Biomedical Engineering Reference
In-Depth Information
analogous experiment using AFM we have some advantages and some disadvantages.
These are summarized below.
Advantages of AFM-based nanoindentation
• High load sensitivity - load sensitivity may be as low as piconewton, although even
for soft materials the required sensitivity is not likely to be greater than a nanonewton.
• Inbuilt ability to measure the indents created, at high resolution in x,
y
and
z
(see Figure 3.17).
• High positioning resolution - i.e. we can choose small regions of a sample, or perform
the experiment on very small samples.
Disadvantages of AFM-based nanoindentation
• Non-perpendicular probe approach - quantitative nanoindentation requires the ind-
enter to approach the sample perpendicularly, which is not the case normally for
AFM. This problem can be overcome, with care.
• Non-linear
z
positioning. Unless the system is equipped with linearization in the
z
-axis this can cause some serious problems.
• The system must be calibrated to extract real forces.
For nanoindentation on hard materials it is necessary to use a very stiff cantilever and a
hard probe. Typically, one might use a cantilever machined from steel, with a diamond tip
glued to the end [168]. Such levers may be appropriate to perform nanoindentation and can
be capable of imaging the sample, but typically give relatively low-resolution images; on
the other hand, they are absolutely necessary to indent hard material such as metals. Many
authors have also carried out nanoindentation with normal AFM probes [168-172], but it
is necessary to characterize the tip radius and cantilever carefully for quantitative results.
One advantage of such an approach is the ability to select from a wide range of spring
constants; the highly stiff nanoindentation cantilevers previously referred to are inappro-
priate for soft samples. One common approach to simplify the problem of tip radius
determination (see Chapter 2) for nanoindentation measurements is to use a colloidal
probe, i.e. to use a normal AFM cantilever without a tip, but with a small spherical particle
in its place [150, 173]. If nanoindentation experiments are carried out in a grid pattern over
the sample surface, then it's possible to determine the spatial variation of hardness and
softness [158, 174, 175]. Data analysis for nanoindentation is often made by modelling the
indentation via the Hertz model, which requires knowledge of the shape of the tip, and
assumes only elastic compressions of the sample take place [162, 176]. For more discus-
sion of data treatment for nanoindentation see references [168, 176, 177].
Applicability
Despite the quantification issues associated with carrying out nanoindentation using AFM,
it has been widely applied. It is particularly useful to look at relative hardness and softness.
For example, it can give an idea about differences in hardness and softness in different
parts of a sample With nanoindentation mapping, the measurements can be made quan-
titative, whereas for many other techniques such as phase imaging (see Section 3.2.3.2), it
is hard to know if differences are due to mechanical or adhesive properties of the sample.
Therefore nanoindentation has been commonly used to study heterogeneous materials
such as polymer composites [158, 181, 182]. Furthermore, the high positioning accuracy
means it's possible to look at small features not possible by traditional nanoindentation,
