Effect of chemical composition and annealing on surface morphology and mechanical properties of NiTi-based shape memory thin films.
Cheong, Jia Wen.
Date of Issue2009
School of Mechanical and Aerospace Engineering
In today’s ever advancing technology world, the component and small scale devices that industry can fabricate is becoming smaller and smaller. This has spark off great interest in researches in nano-sized devices and the way human beings can best utilize their small sized and yet useful property. Shape memory alloy which can be treated as smart material is one such material which can be used in a wide range of nano applications. NiTi-based shape memory alloy which accompany with it great change in mechanical, physical, electrical properties etc during phase transformation is used in application to actuator element in microelectromechanicial systems (MEMS). Ternary NiTi-based shape memory alloy thin film such as NiTiCu which has a narrow temperature hysteresis is suitable for the manufacture of quick response actuation. It also helps to stabilize the alloy during temperature change and mechanical loading. In this study, NiTi thin film and 2 different NiTi-based thin films (1500nm) with different amount of Copper (Cu) content are being fabricated through magnetron sputtering. The mechanical properties of the 2 different NiTi-based amorphous thin films with different amount of Cu content are being studied by nanoindentation and surface morphology such as surface roughness and surface features are being examined by Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM) respectively. The nanoindentation result shows that the increase in Cu content from 3% to 6% lowered the hardness and elastic modulus of the film. AFM results have reflected that the as-deposited amorphous films are smooth with roughness value, Root Mean Square (RMS) to be below 2 nm. The 3 different films are then annealed at 480°C in Conventional Thermal Annealing (CTA) and Rapid Thermal Annealing (RTA) Process for testing with nanoindenter. NiTi is successfully annealed whereas NiTiCu3 and NiTiCu6 delaminate. It is found that the change in chemical composition does have a great influence on the forming of crack-less annealed film and copper content is likely to cause the film to be weaker and not able to withstand the stress during annealing process. This has affects the adherence between the film and the substrate and cause the film to crack. The raising of film thickness has also increase the chance of delamination and cracking due to the increase in force between the film and substrate interface. However in the process, this delaminate thin film has also allow the film to be used for Differential Scanning Calorimeter (DSC) Analysis and from that, its phase transformation temperatures can be determined. In the last part of the project, a newly design and fabricated Hot/Cold stage was used to test for the superelasticity properties of the NiTiCu3 (500nm). It makes use of a peltier module as the heating element and controls the temperature accordingly with the electrical power supplied to it. This sample is a much thinner film that does not crack on both annealing process as mentioned. It is found that this sample has the greatest recovery at room temperature when compared to lower temperature and higher temperature that are tested. This could be the evidence of the sample being in its austenite state and exhibit its superelasticity effect. Since the nanoindentation result of both the rapid and conventional annealing processed samples show similar trend, this shows similarity in these 2 processes; producing films with similar mechanical properties.
DRNTU::Engineering::Materials::Microelectronics and semiconductor materials::Thin films
Final Year Project (FYP)
Nanyang Technological University