PhD Thesis_RuiMSMartins.pdf - RUN UNL

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization
lower values. If one considers that a lower a o value corresponds to a lower compressive
biaxial stress, a film exhibiting the (110) planes of the Ni-Ti B2 phase parallel to the substrate
should possess a lower compressive biaxial stress. It has been suggested that for the
deposition of Ni-Ti on TiN layers without applying V b , the development of the B2(211)
diffraction peak in Bragg-Brentano geometry, for TiN thickness values of ≈ 15 nm and
≈ 80 nm, is the result of a gradual change of the growing direction of the columnar crystals
with increasing thickness. Therefore, for a higher inclination of the (110) planes of the B2
phase, higher residual compressive biaxial stresses are observed (according to the a o values).
Nevertheless, in the different samples, the a o value approaches ≈ 0.304 nm during the
annealing step.
Falub et al. [63] have deposited TiAlN films with the preferred orientation of the
crystallites tilted with respect to the surface of the samples. The observed preferred
orientation was explained by the oblique paths between the substrate and the Ti/Al targets.
They have stated that the inclination of the (200) diffracting planes increased with increasing
residual compressive stress in the coating suggesting that there is interdependence between
intrinsic stress and fibre texture (it should be mentioned that Falub et al. have used a rotating
sample holder). Nevertheless, they concluded that it is still unresolved, whether it is the stress
that influences the texture or inversely the texture developing the stress.
The analysis of the variation of the a 0 values, as calculated from d (110) and d (211) , as a
function of deposition time, for the Ni-Ti B2 phase of the film deposited with a V b of −45 V
(on a TiN buffer layer of thickness ≈ 15 nm) has shown a reasonable stability along the
processing steps (Fig. 3.33). Furthermore, there is an overall decrease of this value (being
≈ 0.303 nm) when compared with the deposition without V b (which has approached a value of
0.304 nm at the end of the annealing step). This indicates that an increase of energy of the
incident ions resulted in enhanced mobility of the adatoms allowing them to diffuse on the
substrate surface and, as a result, the adatoms are no longer kinetically constrained. Most
likely this contributes significantly to the lower biaxial compressive stress state when using
V b (−45 V) during the Ni-Ti deposition on the TiN buffer layer of thickness ≈15 nm.
For the Ni-Ti deposition with V b of −45 V on TiN buffer layers with a thickness of
≈ 215 nm, a continuous decrease of the a 0 value of B2 phase and its approach to ≈ 0.304 nm at
the end of the annealing step, like in the case of the deposition without applying V b was not
observed. In this case, a very small decrease of a 0 at the beginning of the deposition, followed
by a stabilisation near the value of 0.303 nm [Fig. 3.34(b)] was visible. A further increase to
Chapter 4 – Discussion 190