ERI: Friction Stir Processing for Durability of Cobalt-Chromium-Molybdenum Biomaterials

This Engineering Research Initiation (ERI) grant supports research generating new knowledge of advanced manufacturing techniques for biomedical applications, with significant economic and technological benefits, as well as improved patient outcomes. Cobalt-chromium-molybdenum alloys are used as the bearing surfaces of prosthetic joint replacements because of their hardness, toughness, and biocompatibility. However, wear and corrosion of prosthetic implant materials in the human body remain serious problems for many patients. Friction stir processing is an advanced manufacturing technique that uses friction from a rotating tool to alter the metal surface, resulting in mechanical and corrosion properties that are highly desirable for bearing surfaces. Improved knowledge of the friction stir processing advanced manufacturing technique benefits the U.S. economy and medical device manufacturing industry. It also benefits millions of patients who undergo total joint replacement surgeries each year. Extending the durability of a prosthetic joint implant has an enormous impact on the quality of life of these patients and could be the difference between an implant that lasts a lifetime and complicated revision surgeries to replace failed implants. This research is multidisciplinary and provides education and outreach opportunities to encourage increased participation of underrepresented groups in engineering research. Friction stir processing can increase the wear and corrosion resistance of materials by controlling the microstructure through extreme deformation and precise control over the thermal history of the material. This research elucidates how the cobalt-chromium-molybdenum alloy microstructure such as grain size, carbide precipitation, and phase transformations can be controlled, and how they affect both wear and corrosion resistance. Extreme deformation causes dynamic recrystallization and martensitic phase transformations, resulting in fine grains with high hardness and wear resistance. Mechanical stirring and controlled temperature cause micron-sized, uniformly distributed carbides that strengthen passive oxide surface layers to bolster corrosion resistance. The research team performs friction stir processing experiments on the cobalt-based biomaterials with a polycrystalline cubic boron nitride/tungsten-rhenium alloy tool, and determines optimum processing conditions such as rotation speed, traverse speed, and applied load. Pin-on-disk wear tests and potentiodynamic polarization/electrochemical impedance spectroscopy corrosion tests quantify the wear and corrosion resistance of the different processed surfaces using different loads, sliding speeds, and biological lubricants selected to mimic an in-vivo prosthetic hip implant. Numerical simulations capture the gained knowledge of the basic science and physical mechanisms in play during friction stir processing of cobalt-based biomaterials and enable processing plans for medical devices made from this important biomedical alloy. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.