  Room and high temperature mechanical properties of reactive magnetron
sputtered TaSiN coatings were measured using nanoindentation (between 25C and
500C). Fracture toughness was also evaluated at a similar temperature range
using the micropillar splitting method. The influence of the nitrogen
concentration on the evolving phases and microstructure of the TaSiN coatings,
before and after the high temperature testing, were examined by X-ray
diffraction (XRD) and transmission electron microscopy (TEM) analysis. XRD
spectra showed broad peaks with hexagonal $\gamma$-Ta2N as the main phase, with
the cubic $\delta$-TaN phase emerging for higher N contents. Phase composition
remained unchanged before and after the 500C tests. However, after the high
temperature tests, TEM analysis showed the presence of an oxide surface layer,
with a thickness that decreased (from 42 to 15 nm) with N content, due to
residual oxygen diffusion, which replaces nitrogen to form amorphous SiOx.
Beneath the oxide-rich surface layer, coatings exhibited a stable
nanocrystalline columnar microstructure. Hardness and fracture toughness
increased with N content, initially due to the formation of an amorphous Si-N
tissue at grain boundaries, and for even higher N contents, due to the
appearance of the hard cubic $\delta$-TaN phase. Hardness at 500C decreased
only by 15%, while fracture toughness followed the opposite trend, due to
increased plasticity with temperature. The optimum composition turned out to be
Ta55Si10N35, which retained a hardness of 30 GPa at 500C, being also the
toughest. These observations make this system very interesting for high
temperature applications.
