The TiN coating of surgical implant materials

Wisbey, Andrew (1989) The TiN coating of surgical implant materials. University of Southampton, Doctoral Thesis.

Abstract

Potentially harmful metallic elements are known to be released slowly from implant materials like CoCrMo and Ti6Al4V. Since the long term success of implants, such as hip and knee replacements may be affected by the release the present work aims to reduce the dissolution products from CoCrMo and Ti6Al4V via the application of a PVD TiN coating. A HIP CoCrMo alloy is used and a balanced combination of mechanical properties and efficient TiN coating may be obtained by close control over the coating parameters. Wrought Ti6Al4V has also been successfully coated, however, a decrease in the fatigue properties was observed. Electrochemical testing, using potentiostatic polarisation, cyclic voltammetry and rest potential monitoring, indicates that pitting corrosion is unlikely on any of the material combinations examined. CoCrMo exhibited the lowest breakdown potential (0.43V[SCE]), however, this was significantly increased on a polished surface by the application of a TiN coating. On grit-blasted CoCrMo a reduced benefit from TiN coating was observed. The electrochemical behaviour of Ti6Al6V was largely unaffected by TiN coating, however, surface breakdown of TiN coated Ti6Al4V was found at 1.40V(SCE); this is considered to be sufficiently high to maintain the excellent pitting corrosion resistance of Ti6Al4V. Significant quantities of cobalt, chromium and molybdenum were released from unpassivated CoCrMo. The dissolution displayed a high initial release rate which decreased rapidly. Nitric acid passivation reduced the high initial release by between five and ten times. TiN coating further reduced the release of the potentially harmful metal ions from CoCrMo. A continuous release of vanadium was measured from Ti6Al4V; this was substantially reduced by the presence of the TiN coating. Titanium was found to be released at a constant rate from the TiN coatings, C.P.Ti and Ti6Al4V; this is thought to occur by chemical dissolution of the surface oxide. Passivation procedures are shown to effect the titanium release rate;. a tenfold reduction in the titanium release rate was obtained via ageing of the surface oxide in boiling distilled water or thermal oxidation at 400^oC. This ageing behaviour is considered to be associated with a change in the crystal structure of the surface TiO₂ film.

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