NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 249

unrecoverable energy dissipation. Another possible reason may be that
the contact strain across the area of contact beneath a spherical tip is not
constant but has a maximum at the edge of contact. Whereas for a
Berkovich indenter the contact strain is constant across the contact area,
that is, the mean contact strain beneath a Berkovich indenter is greater
than that for a spherical indenter of nominally equivalent contact strain
(tanθ = a / R ).
Where is the extra energy in enamel dissipated? If we initially
consider the apatite crystals, their very small size would have near
theoretical strength, approximately 10% of the shear modulus of
~10 GPa. The mean contact stresses for enamel do not approach this
value and only in the vicinity of the tip in the case of the Berkovich
indenter may such stresses be achieved. Hence, it is not considered that
energy loss by conventional dislocation based plastic deformation of the
inorganic phase is important for enamel. Therefore, the focus of the
discussion is centered on the role of the organic protein components
between and surrounding hydroxyapatite crystallite rods.
Spears cited the elastic modulus of hydroxyapatite crystals and
organic tissues as 114 GPa and 4.3 GPa, respectively, in his FEA
model. 18 Moreover, AFM nanoindentation 6 also reinforced the argument
that the protein rich sheath has a lower elastic modulus and hardness than
crystal containing rods: the nanohardness and elastic modulus of the
sheaths as reported above were 73.6% and 52.7% lower than those of the
prisms, respectively. The nanoindentation curves shown by Ge et al ., 11
although not commented upon, showed that sheaths had more energy
absorption than prisms. White et al . 10 also established that mineral
apatite crystals had a micro-hardness approximately 1.8 times greater
than human enamel. These studies indicate that the minor components of
enamel, namely protein and water, have a profound softening or
plasticizing effect. Other possible mechanisms responsible for the greater
energy loss include: fracture of the apatite crystallites, deformation of the
very thin protein layers between the crystallites within the prisms or by
deformation of the protein and movement of fluid in the nano-scale
porous structure.