Biomedical Engineering Reference
In-Depth Information
a
b
A
A
Profile of AA
5 µm
LCSM micrograph
Fig. 3.17
LCSM micrograph and profi le of wear scar on the enamel surface subjected to 5,000
cycles [
35
]
is rich in protein and mostly a result of the incoherence of combining crystals of
different orientations. More details concerning enamel wear can be seen from the
micrographs in Fig.
3.14
and the profi le in Fig.
3.17
. Some lacunae were observed
to be around the enamel rods on the worn surfaces of enamel (Fig.
3.14c, e
). The
enamel rod was higher than the ambient interrod enamel (shown in Fig.
3.17b
).
Compared with those in the enamel rods, the Young's modulus and hardness were
lower in the interrod enamel probably because of changes in the crystal orientation
and the higher content of soft organic tissue in these areas [
40
]. Therefore, it could
be inferred that the interrod enamel is the weaker phase that can be easily worn out.
However, the protein-rich interrod enamel may act as a stress buffer for the brittle
enamel rods. After a period of time, the surrounding interrod enamel is worn out to
the extent that the toughness of the enamel surface decreases signifi cantly, and then
obvious plastic deformation and delamination of the enamel rods occur by the exter-
nal action of the normal load.
No signifi cant changes in the composition and microstructure of the enamel after
wear testing seemed to occur. The surfaces of enamel before and after 5,000 wear
cycles were examined by XPS and XRD; the results are shown in Figs.
3.18
and
3.19
, respectively. Both the original and worn surfaces of enamel were cleaned with
alcohol before being examined. The basic structural element of enamel is hydroxy-
apatite crystallite composed mainly of Ca, P, and O [
18
,
40
,
41
]. The binding ener-
gies of calcium (Ca), phosphorus (P), and oxygen (O) on the original enamel surface
were 347.0 eV, 133.0 eV, and 530.9 eV, respectively (Fig.
3.18
). After 5,000 wear
cycles, these values almost did not change. The weak characteristic peak of Ti was
also found. According to Fig.
3.19
, characteristic peaks of intrinsic crystal struc-
tures of human tooth enamel appeared between 25 ° and 55 ° before testing. Few
changes in the characteristic peaks were observed after 5,000 cycles. All these
results indicate that the wear of enamel is mainly a process of mechanical removal
without clear changes in both the composition and the crystal structure of enamel
