Microtribology of Human Teeth - Dental Biotribology - page 116

Interrod enamel
Head of rod
Tail of rod

y

x

Occlusal section Axial section

Fig. 5.1 Schematic diagram of human enamel

materials, nanoindentation has been widely used to investigate the mechanical
properties of human teeth [ 2 - 9 ]. Habelitz et al. [ 4 ] reported that the average elastic
modulus and hardness measured in the direction parallel to the axes of enamel rods
were higher than those measured in the direction perpendicular to the axes of
enamel rods. The differences were assigned to the unique alignment of fiber-like
apatite crystals and the composite nature of enamel rods [ 4 ]. Ge et al. [ 5 ] showed
that both the hardness and the modulus of interrod enamel were lower than those of
enamel rod.
Since the nanoscratch technique has a typical working force range of 1 μN to
500 mN and a displacement range of 1 nm to 300 μm, it provides a powerful tool for
the microtribological characterization of tissues and other biomaterials with a
submicron resolution [ 10 - 13 ]. The results can help us to study the role of various
structural and chemical components in providing mechanical strengths. Very
recently, the nanoscratch technique has been used to study the tribological properties
of human teeth [ 14 - 17 ]. Guidoni et al. [ 14 ] presented that the tooth enamel was
brittle at a low load (50 μN) but ductile at a high load (100 μN) by scratching with
a sharp indenter tip (tip radius R = 20-50 nm). Asperity deformation and fatigue
wear dominated the damage during scratching by a rounded conical tip (~1,200 nm
radius), but ploughing and wedge formation were the main damage mechanism by
using a sharp indenter tip (20-50 nm) [ 15 ]. The recovery effects and abrasion resis-
tance were more evident under wet conditions than under dry conditions [ 16 ].
Despite all that, the anisotropy of the wear behavior of tooth enamel was not very
clear and its microtribological behavior was still far from understood.