Improved nanoindentation methods for polymer based multilayer film cross-sections

Nanoindentation (NI) is capable to investigate mechanical properties on a small scale and is also suitable to examine cross-sections of co-extruded or laminated multilayer films with thicknesses in the $\mu$m-regime. The standard Oliver-Pharr (O&P) NI method [1] is typically employed to measure the elastic modulus and hardness. However, this standard method assumes a homogeneous and semi-infinite sample that is rigidly supported. NI on thin, multilayer film cross-sections violate these assumptions because nanoindentations are always near free edges and heterophase interfaces. The structural compliance method was developed by Jakes et al. to correct NI results for edge effects and specimen-scale flexing [2]. The method is based on the discovery, that the effect of both edges and specimen-scale flexing is to introduce a structural compliance (Cs) into the measurement. Applied to a multilayer, the Cs showed a strong position dependence, i.e., the effect was larger near the edges and layer interfaces. The Cs correction had little effect on the hardness values; however, the influence on measured elastic modulus was significant. There, the corrected modulus values tended to be higher than the uncorrected ones in the stiff layers. After the Cs correction, the position dependence of the elastic modulus within a given layer was not observed within experimental uncertainties.