PhD Thesis_RuiMSMartins.pdf - RUN UNL

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization
concomitant high ion flux. A characteristic of this source discharge is a column of plasma,
which leaves the magnetic tunnel region and projects normal to the target surface towards the
substrate position. In consequence, a large flux of charged particles (electrons and ions)
reaches the substrate and, by suitable biasing, the energy of ions impinging on the growing
film can be varied. The micrographs have shown that the microstructure of the films was
influenced by the degree of ion bombardment (V b ). The increase in ion energy by applying a
V b of −45 V led to apparently larger crystals in the form of regular polyhedron visible on the
surface of the film. A further increase of V b to −90 V led to the appearance of surface steps on
the crystals morphology.
The results presented in this section have shown that a TiN buffer layer acts not only
as a diffusion barrier, but there is also a relationship between the TiN substrates and Ni-Ti
texture development (B2 phase):
1) there was a preferential growth of oriented grains of the Ni-Ti B2 phase
from the beginning of the deposition, with a constant growth rate during the whole
deposition, for a Ni-Ti film deposited (without and with V b of −45 and −90V) on
TiN with a topmost layer formed mainly by oriented grains;
2) in the case of Ni-Ti films deposited on top of a TiN layer where a dominating
orientation could not be identified (primarily, and oriented grains of
TiN nucleate and grow), a different behaviour was observed;
- without V b applied, oriented grains of the Ni-Ti B2 phase dominated at
lower thickness values, while, oriented grains took over at higher
thickness values.
- with V b of −45V, the development of the oriented grains of the Ni-Ti
B2 was reduced and the oriented grains dominated since the beginning.
According to the ER results obtained during temperature cycling, the distinct
crystallographic orientations of the Ni-Ti films influence their phase transformation
characteristics. The resistivity increase during R-phase transformation, on cooling and
heating, was higher for Ni-Ti films with a higher fraction of grains of the B2 phase with (211)
parallel to the film surface. The ER measurements have also shown that for the deposition of
Ni-Ti films with V b (−45 and −90 V), the transformation temperatures of the various phases
were shifted to lower values, when compared with the depositions performed without
applying V b .
Chapter 3 – Results 138