Biomedical Engineering Reference
In-Depth Information
Nanoscale Mechanical Evaluation
Nanoindentation test is a promising method for characterization of nanoscale psudoelas-
ticity (PE) (Ma and Komvopoulos, 2003). The psudoelasticity behavior in TiNi-based thin
films demonstrates their intrinsic capacity to undergo large deformations without perma-
nent surface damage, or known as self-healing behavior. Nanoindentation testing with or
without changes in temperature could reveal the different elastic and plastic deformation
behaviors of austenite and martensite, thus is promising for characterization of SME and
thermomechanical properties of the constrained thin films (Shaw et al., 2003; Cole et al.,
2008). During loading and unloading in nanoindentation, there is a large force hysteresis,
that is, large energy dissipation during loading/unloading in the TiNi-based thin films.
During the reverse phase transformation, the martensite variants must overcome the inter-
nal stress generated during phase change to reverse back to the parent austenite matrix.
Therefore, energy is dissipated as friction heat due to the existence of defects in austen-
ite matrix and the martensite. The energy dissipation associated with the pseudoelastic
behavior contributes to the high vibration damping capacity of the TiNi films.
Using a sharp tip, it is difficult to obtain pseudoelastic behavior because the plastic defor-
mation due to dislocation movement is more dominant than the phase transformation
(Zhang et al., 2005). Therefore, spherical-shape tips have been widely used recently to char-
acterize the nanorange SE behavior of TiNi and thin films (Wood et al., 2008). Indentation
using a spherical indenter could avoid large plastic deformation if the indentation force is
not too high. During nanoindentation on SMA thin films using a spherical indenter, Yan
et al. (2006) found that there exist two characteristic points, the bifurcating point and the
returning point in one indentation loading/unloading curve, which are associated with
the forward transformation stress and the reverse transformation stress. They proposed
a method to determine the transformation stresses of SMA films-based on the measured
bifurcating and returning forces.
The SME of those materials at the indented area has been quantitatively studied by AFM
(Zhang et al., 2006a, 2006b; Cinchetti et al., 2005). The two-way shape recovery nature in
indents and indent arrays on SMAs and their thin films explored an alternative approach
for high-density data storage (Figure 9.47) and controllable reversible surface protrusions
(Shaw et al., 2005).
Nanometer scale indentations in TiNi thin films (for example) less than 100 nm in depth
can be fully recovered upon further heating because of thermally induced reverse martens-
itic transformation. Using a spherical indentation method, surface protrusions can be made
on the surfaces, which will disappear upon heating. Reversible circular surface protrusions
can be produced due to two-way SME (Zhang et al., 2006a). SMAs with shape relief ability
can find more optical and mechanical applications in their greater load bearing capacity
and/or better durability than normally used polymers. For example, information storage
technology has undergone a revolution in past years, and magnetic storage is reaching fun-
damental limits of about 100 Gbit/in
2
(or 6500 nm
2
bit
−1
) (Chikazume, 1997). This is because
with the shrinkage of the size of magnetic domains, the fluctuation in temperature could
easily cause the random changing of the moments of the magnetic domains, thus the loss
of the stored data. Recently, the nanoindentation method has been proposed to be used
for high-density mechanical storage applications (Shaw et al., 2005). The storage devices
with capacity 1 Tbit/in
2
are achievable. The write-read or erase-rewrite operations can be
performed with a nanoindenter or atomic force microscope. Information is written into
the martensite TiNi thin film by nanoindentation with probe tips. The indents are then
scanned, and the nanoindentation tip is heated for SME, thus erasing the data recorded.
