SEM of smaller grains with an average

SEM images of the surface
morphologies of the NiTi thin films are shown in Fig.2. In the films that were
deposited at low pressure (0.1 Pa), no grain boundary was observed but some indications of structural
anisotropy exist because of the absence of substrate rotation during sputtering
(Figs. 2a). By increasing the sputtering pressure, the collisions probability
increases, resulting in a decrease in surface diffusion and hence in the
formation of smaller grains with an average size of 20 nm (Fig.2b). According
to XRD patterns, this nano-grain structure is amorphous, as well. Both SEM and
XRD results for as-deposited thin film reveal that the thin film has nano-glassy
structure. Amorphous materials with nano-grained microstructure, today called
nano-glasses, were proposed to display new properties in comparison to metallic
glass (MG) with the same chemical composition 27-28. Structural studies of
nano-glasses have shown that nano-glasses consist of glassy regions, which are
connected by a network of interfaces with a non-crystalline structure that
exhibits an enhanced free volume in comparison to the chemically comparable
glass 29-31. Recently, metallic nano-glasses in the form of the thin films
were generated by magnetron sputtering and the size of the nano-glassy grains
affected by sputtering parameters such as sputtering pressure 32-33.
Nano-glasses have been reported to exhibit new properties 34-36.

By further increasing of the Ar
pressure up to 0.8 Pa, a columnar structure with micro-crack between clusters creates
(Figs. 2c and d). However, all thin films were found to consist of
nanometer-sized glassy clusters, as well. Furthermore, with increasing
sputtering pressures, argon atoms were found to be absorbed in the films and
caused the formation of pores and cracks as was confirmed by the cross-section SEM
studies of the Ni-Ti thin films (Fig. 3).

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At low working pressure, the kinetic
energy of the sputtered atoms increases, because of reduced particle
scattering. In addition, compressive stresses were found to develop most likely
due to the stronger atomic bombardment, which also leads to the formation of
structures of higher density (Figs.3a and b) in comparison to films deposited
at higher Ar pressure (Figs.3c and d). The SEM analysis also revealed that the
thin films deposited at the medium sputtering pressure (0.3 Pa), have a dense
structure with an average grain size of 30 nm (Figs. 2b and 3d) without micro
cracks in the structure.

In addition, at high sputtering
pressure, a columnar grain morphology develops with grains aligned almost perpendicular
to the substrate. The nano-columnar structure in the TiNiCu nano-glass thin
film deposited at high-pressure sputtering (0.9 Pa) was reported by  Sniadecki et al. as well 32. At higher
working gas pressures, the sputtered atoms encounter more collisions with the
Argon atoms leading to a less densely packed structure in the film. The
generation of columnar structure is accompanied by the formation of intrinsic
tensile stresses 32, 37. The nano-columnar growth of amorphous films is
confirmed by multiple processes, including of curvature-driven surface
diffusion, rising of surface mobility because of self-shadowing effect and
energy transfer 32.