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R Hahn, M. Bartosik, R. Soler, C. Kirchlechner, G Dehm, P.H. Mayrhofer:
"Superlattice Effect for Enhanced Fracture Toughness of Hard Coatings";
Talk: Stress 2016, Chicago; 2016-10-02 - 2016-10-05; in: "Stress Evolution in Thin Films and Coatings: From Fundamental Understanding to Control", (2016), 17.

In this contribution, it is shown that coherently grown nanolayered TiN/CrN thin films exhibit a superlattice effect in fracture toughness, similar to the reported effect in indentation hardness. We found -by employing in-situ micromechanical cantilever bending tests on free-standing TiN/CrN superlattice films- that the fracture toughness increases with decreasing bilayer period (Λ), reaching a maximum at Λ ~6 nm. For ultrathin layers (Λ ~2 nm), the fracture toughness drops to the lowest value due to intermixing and loss of superlattice structure. Both, fracture toughness and hardness peak for similar bilayer periods of TiN/CrN superlattices. The hardness enhancement in superlattice thin films is a plasticity driven phenomenon (mainly depending on a difference in the shear moduli of its constituents), and as such, is governed by aggravated dislocation mobility. In contrast, the fracture toughness of the ceramic thin films shows little to none plasticity, suggesting an underlying bilayer-period-dependent property to be responsible for both enhancements. These possible intrinsic superlattice properties, which will be discussed in detail, are: coherency strains due to differing lattice constants a; the average grain size in the individual layers; or spatially oscillating elastic moduli influencing the crack growth.