NiTi shape memory alloy (SAM) thin
films have attracted much attention because of their unique properties such as the
ability to recover large stresses and strains, good corrosion resistance and bio-compatibility. The reversible phase transformation between
the austenite (B2) and martensite (B19?) phases, which can be either
stress-induced (pseudo-elastic effect) or temperature driven (shape memory
effect), is accompanied by changes in the mechanical properties 1,2.
NiTi Thin films deposited by
magnetron sputtering, depending on the deposition temperature, exhibited either amorphous or crystalline. The
thin films deposited at room temperature are amorphous and a post-sputter
annealing of the amorphous NiTi will crystallize it 3, 4, while thin films deposited at high
temperature (above 450 oC), have crystalline structure 5, 6.
The mechanical properties of the
NiTi thin films are essential for practical application in the
micro-electro-mechanical systems (MEMS), such as micro-valves, micro-actuators,
and micro-pumps 7-12.
The most common methods for analysis
of the mechanical properties of thin films are based on advanced methods such
as X-ray diffraction 13,
interferometry 14 and nano-indentation 15,16 and MEMS-based test 17.
These methods often require complicated and advanced tools and aren’t
well compatible with the conventional definition of mechanical properties that
are defined in terms of conventional tensile testing. The direct tension test is
an effective way to measure the mechanical properties as it produces reliable
and easily interpretable results. However, tensile testing is not easy to be
applied for micro- and nano-scale materials like thin films because of the
small dimensions of the samples. In order to overcome this restriction, an
alternative method is to deform thin films that are attached to a thicker
Mechanical properties of the film can be extracted using the rule of the mixture
from the stress-strain curves of thin film/substrate composite. Hou et al. were
the first researchers who tried to deposit NiTi thin films on Polyimide foil at
elevated temperature 21. In
their work, the Polyimide foil was baked at 350 oC for 1h prior
deposition of the NiTi. In addition, Kotnur et al. studied the structure and
phase transformation of NiTi thin films deposited at an elevated temperature on
Polyimide 22, 23.
In the present work, a bi-layer
structure was obtained by depositing Ni-rich NiTi thin films on Kapton, which can
be used as inflexible deformable devices, flexible actuators and for
miniaturized bio-mechanical systems. Furthermore, in
order to be able to control the microstructure and evaluation of its effect on
the mechanical properties of the thin films, we have investigated the
microstructural features of the NiTi thin films as a function of sputtering conditions such as the working gas pressure. If the sputtering parameters were varied, the structure and
properties of deposited thin films were noticed to change considerably. In addition, the mechanical properties of crystalized Ni-rich thin film on
Kapton were evaluated using a universal testing machine with special clamps.