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CHARACTERIZATION OF PLASMA SPRAYED BIOGLASS COATINGS Bioglasses are silicate glasses containing sodium, calcium and phosphate as the main components. There are three key compositional features to these bioactive glasses that distinguish them from traditional soda-lime-silicate glasses less than 60 mol % SiO2, high Na2O and CaO content, and a high CaO/P2O5 ratio. These features make the surface highly reactive. The work on bioglass started in the late sixties at the University of Florida where Hench formulated a new composition of glasses 45 % SiO2, 6 % P2O5, 24.5 % CaO and %24.5 Na2O known as 45S5. Bioactive glasses are not appropriate to be used for load-bearing applications because of their limited content of network formers. This problem can be solved by combining the glass with a fracture tough phase, such as a metal or a polymer. For this purpose various coating techniques have been introduced such as dip coating, electrophoretic deposition, hot isostatic pressing, flame spraying and plasma spraying. Bioceramic based plasma-sprayed on metal substrates have been accepted as implant materials in orthopedic and dental applications because of their favorable biocompatibility and adequate mechanical properties. But this process has not been standardized yet, due to high temperature, resulted in variation of chemical properties and degradation behavior of coating material. Bond coatings are already used widely in many industrial plasma spray applications. It has two specific functions; because the substrate and the main coating have different coefficients of thermal expansion bond coating layer should be used, to provide a good thermal expansion match between these two different layer; and bond coating layers are always thinner than the main coatings and they fit better on the grooves of the main metal. The main coating powder fits on the corrected surface in a similar way and this will result in better mechanical properties. The aim of this study was to determine the effect of bond coating application consist of 60 Al2O3 40 TiO2 on the mechanical properties and microstructural characterization of plasma-sprayed bioglass coatings on titanium and investigate the bioactivity and surface structural changes of 45S5 which was immersed into simulated body fluid. It is possible to coat bio-glass on tiatnium substrate by utilizing present conditions which are being used for hydroxyapatite coating procedure. Application of Amdry 6250 (60 wt% Al2O3 40 wt% TiO2) as bond coat in the plasma spraying of (45S5) bio-glass on titanium substrate has increased the bonding strength about three times. Adhesive bonding characteristic observed at the bio-glass metal interface turned into a cohesive bonding characteristic by the application of bond coat layer. Bioactivity test results indicate that plasma sprayed bioglass coating samples developed an apatite layer on its surface when immersed in SBF. The mechanism of apatite formation on the surface can be interpreted as follows; the calcium ion dissolved from the glass and glass increases the ion activity product of apatite in the SBF, and the hydrated silica on the surfaces of the glasses provides favorable sites for apatite nucleation. Consequently, the apatite nuclei are rapidly formed, they spontaneously grow by consuming calcium and phosphate ions from the SBF. |
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