Bioactive sphene/titanium oxide composite coatings on titanium: Structure and apatite formation mechanism

Sphene/titanium oxide composite coatings with two-layer structure were prepared by a hybrid technique of microarc oxidation (MAO) and heat treatment on titanium. The skin layer of the sphene/titanium oxide composite coatings is formed by the crystallization of the initial MAO coating after heat treatment; while the inner layer is from the oxidation of Ti substrate, containing various titanium oxides with different Ti/O atomic ratios. In addition, the micropores in size of 50∼500 nm with a layer-like distribution are formed in the inner layer due to the oxidation of Ti substrate. The composite coating has good apatite formation ability as compared to the MAO coating, since the sphene in the composite coating has good crystallographic matching relations with hydroxyapatite (HA) on the specific crystal directions and planes, probably providing good sites for apatite nucleation by the epitaxial deposition process.