Use of Surface Mechanical Attrition Treatment for implantable material wear enhancement
Résumé
The Co28Cr6Mo (CCM) is widely used as an articular bearing surface for artificial joints like hip and knee prostheses. Despite many advances to extend lifespan of implants, the improvement of its tribological properties is still a matter of concern. By means of SMAT (Surface Mechanical Attrition Treatment) which generates by severe plastic deformation a nanocrystalline surface layer, the mechanical, tribological, and corrosion properties of materials are enhanced. In our study, we induce a reduced grain size layer on the top surface of a CCM alloy combined with a surface hardening. As a consequence, an optimized hardness gradient up to a depth of few hundreds of microns was established. The impact of this surface modification is examined in terms of tribological properties through scratch tests and also on a dedicated multidirectional pin-on disc apparatus (multi-PoD). Working with high loads and under lubricated conditions, this equipment fulfills the standard test method for wear testing of biomaterials used in total joint prostheses. Scratch tests both with increasing and constant loads and in a multipass mode give some evidence of the benefit of SMAT. A significant decrease of wear track depth (40% in average) is actually noticed and gives some predictive information with regard to the abrasive wear strength of the treated surface. Wear tests on multi-PoD are carried out considering both parts of a conventional bearing bringing PolyEthylene in contact with CCM. A reduction of 15% of UHMWPE mass loss has been provided thanks to SMAT performed on the opponent metallic surface, the latter demonstrating much smaller scratches. Moreover, the addition of a surface mirror polishing step helps to upgrade SMAT leading to an efficient, uncontaminated surface. Finally, SMAT could be considered as an innovative approach to improve implants properties.