Biomedical Engineering Reference
In-Depth Information
9.12d, resulting in partial SE and partial SME by following heating. In the temperature
range
A
Ti,
<
T
<
T
s
, perfect SE appears as shown in Figure 9.12e, where
T
s
stands for the
critical temperature above which the martensitic transformation does not take place and
deformation occurs by slip. If
T
is above
T
s
, plastic deformation occurs as in conventional
metals and alloys as shown in Figure 9.12f.
The stress-strain relationship of SMA is different at different temperature range. Figure
9.13 illustrates the region SME and SE with the critical stress for slip and stress induced
martensitic transformation. In principle, both SME and SE are observable in the same spec-
imen, depending on the testing temperature, as long as the critical stress for slip is high
enough. SME occurs below
A
s
, followed by heating above
A
Ti,
, whereas SE occurs above
A
Ti,
,
where the martensites are completely unstable in the absence of stress.
FilmFabricationandCharacterization
Film Deposition
TiNi-based films are the most frequently used thin film SMA materials and they are typi-
cally prepared using sputtering method. Laser ablation, ion beam deposition, arc plasma
ion plating, plasma spray, and flash evaporation were also reported but with some intrinsic
problems, such as nonuniformity in film thickness and composition, low deposition rate,
and/or nonbatch processing, incompatibility with MEMS process, and so on. Figure 9.14
shows a schematic drawing of a most common radio frequency (r.f.) magnetron sputtering
apparatus (Miyazaki and Ishida, 1999). Ar ions are accelerated into the target to sputter Ti
and Ni atoms which are deposited onto the substrate to form a TiNi film. Transformation
temperatures, shape memory behaviors, and SE of the sputtered TiNi films are sensitive
to metallurgical factors (alloy composition, contamination, thermomechanical treatment,
annealing and aging process, etc.), sputtering conditions (cosputtering with multitargets,
target power, gas pressure, target-to-substrate distance, deposition temperature, substrate
bias, etc.), and the application conditions (loading conditions, ambient temperature and
environment, heat dissipation, heating/cooling rate, strain rate, etc.) (Ishida et al., 1996).
Systematic studies on the detailed effects of all the above parameters are necessary. The
sensitivity of TiNi films to all these factors seems an intrinsic disadvantage, but at the
same time, this sensitivity provides tremendous flexibility in engineering a combination
of properties for intended applications.
Precise control of Ti/Ni ratio in TiNi films is of essential importance, as has been docu-
mented because TiNi film studies started more than two decades ago. The intrinsic prob-
lems associated with sputtering of TiNi films include the difference in sputtering yields
of titanium and nickel at a given sputtering power density, geometrical composition uni-
formity over substrate and along cross-section thickness of the coating, as well as wear,
erosion, and roughening of targets during sputtering (Shih et al., 2001). To combat these
problems, co-sputtering of TiNi target with another Ti target, or using two separate single
element (Ti and Ni) targets, or adding titanium plates on TiNi target are widely used (Ohta
et al., 2000). Substrate rotation, optimal configuration of target position and precise control
of sputtering conditions, etc., are also helpful. Varying the target temperature can produce
the compositional modification: sputtering with a heated TiNi target can limit the loss
of Ti, thus improving the uniformity of film properties (Ho and Carman, 2000; Ho et al.,
