Protective coatings on medical implants by reactive diffusion

Wear resistant and corrosion resistant surfaces are important for medical implants. Wear resistance is especially important for articulating surfaces in load bearing applications such as total hip and total knee replacements. Metal-on- polyethylene bearings result in relative large volumetric wear, ceramics such as alumina have an excellent wear resistance but are brittle, metal-on-metal bearings have good wear resistance, but release nanometric debris and ions that may cause allergy or have toxic effects. Hard ceramic coatings on metal substrates may be a good solution: low wear and low ion release and ductile metal alloy interior—no danger of brittle fracture and easy shaping. Ti alloys such as for example Ti–6Al–4 have the best combination of mechanical properties and biocompatibility, but have very poor wear resistance, Co–Cr–Mo have good wear resistance, but may cause allergy. TiN hard coatings deposited on Ti alloys by PVD have relatively poor adhesion to the substrate as a result limited durability. Hard coatings formed on the surface of Ti alloys, Zr as well as CoCrMo alloys by reactive diffusion have good adhesion to the substrate as a result of gradual change of composition and gradual change of hardness. Reactive diffusion coatings obtained employing developed Powder Immersion Reaction Assisted Coating (PIRAC) on articulating surfaces of Ti alloys and CoCrMo show excellent adhesion to the substrate. Total hip implants show good performance in wear simulator tests as well as in vivo tests in canine and rat models. The thickness of reactive diffusion coatings is proportional to square root from time. It is shown that that thicker TiN coatings can be obtained via 3 stage PIRAC process: nitriding followed by tiatanization and additional PIRAC nitriding. Similar approach in two stages—titanizaton followed by nitriding can be employed for processing on TiN based coatings on CoCrMo alloy. Duplex TiN coatin–TiN PIRAC coating followed by TiN plasma assisted PVD (PAPVD) results in thick coatings with excellent adhesion to the substrate. PIRAC titanization followed by was successfully used for conversion of Ni foam to NiTi Nitinol foam and formation of TiN coating on the surface of Nitinol foam, resulting in 10 fold decrease of Ni ions release into physiological solution. PIRAC nitriding was successfully used also for nitriding of TiNb alloys, Zr and Zr–2.5 Nb alloy. Two stage coating by wear resistant TiN based layer can be applied to steels, BCC metals such as Mo and W and Ni based superalloys.