Friction and Wear Behavior of Human Teeth - Dental Biotribology - page 61

3.4 Effect of Age on the Friction and Wear Behavior

of Human Teeth

In this section, we present results on the friction and wear behavior of human teeth
at different ages against a titanium alloy (TC4) using a reciprocating apparatus con-
taining an artifi cial saliva solution (Fig. 3.1 ) [ 42 ]. The main objective was to under-
stand the effect of age on the friction and wear behaviors of human teeth and help
dentists select appropriate dental restorative materials with better friction-matching
properties.
Flat samples used in this study were freshly extracted human teeth without caries.
Primary teeth were obtained from different children who were 8 years old. Permanent
teeth of young, middle, and old ages came from people who were 18, 35, and 55
years old, respectively. All fl at tooth samples were prepared according to the method
mentioned in Sect. 3.3 . A TC4 ball (its composition is listed in Table 3.2 ) with a
diameter of 40 mm was chosen as a counterpart.
A normal force of 20 N was imposed in the tests. A reciprocating amplitude of
500 m and a frequency of 2 Hz were used for all the tests. Tests up to 5,000 cycles

were conducted with an interval of 30 min at every 1,000 cycles. The normal force
was discharged and the contact surfaces were carefully cleaned by artifi cial saliva
during the period of the intermittence. The composition of artifi cial saliva is listed
in Table 3.1 .
From primary teeth to permanent teeth of the old age, typical variations in the
friction coeffi cient, as a function of the number of cycles, are shown in Fig. 3.20 .
For primary teeth, the friction coeffi cient was low (about 0.11) and relatively stable
before 450 cycles. Then the coeffi cient increased rapidly up to 0.80, fi nally fl uctuat-
ing at about 0.83 (Fig. 3.20a ). For permanent teeth of different ages, the variations
in the coeffi cient were similar to each other (Fig. 3.20b-d ) in the initial wearing
stages. A lower friction coeffi cient lasted for only about 20 cycles and increased to
0.8 after 100 cycles. Also, a notable change in the friction coeffi cient occurred at
later testing stages as the age of the permanent teeth increased to 55 years old.
As seen in Fig. 3.20d , the aged teeth showed strong fl uctuations in the coeffi cient
after about 100 cycles: between 0.80 and 0.93, similar to that of the primary teeth.
For the permanent teeth of young and middle ages, the coeffi cient increased slowly
to a saturation value of 0.91/0.93 after 2,000 cycles. In addition, the data in Fig. 3.20
show that the friction coeffi cient reduced after each interval. The reduction in the
friction coeffi cient could result partly from a change in the lubrication condition.
A comparatively suffi cient artifi cial saliva existed as a lubricant on the interface of
the titanium alloy ball and human teeth specimen at the early stage during each
1,000 cycles of abrasion. However, the artifi cial saliva was displaced gradually from
the contact with the increase in the number of cycles, and the lubrication condition
of the worn surface turned into a boundary lubrication, even a dry friction condition,
at the later stage during each 1,000 cycles of abrasion. Therefore, the friction coef-
fi cient tended to be reduced after each interval.

μ