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

J. Buchinger, A. Wagner, Zhuo Chen, Zaoli Zhang, David Holec, Paul Heinz Mayrhofer, Matthias Bartosik

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6 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)376-386
Number of pages11
JournalActa materialia
Issue number1 January
Early online date1 Nov 2020
Publication statusPublished - 1 Jan 2021

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