Residual stresses in TiPdNi base thin film shape memory alloys
Introduction
Applications for thin film shape memory alloys (SMA) are being developed to take advantage of its inherent properties such as good fatigue properties and large force generation over large displacements. Free standing SMA actuators, such as micropumps [1], [2], [3] and microgrippers [4], [5], must be either mechanically trained for a two way shape memory effect or require an external bias force such as a spring or external pressure in order to induce strain in the material. The introduction of residual stresses in bimorph actuators eliminates the need for external bias and allows for the batch production of SMA actuators [6]. While moderate residual stresses are beneficial in the creation of bimorph actuators, excessive stresses are often generated during annealing resulting in film cracking and delamination. Residual stresses in deposited films include thermal stresses, intrinsic stresses, and phase transformation stresses [7], [8], [9]. Thermal stresses are caused by the difference in the coefficient of thermal expansion between the substrate and the deposited film. Intrinsic stresses stem from film nucleation and growth, while phase transformation stresses are associated with the change from the austenite to martensite crystalline structure in case of TiNi/TiPdNi shape memory alloys. The intrinsic stresses are dependent of deposition conditions, such as gas pressure and film thickness [10], [11], and can be controlled by changing deposition parameters. The mismatch in coefficients of thermal expansion between the film and the substrate produces large stresses, which often lead to film failure.
In this study, TiPdNi films of less than 2 μm thickness were deposited using ion beam assisted deposition (IBAD). Ion bombardment during deposition is capable of improving film properties, such as morphology, density, crystallinity as well as reducing defects and porosity. In addition to improving film microstructure, ion bombardment can influence stress levels in deposited films through increased atomic mobility and atomic peening. SMA films deposited using IBAD are amorphous as-deposited on unheated substrates [9]. In order to achieve the shape memory effect, a crystal structure (in the form of martensite at room temperature and austenite at high temperatures) is required. Crystallization can be achieved either through post-deposition annealing or by deposition on heated substrates. Heating the substrate during deposition, or in situ heat treatment, allows for crystallization at a lower temperature, and therefore is expected to produce less residual stresses. In this study, the evolution of residual stresses in films deposited on unheated substrates followed by annealing was compared to the stresses induced in films deposited on heated substrates in an effort to identify the mechanism of failure for annealed films and further elaborate on the benefits of in situ crystallization during deposition. TiPdNi SMA thin films deposited using IBAD on both heated and unheated substrates were characterized by various techniques including transmission electron microscope (TEM), scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (SEM–EDX), scanning transmission electron microscope equipped with energy dispersive X-ray spectroscope (STEM–EDX), and X-Ray Diffraction (XRD) technique.
Section snippets
Thin film deposition
TiPdNi films were deposited on 50 mm diameter (1 0 0) single crystal silicon wafers using a dual ion beam assisted sputtering system. The schematic of system is shown in Fig. 1. The primary ion source is an 80 mm Kaufman ion source used for sputtering, and the secondary ion source is a 30 mm Kaufman ion source for ion impingement. The fully independent assist beam (from a second ion source) is key in controlling various properties of the deposited film, including stoichiometry, density, grain size,
Results and discussion
Fig. 4 shows the X-ray diffraction patterns of TiPdNi thin films. For the as-deposited film without substrate heating (Fig. 4a), very broad peaks near 33° and 36–47° are present, indicating that the film is mainly composed of an amorphous phase. Cross-sectional SEM image of the as-deposited film (Fig. 5) shows the film is featureless but very dense without any pores or defects. Deposition without substrate heating did not produce crystalline TiPdNi films. Therefore, post-annealing was performed
Summary
It was found that films deposited using ion beam assisted deposition on unheated substrates experienced a mild compressive stress due to atomic peening during deposition. These films were amorphous; therefore, annealing was conducted to induce crystallization in the films. Due to the difference in coefficient of thermal expansion for the film and the substrate, a large tensile stress developed as a result of annealing. This leads to several forms of cracking and film failure such as
Acknowledgment
The authors would like to acknowledge Dr. Jerome Cuomo's special input to this project mainly through accessing his lab facilities without those this work has never been possible.
References (26)
- et al.
Sens. Actuators A
(2001) - et al.
Sens. Actuators A
(2001) - et al.
Sens. Actuators
(2000) - et al.
Surf. Coat. Technol.
(2003) - et al.
Mater. Sci. Eng. A
(2003) - et al.
J. Mech. Phys. Solids
(1996) - et al.
Surf. Coat. Technol.
(2005) - et al.
Surf. Coat. Technol.
(2002) - et al.
Thin Solid Films
(1999) - et al.
Mater. Sci. Eng. A
(1999)
J. Micromech. Syst.
J. Micromech. Microeng.
J. Micromech. Microeng.
Cited by (3)
Investigation of residual stress relaxation on Cr12MoV steel under high-frequency cyclic loading
2019, IOP Conference Series: Materials Science and Engineering