Friction and Wear Behavior of Human Teeth - Dental Biotribology - page 58

Fig. 3.16 Laser confocal
scanning microscopic image
of enamel rods on an occlusal
section [ 35 ]; R rod, IR
interrod
wear scars became smoother after 2,000 cycles than those after 1,000 cycles
(Fig. 3.13a ). With the sliding going on, there were an increasing number of particles
on the worn surface of enamel (Fig. 3.11 ). The worn surface of enamel was almost
covered by a particle layer after 5,000 cycles. The particles were mainly composed
of the debris of enamel as well as a few grains of titanium alloy (Fig. 3.12 ). At the
same time, it was observed that more enamel debris was attached to the worn
surfaces of the TC4 ball after 5,000 cycles than after 1,000 cycles (Fig. 3.15 ). In
addition, Fig. 3.10 also shows that the coeffi cient of friction kept constant after
2,000 cycles. Thus, there may exist two stages in the process of enamel wear. In the
fi rst stage, severe delamination on the surface of enamel makes the enamel surface
very uneven, and then the real contact area decreases, while the contact stress
increases. Therefore, the wear of enamel is intensifi ed; that is, the depth of the wear
scar increases signifi cantly with the number of cycles. As the number of cycles
increases further, the size of the enamel debris becomes smaller due to the recipro-
cating sliding. As a result, a wear-particle layer forms on the worn surface of
enamel. The wear-particle layer causes an increase in the real contact area and then
a decrease in the contact stress. So the wear rate decreases and the wear of enamel
stays in a stable stage, that is, the second stage. Two stages of enamel wear were
also observed in previous experiments with different wear apparatuses [ 30 , 38 ].
Generally, the primary and secondary phases were described as running-in wear and
steady-state wear, respectively. This pattern is commonly seen in engineering con-
tacts and is also consistent with the results of clinical observations [ 39 ]. Clinical
studies showed that the initial stage appeared to last for a period of two years before
transition to a slower second stage.
As shown in Fig. 3.16 , enamel uniquely consists of aligned prisms or rods, which
run approximately perpendicular from the dentin-enamel junction toward the tooth
surface [ 19 , 40 , 41 ]. Each rod consists of tightly packed carbonated hydroxyapatite
crystals covered by a nanometer-thin layer of enamelin. These crystals orient along
the rod axis. The interfacial area between rods is termed the interrod enamel, which