Biomedical Engineering Reference
In-Depth Information
axes. Toward the edge of both the head area and the tail area, they flare laterally,
attaining an almost perpendicular orientation at the interrod enamel. As shown in
Fig.
5.11
, during the nanoindentation loading process, the hydroxyapatite crystal
fibers could be regarded as compression bars with fixed ends. Hence, a force couple
M
F
n
applied to the fibers can be determined from the following equation:
(5.1)
MFD
F
=
,
n
n
where
F
n
is the normal load applied to the free ends of fibers, parallel to the rod
axes, and
D
is the distance
F
n
deviates from the fixed end of the fibers. In the center
of the head area,
F
n
was almost along the direction of the fiber arrangement, causing
a very small value of
D
. Thus, the fibers mainly suffered from axial compression,
resulting in a relatively small deformation and then a shallow indentation depth. The
hardness value and the elastic modulus were high in this area. With the indentation
position moving toward the edge of the head area and the tail area, the fibers showed
an increasing angle to the rod axes, and then the value of
D
gradually increased,
causing an increasing force couple. Hence, significant
lexural deformation
occurred
to the fibers, especially in the tail area, inducing an increased indentation depth. As
a result, the hardness value and the elastic modulus decreased.
5.3
Nanoscratch Along the Vertical and Parallel
Directions to the Enamel Rod Axis
To investigate the microtribological behavior of enamel, the nanoscratch was con-
ducted along the vertical and parallel directions to the enamel rod axis. As shown in
Fig.
5.12
, the third human molars were cut into two halves along the longitudinal
direction with a low-speed saw under constant water irrigation [
18
]. The lingual half
was embedded into the denture base resin with an exposed window of about
4 × 4 mm. They were hand-ground polished using silicon carbide paper with a grit
size of 500 and 1,500 in turn under constant water irrigation, followed by polishing
with 2- and 0.5-μm diamond paste on a rotary polishing machine. To reveal the
microstructure of enamel, the sample was etched with 0.001 M citric acid solution
for 1 min [
4
].
With a nanoscratch tester (CSM Instruments, Peseux, Switzerland), all the nano-
scratch tests were performed by a diamond tip with a nominal radius
R
of 5 μm.
Figure
5.13
shows the SEM image of the diamond tip. During the tests, the scratch
distance was set to 200 μm and the sliding velocity was 200 μm/min. Two kinds of
loading modes were used in the scratch experiments: linearly increasing the load
from 0 to 120 mN, and a constant load. After the scratch tests, the topography of the
scratch-induced damage on the enamel was observed by a scanning electron micro-
scope (SEM, INSPECT, FEI Technology, Eindhoven, the Netherlands) and an
atomic force microscope (AFM, SPI3800N, Seiko, Tokyo, Japan). The AFM tip is
a silicon nitride tip (MLCT, Veeco, New York, NY, USA) with a nominal radius of
10 nm and a normal spring constant of 0.1 N/m.
