Effect of Heating Rate On Surface Composition of Annealed NiTi

Fabrication of medical devices and implants from pseudo elastic or shape memory NiTi requires one or several heat treatments for shape setting and adjusting the specific properties of the alloy. The heat treatments also affect the chemistry and composition at the surface of the material during the formation of an oxide layer. Consequences on the biocompatibility of NiTi and release of Ni are debated controversially, especially since Ni at the very surface was proposed to trigger thrombogenic properties of the material. Until present, adjustment of the surface composition of NiTi by heat treatment relays on laborious and expensive methods, e.g. annealing at low oxygen partial pressure. In a recent study, the growth mechanism of the oxide layer on NiTi during heat treatment was investigated, and a complex mechanism of initial inward and subsequent outward oxide layer growth was identified. The surface composition is set during the initial inward oxide layer growth and thus depends primarily on the early annealing conditions.

In the present work the potential for controlling the surface composition of NiTi and setting the amount of Ni at the surface of the oxide layer by altering the heating rate is investigated. NiTi samples were subject to conventional and induction heating, achieving heating rates from 2.75K/s to 260K/s. The treated samples were characterized using Glow Discharge Optical Emission Spectroscopy (GDOES) and Transmission Electron Microscopy (TEM). For heating rates of 2.75K/s and 260K/s the amount of Ni at the oxide surface changed by ~65% from ~4.5at% to a minimum of ~1.5at%, respectively. The minimum concentration of Ni at the oxide surface was reached for heating rates of >90K/s. The results highlight that controlling the heating rate is a simple and suitable means for adjusting the surface composition of annealed NiTi.