Biomedical Engineering Reference
In-Depth Information
suggests that as the counter ball moves across the tooth surface under the two-body
wear condition, the enamel is plastically deformed and microcracks are generated
within the subsurface. These subsurface microcracks eventually coalesce, resulting
in a crack parallel to the surface and then loss of the wear surface. However, from
the pictures in Fig.
4.5
, we can see that there was no obvious severe damage, such
as deep ploughing, delamination, or bulk loss, on the worn surface under the three-
body wear condition. The whole process of three-body wear can be described as
follows. When the antagonist ball is pressed onto the surface of a tooth, these two
surfaces do not contact directly under the three-body wear condition. The normal
loading is distributed by food particles, so that the maximum stress is much lower
than that under the two-body wear condition, especially for the shear stress and
localized stress on the particles. Three-body wear on the occlusal surface is mainly
caused by the abrasion of the moving food particles. Compared to those in the
enamel rods, the Young's modulus and hardness were reportedly lower in the inter-
rod enamel, which can be attributed to changes in the crystal orientation and the
higher content of soft organic tissue in these areas [
32
]. Therefore, it could be
inferred that during the fi rst stage, the interrod enamel is a weaker phase that can
be easily worn out. But the wear will soon slow down because the prominent enamel
rods may act as a protective shoulder for the surrounding interrod enamel phase. In
this process, the enamel rods are unlikely to be worn themselves by food slurry.
After a period of time, the surrounding interrod enamel is worn to the extent that the
toughness of the occlusal surface decreases signifi cantly, and then the enamel rods
are fi nally worn away by the external action of the food slurry movement and load-
ing force. During the above process, the abrasion from the food slurry is weaker
than delamination under the two-body wear condition. Under the two-body wear
condition, most enamel rods are worn away either with the bulk loss of delamina-
tion or by the very fi rst contact with the antagonist surface. This implies that the
wear resistance of enamel is better under three-body wear conditions than under
two-body wear conditions. Therefore, a lower saturation value of the friction coef-
fi cient (shown in Fig.
4.4b
), a smaller area of wear scars (shown in Fig.
4.6b
), and a
lower wear loss (shown in Fig.
4.7
) were observed under the three-body wear condi-
tion compared with those under the two-body wear condition.
Variations in the friction coeffi cient and the worn volume between different
loads were also evident. It was observed in Fig.
4.4
that under both wear conditions,
with the load increasing, the number of cycles of a low friction coeffi cient, prior to
transition to a higher value, was reduced, although the evolution of the coeffi cient
was similar. Both the length and the depth of wear mark increased with the load
under the two wear conditions, as shown in Fig.
4.6
. In addition, the curves in
Fig.
4.7
exhibited a progressive increase in the worn volume, with the load increas-
ing from 10 to 40 N under both wear conditions. One-way analysis of variance
(ANOVA) revealed that there was a signifi cant difference (
P
< 0.001) in the wear
loss between different loads under either the two-body wear condition or the three-
body wear condition (Table
4.2
). The observations suggest that the enamel is easily
worn out at a higher normal load, and increasing the load could result in more wear.
