Techniques and considerations for nanoindentation measurements of polymer thin film constitutive properties

Modulus and yield strength determinations from nanoindentation experiments of thin poly(methyl methacrylate) (PMMA) polymer films have been investigat ed. The modulus was calculated from Bat punch theory and showed a large eff ect of the substrate, hydrostatic pressure, and surface topography Substrat e corrections were possible in some cases but often resulted in erroneous o r negative values due to large penetration depths with respect to the film thickness and differences in the stiffness of the coating and substrate. Hy drostatic pressures up to 600 MPa were exerted by a conical and cube corner indenter tip and influenced the modulus and yield strength measurements. D ue to the non-linear deformation behavior of the PMMA films, however, hydro static pressure corrections tended to underestimate the modulus. The yield strength was measured based on Tabor's approximation using the unloading cu rve and was compared with measurements using plastic zone predictions and c ompression test data. Yield strength calculations from plastic zone images were lower in every case than those calculated from compliance, showing the viscoelastic nature of the material.