Investigation of nano-mechanical properties of TiNiCu shape memory thin films after annealing.
Foong, Shi Wen.
Date of Issue2009
School of Mechanical and Aerospace Engineering
Shape Memory Alloys (SMAs) are a group of metallic alloys that has the ability to remember and recover to its original shape after heating or loading. This ability is due to a solid state martensitic phase transformation. In recent years, SMA thin films have attracted increasing attention due to their promising applications in Micro-Electro-Mechanical Systems (MEMS) devices, which required efficient actuation and sensing mechanisms. Knowledge of mechanical properties of the SMA thin films at room temperatures, as well as elevated and lowered temperatures, is of great importance when designing for use at different ambient temperatures. In this study, TiNiCu3 SMA thin films were prepared by the co-sputtering of TiNi and Cu targets using a Magnetron Sputtering system at room temperature. The as-deposited 550nm TiNiCu samples were amorphous. As Shape Memory Effect only occurs after crystallization, the films underwent either Rapid Thermal Annealing (RTA 10 seconds, 60 seconds, 120 seconds, and 180 seconds) or Conventional Thermal Annealing (CTA 1 Hour) at 480˚C. The surface features of the annealed samples were studied by Atomic Force Microscope (AFM) and Scanning Electron Microscope. All annealed films were crack-less and suitable for Nanoindentation. It was found that surface roughness of TiNiCu3 thin films increased with increasing RTA time. The higher roughness of RTA as compared to CTA samples might be due their differences in heating mechanisms and ramping rates. Thus, CTA process might be a better annealing method, if surface roughness were a critical concern in industrial applications. Nanoindentation was performed on the annealed samples at room temperature (23˚C) to study their mechanical properties. It was discovered that both the indentation modulus and hardness increased, in a similar trend, with RTA time. The error bars for indentation modulus and hardness were much narrower for fully-crystallized samples (RTA 180 seconds and CTA 1 hour), as compared to amorphous or half-crystallized samples. This is likely due to the more homogenized crystal structure in fully-crystallized samples. Later, to study the Recovery of the crystallized samples (RTA 180 seconds and CTA 1 Hour) at temperatures 0, 13, 23, 33, 43 and 50˚C, a Thermal Stage was designed, fabricated and implemented into the Nanoindentation testing. It was found that the best Recovery occurred at 23˚C, which means the phase present at 23˚C was most likely fully Austenite, and within the temperature range of optimum Superelasticity. Above or below 23˚C, Recovery % of the crystallized samples decreased increasingly. The load-depth curves of RTA and CTA crystallized samples were consistent for all tested temperatures except at 0˚C; the Thermal Stage might not be suitable for Nanoindentation at subzero temperatures yet, due to humidity issues.
Final Year Project (FYP)
Nanyang Technological University