Formation and characterization of hydroxyapatite coating layer on Ti-based metal implant by electron-beam deposition

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Abstract
A hydroxyapatite [HAp; Ca10(PO4)6(OH)2] coating layer was formed on a Ti-based alloy by the electron-beam deposition method. When pure HAp was used as a target for the deposition, an amorphous layer was formed on the metal substrate. By heat treatment in a vacuum at 630 °C, the layer was crystallized into tricalcium phosphate [Ca3(PO4)2]. The crystallization improved the dissolution rate of the layer remarkably; however, at the same time, it deteriorated the bond strength with the substrate. When extra CaO (up to 25 wt%) was added to the target and processed under the same conditions, a layer compositionally close to crystalline HAp was deposited. Before the heat treatment, even though the layer was in amorphous state, the dissolution rate in the physiological solution was extremely low. Furthermore, the bond strength increased remarkably compared to the layer formed by the pure HAp target. Compositional and structural resemblance of the layer with the crystalline HAp was attributed to these improvements in properties.