Tribocorrosion and tribochemistry of cobalt-chromium alloy used for total hip replacement

Cobalt-based metallic biomaterials are mostly employed as bearing materials for Total Hip Replacement (THR) owing to their high strength, their biocompatibility, as well as their wear and fatigue resistance. Cobalt-chromium-molybdenum (CoCrMo) alloys also have excellent passivity characteristics as an oxide layer can form on the surface spontaneously, reducing corrosion. Nevertheless, mechanical interaction, such as friction and wear, can damage the passive oxide layer and also causes metal products used in such replacements to be released into the human body. The repeated removal of the oxide layer due to joint articulation leads to wear and corrosion; a process termed ‘tribocorrosion’.
Due to the complex working environment, as well as the presence of organic elements or species, there remains a lack of knowledge regarding tribocorrosion and the surface film mechanisms. An understanding upon these mechanisms is important since studies have reported that the tribochemical film, a complex metal-protein compound, can enhance the surface protection towards tribocorrosive processes. However, the behaviour is still debatable depending on the working conditions (e.g. temperature, pH, contact load and organic species in the lubricant).
The fundamental mechanisms of wear and corrosion are recognised to solve the longevity problem of the orthopaedic implants. Tribocorrosion and tribochemistry of CoCrMo alloys were investigated in various conditions. A reciprocating tribometer, electrochemical cells and surface analysis techniques were employed to observe the tribocorrosive behaviour of CoCrMo samples tested in protein-containing electrolytes. Conditions were applied to investigate various protein-metal interactions regarding behaviour in the tribocorrosion process: (a) in a range of electrochemical potential, (b) with different organic species, (c) protein conditions and (d) with the addition of metal ions in the bulk electrolytes. Results highlighted that a tribocorrosion and tribochemical reactions occurred during sliding are sensitive to the electrolyte composition. These findings contribute to the pre-clinical understanding of protein-metal interactions occurring in tribofilms’ formation and the system variables effect on the metallic bearing surfaces.