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