CoCrMo alloys in hip and knee replacements application

CoCrMo alloys have been widely used in hip replacements. On the one hand, they have exhibited excellent long-term survival rates, but recently high failure rates have been observed, associated with adverse local tissue reactions. It is still a puzzle why CoCrMo alloys sometimes work very well, while at the other times the wear rate is unacceptably high. There have been several reports that the formation of a surface carbonaceous layer appears to reduce the wear rate. Equally, the formation of surface deformations and nanocrystalline layer has been observed, but it remains unclear whether this is a beneficial or detrimental phenomenon. The application of CoCrMo in hip joints results in mechanical aspects in term of wear and electrochemical aspects in term of corrosion in what is called tribocorrosion. Therefore, the current work was designed to carefully cover some factors that have not been well investigated in the literature using different approaches for data analysis. The study was divided into two major parts; tribological study and tribocorrosion study.
In the tribological study, a direct correlation was found between the surface deformation and the presence of a carbonaceous layer with the wear behaviour. The lowest specific wear rate was associated with the formation of a carbonaceous layer. A wear transition was observed at higher load, related to the loss of the carbonaceous layer and the formation of a thick subsurface nanocrystalline layer. The formation of the carbonaceous layer appeared to be associated with the removal of the surface protective oxide film. Additionally, it was shown that the superior mechanical properties of a thick nanocrystalline sub-surface layer could not protect the surface from a high wear rate. Although the formation of a thick nanocrystalline subsurface layer resulted in the highest surface mechanical properties, it also coincided with the highest wear rate.
In the tribocorrosion part of the study, the mechanical and electrochemical contributions in tribocorrosion were separated using synergistic and mechanistic approaches. The influence of load, load-potential dual effect and sliding speed-potential dual effect were all studied. The results showed a significant effect of re-passivating the worn surfaces on the wear behaviour of the material. At low loads and slow sliding speed, the worn surfaces were able to re-passivate at OCP which in turn had a significant effect on the microstructure of these surfaces and consequently on the mechanical properties and the tribocorrosion behaviour. Furthermore, the results of load-potential and sliding speed-potential showed a correlation between the mechanical contribution of tribocorrosion and the hardness of the worn surfaces that is the harder worn surfaces wear less mechanically at OCP. Such this correlation was not found at anodic potentials.
The results also showed that the surface carbonaceous layer did not form on the surfaces that were covered by an oxide layer and a high cathodic potential was needed to almost entirely remove the oxide film. This is in good agreement with the results of the tribological study of the work which pointed out a certain minimum load is necessary to totally remove the oxide layer and helped in the formation of a carbonaceous layer on the worn surfaces.
A correlation between the COF and the thermodynamic stability of the surface was observed. Throughout the whole study, whenever the system was able to establish new thermodynamic stability (at low load and slow sliding at OCP and at all tested anodic potentials), COF significantly decreased. The study defined two surface status, passivating worn surfaces (the surface tries to re-passivate) with high COF, and passivated surfaces (already passivated) with low COF.