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M. Schlögl, B. Mayer, J. Paulitsch, J. Keckes, C. Kirchlechner, P.H. Mayrhofer:
"Mechanical properties, fracture toughness, and thermal stability of CrN/AlN superlattice and multilayer thin films";
Talk: AVS 59th International Symposium, Tampa; 2012-10-28 - 2012-11-02; in: "Book of Abstracts", (2012).

Transition metal nitrides, such as CrN are highly attractive materials for a wide range of applications due to their outstanding properties like high hardness, excellent corrosion and oxidation resistance. Consequently, many research activities deal with their synthesis-structure-properties-relations. However, it has been reported that CrN/AlN superlattice coatings improve the mechanical properties compared to single CrN especially when keeping the AlN in its metastable cubic phase. Hence, we investigated the influence of the layer thickness of CrN on the stabilization of c-AlN and the critical layer thickness for AlN before transforming into the stable wurtzite phase. Furthermore, stress measurements and thermal stability were accomplished by the in-situ wafer curvature method during vacuum annealing to 700°C, differential scanning calorimetric (DSC) to 1500°C and hardness measurements after annealing up to 1100°C.
The fracture toughness of the coatings is studied by means of in-situ scanning electron microscopy and transmission electron microscopy microbending and microcompression tests. The small test-specimens are prepared by focused ion beam milling of individual free-standing thin films. As generally monolithic coatings with their columnar structure provide low resistance against crack formation and propagation we perform our studies for CrN films, CrN/AlN multilayers and the CrN/AlN superlattice as mentioned above. Especially the multilayers and superlattices provide additional interfaces perpendicular to the major crack-propagation-direction. Adjusting the AlN layer-thicknesses to allow for cubic or wurtzite structure enables to study the influence of the extremely stress sensitive cubic-to-wurtzite AlN phase transformation on the crack propagation.
The microtests clearly demonstrate that the monolithic CrN as well as the CrN/AlN multilayer coating with the wurtzite AlN layers crack with the behavior and features for totally brittle fracture. Contrary, the CrN/AlN multilayer coatings composed of cubic stabilized AlN layers are able to provide resistance against fatal crack propagation. Hence, they allow for significantly higher loads during the microbending and microcompression tests. Detailed structural investigations, in-situ and after the tests, suggest that the cubic AlN layers, which are stabilized by coherency strains in the CrN/AlN multilayer coatings, phase transform with the connected nature expansion when experiencing additional strain fields and thereby hinder crack propagation.

Project Head Paul Heinz Mayrhofer:
Atomistische Untersuchung von metastabilen Phasen