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www.read-wca.comWire & Cable ASIA – March/April 2015
product quality and a huge amount of wasted material,
eg copper and aluminium, which would be unacceptable in
mass production.
A solution to both of the problems has appeared with the
development of the technique of chemical-vapour-depos-
ited (CVD) diamond films. Synthesis and characterisation
of diamond coatings have gained wide spread research
interests
[1]
and this wear resistant coating can be easily
exploited for drawing dies. The hot-filament CVD (HFCVD)
diamond coating on the interior hole surfaces of WC-Co
drawing dies provides particularly good results, having the
same advantages as traditional diamond drawing dies, but
with higher performance in key areas.
For example, super high hardness (70~100 GPa), very low
friction coefficient (~0.1), super high thermo-conductivity
(8~20W/cm
⋅
K) and chemical inertness. Significantly,
the HFCVD techniques provide tremendous economic
advantages at larger bore diameters where traditional
diamond dies show relatively weak economic perfor-
mance. Specifically, nano-dies enjoy spectacular success
replacing TC dies and PCD dies for copper and aluminium
power cable compacting applications up to Ø60mm bore
diameter
[2].
One of the key advantages of HFCVD is production of
diamond coatings with low roughness. This has always
been a big challenge for conventional multi-crystalline
diamond surface films. Because synthetic diamond
films deposited by conventional CVD processes are
multi-crystalline with a large grain size, this results in a
very rough surface, given the very high surface energy of
diamond. Since diamond is the hardest material known,
the polishing methods are difficult to apply and very time
consuming, especially for thin diamond films. The friction
coefficient increases as diamond films grow rougher.
Such surface roughness is not appropriate for many
applications, especially in aluminium conductor drawing
applications which benefit greatly from the very low friction
of process and very high finish of product, both of which
are now provided by HFCVD films.
2 Preparation of nanocrystalline diamond
composite coating dies (nano-dies)
The authors of this paper solved the problem of high
surface roughness of multi-crystalline diamond coatings
by the deposition of nanocrystalline diamond composite
coatings. Composite diamond films with a smooth
surface were deposited by a two-step chemical vapour
deposition procedure, including first the deposition of the
conventional rough multi-crystalline diamond and then
nanocrystalline diamond layers.
Cemented tungsten carbide YG6 (Co 6%) drawing dies
were used as the substrates, which were pre-treated by
various methods including leaching cobalt by dipping in
specific reagents and by scratching the substrate using
diamond powders
[3]
. Conventional and nanocrystalline
diamond films were deposited in a bias-enhanced hot
filament CVD apparatus.
Nanocrystalline diamond thin films were continuously
deposited in situ on 10~15μm thickness conventional
diamond films by adjusting CVD process parameters (such
as gas pressure, hydrocarbon-hydrogen gas mixture ratio
and whether or not bias voltage is applied) to significantly
enhance the secondary nucleation. Composite diamond
films with a smooth surface were achieved by the
deposition of alternate rough multi-crystalline diamond
and smooth fine-grained nanocrystalline diamond layers
[4]
.
A single straight tantalum filament was arranged to lie on
the centre line of the die to be coated (see
Figure 1
). The
tantalum wire is held straight by a high temperature spring
or a kind of specially designed holder in the case of very
small bore diameter dies.
❍
❍
Figure 4
:
Appearance of nanocrystalline diamond composite
coatings die (nano-die)
❍
❍
Figure 3
:
Raman spectra of the multi-crystalline coating (black)
and nanocrystalline diamond coating (red)
intensity (au)
wavenumber (cm
-1
)
underlying MCD film
surface NCD film
❍
❍
Figure 2
:
The plane-view SEM images of the multi-crystalline
and nanocrystalline diamond coating