Some Hard or Soft Coatings to Protect the Pristine Biometallic Substrates under Fretting-Corrosion Solicitations: What Should Be the Best Solution?

Under tribological conditions in aqueous medium, the contact of materials does involve some degradations of materials. Especially friction under small reciprocal displacement, i.e., fretting corrosion, is occurring; this topic has been highlighted since the 80's regarding hip implants. Hip prosthesis is assembled from three parts: femoral stem, neck and head. Fretting corrosion or friction corrosion between metallic parts first involves some degradation of the oxides layers. This step is governed by mechanics and it is related to some few minutes. Afterwards the corrosion occurrs enhanced by mechanical degradation. As well focused some oxides and some metallic ions are related to biocompatibility issues. Some strategies are available in order to avoid metal against metal friction and/or fretting. Some hard coatings and some smooth coatings were investigated. The first one is diamond-like carbon (DLC), and the second is a polyetheretherketone (PEEK), polymeric one. The investigations were focused on fretting corrosion solicitations of Ti-6A1-4V vs. Ti-6A1-4V + coating. DLC as a coating delays the corrosion degradation. The PEEK coating does not promote any corrosion degradation of the metallic counter part and more generally any wear.