Nanoindentation methods for viscoelastic characterization of stiff porous materials

Background
Modifying the mechanical properties of the solid phase of a porous material, in this study calcium-silicate-hydrate, is frequently possible by changing synthesis conditions, but changes in these conditions can also influence porosity, which in turn may affect the mechanical properties of the porous material. Experimental methods to decouple porosity from the viscoelastic properties of the porous material will aid in optimization of the structure of the solid phase to achieve the desired mechanical properties.

Objective
Explore different nanoindentation techniques in order to determine the viscoelastic properties of the solid phase (without the affect of porosity) of a stiff porous material via experimental methods alone.

Methods
Compacted pellets of calcium-silicate-hydrate were prepared with different porosity and subjected to three nanoindentation techniques to determine viscoelastic behavior and the influence of porosity: dynamic, stress relaxation, and creep. Results of the porosity and of the viscoelastic behavior measurements were analyzed with a reverse-micromechanics model to determine viscoelastic properties of the solid phase, which has not been achieved previously for calcium-silicate-hydrate. These methods can be used in development and refinement of materials to determine how changes in the solid phase (e.g. molecular structure) influence viscoelastic behavior while considering the effect of porosity.

Results
Dynamic nanoindentation was found to be unreliable for the stiff material studied in this work. Normalized stress relaxation and creep data were found to be independent of porosity. Reverse micro-mechanics modeling allowed for characterization of the creep modulus that is consistent with other studies that used computational or synchrotron x-ray methods to characterize mechanical properties of the solid calcium-silicate-hydrate phase.

Conclusion
Creep experiments provide more reliable data than dynamic or stress relaxation experiments. When the porosity is known, reverse-micromechanics modeling can be used determine the creep modulus of the solid phase and thus be used to predict creep modulus of a composite with an arbitrary porosity. If the porosity is not known, the viscoelastic properties of the solid phase can still be compared to each other using a normalized creep modulus that is independent of porosity.
Methods
Compacted pellets of calcium-silicate-hydrate were prepared with different porosity and subjected to three nanoindentation techniques to determine viscoelastic behavior and the influence of porosity: dynamic, stress relaxation, and creep. Results of the porosity and of the viscoelastic behavior measurements were analyzed with a reverse-micromechanics model to determine viscoelastic properties of the solid phase, which has not been achieved previously for calcium-silicate-hydrate. These methods can be used in development and refinement of materials to determine how changes in the solid phase (e.g. molecular structure) influence viscoelastic behavior while considering the effect of porosity.