Texture development, microstructure and phase transformation characteristics of sputtered Ni–Ti Shape Memory Alloy films grown on TiN

Abstract

Near equiatomic Ni–Ti films have been deposited by magnetron co-sputtering on TiN films with a topmost layer formed by < 111> oriented grains (TiN/SiO 2/Si(100) substrate) in a chamber installed at a synchrotron radiation beamline. In- situ X-ray diffraction during Ni–Ti film growth and their complementary ex-situ characterization by Auger electron spectroscopy, scanning electron microscopy and electrical resistivity measurements during temperature cycling have allowed us to establish a relationship between the structure and processing parameters. A preferential development of < 110> oriented grains of the B2 phase since the beginning of the deposition has been observed (without and with the application of a substrate bias voltage of −45 and −90 V). The biaxial stress state is considerably influenced by the energy of the bombarding ions, which is dependent on the substrate bias voltage value applied during the growth of the Ni–Ti film. Furthermore, the present work reveals that the control of the energy of the bombarding ions is a promising tool to vary the transformation characteristics of Ni–Ti films, as shown by electrical resistivity measurements during temperature cycling. The in-situ study of the structural evolution of the growing Ni–Ti film as a consequence of changing the Ti:Ni ratio during deposition (on a TiN<111> layer) has also been performed. The preferential growth of < 110> oriented grains of the Ni–Ti B2 phase has been as well observed despite the precipitation of Ti 2Ni during the deposition of a Ti-rich Ni–Ti film fraction. Functionally graded Ni–Ti films should lead to an intrinsic “two-way” shape memory effect which is a plus for the miniaturization of Ni–Ti films based devices in the field of micro-electro-mechanical systems.