Fracture toughness of free-standing nanocrystalline copper–chromium composite thin films

In this paper, a hybrid method of experiments and numerical analyses for measuring the fracture toughness of electron-transparent thin films (∼50–100 nm thick) with a nanocrystalline grain size is presented. Electron-transparent, free-standing copper–chromium composite thin films were produced by electron beam deposition coupled with electron beam lithography and deformed in situ in a transmission electron microscope (TEM) in tension. Crack growth in these nanocrystalline thin films was observed and recorded in situ in the TEM. The recorded crack opening profiles are used to estimate the local as well as the global fracture toughness of the nanocomposite by employing inverse analyses. The yield strength, the plastic hardening modulus and the toughness of the copper matrix are determined by the inverse finite element method by matching numerical crack opening profiles with the experimental counterpart. Knowing the matrix toughness, crack kinking angles at the copper–chromium interfaces are used to estimate the interface toughness. The global composite toughness is then obtained by estimating the bridging forces of crack-face ligaments with a limit analysis. The inverse analyses give the yield stress as ∼800 MPa, the plastic hardening modulus as ∼1 GPa and the local toughness as ∼64 J m⁻² for the nanocrystalline copper matrix. The toughness of the copper–chromium interface is determined to be ∼27 J m⁻²; this weak interface provides crack-face bridging that increases the global toughness of the composite film by ∼38% to ∼89 J m⁻²