Materials Transactions, Vol. 46, No. 12 (2005) pp. 3008 to 3014
#2005 The Japan Institute of Metals
Unusual Wetting of Liquid Metals on Iron Substrate with Oxidized Surface
in Reduced Atmosphere*1
Nobuyuki Takahira1; *2 , Toshihiro Tanaka1 , Shigeta Hara2 and Joonho Lee3
1
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita 565-0871, Japan
Department of Mechanical Engineering, Fukui University of Technology, Fukui 910-8505, Japan
3
Division of Materials Science and Engineering, Korea University, Seoul, Korea
2
The authors found that a liquid Cu droplet wetted and spread very widely on a solid substrate of Fe in a reduced atmosphere after the
surface oxidation of the substrate. The mechanism of the unusual wetting behavior was investigated by using surface-oxidized Fe substrate with
liquid Cu, Ag, Sn, and In. It was found that under a reduced atmosphere condition, fine pores were formed at the surface of the substrate which
had been oxidized, and that the pores were connected to each other continuously over the whole surface. The liquid metals penetrate into these
pores by capillary force to cause the unusual wetting behavior.
(Received July 25, 2005; Accepted October 13, 2005; Published December 15, 2005)
Keywords: wettability, high temperature oxidation, porous structure, capillary phenomenon, copper, silver, tin, indium
1.
Introduction
The recycle processes of various materials have been investigated to realize a recyclable society.1,2) In particular,
many procedures to avoid Cu mixing with Fe have been tried
in an effort to recycle Fe and Cu mixed scrap. After these
metals are mixed, it is very difficult to remove Cu from Fe
through the usual refining processes.3) Even though Cu in
steels can be diluted with raw iron, a little amount of Cu in
the steels causes surface hot shortness.4) This is a phenomenon caused by Cu in the hot rolling process of steels containing
Cu. That is to say, Fe is preferentially oxidized at the steel
surface and liquid Cu droplets appear at the steel/scale interface. The liquid Cu does not have good wettability to the
scale layer, and it infiltrates into grain boundaries during
the hot rolling process to cause cracks and strength degradation of the steels. In this phenomenon, it is important to understand wettability between liquid Cu and scales, or the oxidized surface of solid iron. Some experimental results on
wettability have been reported by Brown et al.,5) Hara et
al.,6) and Mukai et al.,7) and there have been many studies8–10)
about the mechanisms of surface hot shortness. It is generally
known that liquid metals are not wetted with solid oxide and
that there is good wettability between liquid metals and solid
metals.11) However, the authors have found, in experiments
on the wettability of liquid Cu, a peculiar phenomenon in
which liquid Cu unusually wetted on solid Fe substrate with
the oxidized surface. In this paper, we describe the unusual
wetting behavior of liquid metals on surface-oxidized Fe substrate and the mechanism of this unusual wetting behavior.
2.
Unusual Wetting Phenomenon
onto solid Fe substrate, of which the surface had been
oxidized. This phenomenon was concerned with iron oxide
formed on the surface of the iron substrate and therefore
different from typical wetting to the clean iron surface, which
is described later in detail. The experimental procedure was
as follows: Iron rods of 99.5% purity were used. Each rod
was 5 mm in diameter and 50 mm in length. After ultrasonic
cleaning of these rods in acetone and ethanol, some samples
were oxidized at 1073 K for 1 h in air. Two kinds of solid Fe
samples, a pure metallic Fe rod and a surface-oxidized Fe
rod, were contacted with a liquid Cu droplet settled on a
graphite plate at 1423 K in Ar–10%H2 , as shown in Fig. 1.
The rate of heating and cooling was 400 K per hour and the
sample was kept at 1423 K for 20 min.
Iron rod without
oxidized surface
Iron rod with
oxidized surface
liq. Cu
2.1
Liquid Cu wetting on solid Fe substrate with
oxidized surface
The authors found a wetting phenomenon in which liquid
Cu, in reduction atmosphere, unusually wetted and spread
*1This
Paper was Originally Published in Japanese in J. Jpn. Inst. Met. 69
(2005) 465–471.
*2Graduate Student, Osaka University
Fig. 1 Experiment for wettability of liquid Cu on solid iron rods in Ar–
10%H2 atmosphere.
Unusual Wetting of Liquid Metals on Iron Substrate with Oxidized Surface in Reduced Atmosphere
Figure 2 shows the appearances of the samples after the
experiment. The result of using Fe without surface oxidation
is shown in Fig. 2(a). Liquid Cu wetted just at the tip of the
rod where liquid Cu contacted directly. Figure 2(b), on the
other hand, shows that liquid Cu, which contacted at the
(a)
(b)
wetting
wetting
5mm
3009
lower end of the Fe rod with an oxidized surface, climbed up
to the top of the rod. The results of this experiment suggest
that this unusual wetting phenomenon is concerned with the
oxide layer formed on the surface of the Fe sample.
To confirm this unusual wetting behavior of liquid Cu
observed on a solid iron rod, as shown in Fig. 2, further
experiments were carried out by using a screw made of Fe
with a hole as well as a cup-shaped iron rod (purity 99.5%,
10 mm 30 mm) with a hole (5 mm 20 mm). The
experimental procedure is illustrated in Fig. 3. These two
samples were oxidized at 1073 K for 1 h in air. After forming
the oxide layer on the surface, each cup-shaped sample with
Cu shots in the hole was heated up to 1423 K in reduction
atmosphere. The rate of heating and cooling was 400 K per
hour, and the sample was kept at 1423 K for 20 min.
Figure 4 shows the samples after the above experiments
were completed. As shown in Figs. 4(a) and (b), liquid Cu in
the hole of the sample climbed up along the inside wall and
spread over to the lower edge of the screw. Similar behavior
was also seen for the cup-shaped iron rod, as shown in
Fig. 4(c).
2.2
Fig. 2 Wettability of liq. Cu on iron substrate. (a) Iron substrate without
oxidized surface (b) Iron substrate with oxidized surface.
Wettability of various liquid metals for solid iron
substrate in reduction atmosphere
As described in the preceding section, it was found that
Iron screw
Making a hole and
oxidizing in the air
Iron rod
Cu particles put
into the hole
Heating up
in Ar-10%H2
Cup-shaped
iron
Fig. 3 Experiment for wettability of liquid Cu on cup-shaped solid iron.
(a)
(b)
(c)
5mm
Fig. 4
Wettability of liq. Cu on iron substrate: (a), (b) Screw, (c) Iron rod.
3010
N. Takahira, T. Tanaka, S. Hara and J. Lee
Observation of the surface and
cross-section by using SEM
reduction
oxidation
Surface-oxidized
Iron substrate
Reduced iron
iron
Fig. 5
Sample preparation for the observation of the surface and the cross-section.
(a)
(b)
10µm
Fig. 6
SEM images of the reduced iron substrate: (a) surface, (b) cross-section.
liquid Cu wetted unusually on the iron sample with an
oxidized surface. Subsequently, whether the unusual wetting
of other metals also occurs was investigated. Cu, Ag, Sn, and
In, all of which have relatively small solubility of Fe, were
used as liquid metals. The effect of temperature on the
unusual wetting was also investigated by using Sn and In,
which have low melting points. The cup-shaped iron rod
(purity 99.5%, 10 mm 30 mm) with a hole (5 mm 20 mm) was oxidized at 1073 K for 1 h in air. After putting
small metal particles in the hole, the sample was heated up to
the following temperature, which was determined considering the melting points of these metals.
Cu: 1423 K
Ag: 1273 K
Sn and In: 1173, 973, and 773 K
When each surface of the solid iron rod wetted by liquid
metals was observed by EDX, we found that all of the four
metals (Cu, Ag, Sn, and In) spread even to the outside surface
of the rods by the unusual wetting. Thus the unusual wetting
behavior of each liquid metal was confirmed at all of the
experimental temperatures stated above, although some
different wetting behaviors were observed for these metals
due to differences in the solubility of Fe in each liquid metal,
as described later.
3.
Mechanism of Unusual Wetting Behavior
3.1 Surface of reduced iron substrate after oxidization
In the experiments stated in sections 2.1 and 2.2, the
sample of the iron substrate was oxidized before the wetting
experiments in the reduction atmosphere. Therefore it is
meaningful to observe the surface structure of the iron
substrate after the reduction of the oxide layer on the
substrate. An iron substrate (99.5%Fe) with a size of 20 30 1 mm was oxidized at 1073 K for 1 h in air and was
reduced at 1373 K for 1.5 h in Ar–10%H2 . The rate of heating
and cooling was 400 K per hour. Figure 5 illustrates the
above process. The surface and the cross-section of the
substrate were observed by using SEM.
Figure 6 shows SEM images of the surface and the crosssection of the reduced iron substrate. After the oxidization
and the reduction, it was observed that a lot of pores were
formed on the surface, as shown in Fig. 6(a). The upper side
of the photograph corresponds to the surface of the substrate.
Figure 6 demonstrates that a porous layer of approximately
80 mm thickness was formed with many pores on the surface
of the substrate in the above redox condition. It is supposed
that the pores were made by reducing the oxide and removing
the oxygen from the layer. Some pores are found at the
outermost surface of the substrate as micro-size holes, as
shown in Fig. 6(a).
3.2
Porous structure on the surface layer of reduced
iron
The formation of a porous layer with many pores was
found after the reduction of the iron oxide layer, as described
in section 3.1. In order to observe the 3-dimensional porous
structure, we tried to penetrate liquid Ag into the porous
structure and then remove the iron substrate by an acid
Unusual Wetting of Liquid Metals on Iron Substrate with Oxidized Surface in Reduced Atmosphere
aqueous solution. The microstructure penetrated by Ag
corresponds to the porous layer originally formed on the
iron substrate. The reduced iron substrate was prepared in the
procedure stated in section 3.1, and the reduced substrate was
contacted with liquid Ag at 1273 K. The iron sample with Ag
penetrated in the surface layer was soaked in hydrochloric
acid, and a thin film of Ag was obtained after the dissolution
of Fe.
Figure 7 shows the Ag film obtained from the above
procedure. When we observed the microstructure of the film,
it was found that, as shown in Fig. 8, liquid Ag penetrates
into almost all of the porous structure of the iron substrate. In
addition, these pores in the porous layer of the reduced iron
substrate do not exist individually, but are connected to each
other continuously. The average diameter of the pores is 2–
3 mm, as can be seen in Fig. 8(b).
Unusual wetting of various liquid metals on reduced
iron substrate
In order to investigate the wetting behavior of various
liquid metals, wetting experiments were carried out using the
3011
5mm
3.3
(a)
Fig. 7 Ag film obtained from the penetration of liq. Ag in the porous layer
on Fe substrate.
(b)
80µm
5µm
Fig. 8
SEM images of Ag film obtained from the penetration of liq. Ag in the porous layer on Fe substrate.
m
30mm
m
20
Reduced iron
substrate
Heating up
Observation of the surface
and cross-section
by using SEM
Metal plate
(Cu, Ag, Sn, In)
1mm
Fig. 9
Sample preparation in the wetting experiments.
3012
N. Takahira, T. Tanaka, S. Hara and J. Lee
(a)
(a)
(b)
(b)
(c)
20µm
Fig. 10 Cross-section of the reduced iron substrate wetted by liq. Cu:
(a) SEM image, (b) EDX image.
20µm
following four metals at each experimental temperature: Cu
at 1423 K, Ag at 1273 K, Sn at 1273 K and 873 K, and In at
1073 K. The reduced iron substrates were prepared in the way
described in section 3.1. The substrate was vertically placed
in a furnace, with each metal plate attached on the bottom, as
shown in Fig. 9, and heated up to the above temperature in
Ar–10%H2 . The rate of heating and cooling was 400 K per
hour, and the sample was kept at the above temperature for
20 min. The surface and the cross-section of the five samples
fabricated with the four metals were observed by using SEM,
and the concentration distribution of each element was
analyzed with EDX.
3.3.1 Factors of the unusual wetting of liquid Cu
The cross-section of the reduced iron substrate that liquid
Cu wetted was observed by using SEM, and the concentration
of Cu was analyzed by EDX, as shown in Fig. 10. Liquid
metal penetrated into the substrate from left to right. In order
to investigate the forefront of the penetration of liquid Cu into
the porous layer, a little amount of Cu was used to prepare a
substrate in which the liquid Cu had penetrated into half of
the porous layer. Figure 10 shows the forefront of this
penetration of Cu into the layer on the iron substrate. In the
left side of the photograph, Cu was observed at all pores of
the porous layer. Pores are seen in the upper part of Fig. 10(a)
but are not present in the bottom part. Figure 10(b) shows
that liquid Cu penetrated into the pores of the porous layer on
the surface of the reduced iron substrate. We could therefore
conclude that the liquid metal penetrated into these pores
formed in the porous layer by the capillary force to cause the
unusual wetting phenomenon.
3.3.2 Effect of the solubility of Fe in liquid metals on
their wettability with iron substrate
Figure 11 shows the experimental results for the wetting
of liquid Ag, Sn, and In. Each metal existing in the porous
layer is indicated as a white area in Fig. 11. The wetting
Fig. 11 SEM images of the cross-section of the reduced iron substrates
wetted by (a) Ag, (b) Sn, and (c) In.
behaviors of the liquid metals penetrating into the porous
layer of the substrate have almost the same characteristics of
Cu in Fig. 10.
Figure 12 indicates the outermost surfaces of the iron
substrates, in which the above four metals (Cu: 1423 K, Ag:
1273 K, Sn: 1273 K, In: 1073 K) wetted unusually. The way
that each liquid metal wetted to the outermost surfaces
illustrates its own distinctive manner of wetting. Although
liquid Cu and Sn widely cover the surfaces of the iron
substrates, liquid Ag and In only appear in some limited
areas. We guess that these differences depend on the
solubility of Fe in each liquid metal. Under the present
experimental conditions, Fe is solved by about 4.7% into Cu
and 8.0% into Sn, but little into Ag and 0.4% into In.12) As
shown in Fig. 12, liquid metals that have relatively high
solubility of Fe were found to be viable at almost the entire
surface area of iron substrates, but metals with low solubility
of Fe were observed only at pores on the surface or just
around them. Thus, the extent of the wetted surface area of
liquid metals seems to have a rough correlation with the
solubilities of those liquid metals in solid Fe although liquid
metals should have small solubilities in solid Fe. Then, the
solubilities of liquid metals in solid Fe, in other words,
attractive interactions of liquid metals with solid Fe, is
considered to be one of driving forces to control the
spreading of liquid metals on the substrate by the unusual
wetting behavior.
Figure 13 shows the EDX analysis of Sn at the outermost
surface on the substrate obtained from the wetting experiment at 873 K. The white area shows Sn distribution in
Fig. 13. Sn has approximately 0.5% solubility of Fe at
Unusual Wetting of Liquid Metals on Iron Substrate with Oxidized Surface in Reduced Atmosphere
(a) Cu
(b) Ag
(c) Sn
(d) In
3013
10µm
Fig. 12 EDX images of Cu, Ag, Sn, and In on the surfaces of the reduced iron substrates wetted by: (a) Cu at 1423 K, (b) Ag at 1273 K,
(c) Sn at 1273 K, and (d) In at 1073 K.
4.
Sn
10µm
Fig. 13 EDX image of Sn on the surface of the reduced iron substrate
wetted by Sn at 873 K.
873 K, and this solubility is almost the same value as that of
In at 1073 K, as shown in Fig. 12(d). Therefore, though it
had been predicted that liquid Sn would hardly spread in the
present experimental conditions, it in fact wet widely, in
contrast to the case of In. In light of this result, it is supposed
that since Sn is the only metal among the above four metals
which forms an intermetallic compound with Fe,12) then the
wetting proceeds to the extent that the metallic compound is
formed.
Conclusions
In the present study, unusual wetting behavior was found
for liquid Cu on a reduced iron substrate with a pre-oxidized
surface. This wetting investigation leads to the following
conclusions.
(1) The unusual wetting of liquid metals to the iron
substrate is caused by capillary penetration into the
porous structure formed on the surface of the iron
sample that is reduced after oxidized in air.
(2) Except for Cu, unusual wetting was observed by Ag,
Sn, and In, all of which have Fe solubility of only a few
percent.
(3) The wetting behavior at the outermost surface depends
on the solubility of Fe in each metal or the formation of
an intermetallic compound.
Acknowledgments
The authors would like to thank Mr. S. Wakita, Mr. S.
Nakatsuka, and Ms. T. Kitamura for their kind experimental
assistance.
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