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J. Buchinger, N. Koutná, P.H. Mayrhofer, D. Holec, M. Bartosik:
"TiN/WN Superlattice Coatings for Enhanced Toughness";
Poster: METAL 2018, Brno; 2018-05-23 - 2018-05-25.

The most restrictive property of ceramic thin films for a variety of applications is their limited fracture toughness. Recently, the authors have uncovered that the arrangement of ultra-thin and heterogeneous layers of transition metal nitrides (TMN) in a nano-laminated architecture has the potential to enhance the fracture toughness in comparison with their monolithic constituents.
Based on semi-empirical toughness criteria (Pugh, Pettifor, Greaves), we identified TiN/WN superlattice films as a particularly promising system. Density Functional Theory (DFT) simulations were conducted to establish the expected crystal structure and lattice parameter of the superlattice films and their components. The cleavage fracture strength of TiN/WN superlattice films was calculated by atomistic modelling. To provide experimental evidence, we used reactive magnetron sputtering to synthesise TiN/WN superlattice films with bilayer periods ranging from 2.8 to 166.5 nm and a total film thickness of around 2 µm. Monolithic TiN and WN films were grown to obtain reference figures. The synthesis of pure single-phase fcc-WN layers was performed with special diligence, so as to avoid the appearance of other phases, which necessitated careful manipulation of the processing parameters. The films were characterised using X-ray diffraction, scanning electron microscopy, and nanoindentation. The intrinsic fracture toughness was derived from the ratio of hardness to Young´s modulus.
The experimental results, which demonstrate a dependence of the mechanical properties on the bilayer period, are discussed and related to the DFT findings. This study is embedded within a larger project, which aspires to reveal the impact of the difference in structural and mechanical properties between the involved TMNs on the manifestation and extent of the toughness enhancement in TMN superlattices.

Project Head Paul Heinz Mayrhofer:
Hard coating materials