Characterization of a hydroxyapatite sputtered film subject to hydrothermal treatment using FE-SEM and STEM

Hydroxyapatite (HA) was coated onto a titanium substrate using radio frequency magnetron sputtering. The sputtered film was crystallized using a hydrothermal treatment. The films were observed using X-ray diffractometry, field emission scanning electron microscopy (FE-SEM) and scanning transmission electron microscopy (STEM) equipped with energy dispersive X-ray spectroscopy (EDX).It was observed that the surface of the hydrothermally-treated film was covered with globular particles. The FE-SEM observations indicated that these particles were composed of columnar grains with a grain size of 20-50 nm. In the STEM cross-sectional observation of the HA-Ti interface, HA crystalline phase regions were observed, in part, in the non-crystalline phase layer of the as-sputtered film. After the hydrothermal treatment, the HA layer crystallized; the HA crystallization proceeded to a distance of 30 nm above the titanium surface. From an EDX line scan analysis, the titanium oxide layer was not observed at the HA-Ti interface of the as-sputtered film; however, in the hydrothermally-treated film, the titanium oxide layer, with a 15 nm thickness, was observed between the mixing layer and the titanium substrate. The formation of titanium oxide at the HA-Ti interface would contribute to the adhesion improvement of the sputtered film following the hydrothermal treatment.