Surface morphologies SEM images of the NiTi thin films are shown in Fig.2. In the films sputtered 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 Ar pressure, the collisions probability rises, causing a decrease in surface diffusion and hence in the creation 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, recently named nano-glasses, were proposed to show 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 contain 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 fabricated by sputtering method, the average size of the nano-glassy grains affected by sputtering conditions such as sputtering pressure 32-33. Nano-glasses have been presented to display new properties 34-36.
By further increasing of the sputtering pressure up to 0.8 Pa, a columnar structure containing micro-crack between clusters forms (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 led the development of pores and cracks as was confirmed by the cross sectional SEM evaluations of the Ni-Ti thin films (Fig. 3).
At low sputtering pressure, due to the reduction of particle scattering, the kinetic energy of the sputtered atoms rises. Furthermore, compressive stresses were established to develop because of the stronger atomic bombardment, which also results to the creation 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.