Biomedical Engineering Reference
In-Depth Information
This has also been reported by previous researchers [
8
,
33
] and may be related to the
structure of enamel. At lower loads, the high mineral content of enamel and its cor-
responding high hardness result in relatively low wear. At a higher load, however,
the brittle nature of enamel could contribute to its higher wear. In addition, Fig.
4.4a
showed that the saturation value of the coeffi cient decreased with the increase in
normal load under the two-body wear condition. Under the three-body wear condi-
tion, the saturation value of the coeffi cient decreased with the load increasing from
10 to 20 N, but it increased with the normal load increasing from 20 to 40 N, as
shown in Fig.
4.4b
. The phenomenon that the saturation value of the friction coef-
fi cient decreases with the increase in normal load is commonly seen in engineering
contacts and could be explained by Hertzian theory [
34
]. The abnormal increase in
the saturation value of the coeffi cient of friction, with the load increasing from 20 to
40 N under the three-body wear condition (Fig.
4.4b
), may be attributed to the effect
of food particles.
Furthermore, the photographs in Fig.
4.5
, the profi le lines in Fig.
4.6
, and the
plots in Fig.
4.7
demonstrated that the effect of normal load on wear behavior was
different between the two wear conditions. First, with the load increasing, greater
bulk delamination appeared on the worn surface of two-body wear; however, the
morphology of the worn surface was similar for loads of 20 and 40 N under three-
body wear conditions, as shown in Figs.
4.5
and
4.6
. Second, the increasing rate of
wear volume at a high load was lower under the three-body wear condition than
under the two-body wear condition, as shown in Fig.
4.7
. Based on the observations,
it could be inferred that at a higher load, the effect of a normal load is more signifi -
cant on two-body wear than on three-body wear, which is partly attributed to the
different lubrication on the contact surface. For two-body wear, artifi cial saliva can
act as the lubricant at a low load so as to reduce wear. As the load is increased, the
saliva is readily displaced from the contact. As a result, the lubrication condition of
the worn surface at a high load turns into boundary lubrication, or even dry friction
condition, at the later stage during the process of 2,000 cycles. However, for three-
body wear, the scratches on the worn surface of human teeth may act as particle
traps to cover the contact with food particles, which may have acted as a solid lubri-
cant, even at a higher load. Therefore, it has been observed that wear volume
increases sharply at a high load under the two-body wear condition.
Some researchers have pointed out [
35
,
36
] that due to different microstructures
and mechanical properties, wear rates of enamel and dentin showed different
increase tendencies as the load increased. High mineral content and corresponding
hardness result in relatively low wear rates of enamel at lower loads; however, the
brittle nature of enamel contributes to a high wear rate at higher loads. In contrast,
dentin has a higher organic content and relative softness, which makes it less prone
to fracture under oral conditions; as a result, it shows a high wear rate at lower loads
but a low wear rate at higher loads. It was reported that the differential wear rate
between dentin and enamel occurring in areas of exposed dentin may be a cofactor
in the formation of some Class VI lesions [
37
].
