Crystallization kinetics of amorphous equiatomic NiTi thin films: Effect of film thickness

Abstract

We have investigated the crystallization of amorphous equiatomic NiTi thin films sandwiched between two protective silicon nitride barrier films using optical, atomic force, and transmission electron microscopies. Crystallite nucleation occurs homogeneously inside the NiTi films because small composition shifts at the interfaces between NiTi and surrounding layers suppress heterogeneous nucleation at these interfaces. The crystallite growth rate is independent of film thickness for films thicker than 600 nm. Below 600 nm, the growth rate decreases rapidly and has an apparent activation energy that increases with decreasing film thickness. We suggest that diffusion of hydrogen from the film interfaces may be responsible for this unusual behavior.

Crystallization and grain growth characteristics of yttria-stabilized zirconia thin films grown by pulsed laser deposition

“…Additionally, the density of defects acting as nucleation sites is commonly higher at interfaces than in the bulk of a phase, which promotes interfacial nucleation in thin film systems. On the contrary, no thickness dependence of the degree of phase transformation would be expected in the case of homogeneous nucleation since the process could start within the whole volume of the initial amorphous film [82].…”

Characterization and Prediction of Texture in Laser Annealed NiTi Shape Memory Thin Films

“…The actual film thickness is only 1 lm, significantly smaller than the observed grain diameters, and thus the grains are much larger in the in-plane directions than they are in the normal direction. This type of grain shape is common and has been reported for many solid phase crystallized NiTi films [26]. Figure 7 shows the XRD spectra from the as-annealed sample as well as one for the as-deposited amorphous film.…”

“…For partially melted films grain growth occurs from the surface of the remaining solid material. Since solid phase crystallization of amorphous NiTi films has been shown to produce randomly oriented grains that span the thickness of the film [26], the initial grain size in the model is considered to be independent of melt depth and the grain orientations random. Different melt depths can be modeled by varying the number of Monte Carlo steps included in the model.…”

Investigation of the role of Ti oxide layer in the size-dependent superelasticity of NiTi pillars: Modeling and simulation

“…Given a strain history, the stress along the loading direction within each material, σ NiTi and σ O , can be calculated independently using its constitutive relations, Eq. (13), (20), (18) and (19), where the gradient term in the NiTi SMA model is ignored. Fig.…”

“…Another explanation comes from the consideration of the surface Ti oxide layer [13,16], which has been shown to constrain the thermally induced martensitic phase transformation in thin films [17,18]. This Ti oxide layer about 15 nm in thickness does not participate in the phase transformation, and also creates a Ti-depleted zone about 50 nm in thickness [16,17,19], which has limited ability for the phase transformation since the increase in the Ni content stabilizes the austenitic phase [1,20]. For very small pillars, Fig 1, the fixed-thickness Ti oxide layer and Ti-depleted zone take most of the pillar volume, and the suppression of superelasticity can be expected.…”