Plasma Deposition, Plasma Coating, and Ion Implantation to Improve Metallic Implants and Prostheses
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and interesting but imperfect mechanical properties. Stainless steel, namely 316L, has a good price-to-mechanicalproperties ratio, but has the lowest corro sion resistance of the most commonly used metallic biomaterials. An attempt to improve the characteristics of these alloys was conducted by a European consortium within the framework of the European Commission's BriteEuRam program. This study has adapted innovative surface-treatment techniques to the alloys commonly used in surgery and has shown that the resulting implants have better properties than those treated using conventional techniques. The major problem associated with metal implants is inadequate tissueimplant interface properties. 1,2 For loadbearing orthopedic and dental-implant Systems, this is indicated by implant loosening, often requiring revision sur gery. To reduce corrosion and wear rates, as well as to increase fatigue strength, surface treatments such as cold plasma coatings and ion implantation have been investigated. Taking into account the in-
creasing number of research programs dealing with nitrogen implantation of classical orthopedic alloys—Ti-6A1-4V (TAV), 316L stainless steel, etc.3"7—it is important to carefully evaluate and compare t e c h n i q u e s such as ion implan tation, low-temperature diffusion, and physical-deposition processes for improving the bearing surfaces in osteoarticular prostheses. This has not been done in the currently available literature.
Materials and Methods In this program, three alloys com monly used as implant materials—TAV, Ti-5Al-2.5Fe (TAF), and 316L stainless steel—were used as Substrates, and three surface-treatment methods were compared: glow-discharge ion implantation (without mass Separation), sputterdeposited coatings of C-doped stainless steel (SS), and nitrogen plasma diffusion with and without plasma-assisted chemical vapor deposition (PACVD). The results were evaluated by means of wear and fatigue mechanical tests; electrochemical corrosion tests in simulated biological environments; surface analyses by x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), and nuclear reactions; human Osteoblast and fibroblast cell cultures; and in vivo im plantation in rabbits. The methodology used to reach the project goals is outlined in the following sections.
Surface Treatments Ion Implantation. The glow-discharge implantation method, without mass Sepa ration, was chosen in the energy ränge of 30-60 keV. It was decided to focus the work on nitrogen ions (other ions are also possible: C, B, O, etc.) and on the solidsolution concentrations 1016-1018 ions/ cm 2 . Alloys like TiN are easy to realize. This treatment was performed on TAV, TAF, and 316L stainless steel. Sputter-Deposited Coatings. Biomedical-grade stainless steel was treated with two plasma-assisted surface modifications: "carbon-doped" 316L stainless-steel coatings prepared below 200°C by reactive magnetron sputtering in a carbon atmosphere,
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