NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 241

3.1. Elastic modulus and hardness of enamel
Mechanical tests to determine elastic modulus and hardness of enamel
have experienced several stages of development from macro-scale to
micro- and nano-scale over the last several decades. Due to the size limit
of human enamel samples, it has been difficult to perform macro-scale
tests. Therefore, most investigations on the mechanical properties of
human enamel have been done with nanoindentation. A partial listing of
such investigations is listed in Table 8-2 . Also, the range of the results
obtained has reduced considerably, due to the precise control of the test
area.
From the table, we may conclude that, firstly, the large mineral level
(about 96 wt%) as well as the unique microstructure of enamel contribute
to its large mechanical properties. 28-30 The stiffness of a composite
material like enamel is the combination of the stiffness of its multi-phase
components and associated microstructures; the inorganic-hydroxyapatite
crystals and organic tissues which may be considered as a fibrous
composite model. 18
Secondly, current understanding of the mechanical behavior of
enamel at the nano-scale associated with its anisotropic microstructure is
possible with nanoindentation. The hardness and elastic modulus of
enamel were found to be a function of enamel rod alignment. 30,36 The
measured elastic modulus varies with the direction of the load relative to
the apatite crystals of enamel. Higher elastic modulus values are found
when the applied force is parallel to enamel rods as the deflection will be
resisted mostly by the aligned crystals within the rods, while in the
perpendicular directions, the displacement will be influenced more by
the less stiff organic tissue surrounding the crystals thereby resulting in
lower elastic moduli. The rod orientation dependence is also seen in a
uniaxial loading three-dimensional finite element analysis of enamel. 18
This also explains the discrepancy in the elasticity data of enamel
rods and interrods. The interrod was reported to have hardness and
elastic modulus of about only 25 and 50%, respectively of the values of
the rod. 6