Tribochemical reactions in metal-on-metal hip joints influence wear and corrosion

Recent findings indicate the presence of tribochemically gener-ated layers on metal-on-metal (MoM) bearing surfaces. These tribolayers are films of a few-hundred-nanometer thickness and are constituted of carbonaceous material mixed with metal and oxide particles. The purpose of the study was to characterize these tribofilms mechanically and electrochemically. Using a nanoindenter, the local mechanical properties of the tribolayer were measured. On average a hardness of ∼1.0 GPa was determined, which was softer than the underlying metal. The influence of tribomaterial on the electrochemistry of the cobalt-chromium-molybdenum alloy (CoCrMo) was investigated. Bovine calf serum mixture was used as the electrolyte. High- and low-carbon CoCrMo-samples with and without tribolayer were compared using potentiodynamic testing. This corrosive investigation was followed by tribocorrosive tests using a custom made apparatus, where a ceramic ball oscillated against a flat CoCrMo surface. Potential and coefficient of friction were monitored throughout this 100 K cycle test. Electrochemical impedance spectroscopy tests before and after testing were conducted. Weight loss was determined using planimetric analysis. It was found that the tribolayered surface had better corrosion resistance than the corresponding tribolayer-free (polished) surface. The tribolayered surface also exhibited a more noble potential during tribocorrosive testing and demonstrated less wear. High- carbon was the superior alloy compared with low carbon for all surface conditions; however, the differences seemed to equalize in the presence of a tribo- film. There were also differences in tribofilm generation, possibly related to the microstructure of the two alloys.