Assessing Young’s modulus of poly (methyl methacrylate) using nanoindentation: Unraveling ambiguities

In this study, the uncertainty of equivalent Young’s modulus (E_N) estimation from nanoindentation experiments by following the conventional method for poly (methyl methacrylate) (PMMA) is discussed in detail. A constant strain rate load function is followed to perform nanoindentation experiments in a wide range of applied loads. It is well known that the stiffness (S) value is the primitive requirement for E_N estimation which is affected by viscous deformation. Thus, a load function with prolonged holding time (150 s) and fast unloading rate (4500 µN/sec) is followed to minimize viscous effects on unloading data. Thereafter, S values are estimated by fitting with various equations reported in the literature for different ranges of unloading data. In addition, the contact area (A_c) is estimated from the tip area calibration data and the actual tip geometry relationship. The E_N values are calculated from estimated S and A_c, and it is found that the selection of the fitting range for S estimation is not universal to obtain E_N ~ tensile/compression modulus (E). Finally, it is concluded that the selection of the fitting range relies on the applied load, choice of contact area estimation method, and fitting equation.