Biomedical Engineering Reference
In-Depth Information
is less likely to cause failure than in a situation with a sharp interface. Such localized variations in
mechanical properties of mineralized tissues were attributed to the tissues' microstructural variation
such as mineral density, collagen arrangement, and protein content
[35-38]
. There are also some small
structures distributed throughout the volume of the tissues to modify the function of the material and
they may create marked differences in the mechanical properties within the matrix tissue, as in the case
of the highly mineralized peritubular dentin located within the intertubular dentin. The measurements
made with nanoindentation often have a large scatter in the values as a result of the microstructure.
To elucidate the nanoscale differences across the microstructures, extremely precise control
over the location of indentation is required, and this can be done in certain commercial nanoindent-
ers which allow nanoindentation to be performed on the same platform as atomic force microscopy
(AFM). Typically, this involves operating the nanoindenter tip as a (very blunt) AFM tip when in the
microscopy mode. Such a combination was used to identify the thin prism sheaths sandwiched amidst
the prisms, and the measured elastic modulus and hardness values demonstrated a transition across
these two neighboring regions in enamel
[38]
.
Despite the fact that most nanoindentation can be conducted at penetration depths of 100 nm, the
horizontal dimensions are actually much larger at approximately 1 μm due to the large face angle of
the Berkovich tip. In cases where the sizes of the microstructures are smaller than such a resolution
limit of the nanoindenter, a commercial AFM can be operated as a nanoindenter to produce much
smaller indents. The process for AFM indentation is shown schematically in
Figure 16.6
.
To calculate the elastic modulus from the AFM data, the tip-sample interaction and the deflec-
tion of the cantilever of the AFM tip are treated as two elastic springs in series in the early stage of
the indentation, during which the contact between the tip and surface is treated as purely elastic
[39]
.
The ratio
K
between the specimen height and the diode-electric signal, which measures the AFM
cantilever's deflection, can be obtained, and the reduced modulus
E
r
can be calculated as
α
E
(16.14)
r
K A
/
1
Indenting
K
Contact
Loading
Approach
Unloading
Deflection,
δ
(nm)
FIGURE 16.6
Illustration of the photodiode signal-cantilever deflection
curve obtained from an AFM indentation.
