Thermal Oxide Characterization – Heat Treat Modality vs. Composition

The manufacture of Nitinol implants typically includes one or more thermal processing shape setting steps, typically utilizing an air furnace, fluidized media, or molten salt. Variations in the thermal oxide thickness and composition are expected due to both the environment coming into contact with the heated Nitinol and the heating and cooling rates. These differences influence the strategy for removal (e.g. choice of etching chemistry) and/or the biocompatibility of the implant. Sullivan et al (Acta Biomater 2017), and other researchers, have demonstrated variations in the thickness (via Auger electron spectroscopy), corrosion resistance (via ASTM F2129 testing), and biocompatibility (via chronic animal survival studies) resulting from the choice in the upstream heating modality. Efforts to either remove that thermal oxide, or to characterize its in vivo risk requires a further characterization of these oxides beyond those available in the literature today.

In the present study, we grow thermal oxides at conventional manufacturing temperatures ranging from 400 - 800°C using a variety of modalities – air furnace, noble gas furnace, fluidized media, and molten salt. Each thermal oxide layer is then characterized by Focused Ion Beam milling, AES, EBSD, XRD (and micro-XRD), XPS and TEM for a complete qualitative and quantitative analysis of the morphology, thickness, and chemical composition.