Effects of titanium doping on the structure and mechanical properties of diamond-like nanocomposite films

In this paper, we report on comparative studies of mechanical properties of diamond-like nanocomposite (DLN) a-C:H:Si:O films and titanium-doped DLN films, which were focused on (i) nanoindentation and internal stress examination of DLN films of different thickness (up to 10 mu m) and (ii) structural transformations and mechanical properties changes with increasing titanium content in Ti-DLN films. The DLN films were found to combine high hardness of 22-24 GPa, high elastic recovery, and low compressive stresses of 100-160 MPa. For Ti-DLN films, the hardness, elastic modulus, and residual stress were found to exhibit similar behavior vs Ti content: an initial decrease at low Ti contents was followed by gradual increase of the hardness, elastic modulus and compressive stresses. The mechanical properties changes were shown to correlate with the structure modifications induced by titanium incorporation into the DLN matrix. Raman spectra of Ti-DLN films revealed three peaks in the low-frequency region from 200 to 700 cm(-1), attributed to the TiC phase. An additional evidence of the formation of TiC nanocrystals of several nanometers size was obtained using transmission electron microscopy (TEM) of Ti-DLN films. The findings of Raman spectroscopy along with the TEM data have demonstrated the main factors formation of TiC nanocrystals and increase of the sp(2)-carbon content, responsible for the mechanical behavior of Ti-DLN films.