Physical phenomena during nanoindentation deformation of amorphous glassy polymers

TL;DR

This study investigates the physical phenomena during nanoindentation of amorphous glassy polymers, revealing sink-in, pile-up, and volume upflow behaviors, and distinguishes viscoelastic effects from elastic and plastic responses.

Contribution

It adapts the ideal conical indentation framework to VEP polymers, providing new insights into deformation mechanisms and isolating viscoelastic effects from nanoindentation data.

Findings

Sink-in during loading and pile-up after unloading observed.

Volume conserving upflow below the tip identified.

Viscoelastic effects can be separated from elastic and plastic contributions.

Abstract

We identify for visco-elasto-plastic (VEP) glassy polymers, physical phenomena during Berkovich nanoindentation, a locally imposed deformation. Live visuals via in situ nanoindentation indicate mainly sink-in during loading, with pile-up after unloading. Scanning Probe Microscopy (SPM) indicates significant volume conserving upflow below the tip, for these high nu, compliant materials, with compliance correlated high geometric fractional contact (including blunt height, h_b), (h_c+h_b)/(h_m+h_b)~0.86-0.95. We adapt the ideal conical indentation framework to VEP Berkovich nanoindentation, to calculate the contact area and visually depict the upflow and the displacement paths, in the material. The combination of SPM and P-h data, indicates a mixed comparison with uniaxial modulus and yield stress, with conventionally defined hardness, H<3*sig_y, and nanoindentation modulus E_N>E. By rationally removing viscoelastic (VE) effects from the loading P-h data, we find instant, zero-time hardness, H_L0>3*sig_y. We apply the power law model to only the recovery onset, to estimate pure elastic recovery. We then deconvolute the VEP nanoindentation into the conventional EP and elastic contributions, isolating the VE component. Constraint-induced sink-in, pile-up and VE recovery of the highly yielded tip-apex region, mirror the converse constrained deformation effects, governing the trends in conventionally defined H_L0 and E_N for glassy polymers.