Tribo-oxidation of a zirconium nitride multilayer coated knee explant in comparison with an in-vitro tested implant

• Applicant: Dr.-Ing. Julia Rau
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2022 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 514540103

Artificial joints are incessantly subjected to tribological loading and to environmental influences from the human body, such as body fluids and temperature. These conditions ultimately may lead to dramatic changes in surface microstructure and chemistry and with that altered properties. An often-observed mechanism within tribological contacts is tribologically-induced oxidation. Although vastly observed, the exact mechanisms for oxidation are not yet fully understood which prevents a strategic design of (tribo-)oxidation resistant materials. Among the hypotheses for oxide formation while sliding is the presence of defects within the material (e.g. dislocations) that act as high diffusivity pathways and thus accelerate oxide formation. In the past, this was observed in a simple copper-sapphire model system. In the here proposed project, this hypothesis will be tested for a more applied case – an artificial knee joint. It consists of a CoCrMo base material covered with a 7 µm thin multilayer with a final ZrN layer (called AS-layer). When an implant needs to be explanted for various reasons, it is necessary to investigate the origin of this failure to implement corrective measures in the future. The explants surface often shows an increased surface oxygen content which varies for the articulating surface and the locations which underwent no tribological contact. Yet, this behavior and its exact consequences remain elusive. Often, in-vitro tests are performed in the lab to investigate the tribological properties. A detailed (high-resolution) comparison of the microstructures and chemical composition in the near-surface area after in-vitro tests and an explant was, to the best of the applicant’s knowledge, not performed to this day. Three questions will be addressed in this project: 1. Is the theory of dislocation pipes as core pathways for oxygen to enter the material applicable for an AS-coated Co-based explant? 2. What is the difference in composition and microstructure between the articulating surface and the non-articulating surface? 3. Do the in-vitro tested joints exhibit a similar chemistry and microstructural features as the explants? They will be assessed with electron microscopy and atom probe tomography which allow for the necessary resolution to answer the here posed questions. Shedding light on the processes at the explants surface allows to improve coating development (material choice, layer thickness or microstructure). Moreover, the comparison between the explant and the in-vitro samples’ surface allows to draw conclusions on the representative nature of the lab-tests. When proofing that defects serve as high-diffusivity pathways for more complex and applied contacts, more general conclusions for other materials could be made. In the long run, design guidelines for the strategic choice for materials and their microstructures deployed in a tribological contact could be formulated offering lower friction and reduced wear.

• DFG Programme: WBP Fellowship
• International Connection: Sweden
• Host: Professor Dr. Mattias Thuvander