The influence of films thickness on hydrogenation behavior of titanium thin films

Titanium films of different thicknesses were prepared on sapphire substrates in an UHV chamber, by means of ion beam sputter deposition at room temperature, under Ar-atmosphere at the pressure of 1.5·10 E−4 mbar. For electrochemical hydrogen loading, the films were covered by a 30-nm thick layer of Pd in order to prevent oxidation and facilitate hydrogen absorption. In situ stress measurements were conducted during step-by-step electrochemical hydrogen charging of the films. XRD measurements using a Phillips X-Pert diffractometer with a Co-Kα radiation were performed before and after hydrogenation in order to investigate the effect of hydrogen loading on the microstructure. The phase boundaries, as well as the stress and strain development during hydrogen absorption, depend strongly on the thickness of the films. The main characteristics of absorption behavior of hydrogen, as well as the thermodynamics and phase boundaries of titanium–hydrogen thin films are discussed in detail with specific emphasis on the influence of films thickness. The obtained results are also compared to literature data on the widely studied titanium–hydrogen bulk system. Shifted phase boundaries and narrowed two-phase field appear in Ti–H film system, which are mainly attributed to the microstructural contribution, as well as to the large stresses in the GPa-range that built up between the films and their substrate.