Influence of surface quality and heat treatment on functional properties of semi-finished SMA wires for medical applications

Nitinol has been used for medical applications for the past decades due to its pseudoelastic properties and biocompatibility. The material is in direct and often invasive contact with the patient, e.g. as guide wires and stents. In use, the mechanical stress on the material leads to functional and mechanical strain and fatigue. The pseudoelastic properties due to the stress-induced martensitic phase transformation result in large tolerable strains with excellent cyclic fatigue properties.

In this study, pseudoelastic semi-finished NiTi wires manufactured according to ASTM 2063‑18 are investigated. The surface before and after heat treatment is investigated by macroscopic examination and scanning electron microscopy. In addition to the surface quality, the research focuses on the properties of the oxide layers, as these have an influence on the processability and thus the quality of the future medical devices, even though they might be eliminated for the final product. The functional properties are investigated by tensile tests according to ASTM F2516 and differential scanning calorimetry measurements. In this approach, the mechanical and functional characteristic values can be related and correlations can be generated. The evaluation of the overall test results shows the relationship between surface quality, heat treatment and functional properties.