Incidence of nanoscale heterogeneity on the nanoindentation of a semicrystalline polymer: Experiments and modeling

The nanoindentation of semicrystalline polyamide 6 (PA6) specimens was evaluated by atomic force microscopy with an extremely sharp diamond tip whose apex was nominally less than 10 nm in radius. Nanoindentation was performed under dry and wet conditions on PA6 samples injection-molded at different melt temperatures. The bulk Young’s modulus of dry PA6 was found to be in good agreement with the literature, thereby confirming the experimental approach developed in this study. A major finding is the observation of sudden increases in force during the loading portion of the experimental load–displacement curves, which occurred regardless of water absorption. We also found some irregularity in the pile-up morphology around the indents, linked to the heterogeneous nature of the deformation in PA6. Finite element analysis was used to elucidate the phenomenon of force discontinuity by independently considering the variations in elasticity, yield stress and friction coefficient of the crystalline lamellar aggregates in PA6. It is shown that force discontinuities in nanoindented PA6 result from local differences in yield phenomena at the lamellar level. The present investigation sheds some light on the importance of mechanical heterogeneity emerging from contact interactions between an extremely sharp tip and the nanoscale morphology of semicrystalline polymers.