SEM images of the cross sections of the Sn-coated
Al2O3 balls after the mechanical coating at 150 rpm for differ-
ent milling times are shown in Figure 2. It can be seen that
discontinuous Sn coatings were formed when milling time was
4 h. When it came to 8 h, totally continuous Sn coatings were
fo rmed. With further increase of milling time, the thickness of
the coatings was increased. The SEM images of the surface
morphologies of the Sn-coated Al2O3 balls after the mechan ical
coating at 150 rpm for different milling times are given in Fig-
ure 3. As d iscussed in Figure 1 and Figure 2, discontinuous Sn
coatings were formed when milling time was 4 h. Continuous
coatings were formed when it came to 8 h. It can be found that
the surfaces of the coatings were rather uneven and many
bulges of Sn particles were also formed. With further increase
of milling time, the coatings became relatively even and the
bulges of Sn particles seemed not obvious. The morphology
change of the coating should relate to the collision and friction
among the Al2O3 balls co ated with Sn and the inner wall of the
bowl. Under repeated and great collision force and friction
force, Sn bulges were flattened or worn down.
30 40 50 60 70
Al
2
O
3
Sn
Relative intensity (a. u.)
2 Theta (deg)
4 h
8 h
12 h
16 h
20 h
Figure 1. XRD patterns of the Sn-coated Al2O3 balls after the me-
chanica l co ating at 150 rpm for different m ill in g times.
Figure 2. SEM images of the cross sections of the Sn-coated Al2O3
balls after the mechanical coating at 150 rpm for different milling
times: (a) 4 h, (b) 8 h, (c) 12 h, (d) 16 h, (e) 20 h and (f) 30 h.
Figure 3. Surface morphologies of the Sn-coated Al2O3 balls after
the mechanical coating at 150 rpm for different milling times: (a) 4
h, (b) 8 h, (c) 12 h, (d) 16 h, (e) 20 h and (f) 30 h.
L. HAO ET AL.
Copyright © 2012 SciRes. AM PC
128
The evolution of Fe coatings and Zn coatings can be divided
into the following stages: nucleation, formation and coales-
cence of discrete islands, formation and thickening of conti-
nuous coatings, and exfoliation of continuous coatings [8,9].
Although the first three stages were observed for Sn coatings,
the last stage “exfoliation of continuous coatings” was not seen.
The phenomenon is similar to that of Cu coatings [7]. It may
relate to the mechanical properties of metal powders. If the
milling time is prolonged, the last stage may be seen for Cu
coatings and Sn coatings. Therefore, a universal law for the
evolution of metal coatings during mechanical coating can be
concluded as the above four stages of Fe and Zn coatings. For
different metal po wder, the holding time for each st age may be
different and even cert ain stage(s) will not occur if milling time
is not long enough such as Cu coatings and Sn coatings.
3.2. Influence of R otation Spe e d
To investigate the influence of rotation speed on the evolution
and formation of Sn coatings, we performed a series of contrast
experiments in which the rotation speed was set at 100, 150,
200 and 300 rpm. When the rotation speeds was 100 and 300
rpm, continuous Sn coatings were not formed although milling
time was prolonged to 40 h. It can be confirmed that continuous
Sn coatings cannot be formed at 100 and 300 rpm. Figure 4
shows the SEM images of the cross sections of the Sn-coated
Al2O3 balls after the mechanical coating at 200 rpm for differ-
ent milling ti mes. I t can be seen th at d iscr ete isl and s o f Sn were
formed when milling time was 4 h. When it came to 8 h, the
discrete islands connected with each other. With further in-
crease of milling time, continuous coatings were formed when
milling time added up to 20 h. Compared with the case with
150 rpm; the formation of Sn coatings took more time when
rotation speed was 200 rpm.
Figure 4. SEM images of the cross sections of the Sn-coated Al2O3
balls after the mechanical coating at 200 rpm for different milling
times: (a) 4 h, (b) 8 h and (c) 20 h.
From the above discus sion, rot ation speed has great effect o n
the evolution and formation of Sn coatings. Continuous Sn
coatings can be formed during the mechanical coating with a
moderate rotation speed. A similar conclusion can also be
drawn for Fe coatings [8]. However, higher rotation speed can
accelerat e the formation of Zn coatings [9].
The evolution of Sn powder particles during the mechanical
coating was also monitored. Figure 5 shows the SEM images
of Sn powder particles before and after the mechanical coating
at 150 rpm for different milling times. From these images, the
diameter of Sn powder particles became greater with the in-
crease of milling time. These particles became spherical or
lamellar from irregular shape. As discussed in the published
works [8,9], the diameter increase and shape change of the
powder particles should result from the collision and friction
among the Al2O3 balls and the inner wall of the bowl.
During repeated collision, Sn powder particles were trapped
between the Al2O3 balls and the inner wall of the bowl. Under
great impact force, some particles adhered to the surfaces of
Al2O3 balls. That made the formation of Sn coatings on the
surfaces of Al2O3 balls possible. The adhesion between metal
particles and Al2O3 balls will not be involved here since it has
been discussed in our unpublished work [10]. Cold welding
made the particles adhered with each other and hence their
diameter got larger. It should be pointed out that cold welding
may happen only when the strain of metal particles is greater
than a critical value [11]. A greater collision force can be ob-
tained under a higher rotation speed and a greater collision
force can produce a larger strain [9]. Therefore, cold welding
tends to occur during mechanical coating at a higher rotation
speed. It explains why continuous Sn coating was formed at
Figure 5. Evolution of Sn powder particles before and after the
mechanical coating at 150 rpm for different milling time: (a) before
mechanical coating, (b) 4 h, (c) 8 h, (d) 12 h, (e) 16 h and (f) 20 h.
L. HAO ET AL.
Copyright © 2012 SciRes. AM PC
129
150 and 200 rpm but did not form at 100 rpm. However, cold
welding among Sn powder particles was greatly accelerated
when the rotation speed was increased to 300 rpm. It can be
confirmed fro m the si gnificantly in crease of Sn parti cle diame-
ter after mech anical co atin g at 300 rp m. However, t he adhesion
of Sn particles to the surfaces of Al2O3 balls was not accele-
rated due to the increase of rotation speed. Therefore, Sn par-
ticles had grown up before the adhesion of the particles to
Al2O3 balls. It is the reason why the diameter of Sn particles
was largely increased but continuous Sn coatings on Al2O3
balls was not formed when rotation speed was 300 rpm. The
de formation of Sn particles between Al2O3 balls and the inner
wall of the bowl can be regarded as the forging between two
parallel plates because the volume of the trapped Sn particles
was much smal ler t han th e co llid ing bod ies. Therefore, la mellar
particles were formed. Under collision force and friction force,
the sharp corners of the particles were eliminated and hence
large spherical p ar ticles ap peared.
From the above analysis about the influence of rotation speed,
the formation of metal coatings on the surfaces of Al2O3 balls
includes two kinds of interaction: the adhesion of metal par-
ticles to the surfaces of Al2O3 balls and then the cold welding
between metal particles. If we want to prepare metal coatings
on Al2O3 balls, we have to find out the metals which tend to
adhere to the surfaces of Al2O3 and easy to weld with each
other.
4. Collisions
Continuous Sn coatings were fabricated on Al2O3 balls by me-
chanical coating technique. The results of XRD and SEM indi-
cate that the evolution of the coatings follows a universal law
for the evolution of metal coatings which includes the follow-
ing stages: nucleation, formation and coalescence of discrete
islands, formation and thickening of continuous coatings, and
exfoliation of continuous coatings. Continuous Sn coatings can
be prepared during the mechanical coating at a moderate rota-
tion speed such as 150 and 200 rpm but cannot be formed at
higher or lower ones such as 300 and 100 rpm. The influence of
rotation speed should relate to the adhesion of metal powder
particles to the surfaces of Al2O3 balls and the cold welding
among metal powder particles.
5. Acknowledgements
The authors gratefully acknowledge the financial support from
Denshi-Jisso Company in Japan. The authors would like to
thank Fukuda metal foil & powder Co., Ltd. of Japan for pro-
viding us with the Sn powder used in the work.
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