Mechanics of nanoindentation on a monolayer of colloidal hollow nanoparticles.

We explore the collective mechanical behavior of monolayer assemblies composed of close-packed arrays of hollow silica nanoparticles using a spherical nanoindentor. Seven types of well-defined hollow nanoparticles are studied with their radii ranging from 100 to 300 nm and shell thickness ranging from 14 to 44 nm. Micromechanical models reveal the underlying deformation mechanisms during indentation, where the consecutive contacting of the indentor with an increasing number of nanoparticles results in a nonlinear increase in the indentation force with penetration depth. Each contacted hollow nanoparticle successively locally bends, flattens, and then locally buckles. The effective indentation modulus of the monolayer film, which is obtained by a Hertzian fit to the experimental data, is found to be proportional to the elastic modulus of the nanoparticle shell material and scales exponentially with the ratio of particle shell thickness t to radius R to the power of 2.3. Furthermore, we find that for a constant film density with the same (t)/(R) of the constituent nanoparticles, smaller particles with a thinner shell can provide a higher effective indentation modulus, compared to their larger diameter and thicker shell counterparts. This study provides useful insights and guidance for constructing high-performance lightweight nanoparticle films and coatings with potential applications in tailoring stiffness and mechanical energy absorption.