Hydroxyapatite-Zirconia Composite Thin Films Showing Improved Mechanical Properties and Bioactivity

Nano-crystalline hydroxyapatite (HAp) films were formed at the Ti surface by a single-step microarc oxidation (MAO), and HAp-zirconia composite (HZC) films were obtained by subsequent chemical vapor deposition (CVD) of zirconia onto the HAp. Through the CVD process, zero- and one-dimensional zirconia nanostructures having tetragonal crystallinity (t-ZrO2) were uniformly distributed and well incorporated into the HAp crystal matrix to form nanoscale composites. In particular, (t-$ZrO_2$) was synthesized at a very low temperature. The HZC films did not show secondary phases such as tricalcium phosphate (TCP) and tetracalcium phosphate (TTCP) at relatively high temperatures. The most likely mechanism for the formation of the t-$ZrO_2$ and the pure HAp at the low processing temperature was proposed to be the diffusion of $Ca^{2+}$ ions. The HZC films showed increasing micro-Vickers hardness values with increases in the t-$ZrO_2$ content. The morphological features and phase compositions of the HZC films showed strong dependence on the time and temperature of the CVD process. Furthermore, they showed enhanced cell proliferation compared to the $TiO_2$ and HAp films most likely due to the surface structure change.