Effects of carbide precipitation on the microstructural and tribological properties of Co–Cr–Mo–C medical implants after thermal treatment

In this work, the influences of heat treatments on the microstructural features and, consequently, on the mechanical and wear characteristics of Co–Cr–Mo–C alloys commonly used as hip and knee implant materials, are investigated. Specimens of Co–Cr–Mo–C alloy in the as-cast condition were solution treated at 1230 °C for 3 h, and then either were quenched in water or furnace cooled. The achieved microstructures of the heat-treated samples, characterized by fine globular and lamellar-type carbides due to the different thermal-treatment conditions, was proved to affect the material microstructural, mechanical and wear behavior. The wear behavior was evaluated by means of pin-on-disk wear tests, which showed that the wear properties are strongly affected by the carbides shape, distribution, and size. It was proved that both large carbides precipitated in as-cast alloys, and also lamellar-type carbides induced by slow cooling after solubilization caused lower wear resistance than the globular fine carbides that were dispersed in the solution-treated and water-quenched specimens. Moreover, the ε-martensite, formed by the strain-induced martensitic transformation through plastic straining during the sliding of pin on disk, decreases the wear rate mainly due to the lower number of slip systems of the hexagonal structure.