Depth-sensing indentation of low-density brittle nanoporous solids

Many applications of low-density nanoporous dielectrics are limited by their poor mechanical properties. Although nanoindentation is often used to evaluate the mechanical behavior of such materials, the physical meaning of the inelastic parameters measured by various indentation methods is not clear. Here, we study low-density nanoporous silica monoliths (aerogels) by nanoindentation using the most common indenter geometries (spherical, pyramidal and flat punches) and discuss the parameters measured. Results suggest that the deformation of the nanoporous silica monoliths studied in this work is controlled by elastic bending and fracture of nanoligaments with no signs of plasticity. The contact pressure (Meyer hardness) increases with increasing strain and does not represent the foam “crushing pressure”. The critical load for Hertzian fracture obeys the Auerbach law, and the formation of radial cracks and brittleness are strongly suppressed by the presence of nanopores. We also discuss the choice of indenter geometry and provide recommendations that can be used to overcome some typical challenges of indentation studies of low-density nanoporous solids, including very low contact stiffness for indenters with small contact areas, large surface roughness inherent to this class of materials, spatial non-uniformity (skin layers on monoliths), viscoelasticity and elastic nonlinearity.