Why does titanium alloy wear cobalt chrome alloy despite lower bulk hardness: A nanoindentation study?

Titanium-based and cobalt-chrome alloys have been widely used in orthopaedic applications as these materials can significantly enhance the quality of human life as implant materials. The longevity of these materials is highly influenced by their mechanical properties. In some devices cobalt chrome components articulate with titanium alloy counter faces (e.g. in the taper connections of stems and femoral heads in modern modular designs) and damage has been reported of the harder cobalt chrome by the softer titanium alloy component. This study attempts to understand why this might occur by investigating bulk and surface mechanical properties (such as hardness and Young's modulus) of a number of hip implants and test samples using a Hysitron Triboindenter. AFM images were also obtained to determine the contact area and hence, pile-up correction factors.The results were compared for samples before being used in the body, to account for surface mechanical response due to implant manufacture, and after, to account for the materials response to long-term cyclic loads. To assess the effects of oxidation, the alloys were treated electrochemically with Sodium-Chloride (NaCl) solution at body temperature. It was found that titanium oxidised preferentially compared with cobalt-chrome alloys. Furthermore, the oxidised titanium showed significantly higher hardness values therefore damaging the un-oxidised cobalt-chrome material. The implications for device design and manufacture are discussed.