Biomedical Engineering Reference
In-Depth Information
4.2.1
. Surface detection and initial sink-in
Detection of the sample surface typically relies on changes in load
or contact stiffness sensed by the indenter systems.
69
For example, in
some nanoindentation systems, the surface is detected by sensing a
change in the load on the transducer equal to a predetermined setpoint
value. The default setpoint value can be as high as 2
N to minimize the
likelihood of a false surface detection due to noise. In a stiff substrate a
preload of 2
μ
N results in a negligible penetration depth, and does not
have a significant impact on the mechanical properties measured by
indentation. However, with a compliant substrate and a blunt tip there
can be substantial penetration of the tip into the sample even at small
preloads.
68-71
The additional unknown penetration depth due to this
“sink-in” effect can lead to substantial errors in modulus calculation
for compliant materials, as demonstrated by experiments in a poly-
dimethylsiloxane (PDMS) elastomeric sample (
E
~ 1 MPa) that showed
that the apparent modulus value increases with increasing pre-load.
69
In
addition, with a pre-load of 0.1 μN and a Berkovich tip, an error in
displacement of 580 nm was observed for an indent in PDMS with a total
displacement of approximately 1500 nm (based on comparison to an
FEM model).
69
If the maximum depth of the indent is much greater than
the displacement error due to sink-in, then the effect of the preload may
be negligible. However, it is difficult to determine how much “sink-in”
has occurred, since most commercial indentation systems zero out the
displacement in the data files. Hence, there is an unknown amount of
error associated with indents in compliant samples due to this sink-in
effect.
In addition to the problem of sink-in, it can be difficult to even detect
the surface of a compliant sample, a problem that is exacerbated when
samples are submerged in a hydrating fluid during indentation. If the
indent approach is started above the fluid surface, then the change in load
when the tip penetrates the fluid layer may trigger the surface detection
setpoint, resulting in an aborted indent. Even if the tip is already
submerged before the sample approach is started, meniscus and capillary
forces acting on the shaft of the tip may vary as the tip steps towards the
μ
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