L. HAO ET AL.
Copyright © 2012 SciRes. AM PC
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
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.
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.
 C. Suryanarayana, “Mechanical alloying and milling,” Prog.
Mater. Sci. vol. 46, pp. 1-184, January 2001.
 H. Yoshida, Y. Lu, H. Nakayama and M. Hirohashi, “Formation
of Ti O2 film by mech anical coati ng techniqu e and its photocat a-
lyst activity,” J. Alloy. Compd., vol. 475, pp. 383-386, May
 Y. Lu, H. Yoshida, H. Nakayama, L. Hao and M. Hirohashi,
“Formation of TiO2/Ti composite photocatalyst film by 2-step
mechanical coating technique,” Mater. Sci. Forum, vol. 675-677,
pp. 12 29-1232, February 2011
 S. Romankov, W. Sha, S.D. Kaloshkin and K. Kaevitser, “For-
mation of Ti-Al coatings by mechancial alloying method,” Surf.
Coat . Tech., vol. 201, pp. 3235-324 5, D ecem ber 2006.
 G. Gupta, K. Mondal and R. Balasubramaniam, “In situ nano-
crystalline Fe-Si coating by mechanical alloying,” J. Alloy.
Compd., vol. 482, pp. 118-122, August 2009.
 I. Farahbakhsh, A. Zakeri, P. Manikandan and K. Hokamoto,
“Evaluation of nanostructured coating layers form ed on Ni balls
durin g mechani cal allo ying of Cu powder,” Appl. Surf. Sci., vol.
257, pp. 2830-2837, January 2011.
 Y. Lu, L. Hao, K. Toh and H. Yoshida, “Fabrication of TiO2/Cu
composite photocatalyst thin film by 2-step mechanical coating
technique and its photocatalytic activity,” Adv. Mat. Res., vol.
415-417, pp. 1942-1948, February 2012.
 L. Hao, Y. Lu, H. Asanuma and J. Guo, “The influence of the
processing parameters on the formation of iron thin films on
alumina balls by mechanical coating technique,” J. Mater.
Process. Tech., vol. 212, pp. 1169-1176, May 2012.
 L. Hao, Y. Lu, H. Asanuma and J. Guo, “Fabrication of zinc
coatings on alumina balls from zinc powder by mechanical coat-
ing technique and the process analysis,” Powder Technol., vol.
228, pp. 377-384, September 2012.
 L. Hao, Y. Lu, H. Sato, H. Asanuma and J. Guo, “Influence of
metallic properties on formation and evolution of mechanical
coatings during mechanical coating technique,” unpublished.
 L. Lü, M.O. Lai and S. Zhang, “Modeling of the mechani-
cal-alloying process,” J. Mater. Process. Tech., vol. 52, pp.
539-546, June- July 1995.