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/Cr0.37Al0.63N superlattices, in which the involved layers have effectively identical elastic moduli, but sizeably different lattice parameters. Micromechanical bending tests show an enhanced fracture toughness KIC of the SLs (2.5 +/- 0.1 MPa root m) compared with monolithic TiN (2.0 +/- 0.1 MPa root m) and Cr0.37Al0.63N (1.3 +/- 0.1 MPa root m) with only a weak bilayer period (Lambda) dependence. Superimposing an analytical model based on continuum mechanics on the experimental data, we demonstrate that, at low A, 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 Cr0.37Al0.63N and slightly A-shifted in TiN. The associated evolution of coherency strains in the TiN and Cr0.37Al0.63N 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. (c) 2020 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Automated summary

The use of superlattice architectures can enhance the hardness and fracture toughness of ceramic thin films. The mechanisms behind the toughness-enhancing effect are still partially unexplored, but by designing superlattices with identical elastic moduli but different lattice parameters, an enhanced fracture toughness was observed. The understanding of the coherency strains in the nanolayers within the superlattice structure is crucial for the observed toughness enhancement.