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J. Buchinger, N. Koutná, Z. Chen, Z. Zhang, P.H. Mayrhofer, D. Holec, M. Bartosik:
"Toughness Enhanced Transition Metal Nitride Thin Films";
Talk: MSMF9, Brno; 2019-06-26 - 2019-06-28.

Owing to their exceptional chemical, thermal, and mechanical stability, transition metal nitride
(TMN) thin films are widely used and studied as protective coatings for a variety of applications. One
of the most limiting constraints of these compounds, however, remains their low intrinsic fracture
toughness. Motivated by the recently uncovered toughness-enhancing superlattice (SL) effect, as
well as computational studies, predicting a high potential for toughness for WN-related
structures, this study investigates TiN/WN SLs from experimental and theoretical viewpoints.
The sizeable elastic disparities between the involved TiN and WN layers render the resulting
enhancement of all toughness-related quantities particularly pronounced and informative.
We employ Density Functional Theory (DFT) based models to determine stable structural candidates
and to identify TiN/WN polymorphs with the highest capacity for toughness. Building upon this
theoretical basis, we synthesise TiN/WN SLs with various bilayer periods, as well as monolithic TiN
and WN using unbalanced DC reactive magnetron sputtering. Structural and morphological
information is gathered by X-ray diffraction, as well as scanning and transmission electron
microscopy techniques. Nanoindentation and single microcantilever bending tests are used to
evaluate the mechanical properties of our coatings.
Structurally, our TiN/WN SLs all assume a vacancy-stabilised cubic rocksalt configuration. The
investigated mechanical properties all show a distinct dependence on the bilayer period. Highlights
thereof include a tripling of the fracture energy, a maximum fracture toughness of 4.6 MPa√m, a
hardness peak of 36.7 GPa and an indentation modulus minimum of 387 GPa. Our results also
demonstrate that TiN/WN SLs unequivocally outperform their monolithic building blocks with
respect to all toughness-related properties (fracture toughness, fracture energy, elastic deformability).
We partly attribute this pronounced enhancement to the substantial differences of the elastic
properties between the TiN and WN layers. Furthermore, to provide a theoretical perspective on the
experimentally observed trends, we employ the DFT based Grimsditch-Nizzoli model, which
implicates compositional changes in the formation of the bilayer period dependent trends.

Project Head Paul Heinz Mayrhofer:
Hard coating materials