Detailed friction load-displacement response of four distinct metallic surfaces [one beaded porous metal (CTR) and three cast Co-Cr alloy ingrowth mesh surfaces, nonplanar mesh (INX), cast mesh 1 (CM1), and cast mesh 2 (CM2)] on poly-urethane and cancellous bone specimens of six tibiae were measured under different normal stresses (0.1, 0.15, or 0.025 MPa). Bone cubes were obtained from different proximal regions of resurfaced cadaveric tibiae. Both monotonic and cyclic fatigue loadings of up to 4000 cycles at 1 Hz were considered. Comparison of measured results indicated that the friction coefficient was not affected by the magnitude of normal stress and the bone excision site (medial, lateral, anterior, posterior, and central). The CM2 surface showed significantly greater resistance with friction coefficients of more than 0.9 for the bone and 0.8 for the polyurethane. The INX surface yielded the second largest resistance followed by CM1 and CTR surfaces. NO significant difference was found between these latter two surfaces. Fatigue tests of up to 4000 loading-unloading cycles showed about 10% reduction in friction coefficient for CTR and INX surfaces, while negligible reduction was found for CM1 and CM2 surfaces.
The mechanical properties of investment cast Co-Cr-Mo alloy (American Society for Testing and Materials F 75) can be affected to varying degrees by post cast processes such as solution treating (ST), hot isostatic pressing (HIP), sintering used to apply porous coatings, repair welding, abrasive blasting, and laser marking. The mechanical properties of the wrought version of the alloy (American Society for Testing and Materials F 1537) can be influenced by mill practices. Thermo-mechanical processing such as forging, will change the properties of mill products depending on forging practices. Post forging processes such as abrasive blasting and laser marking can affect the mechanical properties to varying degrees. Testing has shown that abrasive blasting has no significant effect on either alloy. Laser marking can reduce the fatigue strength of both alloys. Sintering the cast alloy will reduce the fatigue strength and that HIP will improve the fatigue strength of the sintered cast alloy. Also, the cast alloy can be repair welded with no loss in tensile properties.
The wear characteristics of various Co-Cr alloy combinations were studied using a reciprocating wear machine. Wear test specimens were made from three Co-Cr alloys, cast Co-Cr, low carbon wrought Co-Cr and high carbon wrought Co-Cr alloys. The cast Co-Cr alloy was evaluated in both the as-cast and the solution-treated conditions. All specimens were polished with a surface roughness in the range of 0.01–0.02 μm. The clearance in diameter between the convex and the concave specimens was 100 or 300 μm. The same test conditions were applied to all specimens. Results showed that the best alloy couple was as-cast Co-Cr on as-cast Co-Cr alloy and this couple was found to be superior to the high carbon wrought on the high carbon wrought couple. This finding is also supported by using a hip simulator wear test machine.
A dispersion strengthened version of forged Co-Cr-Mo alloy, GADS Vitallium® alloy, was developed to improve fatigue strength after sintering (1300°C), a required thermal cycle treatment for porous coated products. GADS alloy was made using a powder metallurgy process to create fine dispersed oxides in the alloy. The dispersed fine particles strengthen the alloy and prevent grain growth so that the alloy retains fatigue strength almost twice that of the standard forged alloy (ASTM F-799, Standard Specification for Cobalt-28 Chromium-6 Molybdenum Alloy Forgings for Surgical Implants) following porous coating sintering. The processing development, grain structures, fatigue properties, and corrosion resistance of the gas-atomized dispersion strengthened (GADS) alloy are reported here.
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