Biomedical Engineering Reference
In-Depth Information
15.3.3
Controlling Structure and Morphology
The structure and morphology of the diamond films deposited are highly dependent on a number of
factors, which include the CVD growth and nucleation conditions, substrate type, and substrate prep-
aration. Numerous studies have been carried out onto flat silicon substrates and the general trends are
well established
[57-77]
. However, deposition of adherent high-quality diamond onto substrates such
as cemented carbides, stainless steel, and various metal alloys containing transition elements present
a considerable challenge due to poor adhesion and low nucleation density
[47,48,60-63]
.
As mentioned earlier, high purity diamond films can be grown using HFCVD with a mixture
containing hydrogen (H
2
) and methane (CH
4
). Whilst these highly crystalline diamond films are
exceptionally hard, they are typically very rough and very brittle. For applications requiring smooth
surfaces (for low friction), high hardness (for high wear resistance), and high toughness (for prevent-
ing catastrophic brittle fracture), a more viable solution is needed. This is particularly true for depo-
sition on metallic substrates where large thermally induced residual stresses can be present in the
film, providing a driving force for delamination. Ideally, the film must be well bonded to the substrate
and be able to undergo some plastic deformation without hard elastic-to-brittle fracture occurring.
One promising material is that of nanostructured diamond, generally defined as containing small dia-
mond grains (from about 5 to 100 nm) imbedded in an amorphous carbon phase that will typically
include both sp
2
(graphite-like) and sp
3
(diamond-like) carbon bonds. These films can still be quite
hard (up to 80% that of natural diamond) but can also offer the advantage of much lower surface
roughness and much higher fracture toughness. Several processing routes for these nanostructured
films exist, depending on deposition technique and/or feed gas mixture. Nucleation of diamond is an
important step in the growth of diamond thin films, because it strongly influences the diamond growth
process, film quality, and morphology. The most promising
in situ
method for diamond nucleation
enhancement in this application is negative substrate biasing during the initial stage of deposition.
Complex 3D-shaped dental burs can be biased readily using this method
[67]
. A modified vertical
HFCVD
[64-67,69-82]
was employed and the key process parameters, which affect the structure and
morphology, have also been investigated and summarized.
15.3.3.1 Effects of Temperature
The substrate temperature has a profound effect on the crystal size and morphology of the diamond
films. Using the vertical arrangement, the dental burs were placed concentrically within the coils
of the filament. With this configuration, the temperature varies significantly between the center and
the ends of the filament resulting in the variation of the substrate temperature which in turn affects
the morphology and structure of the resulting diamond films. Therefore, an analysis of temperature
distribution along the coiled filament is required. Filament temperature measurements were carried
out using a two-color optical pyrometer at various positions along the filament (
Figure 15.6
). The
bur substrate temperature was also measured parallel to the positions A, B, and C using a K-type
thermocouple. These measurements show that the substrate temperature varies according to the bur
position within the filament from 950°C at position A to 840°C at position C (
Figure 15.7
). The
diamond growth is as a result also nonuniform with the highest growth rates being at the position
on the burs which was located at the center of the filament during deposition. The films morphol-
ogy at the positions was examined using a scanning electron microscope (SEM).
Figure 15.8
shows
SEM micrographs of the as-grown diamond film at the tip, middle and base of the WC-Co dental bur
(c.f. positions A, B, and C, respectively in
Figure 15.7
).
