Effect of hydrothermal treatment model on the formation of Sr-HA nanorod arrays on microarc oxidized titania coatings

Sr-doped hydroxyapatite (Sr-HA) nanorod arrays were prepared by hydrothermal treatment (HT) on microarc oxidized (MAOed) TiO2 coatings containing Ca2+, Sr2+ and PO43− ions. The effects of HT models, namely the MAOed TiO2 coatings to be hanged up in the autoclave without touching the HT solution (termed as H-HT) and immersed in the HT solution (termed as I-HT), on the growth of Sr-HA nanorod arrays were investigated. It was found that Sr-HA nanoprisms nucleated from Ca0.5Sr0.5TiO3 pre-formed on the MAOed TiO2 and grew in length to nanorods at the expense of Ca2+, Sr2+ and PO43− ions that migrated from the MAOed TiO2 during the HT. The Sr-HA nanorods were compacted, randomly oriented and homogeneous after HT for 24 h in the case of H-HT model, while the Sr-HA nanorods were randomly oriented and homogeneous but sparse after HT for 24 h in the case of I-HT model. The differences in the Sr-HA nanorod arrays were found to be caused by a large quantity of Ca2+, Sr2+ and PO43− ions in the MAOed TiO2 coatings releasing into the HT solution during HT in the case of I-HT model. In addition, Sr-HA nanorods patterned coatings (especially formed in the H-HT model) displayed superhydrophilicity and could firmly adhere to Ti substrates, although HT resulted in a slight decrease in bond strength of the coatings compared to the as-MAOed ones.