A. P. Ehiasarian, P. Eh. Hovsepian, Sheffield Hallam University, Sheffield, United Kingdom
Lifetime and biocompatibility of orthopaedic implants are crucial in meeting the new challenges brought about by the fall in patient age and the ageing population. The high-load surfaces in contact with the biological environment 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 in current use. We report on a novel approach for nitriding of CoCrMo alloys using High Power Impulse Magnetron Sputtering (HIPIMS) discharge. The new nitriding process has been successfully carried out at the National HIPIMS Technology Centre at Sheffield Hallam University, UK in an industrial size Hauzer 1000-4 system enabled with HIPIMS technology. While the nitriding ion flux is controlled by the HIPIMS magnetron plasma source, the ion energy can be independently set via the substrate bias. Implementing the HIPIMS source allows reducing the operational pressure by one order of magnitude compared to conventional dc plasma nitriding (DCPN). Plasma analyses have identified significantly enhanced production of ions of molecular nitrogen (N₂⁺), atomic Nitrogen (N⁺) and N₂H⁺ radicals in the HIPIMS discharge compared to DCPN. Because of the low pressure of operation of the HIPIMS process, the energy of ions is similar to the bias voltage, whereas the high pressures used in DCPN cause severe losses in ion energy due to scattering collisions within the sheath. The high flux and high ion energy are primarily responsible for achieving a fourfold increase in process productivity as compared to state-of-the-art plasma nitriding processes. The nitrided surface layers exhibit excellent mechanical and tribological properties, which bring about significant improvements in hardness, fracture toughness and wear. The protective function of the nitrided layer against corrosion in the aggressive environments of simulated body fluid are remarkably augmented. The barrier properties of the nitrided layer have been demonstrated through a reduction in metal ion release by as much as a factor of 2, 4 and 10 for Co, Cr and Mo respectively.