Modelling of transition zone formation between thin Si or Ta film deposited on TiNi under low-energy electron beam irradiation

Abstract

Nickel titanium or nitinol is very often coated by biocompatible and corrosion-resistant silicon and tantalum coatings. One of the methods for the formation of a continuous transition zone between the coating and nitinol is pulsed or continuous electron beam treatment. Surface heating accelerates diffusion processes that facilitate the formation of new phases. However, the interpretation of experimental results is hindered due to the impossibility of directly observing the processes. Here we model the phenomena accompanying the transition zone formation. The mathematical model used takes into account thermal and diffusion phenomena and the stages of the main chemical reactions in the heated area. Numerical studies revealed that the transition zone for the two types of films is formed differently: in the case of silicon the transition zone grows inside nickel titanium, while in the case of tantalum it is formed mainly in the area of the deposited film. The interaction of competing stages determines the absence of regularities in the oxide phase formation under varying treatment conditions.