Protective coatings on medical implants by reactive diffusion

Abstract

Abstract. 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.

Nitride, Zirconia, Alumina, and Carbide Coatings on Ti6Al4V Femoral Heads: Effect of Deposition Techniques on Mechanical and Tribological Properties

“…To enhance the load‐supporting quality of the coatings and for the prevention of delamination, diffusional techniques that enable modification of the surface to a greater depth have been employed. Such diffusional techniques include plasma nitriding and the PIRAC methods and the literature shows that they lead to improved adhesion and wear resistance . PIRAC has been proposed for coating orthopaedic implants in many studies .…”

“…Bioinert ceramic coatings have predominantly been studied for enhancing hardness, wear, and corrosion resistance of hip joints . Some of the most commonly studied bioinert ceramics for coating femoral heads are nitrides, carbides, alumina, and zirconia because of their high strength, hardness, toughness, wear and corrosion resistance, and low coefficient of friction …”

“…Plasma spraying is a higher temperature technique that enables the deposition of relatively thicker coatings than PVD and CVD . The relative advantages and disadvantages of the most commonly studied techniques are presented in Table …”

“…Moreover, incompatibility between the Young's modulus of the implant and the coating layer may lead to delamination of the coating due to strain development in the interface . The similarity in coefficients of thermal expansion (CTE) of the deposited material and the substrate is also important to obtain a coating with good adhesive strength . Table shows thermal expansion coefficients of bioinert ceramic coatings …”

Influence of regulated modification of nitride layer by oxygen on the electrochemical behavior of Ti–6Al–4V alloy in the Ringer's solution

“…Nowadays, there is a growing interest in thermochemical treatment with diffusion saturation of material surface by the interstitial elements from controlled gas medium . Its advantage is providing high adhesion between phase film and metal matrix due to formation of gradient strengthened diffusion layer.…”