Influence of Microstructure and Processing on the Corrosion Resistance of Medical Device

Nikolaj Gersager Henriksen

Research output: Book/ReportPh.D. thesis

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Abstract

The research work in this thesis revolved around the laser marking of materials used in metallic medical devices, specifically on how the laser marking affects the microstructure and associated properties such as corrosion and fatigue performance. The project was motivated by the increasing demand for traceability in the form of oxidative laser marking applied directly onto medical devices. Medical device manufacturers experienced issues with delamination and corrosion in the laser markings. If the laser marking delaminates or corrodes, it will be considered unreadable and will therefore no longer be fit for application or sale. Laser marking is relatively well-studied regarding the production of high color strength markings, however there is little knowledge regarding the impact on mechanical properties or corrosion resistance. The industry has moved significantly faster than research on the subject.

The impact of laser marking on corrosion resistance of free-machining martensitic stainless steel was investigated to address the current issues in industry. Several sets of laser parameters were investigated regarding the microstructure and the corrosion resistance. It was found that a sub-μm oxide layer forms on the laser marked surface and that the color strength depends largely on the thickness of this oxide layer. The oxide layer was identified to be either Fe3O4 and/or FeCr2O4 and contained chromium. The thickness of the oxide layer depends largely on the heat input (J/area) of the laser treatment. The heat input is also the deciding factor for retention of corrosion resistance, as it was demonstrated that high heat input results in a darker color but also poorer corrosion resistance. This was attributed to chromium depletion in the sub-surface zone and corrosion underneath the oxide layer was identified. It was additionally found that the presence of abundant MnS jeopardizes the corrosion resistance after laser marking. The rod-like MnS inclusions, aligned perpendicular to the surface where laser marking was performed, thermally degraded leaving crater-like features. These craters are thought to act effectively as pits for corrosion initiation. Large tensile residual stresses were found in martensitic stainless steel after laser marking and were the result of a complicated thermal history and altering of the chemical composition during laser marking. The residual stresses on the surface were found to depend on the degree of melting and the hatch spacing of the laser tracks. The chemical composition was severely altered in the HAZ, and a significant chromium depletion could be measured as well as uptake of hydrogen and oxygen. It was discussed that as laser marking alters the chemical composition, resulting in state of tensile residual stress.

The influence of laser marking was also investigated on commercially pure titanium and Ti6Al4V. It was found that laser marking resulted in severe crack-development, such that the cracks develop perpendicular to the surface. Close to the surface the alloys had reached the liquid state during laser marking and distinct microstructural zones were discerned in the depth direction. High oxygen ingress in the melted zone suppressed the liquid → BCC transformation and, instead, the material solidified directly as HCP. The cracks were found to jeopardize the fatigue strength of both types of titanium alloys by up to 80 % relative to unmarked specimen. As cracks are developed before loading of the material, the crack initiation stage is essentially by-passed on subsequent loading.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages177
ISBN (Electronic)978-87-7475-756-6
Publication statusPublished - 2023

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