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R Hahn, M. Bartosik, H. Riedl, H. Bolvardi, S. Kolozsvári, P.H. Mayrhofer:
"Fracture toughness enhancement in superlattice hard coatings";
Talk: 46th International Conference on Metallurgical Coatings and Thin Films, San Diego; 2019-05-19 - 2019-05-24.

Physical vapour deposited (PVD) ceramic hard coatings are widely used in industrial applications as protective, wear reducing coatings. Their combination of good mechanical properties such as high hardness, a low friction coefficient, and their chemical resistance enable the application in harsh environments. However, a strong limitation is the relatively low fracture tolerance (brittle behaviour), depicting especially in cutting applications a major challenge.
In this contribution, we show experimental results of in-situ microcantilever bending tests on nanolayered TiN-CrN coatings, referred to as superlattices, overcoming this unfavourable behaviour. We found a maximum in fracture toughness (KIC) at bilayer periods of ~6 nm [1], similar to the well-known peak for the indentation hardness reported by Helmersson et al. [2]. For both, KIC and the hardness, we observe an increase by ~50 % compared to the rule of mixture of the constituents. The beneficial effect of a careful structural design on the fracture toughness will be shown for reactive magnetron sputtered as well as arc evaporated superlattice coatings. Importantly, the coatings synthesized in the industrial scale arc evaporation plant (Oerlikon Balzers Innova) show an even more pronounced superlattice effect and thus unite high hardness with reasonable toughness.
While mechanisms based on dislocation activity explain the increase in hardness, the linear elastic behaviour during our micromechanical tests suggests a different mechanism responsible. To describe this, we conducted density functional theory (DFT) calculations as well as finite element studies.
Complementary, we studied the microstructure of our coatings by X-ray diffraction experiments, scanning electron microscopy and high-resolution transmission electron microscopy. The thermal stability of our films was investigated by annealing in vacuum and ensuing experiments (XRD, hardness and fracture toughness) [3] along with differential scanning calorimetry (DSC) investigations.
[1] R. Hahn, M. Bartosik, R. Soler, C. Kirchlechner, G. Dehm, P.H. Mayrhofer, Superlattice effect for enhanced fracture toughness of hard coatings, Scripta Mat. 124 (2016) 67.
[2] U. Helmersson, S. Todorova, S.A. Barnett, J.-E. Sundgren, L.C. Markert, J.E. Greene, Growth of single‐crystal TiN/VN strained‐layer superlattices with extremely high mechanical hardness, J. Appl. Phys. 62 (1987) 481.
[3] R. Hahn, M. Bartosik, M. Arndt, P. Polcik, P.H. Mayrhofer, Annealing effect on the fracture toughness of CrN/TiN superlattices, Int. J. Refract. Met. H. 71 (2018) 352-356.

Project Head Helmut Riedl:
CDL-SEC