title: The effect of heating rate on the surface chemistry of NiTi.
abstract: The impact of the heating rate on the Ni content at the surface of the oxide layer of biomedical NiTi is explored. Heat treatment emulating common shape-setting procedures was performed by means of conventional and inductive heating for similar annealing time and temperature, applying various heating rates from ~0.25 K s(-1) to 250 K s(-1). A glow discharge optical emission spectroscopy method was established and employed to evaluate concentration profiles of Ni, Ti and O in the near-surface region at high resolution. The Ni content at the surface of the differently treated samples varies significantly, with maximum surface Ni concentrations of ~20 at.% at the lowest and ~1.5 at.% at the highest heating rate, i.e. the total amount of Ni contained in the surface region of the oxide layer decreases by >15 times. Consequently, the heating rate is a determinant for the biomedical characteristics of NiTi, especially since Ni available at the surface of the oxide layer may affect the hemocompatibility and be released promptly after surgical application of a respective implant. Furthermore, apparently contradictory results presented in the literature reporting surface Ni concentrations of ~3 at.% to >20 at.% after heat treatment are consistently explained considering the ascertained effect of the heating rate.
authors: Otto Schott Institute of Materials Research, Friedrich Schiller University, Löbdergraben 32, 07743 Jena, Germany. Electronic address: Andreas.Undisz@uni-jena.de.
authors: Andreas
authors: A
authors: Undisz
authors: Otto Schott Institute of Materials Research, Friedrich Schiller University, Löbdergraben 32, 07743 Jena, Germany.
authors: Robert
authors: R
authors: Hanke
authors: Otto Schott Institute of Materials Research, Friedrich Schiller University, Löbdergraben 32, 07743 Jena, Germany.
authors: Katharina E
authors: KE
authors: Freiberg
authors: IFW Dresden, Institute for Complex Materials, PO Box 270116, 01171 Dresden, Germany.
authors: Volker
authors: V
authors: Hoffmann
authors: Otto Schott Institute of Materials Research, Friedrich Schiller University, Löbdergraben 32, 07743 Jena, Germany.
authors: Markus
authors: M
authors: Rettenmayr
mesh_terms: D006361:Heating; D009532:Nickel / Q000737:chemistry*; D010087:Oxides / Q000737:chemistry; D013057:Spectrum Analysis; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry*
chemical_list: D010087:Oxides; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium
keywords: Depth profiling; Glow discharge optical emission spectroscopy; Heating rate effect; Nickel–titanium alloy; Surface characterization