Biomedical Engineering Reference
In-Depth Information
a
F
n
F
n
D
D
IR
R
IR
R
F
n
F
n
b
D
D
IR
R
IR
R
Fig. 4.14
Schematic diagrams of the enamel wear process in different wear media [
38
] (
R
rod,
IR
interrod enamel): (
a
) in the citric acid solution; (
b
) in the artifi cial saliva
dissolution of the mineral phase within the enamel in the citric acid solution, and
then a honeycomb-like structure appeared on the surface of enamel. According to
the study from Cheng et al. [
46
], the honeycomb-like structure should be a multi-
layer structure composed of the enamel loss layer (which is completely dissolved
and leaves a serration-shaped structure), the softened layer (where the mineral is
partly eroded), and the transition layer (where the acid solution macerates into the
inner enamel along the organic sheaths). Both the depth of the enamel loss layer and
the thickness of the softened layer increased with the erosion time, but the increas-
ing rate of the thickness of the softened layer was much greater than that of the
depth of enamel loss. The thickness of the softened layer could reach approximately
1
m
after 2,000 cycles under a load of 10 N (Fig.
4.9
). Obviously, under a lower loading
level, the softened layer had a remarkable infl uence on the wear behavior of enamel.
The wear resistance of enamel was decreased signifi cantly, and the wear mechanism
was mainly characterized by adhesion delamination as the counter ball moved
across the tooth surface. As a result, compared with the artifi cial saliva, more sig-
nifi cant delamination traces and a much higher wear loss appeared on the worn
surface of enamel in the citric acid solution at 10 N. With the normal load increas-
ing, the contribution ratios of the external loading force and the brittle nature of
enamel to the wear behavior of enamel increased gradually. Therefore, the wear loss
gap between the enamel specimens in these two wear media became smaller. As
shown in Fig.
4.10
, at 40 N, the wear depth of enamel in the citric acid solution was
about 16
μ
m after 5-min of erosion [
46
], while the wear depth of enamel was about 4
μ
m, and the value was much higher than the thickness of the softened
layer. This seems to suggest that under a high loading level, the contribution of the
external loading force and the brittle nature of enamel to the wear of enamel far
exceeded that of the softened layer by acid attack in the citric acid solution. The
μ
