Tuning Surface Integrity of Biomedical Nitinol Alloy by Low Plastic Burnishing

Nitinol alloys have received considerable attentions in biomedical and aerospace applications. Surface integrity of Nitinol devices by various fabrication processes is crucial for their functionality and performance. In this study, surface integrity of Nitinol SE508 processed by low plastic burnishing (LPB) has been explored to tune the mechanical behavior and microstructures. LPB is very promising to modify surface integrity due to its unique capability to mechanically affect the material down to very deep subsurface on the order of a few millimeters. The effects of LPB parameters, i.e. burnishing pressure, feed, speed, number of path, and pattern on surface integrity characteristics such as roughness, microhardness, surface topography, microstructure, and residual stresses are investigated. The burnished surfaces are smoother than the as-machined ones. Strain-induced phase transformation from austenite to martensite can be controlled by controlling the burnishing pressure. Significant strain hardening occurs on the burnished surface. Highly compressive residual stresses on the surface and in the subsurface are also measured, which is expected to significantly increase fatigue life of vascular stents. 

Keywords: Nitinol, burnishing, surface integrity, fatigue