There is an increasing demand for prosthesis made of metal in the last decade due to its low wear rate and excellent formability. The current generation of metal-on-metal prosthesis is made of Co-Cr-Mo alloys (ASTM 1547), with ~60% Co, 26% Cr and 5-7% Mo. The alloy can be as cast or wrought, the grain size ranging from a few microns to millimeters. The Co-Cr-Mo alloy adopts a metastable f.c.c. matrix at room temperature, with some h.c.p.-martensite formed through strain-induced transformation. In addition, a small amount of M23C6 (M: Cr, Co) carbides are present in the matrix.

The current research in our group is to understand the wear mechanism of metal-on-metal joint replacement. Of interest is the surface structure, which consists a thin nanocrystalline layer formed through recrystallization. Recent study shows that a mixture containing denatured protein and metal/metal oxide is generated at surface, and interestingly, penetration of such mixture into the surface is observed. The primary goal of our research is to exploit state-of-the-art analytical electron microscopy (TEM/STEM/EDS/EELS) to characterize the origin of such mixture, and it is (beneficial) effects to the wear process. The structural and mechanical properties of the up-most surface from nano to micro-scale will also be identified using nanoindentation, XPS and other surface characterization techniques. This research will illustrate the wear process of metal-on-metal prosthesis operated in synovial fluid, and provide us the insight that is essential for designing next generation of joint replacement materials.  

We closely collaborate with Dr. Jacobs and Dr. Wimmer's group at Rush university, Dr. Fischer's group at Germany, and Dr. Shull's group at Northwestern university.

1. Graphitic Tribological Layers in Metal-on-Metal Hip Replacements
   Y. Liao, R. Pourzal, M. A. Wimmer, J. J. Jacobs, A. Fischer and L. D. Marks
   Science 334 (2011) 1687
   , and Supporting Info