Analysis On Growth Behavior And Microstructure Of TiNi Shape Memory Alloy Thin Film

Recently, shape memory alloy (SMAs) have attracted tremendous interest. As a functional material, TiNi shape memory alloy thin film shows the characteristics of large stress, strain, high corrosion resistance and high work density. It is a perfect micro-actuator candidate for the micro-electrical mechanical system (MEMS). Research and exploitation on TiNi memory alloy will provide an effective tool for understanding the micro-field. The growth behavior of TiNi thin film plays an important role in its quality. It is well known that precipitates can strongly affect alloy's shape-memory characteristics in TiNi thin films, such as changing the type of martensitic transformation temperature or mechanical properties of TiNi thin films.Since 1990, TiNi thin films have been prepared by direct current (DC) magnetron sputtering for the usage in MEMS system. Most of the studies on TiNi films concentrate on the behavior of martensitic transformation, large stress, strain, shape memory effect and super-elasticity. However, the growth behavior of TiNi films is less concerned. Furthermore, there are still some disputations in martensitic transformation and microstructure for TiNi films. On the other hand, the growth behavior of thin films grown by magnetron sputtering have been widely investigated, hence we can have a study of the roles for TiNi films'growth and the factors that influence the surface characteristics according to the conclusions of these investigations. Furthermore, we can detect the effect of annealing conditions on the microstructure of TiNi films through the study on the microstructure.In this thesis, the Ni-rich TiNi thin films were deposited on Si substrate by DC magnetron sputtering system, the as-deposited thin films were treated under heat treatments with different conditions. The growth behavior and surface characteristics as a function of the parameters of DC magnetron sputtering in TiNi films has been investigated by Atomic Force Microscopy (AFM) in this work. The microstructures of TiNi films have been studied by means of X-Ray Diffraction (XRD) and Transmission Electron Microscope (TEM); Nano Hardness Tester has studied mechanical properties. Some other instruments were also used in this work to study the properties of TiNi films, such as DSC, EDX, and so on. The conclusions are summarized as follows:1. Three stages are found in the forepart of TiNi films'growth. In the first stage, which is called random growth stage, the intrinsic defects on Si substrates may be responsible for the surface roughness and the growth behavior of TiNi films, and the growth indexβis 0.42. In the second stage, surface roughness and deposition rate may result from the bombardment of energetic ions on the surface of Si substrates and shadow effect. The density of defects in the films increase, andβ=0.58. In the third stage, Si substrates are covered with TiNi films. The deposition rate as well as the growth index of thin films decreases rapidly (β=0.133). Surface roughness changes in small range and keeps steady gradually. Because the differences in the heights of islands increase with the growth of thin film, smoothness effect plays an important role in the growth behavior of film. Surface atoms move among surface islands with different height under the surface tension. TiNi films are deposited on Si substrate at ambient and 420℃. In the initial deposition stage, the sputtered atoms are dispersed on the cold substrate, whereas the deposited atoms are accumulated on a heated substrate; In the stage of steady growth, the film deposited on a substrate at ambient temperature has a smooth surface with a root mean square roughness of1.42nm, the rms roughness calculated for the film deposited at 420℃is 2.75nm. As the temperature of the substrate increases, the surface of film becomes coarse. In addition, we detect columnar growth in TiNi films in our experiment.2. The surface roughness of TiNi film increases with the increment of sputtering pressure. The reason is that the collision of Ar+ increases, which induce that the energy of sputtering atoms decreases and then restricts smooth effect, which is induced by surface diffusion. The surface roughness of TiNi film increases with the increment of DC magnetron sputtering power. The competition between the quick rate of film growth and surface diffusion effect appears in the initial stage of increasing DC power, and the alteration of surface roughness of film is unconspicuous. When the DC power increases further, the surface roughness will increases rapidly. The surface roughness of TiNi films increases with the increase of annealing temperature, because the size and growth of grain appears different at different annealing temperature. Additionally, the change in the size of the grain results in the change of the surface roughness.3. The Ni-rich as-deposit film is amorphous. The crystallization occurs at 682K, and finishes at 807K. There is Ti3Ni4 precipitation in the crystallized film, but we do not find Ti-rich precipitation. The size of B2 phase increases in crystallization process and Ti3Ni4 phase precipitate simultaneity, whose grain size also increases gradually. When the film is annealed at 550℃, we can observe martensitic phase in this film at ambient temperature, which proves that the temperature of martensitic transformation increase with the increase of annealing temperature. 4. Ti3Ni4 phase is metastable, and it can decompound to TiNi3 phase, which is stable in Ni-rich TiNi film. The content of Ti3Ni4 phase, which has decompounded at 550℃, is associated with the annealing temperature. Keeping annealing at 550℃, the grain size of Ti3Ni4 increases with the increase of annealing time.5. The mechanical properties are relative to the conditions of annealing, as Ti3Ni4 phase emergence and decomposing, in the stage of annealing from 525℃to 600℃, nano-hardness and elastic modulus decreases firstly and increases steadily, which is attributed to the precipitation harden induced by Ti3Ni4 and TiNi3. The surface hardness is higher than the interior hardness of the film, and the reason of which is that a layer of TiO2 forms on the surface of TiNi thin film. Furthermore, with the increasing of annealing temperature, the oxidation layer becomes denser, which induce the increase of the hardness.