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

Superlattice (SL) thin films composed of refractory ceramics unite extremely high hardness and enhanced fracture toughness; a material combination often being mutually exclusive. While the hardness enhancement obtained whentwo materials form a superlattice 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 enhancement in (semi-)epitaxial nanolayers due to coherency stresses and formation of misfit dislocations. We exemplarily study a superlattice structure composed of two cubic transition metal nitrides (TiN, CrN) on a MgO (100) single-crystal substrate. Minimization of the overall strain energy, each time a new layer is added on the nanolayered stack, allows estimating the density of misfit dislocations formed at the interfaces. The evolving coherency stresses, which are partly relaxed by the misfit dislocations, are then used to calculate the apparent fracture toughness of respective SL architectures by applying the weight function method. The results show that the critical stress intensity increases steeply with increasing bilayer period for very thin (essentially dislocation-free) SLs, before the K_IC values decline more gently along with the formation of misfit dislocations. The characteristic K_IC vs. bilayer-period-dependence nicely matches experimental trends. Importantly, all critical stress intensity values of the superlattice films clearly exceed the intrinsic fracture toughness of the constituting layer materials, evincing the importance of coherency stresses for increasing the crack growth resistance.