Effect of strain rate on the deformation of hollow CoS nanoboxes and doubly porous self-assembled films

Abstract

Nanomaterials with multiscale porosity are attractive as lightweight structural materials because unique deformation mechanisms can be programmed at different length scales. Here, we explore this concept by investigating the mechanics of hollow nanoboxes, as well as films self-assembled from nanoboxes. Hollow CoS nanoboxes with lengths of ∼900 nm and <30 nm wall thickness are synthesized using colloidal methods. Individual nanoboxes are compressed inside a scanning electron microscope at strain rates of 0.001 s−1, 0.01 s−1, and 0.1 s−1. Nanoboxes are found to have a first peak stress of ∼15 MPa and loading modulus of ∼240 MPa regardless of strain rate. The nanoboxes deform through ductile, plastic bending of side walls at 0.001 s−1 strain rate, while brittle fracture occurs at the higher strain rates. Self-assembly of these nanoboxes results in films with relative density of <0.1. Nanoindentation shows strain rate independent behavior with average modulus and hardness of ∼15 MPa and ∼230 kPa, respectively. The mechanical behavior of the film is compared to that of the nanobox building blocks, cellular foams and granular materials.