Transition metal nitrides have found widespread applications in the cutting- and machining-tool industry due to their extreme hardness, thermal stability, and corrosion resistance. The increasing demand for these nitrides requires an in-depth understanding of their structures at the atomic level. This has led to numerous experimental and theoretical research [1-3]. Using advanced electron microscopy techniques, our understanding of this type of material's structure and property relationships has been significantly advanced.

Firstly, we report a multilayered structure comprising rock-salt (rs) structured CrN layers of constant thickness and AlN layers of varying thicknesses, which surprisingly enables the growth of metastable zinc-blend (zb) AlN layers for certain layer-thickness combinations. The multilayer exhibits an atomic and electronic structure gradient as revealed using advanced electron microscopy and electron spectroscopy. A combined experimental analysis based on valence electrons and inner shell electrons allowed the mapping of mechanical properties (bulk modulus) of the multilayer at the nanometre scale. Further, we found that the presence of oxygen impurities causes a remarkable reduction of the bulk modulus of rs-CrN while having no significant effect on the bulk modulus of the stable wurtzite structure wz-AlN layers. The findings are unambiguously validated by theoretical calculations using density functional theory [1].

Secondly, we will present an example of a nitride multilayer (TiN/AlN) study through coupling uses of different microscopy techniques. A surprising intermixing phenomenon in nano-scale nitride multilayers under loads has been detected by detailed advanced TEM investigations. Close examination reveals that a new phase could be created during deformation, which could be visualized via mapping the fine structure difference (i.e., Ti-L2,3). Using spherical aberration-corrected HRTEM and HAADF-STEM, we further corroborated that such a homogeneous solid-state phase has formed in the process of applied loads. Atomic-resolution EDXS analysis exhibits homogeneous elemental distributions after intermixing. The present study provides atomic-scale insights into the extraordinary strength mechanisms pertaining to this nanoscale multilayer [3].


References
[1] Zaoli Zhang et al., Acta Materialia, 2020, 194, 343
[2] Zaoli Zhang et al., Materials & Design, 2021,207,109844.
[3] Zhuo Chen et al., Acta Materialia, 2021, 214, 117004.

Acknowledgment: This work is financially supported by the Austrian Science Fund (FWF P33696-N). The authors thank Lukas Löfler, Ganesh Kumar Nayak, Nikola Koutna, Oliver Renk, David Holec, Matthias Bartosik, Paul H. Mayrhofer, Christian H. Liebscher, and Gerhard Dehm for contributing to the calculations, depositing the multilayers, and discussion.