Study Of The Bio-tribocorrosion Behaviors Of Cobalt-based Biomedical Alloy

For the combination of excellent mechanical,wear resistance,corrosion resistance and biocompatible performances,Co-based alloys have been widely used in biomedical areas to make artificial joints,artificial teeth and stents.The alloys which were used as artificial joints would suffer from bio-tribocorrosion degradation during body motions.The debris and metallic ions would be released in bio-tribocorrosion processes and could lead to periprosthetic tissue reaction,osteolysis and,eventually,late aseptic loosening and may have long-term toxicological effect.This issue has seriously threatened the safety and long-term use of artificial joints and body health.The bio-tribocorrosion was occurred under the coupling action of mechanical wear,electrochemical corrosion and biological environment.Due to the complex process,the bio-tribocorrosion degradation mechanism is still remained unclear.The subsurface microstructure could be changed under tribological contact.This provides a window to investigate the degradation mechanism by observing the gradient subsurface microstructure which was produced in the bio-tribocorrosion process.This could guide the design,use and improvement of materials.The action mechanisms of biological environment,electrochemical corrosion,mechanical wear and the coupling behavior in bio-triboccorrosion have been investigated by observing the subsurface microstructure evolution of the CoCrMo alloy in this study.The effect of subsurface microstructure change on the bio-triboccorrosion behavior was also studied.The main results are shown as follows:(1)The effect of protein on the subsurface microstructure evolution of the CoCrMo alloy in bio-tribocorrosion processes has been studied.The CoCrMo alloys had different subsurface microstructures after worn in protein-containing or protein-free simulated body fluid.The protein could lead to the formation of a multilayer structure and cause severer subsurface deformation.The tribo-film which was transformed from the denatured protein could protect the passive film from scrapping,and then the passive film could reduce or even suppress the stacking faults annihilation by blocking the access the alloy surface.It led to the stacking faults being diffused towards the deeper area and a strain accumulation in the subsurface,before inducing a severer deformation.On the other hand,the protein resulted in the location changing from the top surface to be underneath the top surface,where the maximum frictional shear stress occurs.This can cause a deeper deformation.(2)The action mechanism of electrochemical corrosion and the interactions between it and biological environment/mechanical wear in bio-tribocorrosion were investigated by observing the subsurface microstructure of CoCrMo alloy which was worn under different corrosion conditions by changing applied potential.The increase in the applied potential from cathodic to anodic could lead to a severer subsurface deformation on the CoCrMo alloy.The higher applied potential resulted in a thicker,serverer plastic deformation layer with a higher stacking faults density and finer grains near the top surface.The increase in the applied potential could decrease the amount of adsorbed protein on the CoCrMo alloy surface,and this would decrease the coverage fraction and the thickness of tribo-film,and then increased the COF.These could increase the shear stress and aggravate subsurface deformation.The presence of a passive film at high applied potential could reduce or even suppress the stacking faults annihilation by blocking the alloy surface,which also lead to a serverer subsurface deformation.(3)With the aggravation of the CoCrMo alloy subsurface deformation,the amount of adsorbed protein on alloy surface would increase and the corrosion resistance of alloy would be enhanced.The subsurface microstructure could affect the protein adsorption and alloy corrosion resistance by changing the properties of the passive film which was formed on the alloy surface.The introduction of residual stress and nanocrystal on the subsurface could effectively increase the activity of metal atoms and improve the element diffusion to forming a more compact passive film.It could increase the contents of oxides on the surface,and then enhanced the electrostatic force and increased the surface's positive charge density as well as adsorption site.These facilitated the adsorption of the negatively charged carboxylate group of protein onto the CoCrMo alloy surface.The compact passive could increase the charge transform resistance to enhance the alloy corrosion resistance.(4)The behavior of carbides in the tribocorrosion processes of CoCrMo alloy was investigated.The carbides could inhibit and block the movement of the stacking fault under plastic deformation.Carbides would be torn from the matrix during the tribocorrosion,due to the preferential dissolution of the boundary area under the action of galvanic corrosion and tribological contact.