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R Hahn, M. Bartosik, P.H. Mayrhofer:
"Superlattice effect for enhanced Fracture Toughness of Hard Coatings and its Intrinsic Origins";
Talk: XI. Seminar for CE PhD Students in Materials Science, Wien; 2017-11-09 - 2017-11-10.

Ceramic hard coatings, for instance transition metal nitrides like TiN, protect tools from severe loads and harsh environments and thus enhance their lifetime. In this contribution, we show that TiN/CrN superlattice structures exhibit a peak in fracture toughness, similar to the well-known effect in indentation hardness [1].
We found -using micromechanical cantilever bending tests- that the fracture toughness increases with decreasing bilayer period, reaching a maximum at Λ ~ 5 nm (depending on the substrate material and the specific growth conditions). Thinner layers lead to intermixing and a subsequent loss of its unique superlattice properties.
The hardness enhancement in superlattice systems is a plasticity driven phenomenon, and thus determined by inhibition of dislocation movement [2]. In contrast, the fracture toughness follows a strictly linear elastic behaviour until fracture with no signs of plasticity. Hence, we assume underlying bilayer period dependent intrinsic effects to be responsible for the fracture toughness enhancement, such as coherency stresses and oscillating young´s moduli across the individual layers.


[1] R. Hahn, M. Bartosik, R. Soler, C. Kirchlechner, G. Dehm, and P.H. Mayrhofer, (2016) Scripta Materialia, 124, 67 - 70.
[2] X. Chu, S.A. Barnett, (1995) Journal of Applied Physics, 77, 4403.