טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKabla Meni
SubjectFabrication, Characterization, and Implementation of
Actuators Based on Thin Shape Memory Alloy Films
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Doron Shilo
Full Thesis textFull thesis text - English Version


Abstract

Shape Memory alloys (SMA) are a unique class of materials which exhibit strongly nonlinear thermo-mechanical behaviors associated with a martensitic phase transformation. The shape-memory effect (SME) is often used in actuation applications, having the advantage of producing the highest work-output per unit volume among all other actuation methods. Therefore, thin films of NiTi SMA have the potential to be integrated into micro electro mechanical systems (MEMS) for the fabrication of powerful actuators.

This thesis includes three main parts, each serve a different research objective. The first part is focused on the development of a deposition and heat treatment processes for NiTi thin films with precise composition control. In particular, our results indicate the existence of a critical grain size of approximately 50-100 nm, below which a significant inhibition of the martensitic transformation occurs. Further, we study the effect of deposition conditions and show that a desired uniform martensitic film can be achieved by decreasing the impact energy of the deposited atoms. 

In the second part of this study we demonstrate the usefulness of nano-DMA in the characterization of superelastic properties of shape memory alloys. Measurements of reduced modulus as a function of the indentation depth reveal a transition, which is associated with the finish of the martensitic transformation at the region right beneath the tip. The new developed measurement method can be used for comparing between different samples prepared at different sputtering conditions.

The third part of the study describes a novel concept for micro-actuators that produce in-plane motion based on a thin free-standing SMA film. The presented design rules can be used for designing a variety of actuators that can provide a combination of large strokes and forces for linear and rotary motions. A prototype actuator demonstrated a displacement of 45 µm, related to 4.5 % of the SMA film length, under a force of up to 115 mN, related to a stress of 230 MPa in the SMA film, without plastic deformations. These capabilities allow the actuator to work against stiff springs which are essential for the device ability to sustain vibrations, impacts, and accelerations.

In summary, the thesis contributes to the study of SMA thin films in three parts which are all required together for integrating SMA films into MEMS devices. The first is the expansion of the knowledge about the relationship between sputtering deposition condition, microstructure and thermo-mechanical properties. This knowledge is important for the production of thin NiTi films with desired properties. The second contribution is the development of a new experimental method based on nano-DMA that can be used for comparing the mechanical properties between different samples of NiTi thin films. The third contribution is the development of a novel SMA MEMS in-plane actuator that can provide a combination of large strokes and forces for linear and rotary in-plane motions.