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 mu N/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 similar to 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.