Enhanced fracture toughness in ceramic superlattice thin films: On the role of coherency stresses and misfit dislocations

Antonia Wagner, David Holec, Paul Heinz Mayrhofer, Matthias Bartosik

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2 Citations (Scopus)


Superlattice (SL) thin films composed of refractory ceramics unite extremely high hardness and enhanced fracture toughness; a material combination which is often mutually exclusive. While the hardness enhancement is well described by existing models based on dislocation mobility, the underlying mechanisms behind the increase in fracture toughness are yet to be unraveled. Here, we provide a model based on linear elasticity theory to predict the fracture toughness in (semi-)epitaxial nanolayers. As representative of cubic transition metal nitrides, a TiN/CrN superlattice structure on MgO (100) is studied. The density of misfit dislocations is estimated by minimizing the overall strain energy, each time a new layer is added on the nanolayered stack. The partly relaxed coherency stresses are then used to calculate the apparent fracture toughness (K app) by applying the weight function method. The results show that K app increases steeply with increasing bilayer period for very thin SLs, before the values decline more gently along with the formation of misfit dislocations. The characteristic K app vs. bilayer-period-dependence nicely matches experimental trends. Importantly, all critical stress intensity values of the SLs clearly exceed the intrinsic fracture toughness of the layer materials, evincing the importance of coherency stresses for increasing the crack growth resistance.

Original languageEnglish
Article number109517
Number of pages10
JournalMaterials and Design
Issue number202
Early online date27 Jan 2021
Publication statusPublished - Apr 2021

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