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 investigated. The modulus was calculated from flat punch theory and showed a large effect of the substrate, hydrostatic pressure, and surface topography.Substrate corrections were possible in some cases but often resulted in erroneous or negative values due to large penetration depths with respect to the film thickness and differences in the stiffness of the coating and substrate. Hydrostatic pressures up to 600 MPa were exerted by a conicaland cube corner indenter tip and influenced the modulus and yield strength measurements. Due to the non-linear deformation behavior of the PMMA films, however, hydrostatic pressure corrections tended to underestimate the modulus. The yield strength was measured based on Tabor's approximationusing the unloading curve and was compared with measurements using plastic zone predictions and compression 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.