Nickel Titanium Thin Films Research Articles

Finite-thickness effect on crystallization kinetics in thin films and its adaptation in the Johnson–Mehl–Avrami–Kolmogorov model

The Johnson–Mehl–Avrami–Kolmogorov (JMAK) model is widely used to quantify the isothermal crystallization kinetics. The present work reports an analytical solution for the crystallization kinetics in the special case of plate-shaped samples with a finite thickness. As a result, we obtained an adapted JMAK model revealing the thickness range which influences the crystallization kinetics mode significantly. The analytical solution also provides theoretical bounds for the film thickness, where the assumption of 2D or 3D kinetics is accurate. Finally, the conclusions related to amorphous silicon and amorphous nickel-titanium thin films are reported.

Influence of annealing on structural, morphological, compositional and surface properties of magnetron sputtered nickel–titanium thin films

Thin films of NiTi were deposited by DC magnetron sputtering from an equiatomic alloy target (Ni/Ti: 50/50at.%). The films were deposited without intentional heating of the substrates. The thickness of the deposited films was approximately 2μm. The structure and morphology of NiTi films annealed at different temperatures were analyzed in order to understand the effect of annealing on physical properties of the films. The compositional investigations of fresh and annealed films were also evaluated by energy dispersive X-ray spectroscopy (EDS) and X-ray photo-electron spectroscopy (XPS) techniques. X-ray diffraction (XRD) studies showed that as-deposited films were amorphous in nature whereas annealed films were found to poly-crystalline with the presence of Austenite phase as the dominant phase. AFM investigations showed higher grain size and surface roughness values in the annealed films. In annealed films, the grain size and film roughness values were increased from 10 to 85nm and 2–18nm. Film composition measured by EDS were found to 52.5 atomic percent of Ni and 47.5 atomic percent of Ti. XPS investigations, demonstrated the presence of Ni content on the surface of the films, in fresh films, whereas annealed films did not show any nickel. From HR-XPS investigations, it can be concluded that annealed NiTi films have higher tendency to form metal oxide (titanium dioxide) layer on the surface of the films than fresh NiTi films.

Strengthening metals by narrowing grain size distributions in nickel-titanium thin films

Abstract

Patterning of martensitic nanotwins

We perform the atomistic Monte Carlo simulations of formation of nanostructured martensites in nickel–titanium thin films . The results reveal the atomistically resolved patterning of martensitic nanotwins, including coarsening, branching and twinning within twins, as mediated by temperature and substrate constraints. A scaling law is derived to elucidate the physical effects governing the characteristic nanostructure length scale of the twin width. Our modeling approach opens up possibilities for the atomistically based design of martensitic nanostructures to produce advanced alloys with optimized properties.

Incomplete martensitic transformations in NiTi thin films

Nickel–titanium thin films were deposited onto silicon substrates and subjected to incomplete martensitic transformations . With wafer curvature methods, we observed a stress discontinuity that seems analogous to the splitting of the endothermic peaks measured by differential scanning calorimetry due to the temperature memory effect. The martensite start temperature ( M s ) remains constant during incomplete forward transformations, resulting in a constant-stress range between the arrest temperature and M s . However, incomplete reverse transformations start immediately at the arrest temperature. X-ray diffraction measurements confirm these path-dependent behaviors, which are consistent with the thermodynamic model of martensitic transformations.

A Microstructural Map of Crystallized NiTi Thin Film Derived from <I>In Situ</I> TEM Methods

Sputtered-deposited nickel titanium thin films are commonly amorphous when synthesized and require annealing to crystallize them. The resulting microstructures, which are governed by nucleation and growth kinetics, dictate the actuation properties. The evolution of these microstructures was studied using in situ transmission electron microscopy (TEM) heating methods. The experimentally-determined kinetic values of nucleation and growth were inserted into a mathematical expression derived from the Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory, which predicts the average grain size over a broad range of temperatures.

Indentation stress dependence of the temperature range of microscopic superelastic behavior of nickel-titanium thin films

The microscopic superelastic behavior of thin-film NiTi is investigated by instrumented indentation experiments conducted at different temperatures. The indentation-induced superelastic effect is found to be persistent to about 100K above the austenite transformation finish temperature (Af). In contrast, the upper temperature where superelastic effect exists is only around Af plus 40K in uniaxial tension and compression tests, beyond which the plasticity of the austenite phase overwhelms the transformation-induced superelasticity. By combining the Clausius-Clapeyron equation and spherical cavity model for indentation, we show that the high hydrostatic pressure under the indenter is capable of elevating the transformation temperatures and increase the upper temperature limit of indentation-induced superelastic behavior.

Crystallization and phase transformations in amorphous NiTi thin films for microelectromechanical systems

Amorphous sputtered nickel–titanium thin films were deposited onto micromachined silicon-nitride membranes and subjected to heating and cooling conditions. Their associated microstructure was monitored directly and simultaneously with in situ transmission electron microscopy. These electron-transparent membranes constrained the NiTi films and rendered it possible for observation of the complete transformation cycle, which includes: the crystallization of the amorphous phase to austenite phase (cubic B2 structure) with heating; and the conversion of austenite (B2) to martensite (monoclinic B19′ structure) with cooling. Electron micrographs show the nucleation and growth of grains occurs at a temperature of 470°C and at a rate that indicates a polymorphic transformation. The onset of martensitic transformation occurs between 25 and 35°C. Calorimetric measurements are consistent with the observed crystallization.

Shape memory effect in nanoindentation of nickel–titanium thin films

In this study, a series of nanoindentations was made on NiTi shape memory alloy thin films at millinewton loads with a Berkovich indenter. Mapping of the indentation topography using atomic force microscopy reveals direct evidence that the thermally induced martensitic transformation of these films allows for partial indent recovery on the nanoscale. Indeed, recovery is nearly complete at indentation depths of less than 100 nm. A hemispherical cavity model is presented to predict an upper limit to shape memory recovery of sharp indentations.

Residual Stress, Adhesion and Crystallization of Ion-Sputtered and Ibed Processed NiTi Films

AbstractWhen conventional sputtering is used to deposit nickel-titanium thin films at temperatures below ∼623 - 723 Kelvins, the resultant structures are amorphous and the films must be annealed to form the requisite B2 parent ordering. This invites complications related to interface diffusion and chemical reaction with components of the substrate. For the present work, thin films of near-equiatomic NiTi were prepared on potassium chloride, (100) single-crystal silicon, and Si3N4 passivated silicon substrates by ion sputtering and by ion beam enhanced deposition (IBED). We have investigated residual stress levels produced by these processes, evaluated substrate adhesion levels, and explored the crystallization behavior of NiTi films grown under conditions of concurrent low-energy inert gas ion bombardment. Residual stresses of ionsputtered and IBED films were measured using profilometric techniques. Film crystallization behavior was studied by transmission electron microscopy of as-deposited films. Films produced by unassisted ion sputtering showed compressive residual stresses in the as-deposited amorphous state, which became highly tensile after annealing, leading to spontaneous cracking and delamination. Preliminary results from IBED experiments showed a reduction in the asdeposited stress, and improved adhesion. Under certain conditions, ion enhancement of the deposition process promoted film crystallization during deposition at moderate substrate temperature, producing a dispersion of extremely fine particles, indexed as Ni3Ti.

Cross-section TEM observations of amorphous nickel-titanium thin films formed by solid state reaction

There have been conflicting results concerning solid state amorphizing reactions (SSAR) in thin films consisting of alternating multiple layers of nickel and titanium. Clemens first reported evidence for SSAR in Ni-Ti using X-ray diffraction, Auger depth-profiling, and electrical conductivity measurements. The thickness of the Ni and Ti layers was 200 close-packed atom planes each in those samples in which SSAR was observed, although the reaction did not go to completion. Meng, et al. made plan-view TEM observations of films consisting of 10nm layers of Ni and Ti (about 50 atom planes) which had been vacuum- annealed for five hours at 250°C and 300°C. They observed that an intermetallic compound had formed as a result of the anneal, rather than amorphous material. The present study was initiated to explore the apparent conflict and to examine the kinetics of the SSAR if it occurred.