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Technical article
January 2015
42
www.read-eurowire.comthis 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. A single
straight filament located at the axis
centre of the die can maintain uniform
temperature on the surface of the bore
of the drawing die during the deposition
process.
A DC bias is applied between the filament
and the drawing die substrate so as to
enhance the diamond nucleation density.
3 Characterisation
and discussion of
nanocrystalline
diamond composite
coating dies
(nano-dies)
Figures 2a
and
2b
show the surface
morphology
of
conventional
multi-
crystalline and nanocrystalline diamond
coatings. For conventional multi-crystalline
diamond, as shown in
Figure 2a
, the film
displays a well-faceted microcrystalline
diamond surface with grain size ranging
from 2 to 4μm.
The surface is very rough and consists of
a combination of {111} and {110} facets.
For nanocrystalline diamond, as shown
in
Figure 2b
, the film appears very dense
with a fine-grained morphology (grain
size about 50nm). The surface morphology
is non-faceted and much smoother than
multi-crystalline diamond films.
Figure 3
shows the Raman spectrum of the
composite coatings of multi-crystalline
diamond
film
and
nanocrystalline
diamond
film.
For
conventional
multi-crystalline diamond films, the only
sharp characteristic peak for diamond (sp
3
carbon) appears at 1,332cm
-1
.
For nanocrystalline diamond films, a
characteristic peak for diamond (sp
3
carbon) appears near 1,339cm
-1
. And a
broad peak near 1,580cm
-1
corresponds
to amorphous carbon or non-diamond
carbon (sp
2
carbon). It can be inferred
that the conventional multi-crystalline
diamond film contains much less
non-diamond component.
For
nanocrystalline
diamond,
the
diamond band at 1,332cm
-1
is significantly
▲
▲
Figure 2
:
The plane-view SEM images of the multi-crystalline and nanocrystalline diamond coating
(a)
(b)
▼
▼
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