Zhuo Chen1, Yonghui Zheng1, Lukas Löfler2, Matthias Bartosik3, Ganesh Kumar Nayak2, Oliver Renk1, David Holec2, Zaoli Zhang1

1. Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, A-8700 Leoben, Austria

2. Department of Materials Science, Montanuniversität Leoben, A-8700 Leoben, Austria

3. Institute of Materials Science and Technology, TU Wien, A-1060 Vienna, Austria

Numerous efforts have been devoted to design novel multilayers with a small bilayer thickness in order to acquire superior mechanical properties for potential applications. In general, multilayer structures with a bilayer-period thickness of just a few nanometers showed the best combination of hardness, toughness, and elastic modulus. However, the beneficial effect of reduced bilayer thicknesses on mechanical properties is not observed for any layer spacing. It has been reported that, similar to nanocrystalline metals, the hardness and toughness of the multilayer coating decrease again when the bilayer-period thickness is reduced below a certain critical value, being on the order of a few nanometer. For TMNs multilayer, the current understanding for this degradation of properties is that component intermixing already occurs during the thin film deposition and annealing process.

Here[1], we report on an entirely overlooked phenomenon occurring during the indentation of nanolayers, presumably explaining the degradation of properties. Nanoindentation, commonly used to determine properties of hard coatings, is found to disrupt and intermix the multilayer structure due to the deformation imposed (as seen in Figs.1). Detailed electron microscopy studies and atomistic simulations provide evidence for intermixing in an epitaxial transition metal nitride superlattice thin film induced by nanoindentation. The formation of a solid solution reduces the interfacial density and leads to a sharp drop in the dislocation density. Our results confirm that plastic deformation causes the microstructure instability of nitride multilayer, which may further improve our understanding of multilayer strength mechanisms.

Reference:

[1] Z. Chen, et al., Atomic insights on intermixing of nanoscale nitride multilayer triggered by nanoindentation, Acta Materialia 214 (2021) 117004.