The effect of the argon-containing environment on the structural phase transformations in Ni/Ti layered thin films

Abstract

This study is focused on the effect of isothermal annealing in an argon atmosphere (200 Pa) in the temperature range of 200–600 °C on the evolution of diffusion-induced structural phase transformations in Ni/Ti layered thin films with different modulation period but similar total thickness of 60 nm. Changes in the structure and phase composition of nanoscale films have been examined using X-ray diffraction technique, and the chemical composition has been analyzed using secondary ion mass spectrometry. Annealing at 300 °C induces the development of diffusion interaction between the Ni and Ti layers, leading to the formation of intermetallic Ni_xTi_y phases of various stoichiometry at higher annealing temperatures.  It is shown that an increased number of layers promotes a significant activation of diffusion processes between the components of the system, resulting in a change in its phase composition at lower annealing temperatures. Meanwhile, for all investigated samples, annealing at elevated temperatures (>500 °C) causes the agglomeration of Ti on the free surface with the probable formation of a protective oxide layer. It was established that the decisive influence on thermally induced phase formation is exerted by two competing factors: the use of an argon-containing annealing atmosphere contributes to the intensification of mass transfer processes between the materials of metal nanolayers compared to similar heat treatment in a vacuum, while the decrease of the modulation period of the thin-film system promotes the amorphization processes at the interface between metal layers with a corresponding inhibition of structural ordering processes.