Strain hardening in nanolayered thin films

•Metal–ceramic nano-multilayers show measurable plasticity and high strain hardening rate.•A pronounced size effect is dependent on the layer thickness and the layer thickness ratio.•A 3D crystal elastic–plastic model is developed to study mechanical behavior.•The high strain hardening rate is ascribed to the closely spaced dislocation arrays.•The measurable plasticity is ascribed to the high slip activity in nanometer ceramics.

Experimental results indicate that metal–ceramic multilayered thin films have unusual properties such as high strength, measurable plasticity and high strain hardening rate when both layers are nanoscale. Furthermore, the strength and strain hardening rate show a pronounced size effect, depending not only on the layer thickness but also on the layer thickness ratio. We analyze the strain hardening behavior of nanoscale multilayers using a three-dimensional crystal elastic–plastic model (3DCEPM) that describes plastic deformation based on the evolution of dislocation density in metal and ceramic layers according to confined layer slip mechanism. These glide dislocations nucleate at interfaces, glide inside layers and are deposited at interfaces that impede slip transmission. The high strain hardening rate is ascribed to the closely spaced dislocation arrays deposited at interfaces and the load transfer that is related to the layer thickness ratio of metal and ceramic layers. The measurable plasticity implies the plastically deformable ceramic layer in which the dislocation activity is facilitated by the interaction force among the deposited dislocations within interface and in turn is strongly related to the ceramic layer thickness.