Biomedical Engineering Reference
In-Depth Information
small such that the substrate effect can be avoided and the intrinsic film
property can be measured; typically that means the indentation depth
needs to be smaller than about 5% of film thickness. For some biological
membranes such as lipid bilayer whose thickness is a few nanometer, this
requirement may not be feasible. For very thin films, even if very
shallow indentation depths as small as several nm can be made by a
high-end commercial nanoindenter, there are many experimental issues
interfering with measurements at this extremely small length scale. First,
when the contact depth is of the same order as the surface roughness, it is
very difficult to determine the contact area or depth accurately. Second,
adhesion between the indenter tip and specimen may play an important
role and hence the nanoindentation force may not be measured properly.
Third, even a brand new indenter tip is blunt at the nanoscale, with a
radius of curvature typically tens of nanometers; thus, the conventional
sharp indentation analysis cannot be directly applied to the blunt tip at a
small scale. Fourth, when h is well below one micron, size-dependent
indentation effects have been measured 85,86 that are related to size-
dependent plasticity and the effects of strain gradients on strength. 47,87 In
addition, nanoindentation results may vary depending on whether the
experiment is performed within a grain or at a grain boundary. These
effects complicate the measurement significantly and make it more
difficult to extract intrinsic film properties such as the elastic modulus,
yield strength, and work hardening exponent.
In order to circumvent these issues encountered at extremely small
length scales, it is desirable to perform microindentation experiments at
moderate indentation depths, e.g ., depths on the order of a fraction of a
micrometer. Thus, depending on film thickness, it may be inevitable that
the substrate should affect the indentation measurement. Indeed, as the
indenter approaches the interface between film and substrate, the
measured hardness and stiffness are in fact combinations of film and
substrate properties. The key issue is to understand the substrate effect
and then subtract it from the microindentation measurement, so as to
obtain the intrinsic film properties. Although the analysis becomes
somewhat complicated, the approach is straightforward and can be based
on well-established continuum mechanics, without the uncertainties at
the nanoscale. 26,88
Search WWH ::




Custom Search