WearBehaviour Of Diamond-like-carbon Film In Water Environment

Diamond-like-carbon ?DLC? film is considered as the ultimate candidate for artificial joint surface modification due to its good mechanical properties and biocompatability. In vitro studies, DLC can significantly reduce the wear and corrosion of the metal substrates. However, adhesion failure happened when DLC coated artificial joint was used in vivo. There are various kinds of DLC films with different composition and structure. Different DLC films may exhibit different tribological properties. And the body fluid will affect the wear and failure behaviour of DLC film. Until now, it is still unclear which kind of DLC film is the most properone for artificial joint modifiaction. The failure mechanism of DLC film in vivo has not been totally understood. In order to investigate the effect of film composition, structure and friction pair on the wear behavior of DLC film, and research the effect of film/substrate interface on failure behavior of DLC film in liquid environment, the tribological experiments were performed in pure water or salt solution. The influence of composition, structure, film/substrate and friction pair on wear and failure of DLC film was disscussed. N implatation/DLC film composite treatment was tried to modify the CoCrMo alloy and its feasibility using in liquid environment was investigated.The results show when DLC film slides with Al₂O₃ in pure water environment, the hydrogen in hydrogenated DLC film can bond with carbon atoms, restrain the tribochemical reaction and form more -CH groups on the sliding surface. The interaction between -CH groups and water molecules is Van der Waals force, so the friction and wear of hydrogenated DLC film is relatively low. Without the termination of hydrogen, hydrogen free DLC film will suffer severe tribochemical reaction in water environment, forming more oxygen containing groups ?-COH? on the sliding surface. The interaciton between oxygen containg groups and water molecules is hydrogen bond. So hydrogen free DLC film exihibited relatively high friction and wear. The saturated -CH structure can keep stable under friction in water environment, while the unsaturated -CH structure will graphitize under friction, leading to a softer struture and higher wear in water environment. Compared with air environment, pure water environment can reduce the wear of hydrogenated DLC film and promote the wear of hydrogen free DLC film.The adhesion failure of hydrogenated DLC film in water environment is related to the hydrogen at film/substrate interface. Because the hydrogen can inhibit the passivation of Cr, promote the corrosion of the interface, generate more crack sources and induce adhesion failure under friciton conditons in water environment. The adhesion of DLC film on different substrates has relation with the chemical state at film/substrate interface. Ti6A14V substrate contains strong carbide forming elements ?Ti, V?, so the chemical bonds at interface is strong and the adhesion between Ti6A14V and DLC film is tough. The Fe element in 316L stainless steel has relatively weak carbide forming ability, so the adhesion between 316L and DLC film is relatively weak. The failure mode of DLC film on different substrates in PBS is coating spallation. The DLC film on Ti6A14V has thebestwear resistance and long termatability due to its good adhesion and corrosion resistance. The DLC film on 316L stainless steel has the worst wear resistance and long term stability because of its bad adhesion and corrosion resistance.Friction pair materials can influence the wear behavior of DLC film. In air environment, transfer layer may form on CoCrMo counterface containing metal oxides, which reduces the lubrication effect, brings brittleness and enhances the adhesive wear. So CoCrMo causes high, unstable friction as well as high wear of DLC film in air. Al2O3 brings more stable friction and less wear than CoCMo in air environment due tor educed adhesive wear and solid lubrication. In water environment, as a result of water lubrication, reduced adhesive wear and good solid lubrication, extremely low DLC wear arises when DLC film slides with CoCrMo alloy. Al2O3 causes relatively high wear of DLC film in water due to its high hardness and insufficient solid lubrication.N implatation/DLC film composite treatment successfully improved the mechanical properties and wear resistance of CoCrMo substrate. However, the N implatation lowered the corrosion resistance of CoCrMo in long term service, which induced weakened film adhesion for N implatation/DLC film composited systerm. So the N implatation/DLC film composite treatment must be considered carefully when using in artificial joint.