Nano-Sized Hydroxyapatite Coatings on Ti Substrate with TiO2 Buffer Layer by E-beam Deposition
A nano-sized hydroxyapatite (HA) layer was coated on a Ti substrate with a titanium oxide (TiO2) buffer layer by the electron-beam deposition method. The morphological features as well as the mechanical and biological properties of the HA/TiO2-layered coating were noticeably different from those of a conventional HA coating on Ti. The HA on the TiO2 layer replicated the fine grain structure of the TiO2 layer, with grain sizes of just a few tens of nanometers. The TiO2 buffer layer was highly effective in preserving the adhesion strength of the coating layer following the heat treatment at 500°C, which was necessary to crystallize the structure. Moreover, in contrast to the HA single coating wherein severe cracking was observed under moist conditions, the HA/TiO2 coating retained its mechanical stability under the same conditions. The dissolution of the HA/TiO2 coating in a physiological saline solution exhibited a more favorable pattern than that of the HA single coating, with a reduced initial burst and a subsequent steady release rate. Preliminary in vitro cellular tests showed that osteoblastic cells expressed a significantly higher alkaline phosphatase level on the HA/TiO2 coating than on the HA single coating. Conclusively, the nano-sized HA coating with the TiO2 buffer layer holds great promise as a bioactive coating system.
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