Atomic Structure and Compositional Analysis of 316L Stainless Steel Medical Device Materials with the Local Electrode Atom Probe

Abstract

The Local Electrode Atom Probe (LEAP®) is an innovative three-dimensional atom probe microscope developed at Imago Scientific Instruments that provides 3-D atomic-scale imaging with quantum-level detection capability [1,2]. The LEAP microscope functions by taking specimens apart one atom at a time, and thence one atomic layer after another: Individual specimen atoms are ionized from the surface of a needle-shaped specimen by a rapidly pulsed electric field, and are then accelerated to a position-sensitive detector. The location where each ionized atom excites this detector directly maps to its original specimen position by projection microscopy, while time-offlight measurement determines elemental identity. The LEAP microscope provides 3-D images with a nominal magnification of one million times with ~0.5 nm lateral resolution and 0.2 nm axial resolution, in combination with single atom analysis of elemental composition.316L stainless steel alloy is widely used in medicine for applications that include implanted spinal fixation devices, bone screws, cardiovascular and neurological stents, and as critical components of minimally invasive surgical devices. These applications are made possible due to suitable physical and mechanical properties, good corrosion resistance in biological environments, reasonable biocompatibility, and good manufacturability. As medical technology advances towards microsurgical and minimally invasive techniques, there is a drive to produce ever-smaller devices that demand higher material performance and hence enhanced nano and micro-scale control of material structure. The nano-structure and composition of the material surface, typically an oxide, is especially critical since biological responses and corrosion occur at the material-environment interface. Thus, there is an increasing need to understand the 3-D structure and composition of metallic biomaterials at the atomic scale. Three-dimensional atom probe microscopy can uniquely provide such atomic-level structural information.In the present study, two different materials were analyzed. The first were medical device 316L stainless alloy sheets chemically modified at Medtronic, while the second were model specimens obtained as 0.38 mm diameter 1/8 hard 316L wires from a commodity supplier. To determine the bulk structure of the Medtronic sheet specimens, segments were cut from the interior volume and polished smooth. These were then mounted in holders, electropolished with 10% perchloric acid in acetic acid at 8-20 V DC to a nominal end radius of 20-50 nm, in order to provide the necessary needle-shape for atom probe analysis. The bulk atomic composition and structure of this material was then analyzed with the LEAP microscope (Fig. 1). All anticipated elements were located in the specimen including Fe, Cr, Ni, C, P, Si, Mo, and Cu, with a uniform elemental distribution and without precipitates or grain boundaries in the analyzed regions. The 316L wires were also electropolished as described above. In the course of electropolishing and ...

Diffusion Layers with Ti and Ti+Al Formed on 316L Austenitic Steel by a Pack Cementation Procedure

“…Although several types of stainless steels are currently used in implantology [1][2][3], the mostly employed one is the 316L austenitic steel (ASTM F138, F139, grade 2) [4]. This is predominately made of Fe (60-65 % in weight) with additions of alloying elements such as Cr (16-20 %), Ni (10-14 %) and small amounts of Mn, Mo, P, Si and S. Chromium has the main role to form a Cr 2 O 3 film on the steel surface which confers the latter a high resistance against corrosion in acid media.…”