense coating with comparatively small
change in material properties [1-3].
Compactness of deposited particles is one of the most
important properties for thermal sprayed coatings and often
determines their performance in application such as an
impermeable anticorrosion coating. However, the mechanism
of coating formation is very complicated because thermal
spray process contains a stochastic phenomenon and moreover
it has many statistical parameters such as temperature and
in-flight velocity of sprayed particles, surface state and
temperature of target substrate, and so on. Especially in the
HVOF spraying process, the flight velocity of sprayed particles
as well as their temperature is considered to control
compactibility of the deposited particles because sprayed
particles are deposited accompanying with plastic deformation
to a considerable degree depending on the flight velocity.
In this paper, we aimed to clarify key factors to control the
compactibility of HVOF sprayed coatings. For this purpose,
we designed to simplify the thermal spray phenomenon by
using copper as the spray material, size distribution of which
was limited in the narrow range within about 10 µm in
diameter. Copper was selected because of both its high thermal
conductivity and its sufficiently lower melting point than the
flame temperature. In addition, limitation of size distribution
allowed us to control the thermal state of spray particles
homogeneously and uniformly. As for spraying condition,
fuel/oxygen ratio was varied while the combustion pressure
held constant. This condition enabled us to change mainly the
temperature of spray particles with a defined flight velocity.
From our previous research [4], the combustion pressure is
mainly related to the flight velocity of sprayed particles
whereas the fuel/oxygen ratio is mainly related to the
temperature of in-flight particles. We examined the factors,
which decided the compactibility of constituent particles in
copper coatings formed by HVOF thermal spraying. The spray
particles’ state was determined by molten fraction
measurement and by splat observation including its statistical
analysis. The compactibility of coatings was determined by
cross-sectional observation, by through-porosity evaluation
and by corrosion potential measurement.
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Table 1: Spray conditions.
Unit Condition A Condition B Condition C Condition D
Fuel flow rate dm 3·min -1 0.25 0.28 0.32 0.38
Oxygen flow rate dm 3·min -1 1050 968 920 838
Combustion pressure MPa 0.69 0.69 0.69 0.69
Fuel/oxygen ratio* - 0.46 0.56 0.66 0.87
Barrel length mm 203
Powder feed rate g·min -1 75
Torch velocity mm·s -1 700
Spray distance mm 380
Powder feed gas - Nitrogen(N 2 )
Film thickness µm 200
*1.0 corresponds to stoichiometric mixture ratio.
Experimental Method
Spraying and coating
Coatings were prepared with the HVOF thermal spray
equipment (JP5000, TAFA Co., Concord, NH, US) and the
flame was made from kerosene and oxygen. The copper
powder (FUKUDA METAL FOIL & POWDER Co., Kyoto,
JPN) was used as the feedstock and sieved from 63 to 75 µmin
size. The substrate was JIS SS400 low carbon steel and its
surface was blasted by alumina grit and degreased by
ultrasonic cleaning in acetone. Primary spray conditions are
listed in Table 1. The combustion pressure in this table was the
maximum value settable under the condition that all the spray
particles could exist as the unmelted ones upon impingement to
the substrate. The molten state was estimated by capturing
spray particles with a gel target, as described below. The
specified combustion pressure was expected to fix the flight
velocity of spray particles, based on the assumption that the
combustion pressure determines the flight velocity. On the
other hand, the oxygen/fuel ratio was expected to change
mainly the temperature of the spray particles.
Characterization of sprayed particle
We measured in-flight velocity of sprayed particles by the
flight thermal sprayed particle analyzer (TECNAR Co.,
DPV-2000, St-Hubert, Qc, Canada). Its principle and
mechanism was described in detail elsewhere [4]. This
equipment is usually used in order to calculate the surface
temperature of in-flight sprayed particles from their
synchrotron radiation. In addition, the in-flight velocity is also
determined by dividing the interval (160 µm) of two slits by
the time between two radiation signals detected when one
particle passes through the slits. This method based on the
thermal emission is called the Non-Plasma configuration and
abbreviated as NP below. Copper, however, is highly reflective
material, on the contrary poorly emissive, because emissivity =
1 – reflectivity. Therefore, it was difficult for NP to obtain
detectable numbers of thermally radiated particles, especially
in comparatively low temperature of the spray particles. Under
such a “cold” condition, the additional system of Cold Particle
Sensor (abbreviated as CPS below) was effective. This system
enabled us to acquire signals from copper particles on the basis
of reflection of laser, which was irradiated to the in-flight
particles.
Our research group reported that the molten fraction of HVOF
sprayed particles could be evaluated by capturing the sprayed
particles softly with an agar gel as the target material and by
separating melted and unmelted particles at different depths [4].
Gel targets were passed once horizontally by the spray gun and
captured sprayed copper particles. As shown later, cross
sectional views of the sprayed gels showed clearly that the
spray particles were captured by the gel target at shallow and
deep positions separately. Such views could be seen in thin
films with thickness of approximately 50 µm, made by slicing
the gels manually with a knife along the depth direction. When
the molten fraction was determined, the gels were shaved with
a cutter from the sprayed surface forward to the depth direction
and the shavings contained melted or unmelted particles. Such
shavings were put into a different test tube and heated after
addition of distilled water. When heated, copper particles were
precipitated at the bottom of test tube by removing
supernatants containing agar. The precipitated copper was
dissolved by adding 5 ml of concentrated HNO 3 solution (60.0
62.0wt%). After the total volume of such solutions was
adjusted to 100 ml by adding 0.5 mol.dm-3 HCl solution, the
concentration of the copper solutions was determined by
inductively coupled plasma (ICP) atomic emission
spectrometry using an analyzer (SPS 3000, EKO Instruments
Inc. Tokyo JPN). The molten fraction of the sprayed particles
was calculated from the ratio of copper amount in the shallow
and deep parts of each agar gel capturing copper particles.
Deposited particles (splats) were observed by the optical
microscope (Olympus, BX60M, Tokyo, JPN) and by laser
microscope (Lasertec Co. 1LM21, Kanagawa, JPN). Splats
were formed on AISI 304 stainless steel (SUS304) with a
mirror-polished surface. In order to obtain the splats, the spray
gun passed the targets of once horizontally during spraying.
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Characterization of coating
The crystal structure of coatings were characterized by the
X-ray diffraction measurement (Rigaku Co., RINT 2000,
Tokyo, JPN).
The cross section of the coated specimens were examined by
the optical microscope . The cross section was prepared by
embedding the coated specimen into the epoxy resin, part of
which was removed by abrading and polishing treatments.
Through-porosity of the coatings was evaluated by the
quantitative analysis with the ICP Spectrometry of dissolved
ion from coated specimens immersed in the acid solution [5].
In combination of electrochemically noble copper and less
noble steel, the dissolved Fe ion came from the steel substrate
through the connecting pores of copper coating after formation
by corrosion reaction of the substrate prior to the coating, i.e.
by galvanic corrosion. Accordingly, the dissolution amount and
rate of Fe ion corresponds to the through-porosity of coating.
The experimental procedure of this method was described as
follows. A SS400 specimen coated by HVOF spraying of
copper was cut into square pieces with one side of 2.5cm. A
SUS304 wire was connected to the back surface of the
substrate plate by spot welding. The sprayed area of 2cm 2 left
exposed and the rest of the specimen surface was insulated
with the silicon resin. This specimen was immersed in 0.5
mol·dm-3 HCl solution at 300K for 3 days. During immersion,
5 ml of solution was sampled at a predetermined time and the
amount of dissolved ion was determined by ICP spectrometry.
Simultaneously, corrosion potential of the coatings in the
solution was measured by using the corrosion monitor (Riken
Denshi, Model CT-5, Tokyo, JPN).
Results and Discussion
velocity on the fuel/oxygen ratio. Original data of the velocity
were obtained at each ratio by two measurement methods of
NP and CPS, and their number and weight average were
represented in the figure. The NP method has larger values in
average velocity than the CPS method. This is explained by the
deduction that smaller and lighter particles are heated up to
their thermal radiation temperature in shorter time and
accelerated up to higher velocity, compared to larger and
heavier particles. This deduction can be confirmed by the fact
that the number average velocity of NP is higher than its
weight average, i.e. larger and heavier particles are estimated
to be slower. As for the CPS method, the number and weight
average values were almost identical at each ratio and were
approximately 490 m·s -1 at all ratios. From this result, the
in-flight velocity was constant even at the various ratios of fuel
to oxygen under the constant combustion pressure in this
paper.
Cleanliness of coating
Figure 2 compares the XRD patterns of copper coatings with
that of copper powder as the feedstock. All the peaks of the
coatings prepared under conditions A and D were in
accordance with those of the feedstock powder in 2θ position
and were attributed to those of copper. In general, the
temperature of spray particles increases with the fuel/oxygen
ratio up to 1.0 in the HVOF process because the flame
temperature increases as the fraction of oxygen gas acting as a
cooling medium decreases. However, no additional peaks
could be observed even under the Cond. D although this
condition was expected to cause oxidation of the coating to the
highest degree. Therefore, the XRD results showed that the
coatings prepared under all the conditions in this paper were
not oxidized.
In-flight velocity of sprayed particles
Figure 1 shows dependence of the spray particles’ in-flight
(a )
(1 1 1)
(2 0 0)
(2 2 0)
(3 1 1)
(2 2 2)
700
e d y
a
r
p
s
f
s
m / article p
y
o
600
.
(b )
500
lo
c
it
400
300
(c )
(111)
(2 0 0)
(111)
(2 2 0)
(3 1 1)
(222)
(2 0 0)
(2 2 0)
(3 1 1)
(2 2 2)
I
n
200
0.4 0.5 0.6 0.7 0.8 0.9
Ratio(=Rf/o)
Figure 1: Relationship between in-flight velocity of sprayed
particles and fuel/oxygen ratio: ●: number average (CPS),
■: weight average (CPS), ○: number average (NP), and □:
weight average (NP).
θ / d e g .(C u K α )
2
Figure 2: XRD patterns of (a) supply powder of Cu, and
HVOF sprayed coatings of Cu prepared under (b) Cond A
and (c) Cond D.
2 0 4 0 6 0 8 0 1 0 0
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Compactness of sprayed particles
Figure 3 shows the stacking structure of sprayed particles in
Cond. A
the coatings prepared under 4 conditions. As for the coatings
prepared under the Cond. A and B, deformed particles seemed
to be piled up and boundaries among the particles are observed
clearly. This fact implies the presence of pores and voids in
these coatings. Under the Cond. C, further deformed particles
are observed in the coating and there are few above-mentioned
boundaries, indicating considerable improvement of coating
compactness. Although the coating prepared under the Cond. D
seems similar to that under C in terms of the degree of particle
deformation, the boundaries among the particles are observed
clearly. Taking into account of the highest flame temperature
expected under the Cond. D, these boundaries are not due to
the existence of pore and void in the coating but due to thin
oxide films formed on the sprayed particles.
Cond. B
Substrate
100µm
Typical results of the quantitative analysis of dissolved Fe ion
by ICP spectroscopy are shown in Figs. 4 (a) and 4 (b). The
former figure represents changes in dissolution amount with
immersion time and the latter represents dependence of the
average dissolution rate on the fuel/oxygen ratio. As mentioned
12
(a)
Cond. C
Substrate
100µm
Dissolution amount of substrate
/mgcm -2
10
8
6
4
2
0
0 10 20 30 40 50 60 70
Immersion time / hour
Cond. D
Substrate
100µm
Average dissolution rate of substrate
/molcm -1 h -1
(X10 -6
3.0 )
2.5
2.0
1.5
1.0
0.5
(b)
0
0.4 0.5 0.6 0.7 0.8 0.9
Ratio (=R f/o
)
Substrate
100µm
Figure 3: Cross-sections of Cu coatings under each spray
condition.
Figure 4: (a) Relationship between dissolution amount of
substrate and immersion time■ Cond. A, ▲: Cond. B,
▼:Cond. C and ♦: Cond. D), and (b) average dissolution
rate of substrate under each condition.
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in the experimental section, the dissolution amount and rate of
Fe ion corresponds to through-porosity of the coatings, in other
words, compactness of sprayed particles in the coatings. As
seen in Fig. 4 (b), the dissolution rate decreases with the
fuel/oxygen ratio up to 0.66 (Cond. C), indicating that the
coatings became denser. Over the ratio of 0.66, its decreasing
slope is considerably small, indicating that the coatings became
almost impermeable.
Figures 5 (a) and 5 (b) show changes in corrosion potential of
coatings with immersion time and dependence of corrosion
potential on the fuel/oxygen ratio, respectively. The potential
was an average value between 24 and 72 hours, in which range
the corrosion potential indicates a steady value, as shown in
Fig. 5 (a). The corrosion potentials of the coatings can be
divided clearly into two levels of around –400 mV and –130
mV. This is due to occurrence of two different kinds of primary
corrosion reaction in the following equations.
(1) Fe → Fe
2 +
+ 2e
− (oxidation)
(2) 2H
+
+ 2e
−
→ H ↑ (reduction)
2
(3) Cu → Cu
2+
+ 2e
−
(oxidation)
Cond. A
Cond. B
Cond. C
500µm
500µm
500µm
0
O
-100
Q
W
LD
O
&
(a)
Cond. D
500µm
J
-200
Y
-300
-400
Target surface
Depth direction
Corrosion potencial
/mVvs.Ag/AgCl
-500
0 10 20 30 40 50 60 70
-100
-150
-200
-250
-300
-350
-400
(b)
Immersion time / hour
-450
0.4 0.5 0.6 0.7 0.8 0.9
Ratio (=R f/o
)
Figure 5: (a) Relationship between corrosion potential and
immersion time■: Cond. A, ▲: Cond. B, ▼:Cond. C and
♦: Cond. D) and (b) average corrosion potential under each
condition.
Figure 6: Cross-sections of gel targets with captured
particles under each spray condition.
Molten fraction ( wt% )
100
80
60
40
20
0
Cond.
A B C D
Ratio (Rf/o)
Unmelted
Melted
Figure 7: Molten fraction of particles under each
condition.
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1
(4) O + 2H
+
+ 2e
−
→H
O (reduction)
2 2
2
When there are penetrating paths of connecting pores
through the coating, the corrosive media can permeate the
coating through such a path and the corrosion reaction of
the steel substrate (Eqs. 1 and 2) takes place preferentially
to that of the coating (Eqs. 3 and 4) because of the galvanic
effect and these reactions may occur competitively. Whether
the corrosion reaction becomes preferential or competitive
depends on the through-porosity of the coatings. As for the
copper coated steels prepared under the conditions A and B,
the corrosion of steel substrate (Eq.1) is supposed to take
Cond. A
100µm
Cond. B
100µm
Cond. C
100µm
Cond. D
100µm
Figure 8: Cu splats collected on
polished SUS304 target under each
spray condition
Figure 9: Classification of Cu splats shapes into five types, left: observed and
right: illustrated.
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place in preferential to the corrosion of copper coating (Eq.
3) because the corrosion potential of these coatings were
approximately –400 mV and close to that of the free steel
plate at approximately –450 mV. On the other hand, the
corrosion reaction of coating (Eq. 3) takes place exclusively
and preferentially in case of not any and a slight amount of
penetrating pores, respectively. This is because the corrosion
reaction of steel substrate is negligible. This is the case for the
copper coated steels under the Cond. C and D because their
corrosion potentials corresponded to that of the free copper
plate around –130 mV.
These results revealed that the coating density at the
fuel/oxygen ratio over 0.66 (Cond. C and D) improved more
rapidly than that at the ratio below 0.56 (Cond. A and B). This
fact implies that a certain important factor exists to improve
the coating density, i.e. compactibility of sprayed particles at
the fuel/oxygen ratio over 0.66. Such a factor was discussed in
the next section.
Factors to improve compactibility
The compactness of HVOF spray coating was expected to
depend mainly on the transformation degree of sprayed
particles upon impinging, in other words, amounts of melted
particles and highly deformable particles.
Typical results of capturing spray particles by agar gels are
shown in Fig. 6. The spherical particles at the deeper positions
corresponded to unmelted particles, which plunged into the
agar gel without broken off. On the other hand, the fine
particles at the shallower positions corresponded to melted
particles, which were impinged to the gel target, broken into
fine particles and solidified, resulted in being captured near the
surface. As the fuel/oxygen ratio increases, the unmelted
particles at the deeper positions decrease whereas the melted
particles increase, as seen in Fig. 6. In order to show this
phenomenon quantitatively, molten fractions of the sprayed
particles were determined and represented in Fig. 7. At the
ratio of 0.46 under the Cond. A, the molten fraction was 0 wt%.
(
Abundance ratio (%
100
80
60
40
20
TYPE2
TYPE1
TYPE4
TYPE3
0
Cond. A B C D
TYPE5
Figure 10 Abundance ratios of five types splats under each
condition.
As the ratio increased, the molten fraction increased, especially
considerably at the ratio of 0.87 (Cond. D). This rapid increase
at the ratio of 0.87 (Cond.D) did not agree to the some
experimental results that the compactibility of sprayed particles
improved rapidly at the ratio of 0.66 (Cond. C), as mentioned
above. Accordingly, the molten fraction was not a primary
factor to improve the coating compactness under the HVOF
spray conditions in this paper although the molten fraction
must play an important role on making a dense coating.
The splat morphology is related to deformability of sprayed
particles. Figure 8 shows microscopic images of splats
prepared under four spray conditions. The splats could be
classified into some types of shapes such as lump, bullet
wound and splash. As the fuel/oxygen ratio increased, the
number of lump and bullet wound decreased whereas that of
splash increased. In order to estimate the sprayed particle
deformability quantitatively, three-dimensional information of
the splats was collected by the laser microscopy. According to
the height (or depth) and the area, the splats were classified
into five types of shapes, as shown in Figure 9. In this figure,
observed images are presented at the left side and their
schemas are illustrated at the right side. The five types of splats
are called as TYPE X for convenience below. The shape of
TYPE 1 looks like bullet wound and is supposed to be formed
by impinging of unmelted hard particles and by their falling
apart. TYPE 2 is considered to be part of such hard particles
sticking to the target. The shapes of both TYPE 3 and TYPE 4
were caused by the unmelted particles soft enough to deform
plastically and their deformation degrees may depend on
temperature and impact force of the spray particle. The shape
of TYPE5 is an extremely thin and flat and this is due to
melting of particles. About 100 of splats were sampled
randomly on the surface of test target sprayed under each
condition. The numbers of splats were counted according to the
above-mentioned classification and their abundance ratio was
determined and shown in Fig. 10. The increase in abundance
ratio of TYPE 5 is good agreement with the increase in molten
fraction of melted particles at the ratio of fuel to oxygen of
0.66 (Cond. D). This increase in the number of melted particles
may be related to the result that the higher value was obtained
especially under Cond. D on measuring the flight velocity of
sprayed particles (see Fig. 1). When Cond. B is compared to
Cond. C in terms of the abundance ratio, both TYPE 4 and
TYPE 5 increases under Cond. C. However, the increasing rate
for TYPE 4 is larger than that for TYPE 5. Therefore, the
reason why the compactibility of sprayed particles was
improved rapidly under Cond. C is the increase of the
unmelted particles soft enough to deform plastically, in other
words, with the sufficiently high plastic deformability upon
impinging.
Conclusion
In this study, we revealed that the key factors determining the
compactness of sprayed particles in HVOF sprayed copper
coatings. It is natural that the molten fraction of sprayed
particles is important and the higher fraction leads to form
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denser coatings. In addition, the larger numbers of spray
particles with highly plastic deformability is necessary to
improve the compactibility of sprayed particles considerably.
Acknowledgements
We express our sincere appreciation to Dr. T. Fukushima, Dr. T.
Sundararajan, Mr. M. Komatsu and Mr. A. Kishi at National
Institute for Materials Science and Mr. H. Yamada (now at the
SONY Co., Ltd.) for their useful and technical assistance.
We greatly appreciate to Dr. Y. Kobayashi at National Institute
for Materials Science for providing a chance to use the
analyzer for the ICP emission spectrometry.
References
1. S. Kuroda, T. Fukushima, M. Sasaki and T. Kodama,
Microstructure and corrosion resistance of HVOF sprayed
316L stainless steel and Ni base alloy coatings, Proc.
ITSC2000, C.C. Berndt (Ed.), ASM International,
Montreal, Canada, 2000, p 455-462.
2. T. Fukushima and S. Kuroda, Oxidation of HVOF sprayed
alloy coatings and its control by a gas shroud, Proc.
ITSC2001, C.C. Berndt (Ed.), ASM International,
Singapore, 2001, p 527-532.
3. T. Fukushima, H. Yamada, J. Kawakita and S. Kuroda,
Correlation between the in-flight conditions of HVOF
sprayed alloy particles and the coating structure, Proc.
ITSC2002, E. Lugscheider (Ed.), DVS-verlag GmbH,
Essen, Germany, 2002, p 912-917.
4. H. Yamada, S. Kuroda, T. Fukushima and H. Yumoto,
Capture and evaluation of HVOF thermal sprayed particles
by a gel target, Proc. ITSC2001, C.C. Berndt (Ed.), ASM
International, Singapore, 2001, p 797-803.
5. J. Kawakita, S. Kuroda and T. Kodama, Evaluation on
through-porosity of HVOF sprayed coatings, Proc.
ITSC2002, E. Lugscheider (Ed.), DVS-verlag GmbH,
Essen, Germany, 2002, p 681-685.
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