A. G. Ramirez, X. Huang, Yale University, New Haven, CT
Sputter-deposited NiTi films are promising actuator materials in microelectromechanical systems (MEMS). In order for these films to be useful to MEMS, they must be crystallized at high temperature to induce their shape memory effect. This presentation will discuss the effects of structural relaxation on the crystallization and phase transformation behavior of NiTi thin films. Heat treatments below the glass transformation temperature increases the crystallization temperature by a few degrees, and produces films with a modulus and hardness greater than as-deposited films. The reduction in free volume occurring during film relaxation plays a role. Using scanning electron microscopy, structural relaxation was found to decrease the overall crystallization time and increase the nucleation rate, thus modifying the resulting microstructures. Structural relaxation had little effect on the phase transformation temperatures of fully crystallized films, but slightly increased the resulting actuation force during transformations.
Summary: In summary, the effect of structural relaxation on the mechanical properties, crystallization and phase transformation behavior of NiTi thin films was investigated. After relaxation treatments, the Young’s modulus and hardness increased, which is attributed to the free volume reduction. Additionally, the nucleation rate in the structurally-relaxed sample is much greater than that in the as-deposited sample, leading to a homogeneous grain size distribution. Moreover, structural relaxation has almost no effect on the transformation temperature of fully crystallized films although it affects the grain size distribution. These observations shed light on methods to optimize the performance of devices based on NiTi thin films.
