New insights into hard phases of CoCrMo metal-on-metal hip replacements

Abstract

The microstructural and mechanical properties of the hard phases in CoCrMo prosthetic alloys in both cast and wrought conditions were examined using transmission electron microscopy and nanoindentation. Besides the known carbides of M23C6-type (M=Cr, Mo, Co) and M6C-type which are formed by either eutectic solidification or precipitation, a new mixed-phase hard constituent has been found in the cast alloys, which is composed of ~100 nm fine grains. The nanosized grains were identified to be mostly of M23C6 type using nano-beam precession electron diffraction, and the chemical composition varied from grain to grain being either Cr- or Co-rich. In contrast, the carbides within the wrought alloy having the same M23C6 structure were homogeneous, which can be attributed to the repeated heating and deformation steps. Nanoindentation measurements showed that the hardness of the hard phase mixture in the cast specimen was ~15.7 GPa, while the M23C6 carbides in the wrought alloy were twice as hard (~30.7 GPa). The origin of the nanostructured hard phase mixture was found to be related to slow cooling during casting. Mixed hard phases were produced at a cooling rate of 0.2 °C/s, whereas single phase carbides were formed at a cooling rate of 50 °C/s. This is consistent with sluggish kinetics and rationalizes different and partly conflicting microstructural results in the literature, and could be a source of variations in the performance of prosthetic devices in-vivo.

Mass Loss and Ion Elution of Biomedical Co–Cr–Mo Alloys during Pin-on-Disk Wear Tests

“…6(a)) through the three-body abrasion. The higher hardness, 29) i.e., slower wear rate, and higher Young's modulus of precipitates as compared to the metallic matrix led their detachment during wear tests in Kokubo solution. The formation of the cavity and discontinuous groove did not depend on the phase of precipitates, which suggests that the difference of hardness and Young's modulus between precipitates and metallic matrix are dominant in cavity and discontinuous groove formation.…”

In-Vivo Corrosion and Fretting of Modular TI-6AL-4V/CO-CR-MO Hip Prostheses: The Influence of Microstructure and Design Parameters

“…For one, there is a severe deficit of standardization in the orthopedic industry. Unlike the high-performance alloys used in the aerospace industry, where every processing step is precisely controlled and standardized, the material standards often cited by prosthesis manufacturers to regulatory bodies and physicians are vague and only specify compositional bounds and lower limits on mechanical properties [16].…”

“…However, the potential difference between passivated cobalt and depassivated titanium has been reported by various groups to be between 600mV[44] and an astonishing 3.5V [59], depending on the medium tested. A scenario where passivated cobalt exists alongside depassivated titanium is conceivable given the higher bulk hardness of Co-Cr and the even harder carbides present in as-cast and HC wrought Co-Cr [16]. Through micromotion these carbide asperites are more than capable of disrupting the softer titanium oxide layer[60].…”

A Preliminary Study to Enhance the Tribological Performance of CoCrMo Alloy by Fibre Laser Remelting for Articular Joint Implant Applications

“…Fine carbide precipitates within the grains can increase the strength, whereas coarse carbides at grain boundaries can embrittle the material. Further, the findings of Liao et al indicated that the homogeneity of the carbides in CoCrMo is dependent on the cooling rate in the solidification process, with the higher cooling rate favouring the formation of more homogeneous and harder carbides [47]. The CoCrMo alloy underwent rapid cycles of melting and solidification processes during the laser remelting.…”