Mechanical Characterization of Polymers on a Nanometer Scale through Nanoindentation. A Study on Pile-up and Viscoelasticity

Abstract

The analysis of nanoindentation force curves collected on polymers through the common Oliver
\nand Pharr procedure does not lead to a correct evaluation of Young’s modulus. In particular, the estimated elastic
\nmodulus is several times larger than the correct one, thus compromising the possibility of a nanomechanical
\ncharacterization of polymers. Pile-up or viscoelasticity is usually blamed for this failure, and a deep analysis of
\ntheir influences is attempted in this work. Piling-up can be minimized by indenting on a true nanometer scale,
\ni.e., at penetration depth smaller than 200 nm. On the other side, it is common knowledge that fast indentations
\nminimize the effect of viscoelasticity. However, changing the indentation time in a broad range of contact time
\n(fractions of second up to hundreds of seconds) did not allow the correct estimation of Young’s modulus for the
\npolymers used in this work. The final result is that the Oliver and Pharr procedure as well as any other procedure
\nanalyzing the unloading curve with elastic contact mechanics models cannot be employed to measure Young’s
\nmodulus of polymers because its application is incorrect from a theoretical point of view, unless the analysis is
\nlimited to the very first nanometers of penetration depth when the contact is perfectly elastic. Viscoelastic contact
\nmechanics models should instead be employed to characterize these materials.