Patel, B; (2012) Novel Processing, Microstructure and Properties of a Cobalt-Chromium-Molybdenum Orthopaedic Alloy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The annual rise of hip procedures occurring year upon year is a major concern, especially with revision rates increasing. There is need for improvements for orthopaedic materials to compensate for this problem with higher success rates in the long term. The main failure of current metallic materials used in hip replacement devices is due to wear and the metallic ions being leached out. Cobalt-based alloys are one of the best bearing surface materials used in hip replacement, however, a better understanding of the microstructure and mechanical properties is required to enhance its properties and also the need for new fabrication techniques to be explored to develop materials that can reduce the number of revision surgeries. The most common cobalt based alloy (F75) used in orthopaedics is investigated and a novel route to manufacture the alloy is conducted. Heat treatments via annealing and normalising are analysed. In the annealed alloy up to 1100°C there is an increase in hardness and carbide content. Above this temperature, there is a significant decrease in both properties due to carbide dissolution. XRD analysis identifies the phases present and how they varied with temperature. The production of the F75 alloy via spark plasma sintering has yielded an alloy with carbide free microstructures. The grains are finer than the conventional methods of fabrication (cast and wrought) and the hardness is significantly higher even in the absence of the carbides. The hardness has been attributed to the formation of oxide phases within the microstructure and chromium and molybdenum rich phases that act as solid solution hardeners. The oxide in the microstructure is identified as chromium oxide formed by a redox reaction between cobalt oxide found on the surface of cobalt particles and chromium. Tribological performance has been investigated upon this newly manufactured alloy (SPS alloy) against two commercial medical grade cobalt based alloys (F75 and F1537) used in hip replacement devices. The SPS alloy had higher hardness, which resulted in the lowest wear rate and friction coefficient, with lower amounts of chromium and molybdenum detected from the wear debris compared to the F75 and F1537 alloys. The wear debris size and size distribution generated from the SPS alloy were very small and the shape was more spherical. The element leaching is conducted upon these alloys, with the SPS alloy forming an oxide layer upon the surface that could be beneficial for limiting ion leaching and for tribological performance.

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