In situ observation of surface oxide layers on medical grade Ni‐Ti alloy during straining

Abstract

Medical grade Ni-Ti alloys with shape memory or pseudo-elastic behavior exhibit good biocompatibility because of an electrochemically passive oxide layer on the surface. In this work, the mechanical stability of surface oxide layers is investigated during reversible pseudo-elastic deformation of commonly applied medical grade Ni-Ti wires. Surface oxide layers with varying thickness were generated by varying annealing times under air atmosphere. The thicknesses of the surface oxide layers were determined by means of Rutherford backscattering spectrometry. In situ scanning electron microscopy investigations reveal a damage mechanism, which is assumed to have a significant influence on the biocompatibility of the material. The conditions that lead to the appearance of cracks in the surface oxide layer or to the flaking of surface oxide layer particles are identified. The influence of the thickness of the surface oxide layer on the damage mode is characterized. The possible impact of the damaged surface oxide layer on the material's biocompatibility and the potentials to reduce or avoid the damage are discussed.

Response of monocyte-derived dendritic cells to rapidly solidified nickel-titanium ribbons with shape memory properties

“…The reason for this is because ceramic materials are held together by ionic and covalent bonds, which tend to fracture before any plastic deformation (Shabalovskaya et al, 2009, additional reference). Several papers reported that pseudoelastic Ni-Ti SMA samples deformed 0.6-3 %, strain developed cracks in the relatively thick protective TiO 2 layer and exposed Nirich phases leading to increased Ni ion release (Undisz et al, 2009;Zhu et al, 2004, additional references). On the other hand, as shown by Kimura and Sohmura (1987, additional reference), a thin oxide layer is preferable to maintain the integrity of the surface layer to sustain the large deformation induced by the shape memory effect.…”

Corrosion of Shape Memory and Superelastic Alloys

“…This Ni enrichment has been observed on thermally oxidized NiTi where Ni 3 Ti has been found at the metal-oxide interface and islands of metallic Ni have been found in the TiO 2 passive film [38,42] and on wires with very thick oxide [43]. Undisz et al have also observed a concentration of nickel (up to 65 at %) beneath a thermally grown oxide [44].…”

“…Below about 100 C, NiTi remains shiny in air, but at higher temperatures, the oxide surface slowly thickens, giving interference colors [39,44]. A gold color is seen for 30 nm oxide (2-min anneal at 540 C), blue color for 45 nm (5-min anneal at 540 C), turning back to golden after annealing to 30 min (125 nm) [44].…”

“…A gold color is seen for 30 nm oxide (2-min anneal at 540 C), blue color for 45 nm (5-min anneal at 540 C), turning back to golden after annealing to 30 min (125 nm) [44]. Melton and Harrison reported that oxidation above 700 C is more like stainless steel than Ti since no internal oxidation or absorption of oxygen occurs.…”

“…Undisz et al investigate the mechanical stability of surface oxide layers on NiTi when subjected to the large reversible pseudoelastic strains (up to 8% strain) [44]. Such cracks in the protective oxide layers could have a significant effect on the corrosion and biocompatibility behavior.…”