Biomedical Engineering Reference
In-Depth Information
0.3
0.2
0.1
30 min
0.0
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10 min
0.2
0.1
0.0
0.3
5 min
0.2
0.1
0.0
0.3
3 min
0.2
0.1
0.0
0.3
1 min
0.2
0.1
0.0
0.3
Before erosion
0.2
0.1
0.0
0
0.1
0.2
Displacement/mm
Fig. 4.20
Variation in the friction coeffi cient with displacement subjected to different erosion
times [
49
]
of the acid attack, the erosive lesion was mainly characterized by continuous loss of
the outer enamel with the increasing time of erosion, causing the composition and
hardness of the remaining enamel surface to be almost constant with time except for
some layer-to-layer variations. This inference was supported by the results from
EDX examinations and hardness tests. The EDX examinations on the enamel sur-
face showed that the contents of calcium and phosphate ions decreased slowly at the
late stage (Fig.
4.17
). At the same time, the hardness of the enamel surface exhibited
a similar trend (Fig.
4.18
). Since the contents of calcium and phosphate ions as well
as the hardness were reported to decrease slightly from the outer to the interior of
enamel [
51
,
52
], their slow decrease with erosion time should be attributed to the
layer-to-layer variations caused by the continuous loss of the outer enamel.
From the original enamel surface to the acid-eroded enamel surfaces subjected to
different erosion times, friction curves describing the variations in the coeffi cient of
friction versus displacement are shown in Fig.
4.20
. Figure
4.21
gives the variation
in the coeffi cient of friction as a function of erosion time. Figure
4.22
shows the
variations of the depth and width of nanoscratch with erosion time.
Increased acidity in the mouth could decrease both the hardness and the elastic
modulus of enamel [
53
,
54
]. Lussi et al. [
55
] reported that acid-eroded enamel was
more susceptible to abrasion and attrition than intact enamel [
55
]. However, none of
the previous studies has related the tribological behavior of eroded enamel to its
erosion time. The plots in Figs.
4.20
,
4.21
, and
4.23
demonstrate the relationships
