A nitrogen doped TiO2 layer on Ti metal for the enhanced formation of apatite

Biomedical titanium metals subjected to gas under precisely regulated oxygen partial pressures (PO2) from 10−18 to 105 Pa at 973 K for 1 h were soaked in a simulated body fluid (SBF), whose ion concentrations were nearly equal to those of human blood plasma, at 36.5°C for up to 7 days. The effect of oxygen partial pressures on apatite formation was assessed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) measurements. After heating, the weight of the oxide layer (mainly TiO2) formed on the titanium metal was found to increase with increased oxygen partial pressure. Nitrogen (N)-doped TiO2 (Interstitial N) was formed under a PO2 of 10−14 Pa. At lower PO2 (10−18 Pa), only a titanium nitride layer (TiN and Ti2N) was formed. After soaking in SBF, apatite was detected on heat-treated titanium metal samples. The most apatite was formed, based on the growth rate calculated from the apatite coverage ratio, on the titanium metal heated under a PO2 of 10−14 Pa, followed by the sample heated under a PO2 of 10 and 104 Pa (in N2). The titanium metal heated under a PO2 of 105 Pa (in O2) experienced far less apatite formation than the former three titanium samples. Similarly, very little weight change was observed for the titanium metal heated under a PO2 of 10−18 Pa (in N2). During the experimental observation period (5 days, 36.5°C, SBF), the following relationship held: The growth rate of apatite decreased in the order PO2 of 10−14 Pa > PO2 of 10 Pa ≥ PO2 of 104 Pa > PO2 of 105 Pa > > PO2 of 10−18 Pa. These results suggest that N-doped TiO2 (Interstitial N) strongly induces apatite formation but samples coated only with titanium nitride do not. Thus, controlling the formation of N-doped TiO2 is expected to improve the bioactivity of biomedical titanium metal.