On the nano-glass formation of the Ni–Ti thin films and related micro-structure and mechanical properties by controlling sputtering conditions

Abstract

In this research, glassy Ni–Ti thin films were sputtered on a flexible substrate (Kapton) using direct current (DC) magnetron sputtering system at ambient temperature. The influence of the sputtering pressure on the microstructure, chemical composition, thickness, mechanical properties and the ability to form nano-glass structure was investigated. Micro-structural characterizations of the thin films by x-ray diffraction, scanning and transmission electron microscopy (SEM and TEM) indicate that the sputtered thin films at a minimum pressure of 0.3 Pa have an amorphous structure consisting of nano-glassy grains. Furthermore, as the pressure increasing from 0.1 up to 0.8 Pa, the difference between the chemical composition of the thin films and the target increases. This difference is due to increasing of interatomic collisions between Ar and sputtered atoms that reduces the kinetic energy of sputtered atoms and rises to the scatter of atoms. In addition, the formation of micro-cracks in the films is also encouraged by increasing the sputtering pressure. For evaluation of the mechanical properties of the crystallized Ni–Ti/Kapton composite thin films, the tensile test was used and the results show that the partially crystallized Ni–Ti thin film sputtered at 0.3 Pa has the optimum ultimate tensile strength and toughness due to its composite (amorphous/nano-crystalline) structure.