Biomedical Engineering Reference
In-Depth Information
(a)
(b)
410.0
e
500
Cooling
320.0
300
f
100
c
230.0
1 Cyo
20 Cyo
100 Cyo
600 Cyo
1600 Cyo
2000 Cyo
−100
d
a
140.0
−300
b
Heating
−500
−100
50.0
100
300
500
700
290
310
330
350
370
390
Temperature (
º
C)
Temperature (K)
FIGURE 9.40
(a) Stress evolution of TiNiCu film annealed up to 650°C using curvature measurement method. (b) Hysteresis
evolution of Ti52.5Ni film on Si substrate after thermal cycling in different cycles and become stable after 2000
cycles.
decreasing temperature. At e, martensitic transformation starts (
M
s
). With further decrease
in temperature, significant decrease in stress occurs (from e to f). Figure 9.40a clearly shows
that an appropriate annealing temperature is needed to promote the film crystallization,
thus the phase transformation can occur above room temperature and the large thermal
stress generated during cooling can be released significantly (Miyazaki and Ishida, 1999).
Fu et al. (2004) studied the fatigue of the constrained TiNi films using the curvature method
by investigating the changes in recovery stress during thermal cycling. Results show that
the recovery stress of the TiNi films from curvature measurement decreases dramatically
in the first tens of cycles and becomes stable after thousands of cycles (with one example
shown in Figure 9.40b).
This reduction of the recovery stress is believed to result from the
dislocation movement, grain boundary sliding, void formation, or partial debonding at the
film/substrate interfaces, nonrecoverable plastic deformation, changes in stress, and so on.
Transformation temperatures also change dramatically during cycling. The repeated phase
changes will alter the microstructure and hysteresis of the transformation and in turn lead
to changes in transformation temperatures, recovery stresses, and strains.
The stress evolution could have significant effect on the film surface morphology evolu-
tion. Significant surface relief (or surface upheaval), caused by the martensitic transforma-
tion, is commonly observed in TiNi bulk materials and has recently also been reported in
the sputtered TiNi thin films (He et al., 2004). During the martensitic transformation, the
atomic displacement introduces stacking faults that lead to surface relief morphology on
the film surface. A flat surface in austenite transforms to twinned martensite upon cooling
and becomes rough, without a macroscopic shape change, and vice versa. Fu et al. (2006)
reported a phenomenon of film surface morphology evolution between wrinkling and sur-
face relief during heating/cooling in a sputtered TiNiCu thin film (see Figure 9.41). In situ
optical microscopy observation upon heating revealed that the interweaving martensite
plate structure disappeared. However, many radial surface wrinkles formed within the
original martensitic structure. Further heating up to 300°C did not lead to much change in
these wrinkling patterns. On subsequent cooling to room temperature, the twinned mar-
tensite plates or bands reformed in exactly the same wrinkling patterns as those before
thermal cycling. After post annealing, a partially crystallized TiNiCu films at 650°C,
