Adherence of HDPE powder coating on Co-Cr surfaces: effect of substrate preparation and gas sterilization

As part of the development of a hemiarthroplasty implant with a compliant surface layer, the effect of surface preparation of a cast Co-Cr alloy substrate on the adhesion of a polymeric coating was studied. High-density polyethylene (HDPE) coatings were deposited on Co-Cr alloys without adhesive by spraying powder onto the surface of specimens maintained at a controlled temperature. Prior to deposition, two different types of surface preparation were carried out: chemical etching and grit-blasting. Adherence tests were performed in order to study the effect of the surface preparation of a Co-Cr alloy on the adherence of the HDPE film. The etching process enhanced the adherence by a factor of 10 compared to the grit-blasting process. For coatings deposited on grit-blasted Co-Cr surfaces, rupture occurred at the interface (adhesive mode) as well as in the coating (cohesive mode). A full cohesive mode was observed for films deposited on etched surfaces. Gas sterilization with an ethylene oxide and Freon 12 solution did not change significantly the adherence of the film deposited on etched surfaces. Examination of the surfaces indicated that the etching process created a specific type of porosity consisting of dendritic cavities whereas grit-blasting produced surface irregularities. Cavities produced by etching were larger that the polymer powder particles and could be easily filled. The coating of an etched Co-Cr alloy substrate with a layer of HDPE provided an interface that could probably sustain the physiological loading in a normal synovial joint.