Fracture toughness trends of modulus-matched TiN/(Cr,Al)N thin film superlattices

Through superlattice (SL) architectures, the hardness as well as the fracture toughness of ceramic thin films can be enhanced. The hardness-related SL effect is reasonably well understood, however, the mechanisms driving the toughness-enhancing effect are still partially unexplored. To isolate the effect of the lattice mismatch from the elastic moduli mismatch on the toughness-related properties, we designed TiN/Cr _0.37Al _0.63N superlattices, in which the involved layers have effectively identical elastic moduli, but sizeably different lattice parameters. Micromechanical bending tests show an enhanced fracture toughness K _IC of the SLs (2.5±0.1 MPa√m) compared with monolithic TiN (2.0±0.1 MPa√m) and Cr _0.37Al _0.63N (1.3±0.1 MPa√m) with only a weak bilayer period (Λ) dependence. Superimposing an analytical model based on continuum mechanics on the experimental data, we demonstrate that, at low Λ, the nanolayers within the SL exhibit strong coherency strains, as misfit dislocation formation is energetically unfavourable. With increasing layer thicknesses, misfit dislocations start to form in the two layer materials – first in Cr _0.37Al _0.63N and slightly Λ-shifted in TiN. The associated evolution of coherency strains in the TiN and Cr _0.37Al _0.63N layers causes the observed bilayer-period-dependent toughness enhancement beyond the constituent materials. Supporting structural, morphological, chemical, and mechanical analyses are provided by X-ray diffraction, electron microscopy techniques, and nanoindentation.