NiTi SHAPE MEMORY ALLOY THIN FILMS AND HETEROSTRUCTURES

Abstract

Shape memory alloy (SMA) thin films of Nickel-Titanium (NiTi) have attracted much attention in recent years as intelligent and functional materials due to their unique properties i.e. superelasticity, and shape memory effect, which enable them to be widely used in aerospace, micro-electromechanical systems (MEMS) and various biomedical applications. The phase transformation in SMA thin film is accompanied by significant changes in the mechanical, physical, electrical and optical properties, which could be made use in the design and fabrication of microsensors and micro-actuators. However, there are still some concerns for the wide application of SMA thin films because of their unsatisfactory mechanical and tribological performances, chemical resistance and biological reliability. High nickel content in NiTi alloys often stimulated suspicions for their medical use. The limited hardness and wear resistance of NiTi make it difficult to be used in orthodontic and MEMS applications. Therefore, there is the need to search for stable, corrosion resistance and biocompatible protective coating for biomedical and MEMS applications of NiTi SMA thin films. The main aim of the present work was to synthesize high quality nanostructured NiTi thin films and TiN/NiTi heterostructures on silicon substrate by magnetron sputtering process in order to (i) study the influence of grain size and film thickness on the texture, surface morphology and phase transformation behavior of NiTi thin films; (ii) study the effect of crystallographic orientation of nanocrystalline TiN thin film on structural, electrical and mechanical properties of TiN/NiTi heterostructures and (iii) demonstrate the applications of TiN/NiTi heterostructures in bio-molecule sensing. A chapter- wise summary of the thesis is given below: Chapter 1 gives an overview of shape memory alloys and material background. The chapter includes the discussions on synthesis and properties of NiTi SMA thin films. The proper passivation to prevent surface layer degradation of NiTi thin films has also been discussed. iii Chapter 2 presents the details of experimental techniques, which we have used for the synthesis and study of the properties of SMA thin films. Section 2.1 -Synthesis of thin films in present thesis has been carried out by dc magnetron sputtering technique. We have developed the setup of dc magnetron co-sputtering and optimized various sputtering parameters to obtain good quality equiatomic NiTi thin films. Section 2.2 - Gives an overview of characterization techniques used for present work. X-Ray Diffractometer has been used for the phase identification and texture analysis. Surface morphology and microstructure were studied using Field Emission Scanning Electron Microscopy (FE-SEM) and Atomic Force Microscopy (AFM). Transmission Electron Microscopy (TEM) was used for phase identification and high resolution imaging. Phase transformation behavior of these films was studied using four probe electrical resistivity set up. Further mechanical and electrochemical properties of the films were studied using nanoindenter and voltammetric analyzer. Chapter 3 describes the growth and characterization of NiTi thin films prepared by dc magnetron sputtering technique. This chapter is divided into two sections. The first section (Section 3.1) mainly describes the effect of grain size on structural, electrical and mechanical properties of NiTi thin films. The grain size and the crystallization extent increased with increase in substrate temperature. Electrical resistance versus temperature plots show that grain size of NiTi films plays an important role in their electrical properties. The films with grain size — 20 nm exhibited negative TCR value and non metallic behavior while the film with grain size >33 run showed metallic behavior. An interesting martensite to austenite phase transformation was observed as crystal structure changes from monoclinic to cubic upon heating close to room temperature. Nanoindentation studies revealed relatively low surface roughness, high hardness, high reduced elastic modulus and better wear behavior for the film exhibited austenite structure at room temperature in comparison to that exhibited martensitic structure. Section 3.2- describes the influence of film thickness on phase transformation behavior of NiTi thin films. XRD results revealed the presence of austenitic (110) reflection from the beginning that could be due to the minimum surface energy of (110) plane in bcc structures. Reflection from austenitic iv (211) plane was also observed in the films of higher thickness (> 2.3 gm) because of strain energy minimization with increasing thickness. Ni3Ti precipitate formation was initiated as the film thickness reached to 2.3 pm and the fraction of precipitate formation increased with increasing thickness. AFM results indicated that even with increasing the film thickness, grains follows the Gaussian distribution. Electrical resistance versus temperature curves showed that the film with thickness < 300 nm experiences resistance force due to film and substrate inter-diffusion and small grain size, which affects the phase transformation behavior in these films. However, the films with thickness 634 nm and 1.2 pm showed the martensite H austenite phase transformation via R-phase, which are the suitable candidates for actuators applications. The 2.3 pm thick film displayed the phase transformation behavior with shift in transformation temperatures towards lower temperature values because of the Ni3Ti precipitate formation and the film of 3.4 gm thickness showed suppression of shape memory behavior that could be due to the increased fraction of precipitate formation. Chapter 4 describes the deposition of nanocrystalline TiN/NiTi thin films on silicon substrate by dc magnetron sputtering to improve the surface and mechanical properties of NiTi thin films without sacrificing the phase transformation effect. The preferential orientation of the TiN films was observed to change from (111) to (200) with change in nature of sputtering gas from 70% Ar + 30% N2 to 100% N2. It was observed that TiN (200)/NiTi films exhibited high hardness, high elastic modulus, and thereby better wear resistance as compared to pure NiTi and TiN (111)/NiTi films. In addition the presence of TiN (200) improves the top surface quality of NiTi films while retaining the phase transformation effect. Chapter 5 describes the electrochemical properties of TiN/NiTi heterostructures and their applications in electrochemical sensing. In the present investigation, the prepared TiN/NiTi heterostructures have been tested for first time as working electrode for dopamine sensing. Dopamine is a catecholamine neurotransmitter generated in various parts of central and peripheral nervous system, hence, careful monitoring of dopamine concentration is considered necessary. Parkinson's disease, associated with tremor, rigidity, bradykinesia and postural instability, is one of the most dreadful neurodegenerative disorders of central nervous system (CNS). The disease occurs when dopaminergic neurons decrease or malfunction which is accompanied by a sharp decline in dopamine level. TiN (200)/NiTi coated silicon electrode showed straight line calibration in dopamine concentration range 1-10 pA4 with correlation coefficient of 0.995. Further electrochemical test reveals that TiN coated NiTi film exhibited better corrosion resistance. Chapter 6 presents the summary and conclusion of the entire work presented in the thesis and also proposes the future directions in which these studies can be extended. vi