Influence of the oxygen flow rate on the properties of reactively sputtered Ti–B–O films

In this article, the effect of the oxygen flow rate on the structure and properties of reactively sputtered Ti–B–O films are investigated. At low oxygen flow rate, the films are nanocomposite coatings: TiB2 nanograins embedded in amorphous titanium and boron oxides. For higher values of the oxygen flow rate, the films are composed of amorphous titanium and boron oxides. Introduction of oxygen in titanium-boron based coatings induces an increase of their electrical resistivity. The thermal behaviour of the filmʹs resistance is measured using a two-probe configuration. TiB2 films exhibit a linear dependence of the electrical resistance with temperature that is characteristic of the metallic properties of TiB2. On the other hand, the temperature dependence of the electrical resistance of the nanocomposite Ti–B–O coatings does not match with metallic or semiconducting behaviour. High resolution UV photoemission spectroscopy results show a strong decrease of the Ti-3d states density at the Fermi level as a function of the oxygen flow rate, which is characteristic of a metal-insulator transition in Ti–B–O films. If the oxygen concentration in the films increases, the coatings become more and more transparent in the visible region due to the occurrence of the amorphous phase. The refractive index of amorphous films is also determined. Finally, the reduction of the filmʹs hardness with the increase of the oxygen flow rate is discussed in connection with the film structure.