Compressive flow behavior of Al–TiN multilayers at nanometer scale layer thickness

The mechanical behavior of nanoscale Al–TiN multilayered composites was studied using micropillar compression experiments, which were compared with previously performed nanoindentation experiments, and the microstructure was investigated using scanning and transmission electron microscopy. It was found that at extremely small layer thicknesses (<5 nm) the nanoscale multilayers acquire remarkable hardness (∼6 GPa), high flow strengths (∼4.5 GPa maximum) and high compressive deformability (5–7% plastic strain). These high strengths were accompanied by extraordinarily high strain-hardening rates in the Al layers, which were of the order of 16–35 GPa (∼E/4–E/2) in “regime II” of the stress–strain curve of the compression tests in the 2–4 nm layered films and 5–9 GPa in the Al-18 nm–TiN-2 nm layered films, where Al layers deform plastically and TiN layers are deformed elastically. The high strengths and high work-hardening rates are discussed and analyzed using the concepts of dislocation motion and interactions within the confined nanoscale Al layers.

► Al–TiN multilayers deposited: layer thickness – Al-2 nm–TiN-2 nm (A) and Al-18 nm–TiN-2 nm (B).
► High peak strength 4.5 GPa (sample A) and 1.8 GPa for sample B.
► Both multilayers showed high ductility – (5–10% plastic strain).
► Al(soft metal) and TiN (hard ceramic) layers showed co-deformability for sample A.
► Al layers showed very high hardening rate, explained by confined layer slip model.