A novel plasma nitriding process utilising HIPIMS discharge for enhanced tribological and barrier properties of medical grade alloy surfaces

The demand for improvement of lifetime and biocompatibility of medical implants is ever growing. The materials used in this challenging application must display enhanced tribological properties, biocompatibility and reduced metal ion release in long-term clinical performance. Surface modification techniques such as nitriding, can significantly improve the in-service behaviour of the medical grade alloys used for implants. This communication reports on a novel approach for nitriding of CoCrMo alloy using High Power Impulse Magnetron Sputtering, (HIPIMS) discharge for the first time. The new nitriding process has been successfully carried out in an industrial size Hauzer 1000-4 system enabled with HIPIMS technology at the National HIPIMS Technology Centre at Sheffield Hallam University, UK. Due to the significantly enhanced production of molecular (N2+) and atomic Nitrogen (N+) ion species in the HIPIMS discharge, which are primarily responsible for the nitrided layer formation, a fourfold increase in the process productivity as compared to the state of the art nitriding process was achieved. The surface layers produced exhibit excellent mechanical and tribological properties such as high hardness, high fracture toughness and wear resistance. The protection properties of the nitrided layer against corrosion in the aggressive environments of simulated body fluid (Hank’s solution) are remarkably augmented. Furthermore, the data showed that the amount of metal ions released from the HIPIMS nitrided samples was reduced by a factor of 2, 4 and 10 for Co, Cr and Mo ions respectively thus demonstrating the reliable barrier properties of the nitrided layer.