Biomedical Engineering Reference
In-Depth Information
50
µ
m
50
Vertical
Parallel
40
30
20
10
0
0
30
60
90
120
0
30
60
90
120
Fig. 5.14
SEM images (
top
) and the friction force versus normal load curves (
bottom
) of the
scratches along the directions vertical and parallel to the enamel rod's axis
along the two directions showed a similar variation. They kept a stable increase
when the normal load was below 40 mN. When the normal load was higher than
40 mN, a larger fluctuation in the friction was observed from the friction curve when
scratching along the vertical direction to the axis of the enamel rod. The variation in
the friction force was suggested to relate to different wear mechanisms of enamel,
which was mainly induced by the interval structure of the enamel rod and the inter-
rod enamel [
14
,
20
]. The wear behavior of human enamel was noted to be associ-
ated with the diameter of the scratching tip. Guidoni et al. [
15
] reported that the
sharp tip (~20-50-nm radius) cut into and ploughed the enamel, creating a wedge or
ridge of material ahead of itself. While using a rounded conical tip (~5-μm radius)
in the present research, the debris was mainly located on both sides of the scratches.
To further understand the effect of HA crystallites' orientation on the wear
behavior of tooth enamel, the scratches were made along the parallel and vertical
directions to the axis of the enamel rod at various constant loads. Figure
5.15
shows
the SEM images of the scratches under constant loads along two directions. The
AFM images and cross-sectional profiles of these scratches are shown in Fig.
5.16
[
18
]. At a low load of 5 mN, no perceptible difference was observed on the scratches
along two different directions, which was consistent with the result of the friction
forces in Fig.
5.14
. However, as the load increased to 20 mN, a slight pileup of
materials began to generate beside the scratch along the vertical direction to the axis
of the enamel rod. Finally, when the load was above 50 mN, evident surface debris
was created on two sides of the scratch along the vertical direction, but only a little
pileup was found beside the scratch along the parallel direction. Also, the debris
was apt to generate at the locations of the interrod enamel. As shown in Fig.
5.16
,
the scratch width increased from about 1.5 to 5 μm when the normal load increased
from 5 to 80 mN.
