Biomedical Engineering Reference
In-Depth Information
(a)
(b)
100.00
(nm)
100.00
(nm)
100.00
(nm)
100.00
(nm)
0.00
0.00
5.00
5.00
15.0
15.0
10.00
10.00
10.00
10.00
15.00
15.00
5.00
5.00
0.00
0.00
0.00
0.00
FIGURE 9.16
AFM surface morphologies of TiNiCu films (a) low temperature in martensite state and (b) high temperature in
austenite state. (From Fu et al,
Sens. Actuat.
112, 395-408, 2004, with permission from Elsevier.)
micrographs of AFM surface morphology of TiNi films on Si substrate at a low tempera-
ture (martensite) and a high temperature (austenite), respectively. The surface roughness
of the martensite is much higher than that of the austenite. With the change in tempera-
ture, the surface roughness changes drastically during transformation between the mar-
tensite and austenite, thus clearly revealing the occurrence of phase transformation. The
advantages of this method are its nondestructive nature and applicability to very small
size films (down to nanometers). Moreover, the changes in optical reflection caused by the
changes in the surface roughness and reflective index can also be used to characterize the
transformation behaviors of TiNi films (Wu et al., 2006).
There are usually some discrepancies in transformation temperatures obtained from dif-
ferent characterization methods (Fu et al., 2003). The possible reasons include: (1) the phase
transformation and mechanical behaviors of constrained TiNi films could be different from
those of freestanding films, due to substrate effect, residual stress, strain rate effect, stress
gradient effect, and temperature gradient effect; (2) the intrinsic nature of testing method
(thus the changes in physical properties will not start at exactly the same temperatures);
(3) differences in testing conditions, for example, heating/cooling rate; (4) nonuniformity
of film composition over whole substrate and along cross-section thickness of coating.
Therefore, it is necessary to identify whether the application is based on the freestanding
film or constrained film/substrate system, so that a suitable method can be chosen.
In the following sections, we will discuss the characterization results of TiNi films as
well as Ti-rich and Ni-rich films.
TiNi Binary Alloy Thin Films
As mentioned above, the as-sputtered TiNi thin films are amorphous if the substrate is
not heated during deposition. As a consequence, they should be crystallized by heating at
973 K, which is higher than the crystallization temperature, followed by aging at 773 K for
various times. The crystalline structures of the Ti-51.9 at.% Ni alloy thin film age-treated
for 36 ks were determined at three different temperatures, that is, 300, 270, and 200 K, by
x-ray diffraction. The parent (B2) phase, R-phase, and martensitic (M)0-phase exist inde-
pendently at these temperatures, respectively, as shown in Figure 9.17. The crystal structure
of the parent phase was determined to be B2, whereas those of the R-phase and M-phase
were rhombohedral and monoclinic, respectively. The lattice parameters of each phase are
