Combining Atomic Force Microscopy and Depth-Sensing Instruments for the Nanometer-Scale Mechanical Characterization of Soft Matter

Complex materials exhibit a hierarchical structure where a gradient of features on nanometer scale is induced by the synthetic route eventually enhanced by the loading condition. The nanometer scale at which individual components arrange, determining their properties, is a current challenge of mechanical testing. In this work, a survey on nanoindentation is outlined based on the comparison of results obtained by Atomic Force Microscopy and Depth-Sensing Instruments and their combination. An Atomic Force Microscope equipped with a Force Transducer gives indeed the possibility to scan the sample surface in contact mode, thereby allowing one to choose a suitable position for the nanoindentation, as well as imaging the residual imprint left on the sample. The analysis of the applied load vs. penetration depth curve, also called force curve, shows the limitations of current approaches to determine elastic moduli of compliant viscoelastic materials. Significant deviations from the expected values are observed even after optimizing testing conditions, so as to minimize the artifacts like viscoelastic effects or pile-up. As rigorous approaches are yet to be applied to the interpretation of force curves accounting also of viscoelastic material behavior, an empirical calibration recently proposed by the authors is verified against a set of data on model samples spanning a range of moduli, typical of compliant materials and close to each other, so as to challenge the resolution potential of this method, as well as others in use in the literature.