Papers by Keyword: Co-Cr-Mo

Abstract: Co-Cr-Mo alloys are currently applied as major materials for orthopedic implant because of their excellent wear resistance. The main strengthening mechanism of this alloy is the transformation of γ-phase Co-Cr-Mo alloys to the ε-phase. In this study, the evolution of the microstructure of a Co-28Cr-6Mo-0.08C-0.2N alloy has been investigated during isothermal aging. Solution treatment at 1275°C for 15 hours was carried out for as-cast alloy, followed by aging treatments at 700°C and 800°C for up to 15 hours. Microstructure evaluation, XRD analysis and micro-hardness test were carried out for both as-cast and aged alloys. From XRD analysis showed that the transformation of γ-phase to ε-phase occurred during the isothermal aging. The amount of ε-phase increased with increasing aging temperature and time, while the hardness of the alloy increased with increasing the amount of ε-phase. This led to the increasing of the hardness of alloy at higher aging temperature and time. In addition, the very fine precipitation in the cobalt matrix was observed in the aged-specimens. Aging at 800°C caused the progressive formation of very fine precipitates along intra-granular striation and the matrix. It is suggested that the precipitation was took place in the grain on the dislocations and the stacking faults.

Abstract: This paper presents the novel microstructure design, called Harmonic Structure, which gives structural metallic materials outstanding mechanical properties through an innovative powder metallurgy process. Homogeneous and ultra-fine grain (UFG) structure enables the materials high strength. However, such a “Homo-“ and “UFG” microstructure does not, usually, satisfy the need to be both strong and ductile, due to the plastic instability in the early stage of the deformation. As opposed to such a “Homo-and UFG“ microstructure, “Harmonic Structure” has a heterogeneous microstructure consisting of bimodal grain size together with a controlled and specific topological distribution of fine and coarse grains. In other words, the harmonic structure is heterogeneous on micro-but homogeneous on macro-scales. In the present work, the harmonic structure design has been applied to pure metals and alloys via a powder metallurgy route consisting of controlled severe plastic deformation of the corresponding powders by mechanical milling or high pressure gas milling, and subsequent consolidation by SPS. At a macro-scale, the harmonic structure materials exhibited superior combination of strength and ductility as compared to their homogeneous microstructure counterparts. This behavior was essentially related to the ability of the harmonic structure to promote the uniform distribution of strain during plastic deformation, leading to improved mechanical properties by avoiding or delaying localized plastic instability.

Abstract: Cobalt-chromium alloys are commonly used for surgical implants because of their high strength, superior corrosion resistance, non-magnetic behavior, and biocompatibility. Cobalt-Chromium-Molybdenum (Co-Cr-Mo) applications include prosthetic replacements of hips. This paper presents the attempt to produce metallic implant using Co-Cr-Mo powder by MIM process, focusing on the effects of different heating rate during sintering process at 1380°C. Co-Cr-Mo powder were mixed homogeneously with palm oil and conventional binders respectively with powder loading 65 vol% and was injection molded using vertical injection molding machine with the nozzle temperature of 160°C to produce green compacts. The binders then was removed by solvent extraction process and sintered in vacuum condition at atmosphere 10^-5 mbar at temperature 1380 °C with varied heating rate; 0.5°C/min, 1.0°C/min and 3.0°C/min . Results indicated that sintered density and tensile strength varied from 8.100 gcm^-3 to 8.200 gcm^-3 and 546.971 MPa to 798.767 MPa respectively. The mechanical properties comply with the international standard (ASTM F75).

Abstract: Powder injection molding (PIM) is a powder metallurgy process currently used for the production of complicated and near net shape parts of high performance materials [. This technique basically combines the advantages of plastic injection molding and the versatility of the conventional powder metallurgy technique. The process overcomes the shape limitation of powder compaction, the cost of machining, the productivity limits of isostatic pressing and slip casting, and the defect and tolerance limitations of conventional casting [1, 2, . According to German and Bose [, the technology of metal injection molding (MIM) is more complicated than that of the plastic injection molding, which arises from the need to remove the binder and to densify and strengthen the part. The process composed of four sequential steps: mixing of the powder and organic binder, injection molding, debinding where all binders are removed and sintering [1, 2, 3, 4]. If it necessary, secondary operations such as heat treatments after sintering can be performed [1, 2, 3, 4, .

Abstract: Wear on Co-Cr-Mo biomedical implants is still a major issue especially for applications in articulation joints like in total ankle, knee and hip arthroplasty. Generation of excessive wear particles can coagulate in body tissues which later cause inflammation, bone loss and necrosis. Modification of implant surfaces is a common technique for increasing the hardness and thus minimizing these effects. In this study, thermal oxidation method was carried out on the Co-Cr-Mo to investigate the effects of different pretreatment processes and surface roughness on the hardness of oxide layer formed. Prior to oxidation process, all samples were annealed and pickled to remove residual stress and oxide scales respectively. The oxidation process was done inside furnace under atmospheric condition for 3 hours at 1160 °C. The metallic compositions, surface morphology and hardness of the oxide layer formed on the substrate were verified using X-ray diffraction (XRD), scanning electron microscope and micro-Vickers hardness analysis respectively. It is found that mechanical pretreatment provides oxide/carbide layer with higher hardness than chemical pretreatment method. It is believed that remnants of polishing diamond pastes trapped in roughness valleys react with metal matrix and later transform into carbides during oxidation process. In contrast, initial surface roughness of the substrate has no significant effect on the hardness of oxide/carbide layer.