TERNARY NiTi BASED SHAPE MEMORY ALLOY THIN FILMS

Abstract

Nickel-Titanium (NiTi)-based shape memory alloy (SMA) thin films have attracted much attention in recent years due to their unique properties i.e. shape memory effect, pseudoelasticity and biocompatibility, which enables them to be widely used as actuators and vibration damping devices in microelectromechanical systems (MEMS). Although, NiTi SMA has many advantages over traditional functional materials, but do suffer from various limitations such as wide hysteresis, fatigue and low mechanical hardness. The addition of a third element by relative replacement of either Ni or Ti opens wide possibilities for adapting NiTi thin films towards more specific needs of applications as it effects its shape memory characteristics, hysteresis, strength, ductility and most importantly phase transformation sequence. Hence there is need to develop and study ternary shape memory alloys. The main objectives of the thesis are (i) to synthesize ternary NiTiW thin films by DC magnetron co-sputtering technique to achieve grain refinement in NiTi thin films by tungsten (W) addition, (ii) to investigate the effect of Cu3N protective layer on NiTiCu surface in Cu3N/NiTiCu thin films and (iii) to fabricate NiTiCu/AlN/NiTiCu thin film heterostructures for vibration damping. The thesis is divided into three sections, each related to different aspect of ternary shape memory alloy thin film. An overview and introduction to each section will be given at the beginning of the according section. A chapter- wise summary of the thesis is given below: Chapter 1 gives an overview of ternary alloying on phase transformation, mechanical, corrosion, biocompatibility and antibacterial properties of NiTi shape memory alloys and thin films. The influence of grain size refinement on NiTi based SMAs has also been discussed along with details of grain refinement methods. The fabrication, properties and applications of NiTi based thin film heterostructures with transition metal nitrides and piezoelectric materials are also discussed in a view to extend application areas of NiTi. Chapter 2 presents the details of experimental techniques, which we have used for the synthesis and charaterization of NiTiW, NiTiCu, Cu3N/NiTiCu and NiTiCu/AlN/NiTiCu thin films and heterostructures. The synthesis of thin films in present thesis has been carried out by DC/RF magnetron sputtering technique. Additionally reactive sputtering was used to deposit Cu3N and AlN thin films by chemical reaction between the target material and nitrogen. The X-ray diffraction was used for investigating the crystallographic phase, presence of impurities, ii particle size etc. Further the surface morphology and microstructure of thin films were studied using atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). The film thickness was measured using cross-sectional FESEM. Shape memory properties were measured by monitoring temperature dependent resistance (R-T). The electrochemical experiments were performed with BAS (Bioanalytical Systems, West Lafayette, IN, USA) CV-50W Voltametric analyser. Further, the amount of Ni or Cu ions released from the thin films in simulated biological fluid (SBF) were detected using atomic absorption spectroscopy. The antibacterial activity and cytotoxicity of thin films was investigated using green fluorescent protein expressing E. coli bacteria and human embryonic kidney cells ((HEK) 293 cells), respectively. The nanoindentation was used for mechanical characterization of thin films and heterostructures. It involved load vs. depth and creep tests to determine Hardness (H), Elastic Modulus (Er), elastic recovery ratio (ER), resistance to plastic deformation (H3/Er2) and creep deformation of thin films. Moreover the creep testing method was extended for the vibration damping evaluation. Chapter 3 describes a novel approach to achieve grain refinement in NiTi shape memory alloy thin films by adding tungsten (W) in matrix of NiTi to improve mechanical and creep properties. The grain size of B2-NiTi decreases with increasing W content, due to the immiscible W layer obstructing its grain growth. This chapter is divided into two sections. The first section (Section 3.1) describes the effect of grain size refinement on hardness (H), elastic modulus (Er), elastic recovery ratio (ER), resistance to plastic deformation (H3/Er2) while (Section 3.2) discusses the same effect in creep along with their dominant mechanism. With W content ranging from 2.6 at.% to 4.5 at.%, the films are strengthened and can reach highest hardness and elastic modulus of 32.8 ± 2.7 GPa and 167.83 ± 8.64 GPa, respectively and much reduced strain rate 𝜖 = 6.76×10-4 s-1 indicating highest creep resistance. Moreover addition of W induces the B2-R single step transformation by suppressing thermally induced martensite phase due to grain size refinement below 40 nm. With further increase in W content beyond 4.5 at.% strain rate increases and therefore creep resistance of the films decrease leading to decrease in mechanical hardness and modulus. This behaviour can be explained in terms Hall –petch theory and lattice distortion of NiTi crystals with increasing the W content. The stress exponents were calculated from the loading curves and describe the dominant creep mechanism. The results show that stress exponent for NiTi was 8.2 and increased to 20.5 for NiTiW (2.6) and 22.9 for NiTiW (4.5) and decreased rapidly to 9.5 after increasing the W iii concentration from 9.1 to 33.6 at.%. The mechanism for the room temperature creep is discussed in the framework of dislocation dynamics. Grain boundaries play an important role in creep behaviour. In Chapter 4, NiTiCu shape memory alloy thin films were chosen for their improved structural, phase transformation and biocompatibility as compared to binary NiTi. Further, Cu3N/NiTiCu heterostructures were fabricated in order to improve surface properties of NiTiCu to make them suitable for biomedical applications. The first section (Section 4.1) of Chapter 4 describes the influence of Cu3N protective coating on structural, biological and shape memory properties of NiTiCu thin films. The Cu3N thin films were deposited in a temperature ranging from RT-450°C on the surface of 2-μm-thick NiTiCu shape memory thin films. Interestingly, the phase transformation from martensite phase to austenite phase has been observed in Cu3N/NiTiCu heterostructures with corresponding change in texture and surface morphology of top Cu3N films. Field emission scanning electron microscopy and atomic force microscope images of the heterostructures reveals the formation of 20-nm-sized copper nanodots on NiTiCu surface at higher deposition temperature (450 °C) of Cu3N. Cu3N passivated NiTiCu films possess low corrosion current density with higher corrosion potential and therefore, better corrosion resistance as compared to pure NiTiCu films. The concentration of Ni released from the Cu3N/NiTiCu samples was observed to be much less than that of pure NiTiCu film. It can be reduced to the factor of about one-ninth after the surface passivation resulting in smooth, homogeneous and highly corrosion resistant surface. The antibacterial activity and cytotoxicity of Cu3N coated NiTiCu thin films were investigated through green fluorescent protein expressing E. coli bacteria and human embryonic kidney cells. The results show the strong antibacterial property and non cytotoxicity of Cu3N/NiTiCu heterostructure. In second section (Section 4.2) of Chapter 4, the thickness of nanocrystalline Cu3N was varied from 200 nm to 415 nm and effect of Cu3N layer thickness on structural, phase transformation, morphological, corrosion and Ni release properties of Cu3N/NiTiCu heterostructures was studied. The Cu3N/NiTiCu heterstructures exhibit shape memory effect even after depositing Cu3N protective layer. Cu3N (200, 305 nm)/NiTiCu thin films possess low corrosion current density with higher corrosion potential and therefore exhibit better corrosion resistance as compared Cu3N(415nm)/NiTiCu film. The amount of Ni ions released in SBF solution was almost not detectable in case of 200, 305 nm thin Cu3N layer but increased significantly on increasing the thickness of Cu3N layer to 415 nm. Cu3N (415nm)/NiTiCu iv heterostructure exhibits much reduced corrosion resistance and Ni ion release impeding capability. This can be explained by decrease in adherence of Cu3N (~415nm) layer on NiTiCu thin film due to its increased thickness. This study resulted in determining optimum thickness of Cu3N layer (~200 nm) for passivating NiTiCu surface against corrosion and Ni release. In Chapter 5, NiTiCu, NiTiCu/AlN and NiTiCu/AlN/NiTiCu thin films and heterostructures have been deposited on Si substrate using magnetron sputtering technique. By the use of the interfaces and shape memory effect provided by NiTiCu layers, the damping capacity can be increased along with increase in stiffness and mechanical hardness. NiTiCu/AlN/NiTiCu heterostructure was found to possess high hardness (38 GPa) and elastic modulus (187 GPa). The energy dissipated during nanoindentation load vs. depth curve increases with inserting piezo AlN layer alongwith NiTiCu and exhibit maxima for NiTiCu/AlN/NiTiCu heterostructure, indicating its increased damping capability. AlN is found to play a dual role in the heterostructure. Apart from providing mechanical strength to the structure, it also improved damping due to its intrinsic piezoelectricity. The damping capability could be attributed to the shape memory effect of NiTiCu, intrinsic piezoelectricity of AlN and increased number of interfaces in heterostructure that help in dissipation of mechanical vibrations. Enhanced energy dissipation in NiTiCu/AlN/NiTiCu is confirmed by its high value of damping capacity (tan δ~0.052) and figure of merit (FOM~0.62). 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.