Investigation of nanocomposite formation in TiSiN coatings using atom probe tomography

Owing to their superior mechanical properties, TiSiN nanocomposite coatings are known as excellent protective layers for cutting tools. In the present study, isotopic substitution of N is applied to investigate the nanostructure of TiSiN coatings with varying Si contents using atom probe tomography (APT). TiSiN coatings containing ∼2, 5, and 10 at% Si were sputter-deposited using 15N-enriched nitrogen. The APT results show that Si enriches at assumed grain boundaries forming an amorphous phase for all investigated coatings. This result is corroborated by complementary performed high resolution scanning transmission electron microscopy investigations. Further, no Si-free regions could be observed by APT, evidencing that for all coatings Si is also incorporated into a Ti1-xSixN solid solution. In addition, also no Ti-free regions are present, indicating that the amorphous phase can be assigned as a-SiaTibNc. With increasing the overall Si content from ∼2 to 10 at% the Si incorporated into the solid solution increases from ∼2 to ∼9 at%, while the Si-enrichment in the amorphous phase fraction is rather constant at ∼4 to 6 at%. The highest hardness of 38.5 ± 1.4 GPa, as determined by nanoindentation, was obtained for the coating containing ∼5 at% Si. This study demonstrates that isotopic substitution of nitrogen is a suitable tool to study the nanocomposite structure of TiSiN coatings in unprecedented detail.